{"id":22,"date":"2017-10-26T15:10:37","date_gmt":"2017-10-26T15:10:37","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-dutchess-introbio\/chapter\/themes-and-concepts-of-biology\/"},"modified":"2021-02-23T20:11:18","modified_gmt":"2021-02-23T20:11:18","slug":"themes-and-concepts-of-biology","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-dutchess-introbio2\/chapter\/themes-and-concepts-of-biology\/","title":{"raw":"Themes and Concepts of Biology","rendered":"Themes and Concepts of Biology"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objective<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Identify and describe the properties of life<\/li>\r\n \t<li>Describe the levels of organization among living things<\/li>\r\n \t<li>List examples of different sub disciplines in biology<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-idm71358960\">Biology is the science that studies life. What exactly is life? This may sound like a silly question with an obvious answer, but it is not easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. It turns out that although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life.<\/p>\r\n<p id=\"fs-idm11911600\">From its earliest beginnings, biology has wrestled with four questions: What are the shared properties that make something \"alive\"? How do those various living things function? When faced with the remarkable diversity of life, how do we organize the different kinds of organisms so that we can better understand them? And, finally\u2014what biologists ultimately seek to understand\u2014how did this diversity arise and how is it continuing? As new organisms are discovered every day, biologists continue to seek answers to these and other questions.<\/p>\r\n\r\n<section id=\"fs-idp10605856\">\r\n<h1>Properties of Life<\/h1>\r\n<p id=\"fs-idm15862480\">All groups of living organisms share several key characteristics or functions: order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. When viewed together, these eight characteristics serve to define life.<\/p>\r\n\r\n<section id=\"fs-idm48835200\">\r\n<h2>Order<\/h2>\r\n<p id=\"fs-idm29231504\">Organisms are highly organized structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex. Inside each cell, atoms make up molecules. These in turn make up cell components or organelles. Multicellular organisms, which may consist of millions of individual cells, have an advantage over single-celled organisms in that their cells can be specialized to perform specific functions, and even sacrificed in certain situations for the good of the organism as a whole. How these specialized cells come together to form organs such as the heart, lung, or skin in organisms like the toad shown in <a class=\"autogenerated-content\" href=\"#fig-ch01_01_01\">[Figure 1]<\/a> will be discussed later.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"325\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_01.jpg#fixme#fixme\" alt=\"A photo shows a light-colored toad covered in bright green spots.\" width=\"325\" height=\"354\" \/> Figure 1: A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems. (credit: \"Ivengo(RUS)\"\/Wikimedia Commons)[\/caption]\r\n\r\n<\/section><section id=\"fs-idm29469440\">\r\n<h2>Sensitivity or Response to Stimuli<\/h2>\r\n<p id=\"fs-idm48163904\">Organisms respond to diverse stimuli. For example, plants can bend toward a source of light or respond to touch (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_02\">[Figure 2]<\/a>). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_02\"><figcaption><\/figcaption>&nbsp;\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"439\"]<img class=\"\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_02.jpg#fixme#fixme\" alt=\"A photograph of the Mimosa pudica shows a plant with many tiny leaves.\" width=\"439\" height=\"330\" \/> Figure 2: The leaves of this sensitive plant (Mimosa pudica) will instantly droop and fold when touched. After a few minutes, the plant returns to its normal state. (credit: Alex Lomas)[\/caption]<\/figure>\r\n<div id=\"fs-idm74852384\" class=\"note interactive non-majors\">\r\n<div class=\"textbox shaded\">Watch this <a href=\"http:\/\/openstaxcollege.org\/l\/thigmonasty\" target=\"_window\">video<\/a> to see how the sensitive plant responds to a touch stimulus.<\/div>\r\n<\/div>\r\n<\/section><section id=\"fs-idm127740192\">\r\n<h2>Reproduction<\/h2>\r\n<p id=\"fs-idm127803200\">Single-celled organisms reproduce by first duplicating their DNA, which is the genetic material, and then dividing it equally as the cell prepares to divide to form two new cells. Many multicellular organisms (those made up of more than one cell) produce specialized reproductive cells that will form new individuals. When reproduction occurs, DNA containing genes is passed along to an organism\u2019s offspring. These genes are the reason that the offspring will belong to the same species and will have characteristics similar to the parent, such as fur color and blood type.<\/p>\r\n\r\n<\/section><section id=\"fs-idm59408480\">\r\n<h2>Adaptation<\/h2>\r\n<p id=\"fs-idm83575392\">All living organisms exhibit a \"fit\" to their environment. Biologists refer to this fit as adaptation and it is a consequence of evolution by natural selection, which operates in every lineage of reproducing organisms. Examples of adaptations are diverse and unique, from heat-resistant Archaea that live in boiling hot springs to the tongue length of a nectar-feeding moth that matches the size of the flower from which it feeds. All adaptations enhance the reproductive potential of the individual exhibiting them, including their ability to survive to reproduce. Adaptations are not constant. As an environment changes, different characteristics may become more advantageous and promote a better fit to the environment.<\/p>\r\n\r\n<\/section><section id=\"fs-idm65258208\">\r\n<h2>Growth and Development<\/h2>\r\n<p id=\"fs-idm61791968\">Organisms grow and develop according to specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species\u2019 young (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_03\">Figure 3<\/a>) will grow up to exhibit many of the same characteristics as its parents.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_03\"><figcaption><\/figcaption>&nbsp;\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"255\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_03.jpg#fixme#fixme\" alt=\"A photograph depicts four kittens: one has an orange and white tabby coat, another is entirely black, the third and fourth have a black, white and orange tabby coat but with different patterning.\" width=\"255\" height=\"170\" \/> Figure 3: Although no two look alike, these kittens have inherited genes from both parents and share many of the same characteristics. (credit: Pieter &amp; Ren\u00e9e Lanser)[\/caption]<\/figure>\r\n<\/section><section id=\"fs-idm73379600\">\r\n<h2>Regulation<\/h2>\r\n<p id=\"fs-idm22855200\">Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, such as the transport of nutrients, response to stimuli, and coping with environmental stresses. For example, organ systems such as the digestive or circulatory systems perform specific functions like carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.<\/p>\r\n\r\n<\/section><section id=\"fs-idm44083744\">\r\n<h2>Homeostasis<\/h2>\r\n<figure id=\"fig-ch01_01_04\">\r\n<div id=\"post-1876\" class=\"standard post-1876 chapter type-chapter status-publish hentry\">\r\n<div class=\"entry-content\">\r\n\r\nLiving Things Must Maintain Homeostasis.\u00a0<strong>Homeostasis <\/strong>means <strong>\u201csteady state\u201d.\u00a0<\/strong>Homeostasis is the tendency of an organism or cell to <strong>maintain a constant internal environment<\/strong>. Living things constantly adjust to internal and external changes. Homeostasis means to maintain <strong>dynamic equilibrium<\/strong> in the body<strong>.<\/strong> It is dynamic because it is constantly adjusting to the changes that the body\u2019s systems encounter. It is equilibrium because body functions are kept within specific ranges or normal limits.\r\n\r\nMaintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A\u00a0<strong>set point<\/strong>\u00a0is the physiological value around which the normal range fluctuates. A\u00a0<strong>normal range<\/strong>\u00a0is the restricted set of values that is optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37\u00b0C (98.6\u00b0F) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point.\u00a0<span style=\"font-size: 1rem;text-align: initial\">Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.\u00a0An inability to maintain\u00a0homeostasis\u00a0may lead to\u00a0death\u00a0or a disease, a condition known as\u00a0<\/span><strong style=\"font-size: 1rem;text-align: initial\">homeostatic\u00a0imbalance.<\/strong>\r\n\r\n<span style=\"font-size: 1rem;text-align: initial\">Homeostasis, in a general sense, refers to stability, balance, or equilibrium. Physiologically, it is the body\u2019s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change.<\/span>\r\n\r\n[caption id=\"attachment_4722\" align=\"alignleft\" width=\"960\"]<a href=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3148\/2019\/02\/19024101\/Slide2.png\"><img class=\"wp-image-4722 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3148\/2019\/02\/19024101\/Slide2.png\" alt=\"\" width=\"960\" height=\"720\" \/><\/a> Figure 2: Components of a feedback loop[\/caption]\r\n\r\n<span style=\"font-size: 1rem;text-align: initial\">Adjustment of physiological systems within the body is called <\/span><strong style=\"font-size: 1rem;text-align: initial\">homeostatic regulation<\/strong><span style=\"font-size: 1rem;text-align: initial\">, which involves <\/span><strong style=\"font-size: 1rem;text-align: initial\">three parts or mechanisms:<\/strong>\r\n<ol>\r\n \t<li><strong>Sensory receptor<\/strong><\/li>\r\n \t<li><strong>Control center - Brain<\/strong><\/li>\r\n \t<li><strong>Effector organ\u00a0<\/strong><\/li>\r\n<\/ol>\r\n<\/div>\r\n<div class=\"entry-content\">\r\n<ul>\r\n \t<li>The <strong>sensory<\/strong>\u00a0<strong>receptor<\/strong>\u00a0receives information that something in the environment is changing.\u00a0A change in the internal or external environment is called a <strong>stimulus<\/strong> and is detected by a receptor. sensory receptor monitors a physiological value.\u00a0This value is reported to the control center.<\/li>\r\n \t<li>The\u00a0<strong>control center\u00a0or integration center (Brain)<\/strong> receives and processes information from the receptor.<\/li>\r\n \t<li>The\u00a0<strong>effector<\/strong>\u00a0<strong>organ<\/strong> responds to the commands of the control center by either opposing or enhancing the stimulus. The effector is a <strong>muscle<\/strong> (that contracts or relaxes) or a <strong>gland<\/strong> that secretes.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"entry-content\">\r\n<h2>Maintenance of\u00a0 Homeostasis by Feedback loops<\/h2>\r\nWhen a change occurs in an animal\u2019s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector.\u00a0\u00a0Homeostasis is controlled by the nervous and endocrine system of mammals.\u00a0<strong>Homeostasis is maintained by negative feedback loops.<\/strong> Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Positive feedback mechanisms are as important for life maintenance as negative feedback mechanisms.\r\n\r\n<span style=\"text-decoration: underline\"><strong>Types of homeostatic\u00a0 feedba<\/strong><\/span><strong><span style=\"text-decoration: underline\">ck systems \/ mechanisms<\/span>:<\/strong>\r\n<ul>\r\n \t<li>Negative Feedback Mechanism<\/li>\r\n \t<li>Positive Feedback Mechanism<\/li>\r\n<\/ul>\r\n<table style=\"border-collapse: collapse;width: 100%;height: 62px\" border=\"1\">\r\n<tbody>\r\n<tr style=\"height: 14px\">\r\n<td style=\"width: 50%;height: 14px\">\r\n<h2 class=\"textbox shaded\" style=\"text-align: center\"><strong>Negative Feedback Mechanism maintains<\/strong><\/h2>\r\n<\/td>\r\n<td style=\"width: 50%;height: 14px\">\r\n<h2 class=\"textbox shaded\" style=\"text-align: center\"><strong>Positive Feedback Mechanism promote<\/strong><\/h2>\r\n<\/td>\r\n<\/tr>\r\n<tr style=\"height: 12px\">\r\n<td style=\"width: 50%;height: 12px;text-align: center\">\r\n<div style=\"text-align: center\"><\/div>\r\n<div style=\"text-align: center\"><span style=\"font-family: inherit;font-size: inherit\">Body Temperature<\/span><\/div><\/td>\r\n<td style=\"width: 50%;height: 12px;text-align: center\">Childbirth<\/td>\r\n<\/tr>\r\n<tr style=\"height: 12px\">\r\n<td style=\"width: 50%;height: 12px\">\r\n<div style=\"text-align: center\"><\/div>\r\n<div style=\"text-align: center\">Blood pressure<\/div><\/td>\r\n<td style=\"width: 50%;height: 12px;text-align: center\">Blood Clotting<\/td>\r\n<\/tr>\r\n<tr style=\"height: 12px\">\r\n<td style=\"width: 50%;height: 12px\">\r\n<div style=\"text-align: center\"><\/div>\r\n<div style=\"text-align: center\">Blood Sugar<\/div><\/td>\r\n<td style=\"width: 50%;height: 12px;text-align: center\">Ovulation<\/td>\r\n<\/tr>\r\n<tr style=\"height: 12px\">\r\n<td style=\"width: 50%;height: 12px\">\r\n<div style=\"text-align: center\"><\/div>\r\n<div style=\"text-align: center\">Blood pH<\/div><\/td>\r\n<td style=\"width: 50%;height: 12px;text-align: center\">\u00a0Lactation<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/div>\r\n<div id=\"post-1876\" class=\"standard post-1876 chapter type-chapter status-publish hentry\">\r\n<div class=\"entry-content\">\r\n<div class=\"textbox shaded\" style=\"text-align: center\">Table 1: Examples for Homeostasis<\/div>\r\n<h3>Negative Feedback Mechanisms<\/h3>\r\nAny homeostatic process that changes the direction of the stimulus is a\u00a0<b>negative feedback loop<\/b>. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback.\r\n\r\n<strong>Thermoregulation:<\/strong>\r\n\r\nThermoregulation\u00a0is a process that allows your body to maintain its core internal temperature.\u00a0\u00a0The set point for normal human body temperature is\u00a0approximately 37\u00b0C (98.6\u00b0F). Body temperature affects body activities. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50\u00baC for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions.\r\n\r\nDuring body temperature regulation, <strong>temperature receptors in the skin<\/strong>\u00a0 <strong>(sensory receptor)<\/strong> communicate information to the <strong>brain (the control center)<\/strong> which signals the <strong>\u00a0blood vessels and sweat glands in the skin (effectors).<\/strong> As the internal and external environment of the body are constantly changing, adjustments must be made continuously to stay at or near a specific value: the\u00a0<strong>set point<\/strong>. (approximately 37\u00b0C \/ 98.6\u00b0F)\r\n\r\n[caption id=\"attachment_1882\" align=\"aligncenter\" width=\"601\"]<img class=\"wp-image-1882\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/30234617\/Figure_33_03_04.png\" alt=\"Flow chart shows how normal body temperature is maintained. If the body temperature rises, blood vessels dilate, resulting in loss of heat to the environment. Sweat glands secrete fluid. As this fluid evaporates, heat is lost form the body. As a result, the body temperature falls to normal body temperature. If body temperature falls, blood vessels constrict so that heat is conserved. Sweat glands do not secrete fluid. Shivering (involuntary contraction of muscles) releases heat which warms the body. Heat is retained, and body temperature increases to normal.\" width=\"601\" height=\"418\" \/> Figure 3: Flow chart shows how normal body temperature is maintained.[\/caption]\r\n\r\n<strong>When body temperature exceeds its normal range:<\/strong>\r\n\r\nWhen the brain\u2019s temperature regulation center receives data from the sensors indicating that the body\u2019s temperature exceeds its normal range, the brain (the control center) which signals the effectors: blood vessels and sweat glands in the skin.\u00a0This stimulation has three major effects:\r\n<ul>\r\n \t<li>Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the surface of the skin allowing the heat to radiate into the environment.<\/li>\r\n \t<li>As blood flow to the skin increases, sweat glands are activated to increase their output. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it.<\/li>\r\n \t<li>The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.<\/li>\r\n<\/ul>\r\n<strong>When body temperature reduces below\u00a0 its normal range:<\/strong>\r\n\r\nIn contrast, activation of the brain\u2019s heat-gain center by exposure to cold\r\n<ul>\r\n \t<li>reduces blood flow to the skin, and blood returning from the limbs is diverted into a network of deep veins. This arrangement traps heat closer to the body core and restricts heat loss.<\/li>\r\n \t<li>If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering. The muscle contractions of shivering release heat while using up ATP.<\/li>\r\n \t<li>The brain triggers the thyroid gland in the endocrine system to release thyroid hormone, which increases metabolic activity and heat production in cells throughout the body. The brain also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes the breakdown of glycogen into glucose, which can be used as an energy source. The breakdown of glycogen into glucose also results in increased metabolism and heat production.<\/li>\r\n<\/ul>\r\n[caption id=\"\" align=\"aligncenter\" width=\"537\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/403\/2015\/04\/21031822\/2523_The-Hypothalamus_Controls_Thermoregulation.jpg\" alt=\"\" width=\"537\" height=\"904\" \/> Figure 4: The hypothalamus in Brain controls thermoregulation.[\/caption]\r\n<h3>Positive Feedback Mechanisms<\/h3>\r\n<strong>Positive feedback<\/strong>\u00a0intensifies a change in the body\u2019s physiological condition rather than reversing it, \u00a0maintains the direction of the stimulus, possibly accelerating it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point. Childbirth and the body\u2019s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed.\r\n<h4>Example 1: Labor and Delivery During Childbirth:<\/h4>\r\nThe extreme muscular work of labor and delivery are the result of a positive feedback system (Figure 5). The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains <strong>stretch-sensitive nerve cells\u00a0\u00a0(the sensory receptors)\u00a0<\/strong>that monitor the degree of stretching <strong>.<\/strong> These nerve cells send messages to the brain, which in turn causes the <strong>pituitary gland at the base of the brain ( control center)<\/strong> to release the hormone <strong>oxytocin<\/strong> into the bloodstream. Oxytocin causes stronger contractions of the smooth muscles in of the <strong>uterus (the effectors),<\/strong> pushing the baby further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby comes out of the uterus. At this point, the stretching of the cervix halts, stopping the release of oxytocin.\r\n\r\n<\/div>\r\n<\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"450\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/10\/20082354\/106_Pregnancy-Positive_Feedback.jpg\" alt=\"This diagram shows the steps of a positive feedback loop as a series of stepwise arrows looping around a diagram of an infant within the uterus of a pregnant woman. Initially the head of the baby pushes against the cervix, transmitting nerve impulses from the cervix to the brain. Next the brain stimulates the pituitary gland to secrete oxytocin which is carried in the bloodstream to the uterus. Finally, the oxytocin simulates uterine contractions and pushes the baby harder into the cervix. As the head of the baby pushes against the cervix with greater and greater force, the uterine contractions grow stronger and more frequent. This mechanism is a positive feedback loop.\" width=\"450\" height=\"367\" \/> <strong>Figure 5.\u00a0Positive Feedback Loop.<\/strong>\u00a0Normal childbirth is driven by a positive feedback loop. A positive feedback loop results in a change in the body\u2019s status, rather than a return to homeostasis.[\/caption]\r\n<h4>Example 2: Steps in Blood Clotting<\/h4>\r\nA second example of positive feedback centers on reversing extreme damage to the body. Following a penetrating wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion (penetration of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. <strong>As each step of clotting occurs, it stimulates the release of more clotting substances.<\/strong> This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.\r\n\r\n<span id=\"fs-idm74521424\">\u00a0<\/span><\/figure>\r\n<\/section><section id=\"fs-idm58873920\">\r\n<h2>Energy Processing<\/h2>\r\n<p id=\"fs-idm82045936\">All organisms (such as the California condor shown in (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_05\">Figure 5<\/a>) use a source of energy for their metabolic activities. Some organisms capture energy from the Sun and convert it into chemical energy in food; others use chemical energy from molecules they take in.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_05\"><figcaption><\/figcaption>&nbsp;\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"325\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_05.jpg#fixme#fixme\" alt=\"This photo shows a California condor in flight with a tag on its wing.\" width=\"325\" height=\"406\" \/> Figure 5: A lot of energy is required for a California condor to fly. Chemical energy derived from food is used to power flight. California condors are an endangered species; scientists have strived to place a wing tag on each bird to help them identify and locate each individual bird. (credit: Pacific Southwest Region U.S. Fish and Wildlife)[\/caption]<\/figure>\r\n<\/section><\/section><section id=\"fs-idm72014816\">\r\n<h1>Levels of Organization of Living Things<\/h1>\r\n<p id=\"fs-idm71961520\">Living things are highly organized and structured, following a hierarchy on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by a chemical bond. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by combining smaller units called monomers. An example of a macromolecule is deoxyribonucleic acid (DNA) (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_06\">Figure 6\u00a0<\/a>), which contains the instructions for the functioning of the organism that contains it.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_06\"><figcaption>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"200\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_06.jpg#fixme#fixme\" alt=\"Molecular model depicts a DNA molecule, showing its double helix structure.\" width=\"200\" height=\"691\" \/> Figure 6: A molecule, like this large DNA molecule, is composed of atoms. (credit: \"Brian0918\"\/Wikimedia Commons)[\/caption]\r\n\r\n<\/figcaption><\/figure>\r\n<div id=\"fs-idm8353152\" class=\"note interactive non-majors\">\r\n<div class=\"title\"><\/div>\r\n<div class=\"textbox shaded\">\r\n<div class=\"note interactive non-majors\">\r\n<p id=\"fs-idm17985200\">To see an animation of this DNA molecule, click <a href=\"http:\/\/openstaxcollege.org\/l\/rotating_DNA2\" target=\"_window\">here<\/a>.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<p id=\"fs-idm42027712\">Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells and perform specialized functions. All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled organisms that lack organelles surrounded by a membrane and do not have nuclei surrounded by nuclear membranes; in contrast, the cells of eukaryotes do have membrane-bound organelles and nuclei.<\/p>\r\n<p id=\"fs-idm28612144\">In most multicellular organisms, cells combine to make tissues, which are groups of similar cells carrying out the same function. Organs are collections of tissues grouped together based on a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. For example vertebrate animals have many organ systems, such as the circulatory system that transports blood throughout the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.<\/p>\r\n\r\n<div id=\"fs-idm40888688\" class=\"note art-connection\">\r\n<div class=\"title\">\r\n<div class=\"textbox exercises\">\r\n<h3>Art Connection<\/h3>\r\n<div class=\"note art-connection\">\r\n<div class=\"title\"><\/div>\r\n<figure id=\"fig-ch01_01_07\"><figcaption><\/figcaption>&nbsp;\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"415\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_07.png#fixme#fixme\" alt=\"A flow chart shows the hierarchy of living organisms. From smallest to largest, this hierarchy includes: 1 An atom, with protons, neutrons and electrons. 2 Molecules such as the phospholipid shown, made up of atoms. 3 Organelles, such as Golgi apparatus and nuclei, that exist inside cells. 4 Cells, such as a red blood cell. 5 Tissues, such as human skin tissue. 6 Organs such as the stomach and intestine make up the human digestive system, an example of an organ system. 7 Organisms, populations and communities. In a park, each person is an organism. Together, all the people make up a population. All the plant and animal species in the park comprise a community. 8 Ecosystems: The ecosystem of Central Park in New York includes living organisms and the environment in which they live. 9 The biosphere: encompasses all the ecosystems on Earth.\" width=\"415\" height=\"1200\" \/> Figure 7: From an atom to the entire Earth, biology examines all aspects of life. (credit \"molecule\": modification of work by Jane Whitney; credit \"organelles\": modification of work by Louisa Howard; credit \"cells\": modification of work by Bruce Wetzel, Harry Schaefer, National Cancer Institute; credit \"tissue\": modification of work by \"Kilbad\"\/Wikimedia Commons; credit \"organs\": modification of work by Mariana Ruiz Villareal, Joaquim Alves Gaspar; credit \"organisms\": modification of work by Peter Dutton; credit \"ecosystem\": modification of work by \"gigi4791\"\/Flickr; credit \"biosphere\": modification of work by NASA)[\/caption]<\/figure>\r\n<p id=\"fs-idm44001808\">Which of the following statements is false?<\/p>\r\n\r\n<ol id=\"eip-663\">\r\n \t<li>Tissues exist within organs which exist within organ systems.<\/li>\r\n \t<li>Communities exist within populations which exist within ecosystems.<\/li>\r\n \t<li>Organelles exist within cells which exist within tissues.<\/li>\r\n \t<li>Communities exist within ecosystems which exist in the biosphere.\r\n[reveal-answer q=\"606752\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"606752\"]2[\/hidden-answer]<\/li>\r\n<\/ol>\r\n<\/div>\r\n<p id=\"fs-idm127718976\"><\/p>\r\n\r\n<\/div>\r\nAll the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many white pine trees. All of these pine trees represent the population of white pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the set of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest\u2019s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, or non-living, parts of that environment such as nitrogen in the soil or rainwater. At the highest level of organization (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_07\">Figure 7<\/a>), the biosphere is the collection of all ecosystems, and it represents the zones of life on Earth. It includes land, water, and portions of the atmosphere.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/section><section id=\"fs-idm10829232\">\r\n<h1>The Diversity of Life<\/h1>\r\n<p id=\"fs-idm41986800\">The science of biology is very broad in scope because there is a tremendous diversity of life on Earth. The source of this diversity is evolution, the process of gradual change during which new species arise from older species. Evolutionary biologists study the evolution of living things in everything from the microscopic world to ecosystems.<\/p>\r\n<p id=\"fs-idm25429264\">In the 18th century, a scientist named Carl Linnaeus first proposed organizing the known species of organisms into a hierarchical taxonomy. In this system, species that are most similar to each other are put together within a grouping known as a genus. Furthermore, similar genera (the plural of genus) are put together within a family. This grouping continues until all organisms are collected together into groups at the highest level. The current taxonomic system now has eight levels in its hierarchy, from lowest to highest, they are: species, genus, family, order, class, phylum, kingdom, domain. Thus species are grouped within genera, genera are grouped within families, families are grouped within orders, and so on (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_08\">[Figure 8]<\/a>).<\/p>\r\n\r\n<figure id=\"fig-ch01_01_08\"><figcaption><\/figcaption>\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"566\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_08.jpg#fixme#fixme\" alt=\"A chart shows the eight levels of taxonomic hierarchy for the dog, Canis lupus.\" width=\"566\" height=\"272\" \/> Figure 8: This diagram shows the levels of taxonomic hierarchy for a dog, from the broadest category\u2014domain\u2014to the most specific\u2014species.[\/caption]\r\n\r\n<span id=\"fs-idm125583568\">\u00a0<\/span><\/figure>\r\n<p id=\"fs-idm72098240\">The highest level, domain, is a relatively new addition to the system since the 1990s. Scientists now recognize three domains of life, the Eukarya, the Archaea, and the Bacteria. The domain Eukarya contains organisms that have cells with nuclei. It includes the kingdoms of fungi, plants, animals, and several kingdoms of protists. The Archaea, are single-celled organisms without nuclei and include many extremophiles that live in harsh environments like hot springs. The Bacteria are another quite different group of single-celled organisms without nuclei (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_09\">[Figure 9]<\/a>). Both the Archaea and the Bacteria are prokaryotes, an informal name for cells without nuclei. The recognition in the 1990s that certain \"bacteria,\" now known as the Archaea, were as different genetically and biochemically from other bacterial cells as they were from eukaryotes, motivated the recommendation to divide life into three domains. This dramatic change in our knowledge of the tree of life demonstrates that classifications are not permanent and will change when new information becomes available.<\/p>\r\n<p id=\"fs-idm76440992\">In addition to the hierarchical taxonomic system, Linnaeus was the first to name organisms using two unique names, now called the binomial naming system. Before Linnaeus, the use of common names to refer to organisms caused confusion because there were regional differences in these common names. Binomial names consist of the genus name (which is capitalized) and the species name (all lower-case). Both names are set in italics when they are printed. Every species is given a unique binomial which is recognized the world over, so that a scientist in any location can know which organism is being referred to. For example, the North American blue jay is known uniquely as <em>Cyanocitta cristata<\/em>. Our own species is <em>Homo sapiens<\/em>.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_09\"><figcaption><\/figcaption>\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"816\"]<img class=\"\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_09.jpg#fixme#fixme\" alt=\"Photos depict: A: bacterial cells. B: a natural hot vent. C: a sunflower. D: a lion.\" width=\"816\" height=\"204\" \/> These images represent different domains. The scanning electron micrograph shows (a) bacterial cells belong to the domain Bacteria, while the (b) extremophiles, seen all together as colored mats in this hot spring, belong to domain Archaea. Both the (c) sunflower and (d) lion are part of domain Eukarya. (credit a: modification of work by Rocky Mountain Laboratories, NIAID, NIH; credit b: modification of work by Steve Jurvetson; credit c: modification of work by Michael Arrighi; credit d: modification of work by Frank Vassen)[\/caption]\r\n\r\n<span id=\"fs-idm9076800\">\u00a0<\/span><\/figure>\r\n<div id=\"fs-idm108538560\" class=\"note evolution non-majors\">\r\n<div class=\"title\">\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Takeaways<\/h3>\r\n<section>\r\n<div class=\"note evolution non-majors\">\r\n<div class=\"title\">Evolution in Action<\/div>\r\n<p id=\"fs-idm10904720\">Carl Woese and the Phylogenetic Tree - the evolutionary relationships of various life forms on Earth can be summarized in a phylogenetic tree. A phylogenetic tree is a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both. A phylogenetic tree is composed of branch points, or nodes, and branches. The internal nodes represent ancestors and are points in evolution when, based on scientific evidence, an ancestor is thought to have diverged to form two new species. The length of each branch can be considered as estimates of relative time.<\/p>\r\n<p id=\"fs-idm71571824\">In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and bacteria. The pioneering work of American microbiologist Carl Woese in the early 1970s has shown, however, that life on Earth has evolved along three lineages, now called domains\u2014Bacteria, Archaea, and Eukarya. Woese proposed the domain as a new taxonomic level and Archaea as a new domain, to reflect the new phylogenetic tree (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_10\">[Figure 10]<\/a>). Many organisms belonging to the Archaea domain live under extreme conditions and are called extremophiles. To construct his tree, Woese used genetic relationships rather than similarities based on morphology (shape). Various genes were used in phylogenetic studies. Woese\u2019s tree was constructed from comparative sequencing of the genes that are universally distributed, found in some slightly altered form in every organism, conserved (meaning that these genes have remained only slightly changed throughout evolution), and of an appropriate length.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_10\"><figcaption><\/figcaption>\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"500\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_10.jpg#fixme#fixme\" alt=\"This phylogenetic tree shows that the three domains of life, bacteria, archaea and eukarya, all arose from a common ancestor.\" width=\"500\" height=\"540\" \/> Figure 10: This phylogenetic tree was constructed by microbiologist Carl Woese using genetic relationships. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are organisms without a nucleus or other organelles surrounded by a membrane and, therefore, are prokaryotes. (credit: modification of work by Eric Gaba)[\/caption]<\/figure>\r\n<\/div>\r\n<\/section><section id=\"fs-idm93700768\">\r\n<h1><\/h1>\r\n<\/section><\/div>\r\n<\/div>\r\n<div><\/div>\r\n<h3 class=\"title\">Branches of Biological Study<\/h3>\r\n<\/div>\r\n<\/section><section>\r\n<p id=\"fs-idm128104608\">The scope of biology is broad and therefore contains many branches and sub disciplines. Biologists may pursue one of those sub disciplines and work in a more focused field. For instance, molecular biology studies biological processes at the molecular level, including interactions among molecules such as DNA, RNA, and proteins, as well as the way they are regulated. Microbiology is the study of the structure and function of microorganisms. It is quite a broad branch itself, and depending on the subject of study, there are also microbial physiologists, ecologists, and geneticists, among others.<\/p>\r\n<p id=\"fs-idm75927680\">Another field of biological study, neurobiology, studies the biology of the nervous system, and although it is considered a branch of biology, it is also recognized as an interdisciplinary field of study known as neuroscience. Because of its interdisciplinary nature, this sub discipline studies different functions of the nervous system using molecular, cellular, developmental, medical, and computational approaches.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_11\"><figcaption><\/figcaption>\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"350\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_11.jpg#fixme#fixme\" alt=\"Photo depicts scientists digging fossils out of the dirt.\" width=\"350\" height=\"418\" \/> Figure 11: Researchers work on excavating dinosaur fossils at a site in Castell\u00f3n, Spain. (credit: Mario Modesto)[\/caption]\r\n\r\n<span id=\"fs-idm107121744\">\u00a0<\/span><\/figure>\r\n<p id=\"fs-idm77010944\">Paleontology, another branch of biology, uses fossils to study life\u2019s history (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_11\">[Figure 11]<\/a>). Zoology and botany are the study of animals and plants, respectively. Biologists can also specialize as biotechnologists, ecologists, or physiologists, to name just a few areas. Biotechnologists apply the knowledge of biology to create useful products. Ecologists study the interactions of organisms in their environments. Physiologists study the workings of cells, tissues and organs. This is just a small sample of the many fields that biologists can pursue. From our own bodies to the world we live in, discoveries in biology can affect us in very direct and important ways. We depend on these discoveries for our health, our food sources, and the benefits provided by our ecosystem. Because of this, knowledge of biology can benefit us in making decisions in our day-to-day lives.<\/p>\r\n<p id=\"fs-idm78647248\">The development of technology in the twentieth century that continues today, particularly the technology to describe and manipulate the genetic material, DNA, has transformed biology. This transformation will allow biologists to continue to understand the history of life in greater detail, how the human body works, our human origins, and how humans can survive as a species on this planet despite the stresses caused by our increasing numbers. Biologists continue to decipher huge mysteries about life suggesting that we have only begun to understand life on the planet, its history, and our relationship to it. For this and other reasons, the knowledge of biology gained through this textbook and other printed and electronic media should be a benefit in whichever field you enter.<\/p>\r\n\r\n<div id=\"fs-idm74894912\" class=\"note career non-majors\"><section>\r\n<h2>Forensic Science<\/h2>\r\n<p id=\"fs-idm80622272\">Forensic science is the application of science to answer questions related to the law. Biologists as well as chemists and biochemists can be forensic scientists. Forensic scientists provide scientific evidence for use in courts, and their job involves examining trace material associated with crimes. Interest in forensic science has increased in the last few years, possibly because of popular television shows that feature forensic scientists on the job. Also, the development of molecular techniques and the establishment of DNA databases have updated the types of work that forensic scientists can do. Their job activities are primarily related to crimes against people such as murder, rape, and assault. Their work involves analyzing samples such as hair, blood, and other body fluids and also processing DNA (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_12\">Figure 12]<\/a>) found in many different environments and materials. Forensic scientists also analyze other biological evidence left at crime scenes, such as insect parts or pollen grains. Students who want to pursue careers in forensic science will most likely be required to take chemistry and biology courses as well as some intensive math courses.<\/p>\r\n\r\n<figure id=\"fig-ch01_01_12\"><figcaption><\/figcaption>\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"450\"]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_12.jpg#fixme#fixme\" alt=\"Photo depicts a scientist working in a lab.\" width=\"450\" height=\"379\" \/> Figure 12: This forensic scientist works in a DNA extraction room at the U.S. Army Criminal Investigation Laboratory. (credit: U.S. Army CID Command Public Affairs)[\/caption]\r\n\r\n<span id=\"fs-idm48501008\">\u00a0<\/span><\/figure>\r\n<\/section><\/div>\r\n<\/section><section id=\"fs-idm107479568\" class=\"summary\">\r\n<h1>Section Summary<\/h1>\r\n<p id=\"fs-idm55696240\">Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. Living things are highly organized following a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere. Evolution is the source of the tremendous biological diversity on Earth today. A diagram called a phylogenetic tree can be used to show evolutionary relationships among organisms. Biology is very broad and includes many branches and sub disciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others.<\/p>\r\n&nbsp;\r\n\r\n<\/section>\r\n<div>\r\n<h2>Glossary<\/h2>\r\n<dl id=\"fs-idm17545584\" class=\"definition\">\r\n \t<dt><strong>atom<\/strong><\/dt>\r\n \t<dd id=\"fs-idm41446080\">a basic unit of matter that cannot be broken down by normal chemical reactions<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm19475200\" class=\"definition\">\r\n \t<dt><strong>biology<\/strong><\/dt>\r\n \t<dd id=\"fs-idm60761200\">the study of living organisms and their interactions with one another and their environments<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm19153088\" class=\"definition\">\r\n \t<dt><strong>biosphere<\/strong><\/dt>\r\n \t<dd id=\"fs-idm18996064\">a collection of all ecosystems on Earth<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm61529952\" class=\"definition\">\r\n \t<dt><strong>cell<\/strong><\/dt>\r\n \t<dd id=\"fs-idm28458496\">the smallest fundamental unit of structure and function in living things<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm127697392\" class=\"definition\">\r\n \t<dt><strong>community<\/strong><\/dt>\r\n \t<dd id=\"fs-idm60799520\">a set of populations inhabiting a particular area<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm74280784\" class=\"definition\">\r\n \t<dt><strong>ecosystem<\/strong><\/dt>\r\n \t<dd id=\"fs-idm62480352\">all living things in a particular area together with the abiotic, nonliving parts of that environment<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm80696624\" class=\"definition\">\r\n \t<dt><strong>eukaryote<\/strong><\/dt>\r\n \t<dd id=\"fs-idm59473040\">an organism with cells that have nuclei and membrane-bound organelles<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm58443328\" class=\"definition\">\r\n \t<dt><strong>evolution<\/strong><\/dt>\r\n \t<dd id=\"fs-idm54214224\">the process of gradual change in a population that can also lead to new species arising from older species<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm41117184\" class=\"definition\">\r\n \t<dt><strong>homeostasis<\/strong><\/dt>\r\n \t<dd id=\"fs-idm76254000\">the ability of an organism to maintain constant internal conditions<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm518832\" class=\"definition\">\r\n \t<dt><strong>macromolecule<\/strong><\/dt>\r\n \t<dd id=\"fs-idm74473904\">a large molecule typically formed by the joining of smaller molecules<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm59477568\" class=\"definition\">\r\n \t<dt><strong>molecule<\/strong><\/dt>\r\n \t<dd id=\"fs-idm22070880\">a chemical structure consisting of at least two atoms held together by a chemical bond<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm47135696\" class=\"definition\">\r\n \t<dt><strong>organ<\/strong><\/dt>\r\n \t<dd id=\"fs-idm53274960\">a structure formed of tissues operating together to perform a common function<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm73130992\" class=\"definition\">\r\n \t<dt><strong>organ system<\/strong><\/dt>\r\n \t<dd id=\"fs-idm9181952\">the higher level of organization that consists of functionally related organs<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm7168368\" class=\"definition\">\r\n \t<dt><strong>organelle<\/strong><\/dt>\r\n \t<dd id=\"fs-idm59599344\">a membrane-bound compartment or sac within a cell<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm17991488\" class=\"definition\">\r\n \t<dt><strong>organism<\/strong><\/dt>\r\n \t<dd id=\"fs-idm79676656\">an individual living entity<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm485824\" class=\"definition\">\r\n \t<dt><strong>phylogenetic tree<\/strong><\/dt>\r\n \t<dd id=\"fs-idm107117136\">a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm74536096\" class=\"definition\">\r\n \t<dt><strong>population<\/strong><\/dt>\r\n \t<dd id=\"fs-idm11206032\">all individuals within a species living within a specific area<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm71391472\" class=\"definition\">\r\n \t<dt><strong>prokaryote<\/strong><\/dt>\r\n \t<dd id=\"fs-idm42022320\">a unicellular organism that lacks a nucleus or any other membrane-bound organelle<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idm60819312\" class=\"definition\">\r\n \t<dt><strong>tissue<\/strong><\/dt>\r\n \t<dd id=\"fs-idm71924960\">a group of similar cells carrying out the same function<\/dd>\r\n<\/dl>\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objective<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Identify and describe the properties of life<\/li>\n<li>Describe the levels of organization among living things<\/li>\n<li>List examples of different sub disciplines in biology<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-idm71358960\">Biology is the science that studies life. What exactly is life? This may sound like a silly question with an obvious answer, but it is not easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. It turns out that although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life.<\/p>\n<p id=\"fs-idm11911600\">From its earliest beginnings, biology has wrestled with four questions: What are the shared properties that make something &#8220;alive&#8221;? How do those various living things function? When faced with the remarkable diversity of life, how do we organize the different kinds of organisms so that we can better understand them? And, finally\u2014what biologists ultimately seek to understand\u2014how did this diversity arise and how is it continuing? As new organisms are discovered every day, biologists continue to seek answers to these and other questions.<\/p>\n<section id=\"fs-idp10605856\">\n<h1>Properties of Life<\/h1>\n<p id=\"fs-idm15862480\">All groups of living organisms share several key characteristics or functions: order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. When viewed together, these eight characteristics serve to define life.<\/p>\n<section id=\"fs-idm48835200\">\n<h2>Order<\/h2>\n<p id=\"fs-idm29231504\">Organisms are highly organized structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex. Inside each cell, atoms make up molecules. These in turn make up cell components or organelles. Multicellular organisms, which may consist of millions of individual cells, have an advantage over single-celled organisms in that their cells can be specialized to perform specific functions, and even sacrificed in certain situations for the good of the organism as a whole. How these specialized cells come together to form organs such as the heart, lung, or skin in organisms like the toad shown in <a class=\"autogenerated-content\" href=\"#fig-ch01_01_01\">[Figure 1]<\/a> will be discussed later.<\/p>\n<div style=\"width: 335px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_01.jpg#fixme#fixme\" alt=\"A photo shows a light-colored toad covered in bright green spots.\" width=\"325\" height=\"354\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 1: A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems. (credit: &#8220;Ivengo(RUS)&#8221;\/Wikimedia Commons)<\/p>\n<\/div>\n<\/section>\n<section id=\"fs-idm29469440\">\n<h2>Sensitivity or Response to Stimuli<\/h2>\n<p id=\"fs-idm48163904\">Organisms respond to diverse stimuli. For example, plants can bend toward a source of light or respond to touch (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_02\">[Figure 2]<\/a>). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.<\/p>\n<figure id=\"fig-ch01_01_02\"><figcaption><\/figcaption>&nbsp;<\/p>\n<div style=\"width: 449px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_02.jpg#fixme#fixme\" alt=\"A photograph of the Mimosa pudica shows a plant with many tiny leaves.\" width=\"439\" height=\"330\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2: The leaves of this sensitive plant (Mimosa pudica) will instantly droop and fold when touched. After a few minutes, the plant returns to its normal state. (credit: Alex Lomas)<\/p>\n<\/div>\n<\/figure>\n<div id=\"fs-idm74852384\" class=\"note interactive non-majors\">\n<div class=\"textbox shaded\">Watch this <a href=\"http:\/\/openstaxcollege.org\/l\/thigmonasty\" target=\"_window\">video<\/a> to see how the sensitive plant responds to a touch stimulus.<\/div>\n<\/div>\n<\/section>\n<section id=\"fs-idm127740192\">\n<h2>Reproduction<\/h2>\n<p id=\"fs-idm127803200\">Single-celled organisms reproduce by first duplicating their DNA, which is the genetic material, and then dividing it equally as the cell prepares to divide to form two new cells. Many multicellular organisms (those made up of more than one cell) produce specialized reproductive cells that will form new individuals. When reproduction occurs, DNA containing genes is passed along to an organism\u2019s offspring. These genes are the reason that the offspring will belong to the same species and will have characteristics similar to the parent, such as fur color and blood type.<\/p>\n<\/section>\n<section id=\"fs-idm59408480\">\n<h2>Adaptation<\/h2>\n<p id=\"fs-idm83575392\">All living organisms exhibit a &#8220;fit&#8221; to their environment. Biologists refer to this fit as adaptation and it is a consequence of evolution by natural selection, which operates in every lineage of reproducing organisms. Examples of adaptations are diverse and unique, from heat-resistant Archaea that live in boiling hot springs to the tongue length of a nectar-feeding moth that matches the size of the flower from which it feeds. All adaptations enhance the reproductive potential of the individual exhibiting them, including their ability to survive to reproduce. Adaptations are not constant. As an environment changes, different characteristics may become more advantageous and promote a better fit to the environment.<\/p>\n<\/section>\n<section id=\"fs-idm65258208\">\n<h2>Growth and Development<\/h2>\n<p id=\"fs-idm61791968\">Organisms grow and develop according to specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species\u2019 young (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_03\">Figure 3<\/a>) will grow up to exhibit many of the same characteristics as its parents.<\/p>\n<figure id=\"fig-ch01_01_03\"><figcaption><\/figcaption>&nbsp;<\/p>\n<div style=\"width: 265px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_03.jpg#fixme#fixme\" alt=\"A photograph depicts four kittens: one has an orange and white tabby coat, another is entirely black, the third and fourth have a black, white and orange tabby coat but with different patterning.\" width=\"255\" height=\"170\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 3: Although no two look alike, these kittens have inherited genes from both parents and share many of the same characteristics. (credit: Pieter &amp; Ren\u00e9e Lanser)<\/p>\n<\/div>\n<\/figure>\n<\/section>\n<section id=\"fs-idm73379600\">\n<h2>Regulation<\/h2>\n<p id=\"fs-idm22855200\">Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, such as the transport of nutrients, response to stimuli, and coping with environmental stresses. For example, organ systems such as the digestive or circulatory systems perform specific functions like carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.<\/p>\n<\/section>\n<section id=\"fs-idm44083744\">\n<h2>Homeostasis<\/h2>\n<figure id=\"fig-ch01_01_04\">\n<div id=\"post-1876\" class=\"standard post-1876 chapter type-chapter status-publish hentry\">\n<div class=\"entry-content\">\n<p>Living Things Must Maintain Homeostasis.\u00a0<strong>Homeostasis <\/strong>means <strong>\u201csteady state\u201d.\u00a0<\/strong>Homeostasis is the tendency of an organism or cell to <strong>maintain a constant internal environment<\/strong>. Living things constantly adjust to internal and external changes. Homeostasis means to maintain <strong>dynamic equilibrium<\/strong> in the body<strong>.<\/strong> It is dynamic because it is constantly adjusting to the changes that the body\u2019s systems encounter. It is equilibrium because body functions are kept within specific ranges or normal limits.<\/p>\n<p>Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A\u00a0<strong>set point<\/strong>\u00a0is the physiological value around which the normal range fluctuates. A\u00a0<strong>normal range<\/strong>\u00a0is the restricted set of values that is optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37\u00b0C (98.6\u00b0F) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point.\u00a0<span style=\"font-size: 1rem;text-align: initial\">Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.\u00a0An inability to maintain\u00a0homeostasis\u00a0may lead to\u00a0death\u00a0or a disease, a condition known as\u00a0<\/span><strong style=\"font-size: 1rem;text-align: initial\">homeostatic\u00a0imbalance.<\/strong><\/p>\n<p><span style=\"font-size: 1rem;text-align: initial\">Homeostasis, in a general sense, refers to stability, balance, or equilibrium. Physiologically, it is the body\u2019s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change.<\/span><\/p>\n<div id=\"attachment_4722\" style=\"width: 970px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3148\/2019\/02\/19024101\/Slide2.png\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-4722\" class=\"wp-image-4722 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3148\/2019\/02\/19024101\/Slide2.png\" alt=\"\" width=\"960\" height=\"720\" \/><\/a><\/p>\n<p id=\"caption-attachment-4722\" class=\"wp-caption-text\">Figure 2: Components of a feedback loop<\/p>\n<\/div>\n<p><span style=\"font-size: 1rem;text-align: initial\">Adjustment of physiological systems within the body is called <\/span><strong style=\"font-size: 1rem;text-align: initial\">homeostatic regulation<\/strong><span style=\"font-size: 1rem;text-align: initial\">, which involves <\/span><strong style=\"font-size: 1rem;text-align: initial\">three parts or mechanisms:<\/strong><\/p>\n<ol>\n<li><strong>Sensory receptor<\/strong><\/li>\n<li><strong>Control center &#8211; Brain<\/strong><\/li>\n<li><strong>Effector organ\u00a0<\/strong><\/li>\n<\/ol>\n<\/div>\n<div class=\"entry-content\">\n<ul>\n<li>The <strong>sensory<\/strong>\u00a0<strong>receptor<\/strong>\u00a0receives information that something in the environment is changing.\u00a0A change in the internal or external environment is called a <strong>stimulus<\/strong> and is detected by a receptor. sensory receptor monitors a physiological value.\u00a0This value is reported to the control center.<\/li>\n<li>The\u00a0<strong>control center\u00a0or integration center (Brain)<\/strong> receives and processes information from the receptor.<\/li>\n<li>The\u00a0<strong>effector<\/strong>\u00a0<strong>organ<\/strong> responds to the commands of the control center by either opposing or enhancing the stimulus. The effector is a <strong>muscle<\/strong> (that contracts or relaxes) or a <strong>gland<\/strong> that secretes.<\/li>\n<\/ul>\n<\/div>\n<div class=\"entry-content\">\n<h2>Maintenance of\u00a0 Homeostasis by Feedback loops<\/h2>\n<p>When a change occurs in an animal\u2019s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector.\u00a0\u00a0Homeostasis is controlled by the nervous and endocrine system of mammals.\u00a0<strong>Homeostasis is maintained by negative feedback loops.<\/strong> Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Positive feedback mechanisms are as important for life maintenance as negative feedback mechanisms.<\/p>\n<p><span style=\"text-decoration: underline\"><strong>Types of homeostatic\u00a0 feedba<\/strong><\/span><strong><span style=\"text-decoration: underline\">ck systems \/ mechanisms<\/span>:<\/strong><\/p>\n<ul>\n<li>Negative Feedback Mechanism<\/li>\n<li>Positive Feedback Mechanism<\/li>\n<\/ul>\n<table style=\"border-collapse: collapse;width: 100%;height: 62px\">\n<tbody>\n<tr style=\"height: 14px\">\n<td style=\"width: 50%;height: 14px\">\n<h2 class=\"textbox shaded\" style=\"text-align: center\"><strong>Negative Feedback Mechanism maintains<\/strong><\/h2>\n<\/td>\n<td style=\"width: 50%;height: 14px\">\n<h2 class=\"textbox shaded\" style=\"text-align: center\"><strong>Positive Feedback Mechanism promote<\/strong><\/h2>\n<\/td>\n<\/tr>\n<tr style=\"height: 12px\">\n<td style=\"width: 50%;height: 12px;text-align: center\">\n<div style=\"text-align: center\"><\/div>\n<div style=\"text-align: center\"><span style=\"font-family: inherit;font-size: inherit\">Body Temperature<\/span><\/div>\n<\/td>\n<td style=\"width: 50%;height: 12px;text-align: center\">Childbirth<\/td>\n<\/tr>\n<tr style=\"height: 12px\">\n<td style=\"width: 50%;height: 12px\">\n<div style=\"text-align: center\"><\/div>\n<div style=\"text-align: center\">Blood pressure<\/div>\n<\/td>\n<td style=\"width: 50%;height: 12px;text-align: center\">Blood Clotting<\/td>\n<\/tr>\n<tr style=\"height: 12px\">\n<td style=\"width: 50%;height: 12px\">\n<div style=\"text-align: center\"><\/div>\n<div style=\"text-align: center\">Blood Sugar<\/div>\n<\/td>\n<td style=\"width: 50%;height: 12px;text-align: center\">Ovulation<\/td>\n<\/tr>\n<tr style=\"height: 12px\">\n<td style=\"width: 50%;height: 12px\">\n<div style=\"text-align: center\"><\/div>\n<div style=\"text-align: center\">Blood pH<\/div>\n<\/td>\n<td style=\"width: 50%;height: 12px;text-align: center\">\u00a0Lactation<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n<div id=\"post-1876\" class=\"standard post-1876 chapter type-chapter status-publish hentry\">\n<div class=\"entry-content\">\n<div class=\"textbox shaded\" style=\"text-align: center\">Table 1: Examples for Homeostasis<\/div>\n<h3>Negative Feedback Mechanisms<\/h3>\n<p>Any homeostatic process that changes the direction of the stimulus is a\u00a0<b>negative feedback loop<\/b>. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback.<\/p>\n<p><strong>Thermoregulation:<\/strong><\/p>\n<p>Thermoregulation\u00a0is a process that allows your body to maintain its core internal temperature.\u00a0\u00a0The set point for normal human body temperature is\u00a0approximately 37\u00b0C (98.6\u00b0F). Body temperature affects body activities. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50\u00baC for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions.<\/p>\n<p>During body temperature regulation, <strong>temperature receptors in the skin<\/strong>\u00a0 <strong>(sensory receptor)<\/strong> communicate information to the <strong>brain (the control center)<\/strong> which signals the <strong>\u00a0blood vessels and sweat glands in the skin (effectors).<\/strong> As the internal and external environment of the body are constantly changing, adjustments must be made continuously to stay at or near a specific value: the\u00a0<strong>set point<\/strong>. (approximately 37\u00b0C \/ 98.6\u00b0F)<\/p>\n<div id=\"attachment_1882\" style=\"width: 611px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1882\" class=\"wp-image-1882\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/30234617\/Figure_33_03_04.png\" alt=\"Flow chart shows how normal body temperature is maintained. If the body temperature rises, blood vessels dilate, resulting in loss of heat to the environment. Sweat glands secrete fluid. As this fluid evaporates, heat is lost form the body. As a result, the body temperature falls to normal body temperature. If body temperature falls, blood vessels constrict so that heat is conserved. Sweat glands do not secrete fluid. Shivering (involuntary contraction of muscles) releases heat which warms the body. Heat is retained, and body temperature increases to normal.\" width=\"601\" height=\"418\" \/><\/p>\n<p id=\"caption-attachment-1882\" class=\"wp-caption-text\">Figure 3: Flow chart shows how normal body temperature is maintained.<\/p>\n<\/div>\n<p><strong>When body temperature exceeds its normal range:<\/strong><\/p>\n<p>When the brain\u2019s temperature regulation center receives data from the sensors indicating that the body\u2019s temperature exceeds its normal range, the brain (the control center) which signals the effectors: blood vessels and sweat glands in the skin.\u00a0This stimulation has three major effects:<\/p>\n<ul>\n<li>Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the surface of the skin allowing the heat to radiate into the environment.<\/li>\n<li>As blood flow to the skin increases, sweat glands are activated to increase their output. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it.<\/li>\n<li>The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.<\/li>\n<\/ul>\n<p><strong>When body temperature reduces below\u00a0 its normal range:<\/strong><\/p>\n<p>In contrast, activation of the brain\u2019s heat-gain center by exposure to cold<\/p>\n<ul>\n<li>reduces blood flow to the skin, and blood returning from the limbs is diverted into a network of deep veins. This arrangement traps heat closer to the body core and restricts heat loss.<\/li>\n<li>If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering. The muscle contractions of shivering release heat while using up ATP.<\/li>\n<li>The brain triggers the thyroid gland in the endocrine system to release thyroid hormone, which increases metabolic activity and heat production in cells throughout the body. The brain also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes the breakdown of glycogen into glucose, which can be used as an energy source. The breakdown of glycogen into glucose also results in increased metabolism and heat production.<\/li>\n<\/ul>\n<div style=\"width: 547px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/403\/2015\/04\/21031822\/2523_The-Hypothalamus_Controls_Thermoregulation.jpg\" alt=\"\" width=\"537\" height=\"904\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 4: The hypothalamus in Brain controls thermoregulation.<\/p>\n<\/div>\n<h3>Positive Feedback Mechanisms<\/h3>\n<p><strong>Positive feedback<\/strong>\u00a0intensifies a change in the body\u2019s physiological condition rather than reversing it, \u00a0maintains the direction of the stimulus, possibly accelerating it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point. Childbirth and the body\u2019s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed.<\/p>\n<h4>Example 1: Labor and Delivery During Childbirth:<\/h4>\n<p>The extreme muscular work of labor and delivery are the result of a positive feedback system (Figure 5). The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains <strong>stretch-sensitive nerve cells\u00a0\u00a0(the sensory receptors)\u00a0<\/strong>that monitor the degree of stretching <strong>.<\/strong> These nerve cells send messages to the brain, which in turn causes the <strong>pituitary gland at the base of the brain ( control center)<\/strong> to release the hormone <strong>oxytocin<\/strong> into the bloodstream. Oxytocin causes stronger contractions of the smooth muscles in of the <strong>uterus (the effectors),<\/strong> pushing the baby further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby comes out of the uterus. At this point, the stretching of the cervix halts, stopping the release of oxytocin.<\/p>\n<\/div>\n<\/div>\n<div style=\"width: 460px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/10\/20082354\/106_Pregnancy-Positive_Feedback.jpg\" alt=\"This diagram shows the steps of a positive feedback loop as a series of stepwise arrows looping around a diagram of an infant within the uterus of a pregnant woman. Initially the head of the baby pushes against the cervix, transmitting nerve impulses from the cervix to the brain. Next the brain stimulates the pituitary gland to secrete oxytocin which is carried in the bloodstream to the uterus. Finally, the oxytocin simulates uterine contractions and pushes the baby harder into the cervix. As the head of the baby pushes against the cervix with greater and greater force, the uterine contractions grow stronger and more frequent. This mechanism is a positive feedback loop.\" width=\"450\" height=\"367\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 5.\u00a0Positive Feedback Loop.<\/strong>\u00a0Normal childbirth is driven by a positive feedback loop. A positive feedback loop results in a change in the body\u2019s status, rather than a return to homeostasis.<\/p>\n<\/div>\n<h4>Example 2: Steps in Blood Clotting<\/h4>\n<p>A second example of positive feedback centers on reversing extreme damage to the body. Following a penetrating wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion (penetration of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. <strong>As each step of clotting occurs, it stimulates the release of more clotting substances.<\/strong> This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.<\/p>\n<p><span id=\"fs-idm74521424\">\u00a0<\/span><\/figure>\n<\/section>\n<section id=\"fs-idm58873920\">\n<h2>Energy Processing<\/h2>\n<p id=\"fs-idm82045936\">All organisms (such as the California condor shown in (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_05\">Figure 5<\/a>) use a source of energy for their metabolic activities. Some organisms capture energy from the Sun and convert it into chemical energy in food; others use chemical energy from molecules they take in.<\/p>\n<figure id=\"fig-ch01_01_05\"><figcaption><\/figcaption>&nbsp;<\/p>\n<div style=\"width: 335px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_05.jpg#fixme#fixme\" alt=\"This photo shows a California condor in flight with a tag on its wing.\" width=\"325\" height=\"406\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 5: A lot of energy is required for a California condor to fly. Chemical energy derived from food is used to power flight. California condors are an endangered species; scientists have strived to place a wing tag on each bird to help them identify and locate each individual bird. (credit: Pacific Southwest Region U.S. Fish and Wildlife)<\/p>\n<\/div>\n<\/figure>\n<\/section>\n<\/section>\n<section id=\"fs-idm72014816\">\n<h1>Levels of Organization of Living Things<\/h1>\n<p id=\"fs-idm71961520\">Living things are highly organized and structured, following a hierarchy on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by a chemical bond. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by combining smaller units called monomers. An example of a macromolecule is deoxyribonucleic acid (DNA) (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_06\">Figure 6\u00a0<\/a>), which contains the instructions for the functioning of the organism that contains it.<\/p>\n<figure id=\"fig-ch01_01_06\"><figcaption>\n<div style=\"width: 210px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_06.jpg#fixme#fixme\" alt=\"Molecular model depicts a DNA molecule, showing its double helix structure.\" width=\"200\" height=\"691\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 6: A molecule, like this large DNA molecule, is composed of atoms. (credit: &#8220;Brian0918&#8243;\/Wikimedia Commons)<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<div id=\"fs-idm8353152\" class=\"note interactive non-majors\">\n<div class=\"title\"><\/div>\n<div class=\"textbox shaded\">\n<div class=\"note interactive non-majors\">\n<p id=\"fs-idm17985200\">To see an animation of this DNA molecule, click <a href=\"http:\/\/openstaxcollege.org\/l\/rotating_DNA2\" target=\"_window\">here<\/a>.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<p id=\"fs-idm42027712\">Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells and perform specialized functions. All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled organisms that lack organelles surrounded by a membrane and do not have nuclei surrounded by nuclear membranes; in contrast, the cells of eukaryotes do have membrane-bound organelles and nuclei.<\/p>\n<p id=\"fs-idm28612144\">In most multicellular organisms, cells combine to make tissues, which are groups of similar cells carrying out the same function. Organs are collections of tissues grouped together based on a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. For example vertebrate animals have many organ systems, such as the circulatory system that transports blood throughout the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.<\/p>\n<div id=\"fs-idm40888688\" class=\"note art-connection\">\n<div class=\"title\">\n<div class=\"textbox exercises\">\n<h3>Art Connection<\/h3>\n<div class=\"note art-connection\">\n<div class=\"title\"><\/div>\n<figure id=\"fig-ch01_01_07\"><figcaption><\/figcaption>&nbsp;<\/p>\n<div style=\"width: 425px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_07.png#fixme#fixme\" alt=\"A flow chart shows the hierarchy of living organisms. From smallest to largest, this hierarchy includes: 1 An atom, with protons, neutrons and electrons. 2 Molecules such as the phospholipid shown, made up of atoms. 3 Organelles, such as Golgi apparatus and nuclei, that exist inside cells. 4 Cells, such as a red blood cell. 5 Tissues, such as human skin tissue. 6 Organs such as the stomach and intestine make up the human digestive system, an example of an organ system. 7 Organisms, populations and communities. In a park, each person is an organism. Together, all the people make up a population. All the plant and animal species in the park comprise a community. 8 Ecosystems: The ecosystem of Central Park in New York includes living organisms and the environment in which they live. 9 The biosphere: encompasses all the ecosystems on Earth.\" width=\"415\" height=\"1200\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 7: From an atom to the entire Earth, biology examines all aspects of life. (credit &#8220;molecule&#8221;: modification of work by Jane Whitney; credit &#8220;organelles&#8221;: modification of work by Louisa Howard; credit &#8220;cells&#8221;: modification of work by Bruce Wetzel, Harry Schaefer, National Cancer Institute; credit &#8220;tissue&#8221;: modification of work by &#8220;Kilbad&#8221;\/Wikimedia Commons; credit &#8220;organs&#8221;: modification of work by Mariana Ruiz Villareal, Joaquim Alves Gaspar; credit &#8220;organisms&#8221;: modification of work by Peter Dutton; credit &#8220;ecosystem&#8221;: modification of work by &#8220;gigi4791&#8243;\/Flickr; credit &#8220;biosphere&#8221;: modification of work by NASA)<\/p>\n<\/div>\n<\/figure>\n<p id=\"fs-idm44001808\">Which of the following statements is false?<\/p>\n<ol id=\"eip-663\">\n<li>Tissues exist within organs which exist within organ systems.<\/li>\n<li>Communities exist within populations which exist within ecosystems.<\/li>\n<li>Organelles exist within cells which exist within tissues.<\/li>\n<li>Communities exist within ecosystems which exist in the biosphere.\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q606752\">Show Answer<\/span><\/p>\n<div id=\"q606752\" class=\"hidden-answer\" style=\"display: none\">2<\/div>\n<\/div>\n<\/li>\n<\/ol>\n<\/div>\n<p id=\"fs-idm127718976\">\n<\/div>\n<p>All the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many white pine trees. All of these pine trees represent the population of white pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the set of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest\u2019s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, or non-living, parts of that environment such as nitrogen in the soil or rainwater. At the highest level of organization (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_07\">Figure 7<\/a>), the biosphere is the collection of all ecosystems, and it represents the zones of life on Earth. It includes land, water, and portions of the atmosphere.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section id=\"fs-idm10829232\">\n<h1>The Diversity of Life<\/h1>\n<p id=\"fs-idm41986800\">The science of biology is very broad in scope because there is a tremendous diversity of life on Earth. The source of this diversity is evolution, the process of gradual change during which new species arise from older species. Evolutionary biologists study the evolution of living things in everything from the microscopic world to ecosystems.<\/p>\n<p id=\"fs-idm25429264\">In the 18th century, a scientist named Carl Linnaeus first proposed organizing the known species of organisms into a hierarchical taxonomy. In this system, species that are most similar to each other are put together within a grouping known as a genus. Furthermore, similar genera (the plural of genus) are put together within a family. This grouping continues until all organisms are collected together into groups at the highest level. The current taxonomic system now has eight levels in its hierarchy, from lowest to highest, they are: species, genus, family, order, class, phylum, kingdom, domain. Thus species are grouped within genera, genera are grouped within families, families are grouped within orders, and so on (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_08\">[Figure 8]<\/a>).<\/p>\n<figure id=\"fig-ch01_01_08\"><figcaption><\/figcaption><div style=\"width: 576px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_08.jpg#fixme#fixme\" alt=\"A chart shows the eight levels of taxonomic hierarchy for the dog, Canis lupus.\" width=\"566\" height=\"272\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 8: This diagram shows the levels of taxonomic hierarchy for a dog, from the broadest category\u2014domain\u2014to the most specific\u2014species.<\/p>\n<\/div>\n<p><span id=\"fs-idm125583568\">\u00a0<\/span><\/figure>\n<p id=\"fs-idm72098240\">The highest level, domain, is a relatively new addition to the system since the 1990s. Scientists now recognize three domains of life, the Eukarya, the Archaea, and the Bacteria. The domain Eukarya contains organisms that have cells with nuclei. It includes the kingdoms of fungi, plants, animals, and several kingdoms of protists. The Archaea, are single-celled organisms without nuclei and include many extremophiles that live in harsh environments like hot springs. The Bacteria are another quite different group of single-celled organisms without nuclei (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_09\">[Figure 9]<\/a>). Both the Archaea and the Bacteria are prokaryotes, an informal name for cells without nuclei. The recognition in the 1990s that certain &#8220;bacteria,&#8221; now known as the Archaea, were as different genetically and biochemically from other bacterial cells as they were from eukaryotes, motivated the recommendation to divide life into three domains. This dramatic change in our knowledge of the tree of life demonstrates that classifications are not permanent and will change when new information becomes available.<\/p>\n<p id=\"fs-idm76440992\">In addition to the hierarchical taxonomic system, Linnaeus was the first to name organisms using two unique names, now called the binomial naming system. Before Linnaeus, the use of common names to refer to organisms caused confusion because there were regional differences in these common names. Binomial names consist of the genus name (which is capitalized) and the species name (all lower-case). Both names are set in italics when they are printed. Every species is given a unique binomial which is recognized the world over, so that a scientist in any location can know which organism is being referred to. For example, the North American blue jay is known uniquely as <em>Cyanocitta cristata<\/em>. Our own species is <em>Homo sapiens<\/em>.<\/p>\n<figure id=\"fig-ch01_01_09\"><figcaption><\/figcaption><div style=\"width: 826px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_09.jpg#fixme#fixme\" alt=\"Photos depict: A: bacterial cells. B: a natural hot vent. C: a sunflower. D: a lion.\" width=\"816\" height=\"204\" \/><\/p>\n<p class=\"wp-caption-text\">These images represent different domains. The scanning electron micrograph shows (a) bacterial cells belong to the domain Bacteria, while the (b) extremophiles, seen all together as colored mats in this hot spring, belong to domain Archaea. Both the (c) sunflower and (d) lion are part of domain Eukarya. (credit a: modification of work by Rocky Mountain Laboratories, NIAID, NIH; credit b: modification of work by Steve Jurvetson; credit c: modification of work by Michael Arrighi; credit d: modification of work by Frank Vassen)<\/p>\n<\/div>\n<p><span id=\"fs-idm9076800\">\u00a0<\/span><\/figure>\n<div id=\"fs-idm108538560\" class=\"note evolution non-majors\">\n<div class=\"title\">\n<div class=\"textbox key-takeaways\">\n<h3>Key Takeaways<\/h3>\n<section>\n<div class=\"note evolution non-majors\">\n<div class=\"title\">Evolution in Action<\/div>\n<p id=\"fs-idm10904720\">Carl Woese and the Phylogenetic Tree &#8211; the evolutionary relationships of various life forms on Earth can be summarized in a phylogenetic tree. A phylogenetic tree is a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both. A phylogenetic tree is composed of branch points, or nodes, and branches. The internal nodes represent ancestors and are points in evolution when, based on scientific evidence, an ancestor is thought to have diverged to form two new species. The length of each branch can be considered as estimates of relative time.<\/p>\n<p id=\"fs-idm71571824\">In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and bacteria. The pioneering work of American microbiologist Carl Woese in the early 1970s has shown, however, that life on Earth has evolved along three lineages, now called domains\u2014Bacteria, Archaea, and Eukarya. Woese proposed the domain as a new taxonomic level and Archaea as a new domain, to reflect the new phylogenetic tree (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_10\">[Figure 10]<\/a>). Many organisms belonging to the Archaea domain live under extreme conditions and are called extremophiles. To construct his tree, Woese used genetic relationships rather than similarities based on morphology (shape). Various genes were used in phylogenetic studies. Woese\u2019s tree was constructed from comparative sequencing of the genes that are universally distributed, found in some slightly altered form in every organism, conserved (meaning that these genes have remained only slightly changed throughout evolution), and of an appropriate length.<\/p>\n<figure id=\"fig-ch01_01_10\"><figcaption><\/figcaption><div style=\"width: 510px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_10.jpg#fixme#fixme\" alt=\"This phylogenetic tree shows that the three domains of life, bacteria, archaea and eukarya, all arose from a common ancestor.\" width=\"500\" height=\"540\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 10: This phylogenetic tree was constructed by microbiologist Carl Woese using genetic relationships. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are organisms without a nucleus or other organelles surrounded by a membrane and, therefore, are prokaryotes. (credit: modification of work by Eric Gaba)<\/p>\n<\/div>\n<\/figure>\n<\/div>\n<\/section>\n<section id=\"fs-idm93700768\">\n<h1><\/h1>\n<\/section>\n<\/div>\n<\/div>\n<div><\/div>\n<h3 class=\"title\">Branches of Biological Study<\/h3>\n<\/div>\n<\/section>\n<section>\n<p id=\"fs-idm128104608\">The scope of biology is broad and therefore contains many branches and sub disciplines. Biologists may pursue one of those sub disciplines and work in a more focused field. For instance, molecular biology studies biological processes at the molecular level, including interactions among molecules such as DNA, RNA, and proteins, as well as the way they are regulated. Microbiology is the study of the structure and function of microorganisms. It is quite a broad branch itself, and depending on the subject of study, there are also microbial physiologists, ecologists, and geneticists, among others.<\/p>\n<p id=\"fs-idm75927680\">Another field of biological study, neurobiology, studies the biology of the nervous system, and although it is considered a branch of biology, it is also recognized as an interdisciplinary field of study known as neuroscience. Because of its interdisciplinary nature, this sub discipline studies different functions of the nervous system using molecular, cellular, developmental, medical, and computational approaches.<\/p>\n<figure id=\"fig-ch01_01_11\"><figcaption><\/figcaption><div style=\"width: 360px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_11.jpg#fixme#fixme\" alt=\"Photo depicts scientists digging fossils out of the dirt.\" width=\"350\" height=\"418\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 11: Researchers work on excavating dinosaur fossils at a site in Castell\u00f3n, Spain. (credit: Mario Modesto)<\/p>\n<\/div>\n<p><span id=\"fs-idm107121744\">\u00a0<\/span><\/figure>\n<p id=\"fs-idm77010944\">Paleontology, another branch of biology, uses fossils to study life\u2019s history (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_11\">[Figure 11]<\/a>). Zoology and botany are the study of animals and plants, respectively. Biologists can also specialize as biotechnologists, ecologists, or physiologists, to name just a few areas. Biotechnologists apply the knowledge of biology to create useful products. Ecologists study the interactions of organisms in their environments. Physiologists study the workings of cells, tissues and organs. This is just a small sample of the many fields that biologists can pursue. From our own bodies to the world we live in, discoveries in biology can affect us in very direct and important ways. We depend on these discoveries for our health, our food sources, and the benefits provided by our ecosystem. Because of this, knowledge of biology can benefit us in making decisions in our day-to-day lives.<\/p>\n<p id=\"fs-idm78647248\">The development of technology in the twentieth century that continues today, particularly the technology to describe and manipulate the genetic material, DNA, has transformed biology. This transformation will allow biologists to continue to understand the history of life in greater detail, how the human body works, our human origins, and how humans can survive as a species on this planet despite the stresses caused by our increasing numbers. Biologists continue to decipher huge mysteries about life suggesting that we have only begun to understand life on the planet, its history, and our relationship to it. For this and other reasons, the knowledge of biology gained through this textbook and other printed and electronic media should be a benefit in whichever field you enter.<\/p>\n<div id=\"fs-idm74894912\" class=\"note career non-majors\">\n<section>\n<h2>Forensic Science<\/h2>\n<p id=\"fs-idm80622272\">Forensic science is the application of science to answer questions related to the law. Biologists as well as chemists and biochemists can be forensic scientists. Forensic scientists provide scientific evidence for use in courts, and their job involves examining trace material associated with crimes. Interest in forensic science has increased in the last few years, possibly because of popular television shows that feature forensic scientists on the job. Also, the development of molecular techniques and the establishment of DNA databases have updated the types of work that forensic scientists can do. Their job activities are primarily related to crimes against people such as murder, rape, and assault. Their work involves analyzing samples such as hair, blood, and other body fluids and also processing DNA (<a class=\"autogenerated-content\" href=\"#fig-ch01_01_12\">Figure 12]<\/a>) found in many different environments and materials. Forensic scientists also analyze other biological evidence left at crime scenes, such as insect parts or pollen grains. Students who want to pursue careers in forensic science will most likely be required to take chemistry and biology courses as well as some intensive math courses.<\/p>\n<figure id=\"fig-ch01_01_12\"><figcaption><\/figcaption><div style=\"width: 460px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/OSbioconc\/m45419\/Figure_01_01_12.jpg#fixme#fixme\" alt=\"Photo depicts a scientist working in a lab.\" width=\"450\" height=\"379\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 12: This forensic scientist works in a DNA extraction room at the U.S. Army Criminal Investigation Laboratory. (credit: U.S. Army CID Command Public Affairs)<\/p>\n<\/div>\n<p><span id=\"fs-idm48501008\">\u00a0<\/span><\/figure>\n<\/section>\n<\/div>\n<\/section>\n<section id=\"fs-idm107479568\" class=\"summary\">\n<h1>Section Summary<\/h1>\n<p id=\"fs-idm55696240\">Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. Living things are highly organized following a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere. Evolution is the source of the tremendous biological diversity on Earth today. A diagram called a phylogenetic tree can be used to show evolutionary relationships among organisms. Biology is very broad and includes many branches and sub disciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others.<\/p>\n<p>&nbsp;<\/p>\n<\/section>\n<div>\n<h2>Glossary<\/h2>\n<dl id=\"fs-idm17545584\" class=\"definition\">\n<dt><strong>atom<\/strong><\/dt>\n<dd id=\"fs-idm41446080\">a basic unit of matter that cannot be broken down by normal chemical reactions<\/dd>\n<\/dl>\n<dl id=\"fs-idm19475200\" class=\"definition\">\n<dt><strong>biology<\/strong><\/dt>\n<dd id=\"fs-idm60761200\">the study of living organisms and their interactions with one another and their environments<\/dd>\n<\/dl>\n<dl id=\"fs-idm19153088\" class=\"definition\">\n<dt><strong>biosphere<\/strong><\/dt>\n<dd id=\"fs-idm18996064\">a collection of all ecosystems on Earth<\/dd>\n<\/dl>\n<dl id=\"fs-idm61529952\" class=\"definition\">\n<dt><strong>cell<\/strong><\/dt>\n<dd id=\"fs-idm28458496\">the smallest fundamental unit of structure and function in living things<\/dd>\n<\/dl>\n<dl id=\"fs-idm127697392\" class=\"definition\">\n<dt><strong>community<\/strong><\/dt>\n<dd id=\"fs-idm60799520\">a set of populations inhabiting a particular area<\/dd>\n<\/dl>\n<dl id=\"fs-idm74280784\" class=\"definition\">\n<dt><strong>ecosystem<\/strong><\/dt>\n<dd id=\"fs-idm62480352\">all living things in a particular area together with the abiotic, nonliving parts of that environment<\/dd>\n<\/dl>\n<dl id=\"fs-idm80696624\" class=\"definition\">\n<dt><strong>eukaryote<\/strong><\/dt>\n<dd id=\"fs-idm59473040\">an organism with cells that have nuclei and membrane-bound organelles<\/dd>\n<\/dl>\n<dl id=\"fs-idm58443328\" class=\"definition\">\n<dt><strong>evolution<\/strong><\/dt>\n<dd id=\"fs-idm54214224\">the process of gradual change in a population that can also lead to new species arising from older species<\/dd>\n<\/dl>\n<dl id=\"fs-idm41117184\" class=\"definition\">\n<dt><strong>homeostasis<\/strong><\/dt>\n<dd id=\"fs-idm76254000\">the ability of an organism to maintain constant internal conditions<\/dd>\n<\/dl>\n<dl id=\"fs-idm518832\" class=\"definition\">\n<dt><strong>macromolecule<\/strong><\/dt>\n<dd id=\"fs-idm74473904\">a large molecule typically formed by the joining of smaller molecules<\/dd>\n<\/dl>\n<dl id=\"fs-idm59477568\" class=\"definition\">\n<dt><strong>molecule<\/strong><\/dt>\n<dd id=\"fs-idm22070880\">a chemical structure consisting of at least two atoms held together by a chemical bond<\/dd>\n<\/dl>\n<dl id=\"fs-idm47135696\" class=\"definition\">\n<dt><strong>organ<\/strong><\/dt>\n<dd id=\"fs-idm53274960\">a structure formed of tissues operating together to perform a common function<\/dd>\n<\/dl>\n<dl id=\"fs-idm73130992\" class=\"definition\">\n<dt><strong>organ system<\/strong><\/dt>\n<dd id=\"fs-idm9181952\">the higher level of organization that consists of functionally related organs<\/dd>\n<\/dl>\n<dl id=\"fs-idm7168368\" class=\"definition\">\n<dt><strong>organelle<\/strong><\/dt>\n<dd id=\"fs-idm59599344\">a membrane-bound compartment or sac within a cell<\/dd>\n<\/dl>\n<dl id=\"fs-idm17991488\" class=\"definition\">\n<dt><strong>organism<\/strong><\/dt>\n<dd id=\"fs-idm79676656\">an individual living entity<\/dd>\n<\/dl>\n<dl id=\"fs-idm485824\" class=\"definition\">\n<dt><strong>phylogenetic tree<\/strong><\/dt>\n<dd id=\"fs-idm107117136\">a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both<\/dd>\n<\/dl>\n<dl id=\"fs-idm74536096\" class=\"definition\">\n<dt><strong>population<\/strong><\/dt>\n<dd id=\"fs-idm11206032\">all individuals within a species living within a specific area<\/dd>\n<\/dl>\n<dl id=\"fs-idm71391472\" class=\"definition\">\n<dt><strong>prokaryote<\/strong><\/dt>\n<dd id=\"fs-idm42022320\">a unicellular organism that lacks a nucleus or any other membrane-bound organelle<\/dd>\n<\/dl>\n<dl id=\"fs-idm60819312\" class=\"definition\">\n<dt><strong>tissue<\/strong><\/dt>\n<dd id=\"fs-idm71924960\">a group of similar cells carrying out the same function<\/dd>\n<\/dl>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-22\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Concepts of Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.25\">http:\/\/cnx.org\/contents\/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.25<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.25<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":1,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Concepts of Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.25\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at 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