{"id":121,"date":"2014-10-27T08:18:44","date_gmt":"2014-10-27T08:18:44","guid":{"rendered":"http:\/\/courses.candelalearning.com\/novabiology\/?post_type=chapter&p=121"},"modified":"2018-07-10T18:07:25","modified_gmt":"2018-07-10T18:07:25","slug":"passive-transport","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/chapter\/passive-transport\/","title":{"raw":"Passive Transport","rendered":"Passive Transport"},"content":{"raw":"
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Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n
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  • Explain why and how passive transport occurs<\/li>\r\n \t
  • Understand the processes of diffusion and osmosis<\/li>\r\n \t
  • Define tonicity and distinguish the three solutions formed<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n

    Plasma membranes determine what substances enter and exit a cell.\u00a0 It must balance this movement, preventing harmful material from entering and essential material from leaving. For this reason, plasma membranes are selectively permeable.\u00a0 They permit some substances through but prevent others. If this selectivity was lost, the cell would no longer be able to sustain itself, and it would be destroyed.\u00a0 Cells must have a way of obtaining these materials from the extracellular fluids. This may happen passively when certain materials move back and forth.\u00a0 But cells may have special mechanisms that ensures transport. Most cells expend much of their energy(ATP), to create and maintain this constant transfer.\u00a0 The plasma membrane structure can assist but also inhibit some of this movement.<\/p>\r\n

    The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to expend energy.\u00a0 During passive transport, substances move from an area of higher concentration to an area of lower concentration in a process called diffusion.<\/p>\r\n\r\n

    \r\n

    Selective Permeability<\/h2>\r\n

    \u00a0Changes in environmental conditions greatly impact what enters and exits a cell.\u00a0 The plasma membrane adjusts to these changes in many ways.\u00a0 Integral proteins embedded within the plasma membrane act as channels(pumps) allowing materials to pass through the membrane. Carbohydrates, attached to lipids or proteins found on the exterior surface, help the cell bind to substances needed in the extracellular fluid.<\/p>\r\n

    So, what can enter and exit a cell?\u00a0 Plasma membranes have both hydrophilic and hydrophobic regions. This helps the movement of certain materials through the membrane and hinders the movement of others.\u00a0 Size and solubility are major determining factors in what can pass. Generally, small, uncharged particles have no trouble in passing, while large molecules and ions tend to face issues.\u00a0 In those cases, special mechanisms are used in order to penetrate the plasma membrane. \u00a0 Molecules of oxygen and carbon dioxide pass through quite easily.<\/p>\r\n

    Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane.\u00a0 In these instances, special mechanisms are again necessary.<\/p>\r\n\r\n<\/section>

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    Diffusion<\/h2>\r\n

    Diffusion is a passive process. \u00a0 A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space.\u00a0 For example, what happens when someone opens a bottle of perfume in a room?\u00a0 The perfume is at its highest concentration in the bottle and at its lowest in the room. The perfume vapor will diffuse, or spread away, from the bottle.\u00a0 Gradually, the vapor will spread.\u00a0 Materials move within the cell\u2019s cytosol by this means of transport (Figure 1). Diffusion requires no energy use and stops when equilibrium is reached.\u00a0 The molecules do not stop movement but instead, maintain equilibrium.<\/p>\r\n\r\n\r\n[caption id=\"attachment_1154\" align=\"aligncenter\" width=\"800\"]\"The Figure 1. Diffusion through a permeable membrane follows the concentration gradient of a substance, moving the substance from an area of high concentration to one of low concentration. (credit: modification of work by Mariana Ruiz Villarreal)[\/caption]\r\n

    <\/figure>\r\n

    Each separate substance in a medium has its own concentration gradient, independent of the concentration gradients of other materials. Additionally, each substance will diffuse according to that gradient.<\/p>\r\n

    Several factors affect the rate of diffusion.<\/p>\r\n\r\n

      \r\n \t
    • Concentration gradient:\u00a0 the greater the difference in concentration, the more rapid the diffusion;\u00a0 as equilibrium gets close, diffusion slows<\/li>\r\n \t
    • Molecular mass:\u00a0 larger molecules move more slowly,\u00a0 it is more difficult to move between the molecules of the substance they are moving through<\/li>\r\n \t
    • Temperature:\u00a0 higher temperatures increase the energy and molecular movement, increasing the rate of diffusion<\/li>\r\n<\/ul>\r\n
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      Concept in Action<\/h3>\r\nFor an animation of the diffusion process in action, view this short video on cell membrane transport.\r\n\r\nhttp:\/\/youtu.be\/JShwXBWGMyY\r\n\r\n<\/div>\r\n<\/section>
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      Facilitated transport\/diffusion<\/h2>\r\n

      In facilitated transport, also known as facilitated diffusion, material moves across the plasma membrane from high to low concentration without the need for ATP.\u00a0 However, the use of integral proteins provide the advantage of reaching equilibrium easier and faster. The substances are passed to specific integral proteins that facilitate their passage by forming channels(pores) to allow certain substances to pass through the membrane. The integral proteins involved in facilitated transport are referred to as transport proteins and function as either channels or carriers.\u00a0 Substances that undergo facilitated transport might not pass through otherwise.<\/p>\r\n\r\n

      Osmosis<\/h2>\r\n
      <\/figure>\r\n[caption id=\"attachment_1155\" align=\"alignright\" width=\"399\"]\"Two Figure 2. In osmosis, water always moves from an area of higher concentration (of water) to one of lower concentration (of water). In this system, the solute cannot pass through the selectively permeable membrane.[\/caption]\r\n\r\nOsmosis is the diffusion(high to low) of water through a semipermeable membrane.\u00a0 The membrane limits the diffusion of solutes in the water. Osmosis is a special case of diffusion. Water, like other substances, moves from an area of higher concentration to one of lower concentration. Imagine a beaker with a semipermeable membrane, separating the two sides(Figure 2).\u00a0 The water(solvent) is the same on both sides of the membrane but there are different concentrations on each side of a dissolved substance(solute) that cannot cross the membrane. If the volume of the water is the same, but the concentrations of solute are different, then there are also different concentrations of water on either side of the membrane.\r\n

      A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if possible.\u00a0 Only the material capable of getting through the membrane will diffuse.\u00a0 In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Therefore, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane, osmosis, will continue until the concentration gradient of water goes to zero. Osmosis is actively involved in all living things.<\/p>\r\n\r\n

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      Concept in Action<\/h3>\r\nWatch this video that illustrates diffusion in hot versus cold solutions.\r\n\r\nhttp:\/\/youtu.be\/JShwXBWGMyY\r\n\r\n<\/div>\r\n<\/section>
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      Tonicity<\/h2>\r\n

      Tonicity describes the amount of solute in a solution.\u00a0 Three terms\u2014hypertonic, isotonic, and hypotonic\u2014are used to relate the tonicity of a cell to the tonicity of a cell's extracellular fluid(Figures 3 and 4)\/<\/p>\r\nIn a hypertonic solution, the extracellular fluid contains less water than the cell. Because the cell has a lower concentration of solutes, the water will leave the cell. In effect, the solute is drawing the water out of the cell. In animal cells, this may result in the cell undergoing crenation(shrivel). Plants will undergo the process of plasmolysis(wilt).\r\n\r\nIn an isotonic solution, the extracellular fluid has the same tonicity as the cell. If the concentration of solutes of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell. In animal cells, this the considered the best tonicity. Plants can withstand this tonicity but prefer a hypotonic solution. Blood cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances.\r\n\r\nIn a hypotonic solution, the extracellular fluid has a lower concentration of solutes than the cell. \u00a0 The extracellular fluid has a higher concentration of water than does the cell. In this situation, water will follow its concentration gradient and enter the cell.\u00a0 In animal cells, the cell may swell(lysis).\u00a0 Due to the large, central vacuole and the cell wall, plants prefer this form of tonicity, referred to as turgor.\r\n\r\n \r\n

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      Art Connection<\/h3>\r\n[caption id=\"attachment_1156\" align=\"aligncenter\" width=\"469\"]\"Illustration Figure 3. Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. (credit: modification of work by Mariana Ruiz Villarreal)[\/caption]\r\n\r\n<\/header>
      \r\n
      <\/figure>\r\n

      A doctor injects a patient with what is thought to be an isotonic saline solution. The patient dies.\u00a0 An autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?<\/p>\r\n\r\n<\/section><\/div>\r\n\r\n[caption id=\"attachment_1157\" align=\"aligncenter\" width=\"544\"]\"The Figure 4. The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in. (credit: modification of work by Mariana Ruiz Villarreal)[\/caption]\r\n\r\n<\/section>

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      Section Summary<\/h2>\r\n

      The passive forms of transport, diffusion and osmosis, move material without the need for energy.\u00a0 Substances diffuse from areas of high concentration to areas of low concentration, and this process continues until the substance is evenly distributed in a system. In solutions of more than one substance, each type of molecule diffuses according to its own concentration gradient. Many factors can affect the rate of diffusion, including concentration gradient, molecular size, and temperature.<\/p>\r\n

      In living systems, diffusion of substances into and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane while others are hindered.\u00a0 Their passage is only made possible by protein channels and carriers. The chemistry of living things occurs in aqueous solutions, and balancing the concentrations of those solutions is an ongoing problem.<\/p>\r\nhttps:\/\/www.openassessments.org\/assessments\/649\r\n\r\n<\/section>\r\n

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      Additional Self Check Questions<\/h3>\r\n
      1. A doctor injects a patient with what is thought to be an isotonic saline solution. The patient dies.\u00a0 An autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?<\/section>
      <\/section>
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      2. Why does osmosis occur?<\/p>\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n<\/section>

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      <\/div>\r\n
      \u00a0<\/section><\/div>\r\n<\/section><\/div>\r\n<\/section><\/div>\r\n \r\n\r\n
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      Answers<\/h3>\r\n1. No, it must have been hypotonic.\u00a0 A hypotonic solution would cause water to enter the cells, thereby making them burst.\r\n\r\n2. Water moves through a semipermeable membrane in osmosis because there is a concentration gradient across the membrane of solute and solvent. The solute cannot effectively move to balance the concentration on both sides of the membrane, so water moves to achieve this balance.\r\n\r\n<\/div>\r\n \r\n\r\n<\/section><\/div>\r\n<\/section><\/div>\r\n<\/section><\/div>","rendered":"
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      Learning Objectives<\/h3>\n

      By the end of this section, you will be able to:<\/p>\n

        \n
      • Explain why and how passive transport occurs<\/li>\n
      • Understand the processes of diffusion and osmosis<\/li>\n
      • Define tonicity and distinguish the three solutions formed<\/li>\n<\/ul>\n<\/div>\n<\/div>\n

        Plasma membranes determine what substances enter and exit a cell.\u00a0 It must balance this movement, preventing harmful material from entering and essential material from leaving. For this reason, plasma membranes are selectively permeable.\u00a0 They permit some substances through but prevent others. If this selectivity was lost, the cell would no longer be able to sustain itself, and it would be destroyed.\u00a0 Cells must have a way of obtaining these materials from the extracellular fluids. This may happen passively when certain materials move back and forth.\u00a0 But cells may have special mechanisms that ensures transport. Most cells expend much of their energy(ATP), to create and maintain this constant transfer.\u00a0 The plasma membrane structure can assist but also inhibit some of this movement.<\/p>\n

        The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to expend energy.\u00a0 During passive transport, substances move from an area of higher concentration to an area of lower concentration in a process called diffusion.<\/p>\n

        \n

        Selective Permeability<\/h2>\n

        \u00a0Changes in environmental conditions greatly impact what enters and exits a cell.\u00a0 The plasma membrane adjusts to these changes in many ways.\u00a0 Integral proteins embedded within the plasma membrane act as channels(pumps) allowing materials to pass through the membrane. Carbohydrates, attached to lipids or proteins found on the exterior surface, help the cell bind to substances needed in the extracellular fluid.<\/p>\n

        So, what can enter and exit a cell?\u00a0 Plasma membranes have both hydrophilic and hydrophobic regions. This helps the movement of certain materials through the membrane and hinders the movement of others.\u00a0 Size and solubility are major determining factors in what can pass. Generally, small, uncharged particles have no trouble in passing, while large molecules and ions tend to face issues.\u00a0 In those cases, special mechanisms are used in order to penetrate the plasma membrane. \u00a0 Molecules of oxygen and carbon dioxide pass through quite easily.<\/p>\n

        Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane.\u00a0 In these instances, special mechanisms are again necessary.<\/p>\n<\/section>\n

        \n

        Diffusion<\/h2>\n

        Diffusion is a passive process. \u00a0 A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space.\u00a0 For example, what happens when someone opens a bottle of perfume in a room?\u00a0 The perfume is at its highest concentration in the bottle and at its lowest in the room. The perfume vapor will diffuse, or spread away, from the bottle.\u00a0 Gradually, the vapor will spread.\u00a0 Materials move within the cell\u2019s cytosol by this means of transport (Figure 1). Diffusion requires no energy use and stops when equilibrium is reached.\u00a0 The molecules do not stop movement but instead, maintain equilibrium.<\/p>\n

        \"The<\/p>\n

        Figure 1. Diffusion through a permeable membrane follows the concentration gradient of a substance, moving the substance from an area of high concentration to one of low concentration. (credit: modification of work by Mariana Ruiz Villarreal)<\/p>\n<\/div>\n

        <\/figure>\n

        Each separate substance in a medium has its own concentration gradient, independent of the concentration gradients of other materials. Additionally, each substance will diffuse according to that gradient.<\/p>\n

        Several factors affect the rate of diffusion.<\/p>\n

          \n
        • Concentration gradient:\u00a0 the greater the difference in concentration, the more rapid the diffusion;\u00a0 as equilibrium gets close, diffusion slows<\/li>\n
        • Molecular mass:\u00a0 larger molecules move more slowly,\u00a0 it is more difficult to move between the molecules of the substance they are moving through<\/li>\n
        • Temperature:\u00a0 higher temperatures increase the energy and molecular movement, increasing the rate of diffusion<\/li>\n<\/ul>\n
          \n

          Concept in Action<\/h3>\n

          For an animation of the diffusion process in action, view this short video on cell membrane transport.<\/p>\n