{"id":1053,"date":"2016-04-07T19:41:04","date_gmt":"2016-04-07T19:41:04","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/biologyxwaymakerxmaster\/?post_type=chapter&#038;p=1053"},"modified":"2017-06-16T15:04:50","modified_gmt":"2017-06-16T15:04:50","slug":"the-characteristics-of-life","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology1\/chapter\/the-characteristics-of-life\/","title":{"raw":"The Characteristics of Life","rendered":"The Characteristics of Life"},"content":{"raw":"<h2>List the defining characteristics of biological life<\/h2>\r\nBiology is the science that studies life, but what exactly is life? This may sound like a silly question with an obvious response, but it is not always 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. Consequently, virologists are not biologists, strictly speaking. Similarly, some biologists study the early molecular evolution that gave rise to life; since the events that preceded life are not biological events, these scientists are also excluded from biology in the strict sense of the term.\r\n\r\nFrom its earliest beginnings, biology has wrestled with these\u00a0questions: What are the shared properties that make something \"alive\"? And once we know something is alive, how do we find meaningful levels of organization in its structure?\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>List the properties of life<\/li>\r\n \t<li>Order the levels of organization of living things<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Properties of Life<\/h2>\r\nAll living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, growth and development, regulation, homeostasis, and energy processing. When viewed together, these\u00a0characteristics serve to define life.\r\n<h3>Order<\/h3>\r\n[caption id=\"attachment_1509\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-1509\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02201627\/Figure_01_02_09-e1462220416468.jpg\" alt=\"A photo shows a light-colored toad covered in bright green spots.\" width=\"350\" height=\"188\" \/> Figure 1. A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems.[\/caption]\r\n\r\nOrganisms are highly organized, coordinated 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 organelles and other cellular inclusions.\r\n\r\nIn multicellular organisms (Figure 1), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.\r\n<h3>Sensitivity or Response to Stimuli<\/h3>\r\nOrganisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 2).\r\n\r\n[caption id=\"attachment_1510\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-1510\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02201729\/Figure_01_02_10.jpg\" alt=\"A photograph of the Mimosa pudica shows a plant with many tiny leaves connected to a central stem. Four of these stems connect together.\" width=\"350\" height=\"263\" \/> Figure 2.The leaves of this sensitive plant (<em>Mimosa pudica<\/em>) will instantly droop and fold when touched. After a few minutes, the plant returns to normal. (credit: Alex Lomas)[\/caption]\r\n\r\nEven tiny bacteria can move toward or away from chemicals (a process called\u00a0<em>chemotaxis<\/em>) or light (<em>phototaxis<\/em>). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.\r\n\r\nWatch\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/movement_plants\" target=\"_window\" rel=\"nofollow\">this video<\/a> to see how plants respond to a stimulus\u2014from opening to light, to wrapping a tendril around a branch, to capturing prey.\r\n<h3>Reproduction<\/h3>\r\nSingle-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals. When reproduction occurs, genes containing DNA are passed along to an organism's offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape.\r\n<h3>Growth and Development<\/h3>\r\n[caption id=\"attachment_1102\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-1102\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/04\/07212344\/puppies.jpg\" alt=\"a mother dog nursing approximately five puppies. three are black, one is brown, and the other is pale yellow. The mother is a light brown.\" width=\"350\" height=\"233\" \/> Figure 3. Although no two look alike, these puppies have inherited genes from both parents and share many of the same characteristics.[\/caption]\r\n\r\nOrganisms grow and develop following specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species' young (Figure 3) will grow up to exhibit many of the same characteristics as its parents.\r\n<h3>Regulation<\/h3>\r\nEven the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.\r\n<h3>Homeostasis<\/h3>\r\n[caption id=\"attachment_1511\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-1511\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02201807\/Figure_01_02_12.jpg\" alt=\"A photos shows a white, furry polar bear.\" width=\"350\" height=\"250\" \/> Figure 4. Polar bears (<em>Ursus maritimus<\/em>) and other mammals living in ice-covered regions maintain their body temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin. (credit: \"longhorndave\"\/Flickr)[\/caption]\r\n\r\nIn order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through\u00a0<strong>homeostasis<\/strong> (literally, \"steady state\")\u2014the ability of an organism to maintain constant internal conditions. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure 4), have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat.\r\n<h3>Energy Processing<\/h3>\r\nAll organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food (photosynthesis); others use chemical energy in molecules they take in as food (cellular respiration).\r\n\r\n[caption id=\"attachment_1101\" align=\"aligncenter\" width=\"1024\"]<img class=\"wp-image-1101 size-large\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/04\/07205837\/California-condor-e1460062775102-1024x366.jpg\" alt=\"A california condor perched on a cliff's edge. Its wings are extended in preparation for flight.\" width=\"1024\" height=\"366\" \/> Figure 5. The California condor (<em>Gymnogyps californianus<\/em>) uses chemical energy derived from food to power flight. California condors are an endangered species; this bird has a wing tag that helps biologists identify the individual.[\/caption]\r\n<h2>Levels of Organization of Living Things<\/h2>\r\nLiving things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The <strong>atom<\/strong> is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A <strong>molecule<\/strong> is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are <strong>macromolecules<\/strong>, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure 6), which contains the instructions for the structure and functioning of all living organisms.\r\n\r\n[caption id=\"attachment_1165\" align=\"alignright\" width=\"181\"]<img class=\"size-full wp-image-1165\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/04\/08172216\/ADN_animation.gif\" alt=\"Molecular model depicts a DNA molecule, showing its double helix structure.\" width=\"181\" height=\"313\" \/> Figure 6. All molecules, including this DNA molecule, are composed of atoms. (credit: \u201cbrian0918\u2033\/Wikimedia Commons)[\/caption]\r\n\r\nSome cells contain aggregates of macromolecules surrounded by membranes; these are called\u00a0<strong>organelles<\/strong>. Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made of cells; the <strong>cell<\/strong> 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 the reproductive mechanism of 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. <strong>Prokaryotes<\/strong> are single-celled or colonial organisms that do not have membrane-bound nuclei or organelles; in contrast, the cells of <strong>eukaryotes<\/strong> do have membrane-bound organelles and a membrane-bound nucleus.\r\n\r\nIn larger organisms, cells combine to make\u00a0<strong>tissues<\/strong>, which are groups of similar cells carrying out similar or related functions. <strong>Organs<\/strong> are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An <strong>organ system<\/strong> is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. <strong>Organisms<\/strong> 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.\r\n\r\nAll the individuals of a species living within a specific area are collectively called a\u00a0<strong>population<\/strong>. For example, a forest may include many pine trees. All of these pine trees represent the population of 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 <strong>community<\/strong> is the sum of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest's community. The forest itself is an ecosystem. An <strong>ecosystem<\/strong> consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization (Figure 7), the <strong>biosphere<\/strong> is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent.\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nFrom a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy.\r\n\r\n[caption id=\"attachment_4598\" align=\"aligncenter\" width=\"1023\"]<img class=\"size-full wp-image-4598\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1087\/2016\/04\/14154237\/Figure_01_02_15.jpg\" alt=\"A flow chart shows the hierarchy of living organisms. From smallest to largest, this hierarchy includes: (1) Organelles, such as nuclei, that exist inside cells. (2) Cells, such as a red blood cell. (3) Tissues, such as human skin tissue. (4) Organs such as the stomach make up the human digestive system, an example of an organ system. (5) Organisms, populations, and communities. In a forest, each pine tree is an organism. Together, all the pine trees make up a population. All the plant and animal species in the forest comprise a community. (6) Ecosystems: the coastal ecosystem in the Southeastern United States includes living organisms and the environment in which they live. (7) The biosphere: encompasses all the ecosystems on Earth.\" width=\"1023\" height=\"853\" \/> Figure 7. The biological levels of organization of living things are shown. From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy. (credit \u201corganelles\u201d: modification of work by Umberto Salvagnin; credit \u201ccells\u201d: modification of work by Bruce Wetzel, Harry Schaefer\/ National Cancer Institute; credit \u201ctissues\u201d: modification of work by Kilbad; Fama Clamosa; Mikael H\u00e4ggstr\u00f6m; credit \u201corgans\u201d: modification of work by Mariana Ruiz Villareal; credit \u201corganisms\u201d: modification of work by \"Crystal\"\/Flickr; credit \u201cecosystems\u201d: modification of work by US Fish and Wildlife Service Headquarters; credit \u201cbiosphere\u201d: modification of work by NASA)[\/caption]\r\n\r\nWhich of the following statements is false?\r\n<ol style=\"list-style-type: lower-alpha;\">\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.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"750531\"]<strong>Show Answer<\/strong>[\/reveal-answer]\r\n[hidden-answer a=\"750531\"]Statement b is false: populations exist within communities.[\/hidden-answer]\r\n\r\n<\/div>\r\n<h2><strong>Check Your Understanding<\/strong><\/h2>\r\nAnswer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does\u00a0<strong>not<\/strong>\u00a0count toward your grade in the class, and you can retake it an unlimited number of times.\r\n\r\nUse this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.\r\n\r\nhttps:\/\/assessments.lumenlearning.com\/assessments\/3305","rendered":"<h2>List the defining characteristics of biological life<\/h2>\n<p>Biology is the science that studies life, but what exactly is life? This may sound like a silly question with an obvious response, but it is not always 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. Consequently, virologists are not biologists, strictly speaking. Similarly, some biologists study the early molecular evolution that gave rise to life; since the events that preceded life are not biological events, these scientists are also excluded from biology in the strict sense of the term.<\/p>\n<p>From its earliest beginnings, biology has wrestled with these\u00a0questions: What are the shared properties that make something &#8220;alive&#8221;? And once we know something is alive, how do we find meaningful levels of organization in its structure?<\/p>\n<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>List the properties of life<\/li>\n<li>Order the levels of organization of living things<\/li>\n<\/ul>\n<\/div>\n<h2>Properties of Life<\/h2>\n<p>All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, growth and development, regulation, homeostasis, and energy processing. When viewed together, these\u00a0characteristics serve to define life.<\/p>\n<h3>Order<\/h3>\n<div id=\"attachment_1509\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1509\" class=\"wp-image-1509\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02201627\/Figure_01_02_09-e1462220416468.jpg\" alt=\"A photo shows a light-colored toad covered in bright green spots.\" width=\"350\" height=\"188\" \/><\/p>\n<p id=\"caption-attachment-1509\" class=\"wp-caption-text\">Figure 1. A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems.<\/p>\n<\/div>\n<p>Organisms are highly organized, coordinated 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 organelles and other cellular inclusions.<\/p>\n<p>In multicellular organisms (Figure 1), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.<\/p>\n<h3>Sensitivity or Response to Stimuli<\/h3>\n<p>Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 2).<\/p>\n<div id=\"attachment_1510\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1510\" class=\"wp-image-1510\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02201729\/Figure_01_02_10.jpg\" alt=\"A photograph of the Mimosa pudica shows a plant with many tiny leaves connected to a central stem. Four of these stems connect together.\" width=\"350\" height=\"263\" \/><\/p>\n<p id=\"caption-attachment-1510\" class=\"wp-caption-text\">Figure 2.The leaves of this sensitive plant (<em>Mimosa pudica<\/em>) will instantly droop and fold when touched. After a few minutes, the plant returns to normal. (credit: Alex Lomas)<\/p>\n<\/div>\n<p>Even tiny bacteria can move toward or away from chemicals (a process called\u00a0<em>chemotaxis<\/em>) or light (<em>phototaxis<\/em>). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.<\/p>\n<p>Watch\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/movement_plants\" target=\"_window\" rel=\"nofollow\">this video<\/a> to see how plants respond to a stimulus\u2014from opening to light, to wrapping a tendril around a branch, to capturing prey.<\/p>\n<h3>Reproduction<\/h3>\n<p>Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals. When reproduction occurs, genes containing DNA are passed along to an organism&#8217;s offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape.<\/p>\n<h3>Growth and Development<\/h3>\n<div id=\"attachment_1102\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1102\" class=\"wp-image-1102\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/04\/07212344\/puppies.jpg\" alt=\"a mother dog nursing approximately five puppies. three are black, one is brown, and the other is pale yellow. The mother is a light brown.\" width=\"350\" height=\"233\" \/><\/p>\n<p id=\"caption-attachment-1102\" class=\"wp-caption-text\">Figure 3. Although no two look alike, these puppies have inherited genes from both parents and share many of the same characteristics.<\/p>\n<\/div>\n<p>Organisms grow and develop following specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species&#8217; young (Figure 3) will grow up to exhibit many of the same characteristics as its parents.<\/p>\n<h3>Regulation<\/h3>\n<p>Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.<\/p>\n<h3>Homeostasis<\/h3>\n<div id=\"attachment_1511\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1511\" class=\"wp-image-1511\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02201807\/Figure_01_02_12.jpg\" alt=\"A photos shows a white, furry polar bear.\" width=\"350\" height=\"250\" \/><\/p>\n<p id=\"caption-attachment-1511\" class=\"wp-caption-text\">Figure 4. Polar bears (<em>Ursus maritimus<\/em>) and other mammals living in ice-covered regions maintain their body temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin. (credit: &#8220;longhorndave&#8221;\/Flickr)<\/p>\n<\/div>\n<p>In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through\u00a0<strong>homeostasis<\/strong> (literally, &#8220;steady state&#8221;)\u2014the ability of an organism to maintain constant internal conditions. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure 4), have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat.<\/p>\n<h3>Energy Processing<\/h3>\n<p>All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food (photosynthesis); others use chemical energy in molecules they take in as food (cellular respiration).<\/p>\n<div id=\"attachment_1101\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1101\" class=\"wp-image-1101 size-large\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/04\/07205837\/California-condor-e1460062775102-1024x366.jpg\" alt=\"A california condor perched on a cliff's edge. Its wings are extended in preparation for flight.\" width=\"1024\" height=\"366\" \/><\/p>\n<p id=\"caption-attachment-1101\" class=\"wp-caption-text\">Figure 5. The California condor (<em>Gymnogyps californianus<\/em>) uses chemical energy derived from food to power flight. California condors are an endangered species; this bird has a wing tag that helps biologists identify the individual.<\/p>\n<\/div>\n<h2>Levels of Organization of Living Things<\/h2>\n<p>Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The <strong>atom<\/strong> is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A <strong>molecule<\/strong> is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are <strong>macromolecules<\/strong>, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure 6), which contains the instructions for the structure and functioning of all living organisms.<\/p>\n<div id=\"attachment_1165\" style=\"width: 191px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1165\" class=\"size-full wp-image-1165\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/04\/08172216\/ADN_animation.gif\" alt=\"Molecular model depicts a DNA molecule, showing its double helix structure.\" width=\"181\" height=\"313\" \/><\/p>\n<p id=\"caption-attachment-1165\" class=\"wp-caption-text\">Figure 6. All molecules, including this DNA molecule, are composed of atoms. (credit: \u201cbrian0918\u2033\/Wikimedia Commons)<\/p>\n<\/div>\n<p>Some cells contain aggregates of macromolecules surrounded by membranes; these are called\u00a0<strong>organelles<\/strong>. Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made of cells; the <strong>cell<\/strong> 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 the reproductive mechanism of 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. <strong>Prokaryotes<\/strong> are single-celled or colonial organisms that do not have membrane-bound nuclei or organelles; in contrast, the cells of <strong>eukaryotes<\/strong> do have membrane-bound organelles and a membrane-bound nucleus.<\/p>\n<p>In larger organisms, cells combine to make\u00a0<strong>tissues<\/strong>, which are groups of similar cells carrying out similar or related functions. <strong>Organs<\/strong> are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An <strong>organ system<\/strong> is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. <strong>Organisms<\/strong> 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<p>All the individuals of a species living within a specific area are collectively called a\u00a0<strong>population<\/strong>. For example, a forest may include many pine trees. All of these pine trees represent the population of 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 <strong>community<\/strong> is the sum of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest&#8217;s community. The forest itself is an ecosystem. An <strong>ecosystem<\/strong> consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization (Figure 7), the <strong>biosphere<\/strong> is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent.<\/p>\n<div class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy.<\/p>\n<div id=\"attachment_4598\" style=\"width: 1033px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-4598\" class=\"size-full wp-image-4598\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1087\/2016\/04\/14154237\/Figure_01_02_15.jpg\" alt=\"A flow chart shows the hierarchy of living organisms. From smallest to largest, this hierarchy includes: (1) Organelles, such as nuclei, that exist inside cells. (2) Cells, such as a red blood cell. (3) Tissues, such as human skin tissue. (4) Organs such as the stomach make up the human digestive system, an example of an organ system. (5) Organisms, populations, and communities. In a forest, each pine tree is an organism. Together, all the pine trees make up a population. All the plant and animal species in the forest comprise a community. (6) Ecosystems: the coastal ecosystem in the Southeastern United States includes living organisms and the environment in which they live. (7) The biosphere: encompasses all the ecosystems on Earth.\" width=\"1023\" height=\"853\" \/><\/p>\n<p id=\"caption-attachment-4598\" class=\"wp-caption-text\">Figure 7. The biological levels of organization of living things are shown. From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy. (credit \u201corganelles\u201d: modification of work by Umberto Salvagnin; credit \u201ccells\u201d: modification of work by Bruce Wetzel, Harry Schaefer\/ National Cancer Institute; credit \u201ctissues\u201d: modification of work by Kilbad; Fama Clamosa; Mikael H\u00e4ggstr\u00f6m; credit \u201corgans\u201d: modification of work by Mariana Ruiz Villareal; credit \u201corganisms\u201d: modification of work by &#8220;Crystal&#8221;\/Flickr; credit \u201cecosystems\u201d: modification of work by US Fish and Wildlife Service Headquarters; credit \u201cbiosphere\u201d: modification of work by NASA)<\/p>\n<\/div>\n<p>Which of the following statements is false?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\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.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q750531\"><strong>Show Answer<\/strong><\/span><\/p>\n<div id=\"q750531\" class=\"hidden-answer\" style=\"display: none\">Statement b is false: populations exist within communities.<\/div>\n<\/div>\n<\/div>\n<h2><strong>Check Your Understanding<\/strong><\/h2>\n<p>Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does\u00a0<strong>not<\/strong>\u00a0count toward your grade in the class, and you can retake it an unlimited number of times.<\/p>\n<p>Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.<\/p>\n<p>\t<iframe id=\"lumen_assessment_3305\" class=\"resizable\" src=\"https:\/\/assessments.lumenlearning.com\/assessments\/load?assessment_id=3305&#38;embed=1&#38;external_user_id=&#38;external_context_id=&#38;iframe_resize_id=lumen_assessment_3305\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:400px;\"><br \/>\n\t<\/iframe><\/p>\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-1053\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Original<\/div><ul class=\"citation-list\"><li>Introduction to Characteristics of Life. <strong>Authored by<\/strong>: Shelli Carter and Lumen Learning. <strong>Provided by<\/strong>: Lumen Learning. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/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\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/li><li>fresh litter. <strong>Authored by<\/strong>: Magalie LAbbe. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/flic.kr\/p\/9yeYXd\">https:\/\/flic.kr\/p\/9yeYXd<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/4.0\/\">CC BY-NC: Attribution-NonCommercial<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">Public domain content<\/div><ul class=\"citation-list\"><li>Bufo viridis\/European Green Toad. <strong>Authored by<\/strong>: Ivengo(RUS). <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Bufo_viridis.jpg\">https:\/\/commons.wikimedia.org\/wiki\/File:Bufo_viridis.jpg<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/about\/pdm\">Public Domain: No Known Copyright<\/a><\/em><\/li><li>Young California condor (Gymnogyps californianus) ready for flight. <strong>Provided by<\/strong>: US Fish and Wildlife Service. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/en.wikipedia.org\/wiki\/File:California-condor.jpg\">https:\/\/en.wikipedia.org\/wiki\/File:California-condor.jpg<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/about\/pdm\">Public Domain: No Known Copyright<\/a><\/em><\/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":2,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\"},{\"type\":\"original\",\"description\":\"Introduction to Characteristics of 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