{"id":781,"date":"2017-08-30T14:36:42","date_gmt":"2017-08-30T18:36:42","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/chapter\/3-1-the-cell-membrane\/"},"modified":"2018-06-23T15:46:16","modified_gmt":"2018-06-23T19:46:16","slug":"3-1-the-cell-membrane","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/chapter\/3-1-the-cell-membrane\/","title":{"raw":"3.1 The Cell Membrane","rendered":"3.1 The Cell Membrane"},"content":{"raw":"<div class=\"bcc-box bcc-highlight\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Describe the structure and components of the cell membrane<\/li>\r\n \t<li>Describe the function of the plasma membrane<\/li>\r\n \t<li>Describe the \"fluid mosaic\" model of membrane structure<\/li>\r\n \t<li>Describe how the structure of the cell membrane affects membrane permeability<\/li>\r\n \t<li>Describe and explain the effects of placing erythrocytes in hypertonic, hypotonic, and isotonic solutions<\/li>\r\n \t<li>Describe and clearly distinguish between the processes of diffusion and active transport<\/li>\r\n \t<li>Describe and clearly distinguish between the processes of simple diffusion and facilitated diffusion<\/li>\r\n \t<li>Describe the process of osmosis and explain how it differs from diffusion<\/li>\r\n \t<li>Describe and clearly distinguish between the processes of phagocytosis and pinocytosis<\/li>\r\n \t<li>Explain the importance of each of the above transport processes to the functioning of a cell<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-id1455946\">Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.<\/p>\r\n\r\n<section id=\"fs-id2259833\">\r\n<h1>Structure and Composition of the Cell Membrane<\/h1>\r\n<p id=\"fs-id1859725\">The <strong>cell membrane<\/strong> is an extremely pliable structure composed primarily of back-to-back <strong>phospholipids<\/strong> (a \u201cbilayer\u201d). <strong>Cholesterol<\/strong> is also present, which contributes to the fluidity of the membrane, and there are various <strong>proteins<\/strong> embedded within the membrane that have a variety of functions.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignleft\" width=\"280\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0301_Phospholipid_Structure-3.jpg\" alt=\"This diagram shows the structure of a phospholipid. The hydrophilic head group is shown as a pink sphere and the two tails are shown as yellow rectangles.\" width=\"280\" height=\"609\" \/> Figure 1. Phospholipid Structure. A phospholipid molecule consists of a polar phosphate \u201chead,\u201d which is hydrophilic and a non-polar lipid \u201ctail,\u201d which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails.[\/caption]\r\n\r\nA single phospholipid molecule has a phosphate group on one end, called the \u201chead,\u201d and two side-by-side chains of fatty acids that make up the lipid tails (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_01\">Figure 1<\/a>). The phosphate group is negatively charged, making the head polar and hydrophilic\u2014or \u201cwater loving.\u201d A <strong>hydrophilic<\/strong> molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic\u2014or \u201cwater fearing.\u201d A <strong>hydrophobic<\/strong> molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules. An <strong>amphipathic<\/strong> molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away.\r\n<figure id=\"fig-ch03_01_01\"><figcaption><\/figcaption><\/figure>\r\n[caption id=\"\" align=\"alignright\" width=\"280\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0302_Phospholipid_Bilayer-3.jpg\" alt=\"This diagram shows a phospholipid bilayer. Two sets of phospholipids are arranged such that the hydrophobic tails are facing each other and the hydrophilic heads are facing the extracellular environment.\" width=\"280\" height=\"308\" \/> Figure 2. Phospolipid Bilayer. The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads contact the fluid inside and outside of the cell.[\/caption]\r\n\r\nThe cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_02\">Figure 2<\/a>). Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid.<strong> Intracellular fluid (ICF)<\/strong> is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. <strong>Extracellular fluid (ECF)<\/strong> is the fluid environment outside the enclosure of the cell membrane. <strong>Interstitial fluid (IF)<\/strong> is the term given to extracellular fluid not contained within blood vessels. Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place.\r\n\r\n<\/section><section id=\"fs-id1476432\">\r\n<h1>Membrane Proteins<\/h1>\r\n<p id=\"fs-id1858979\">The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins. Two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral protein (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_03\">Figure 3<\/a>). As its name suggests, an <strong>integral protein<\/strong> is a protein that is embedded in the membrane. A <strong>channel protein<\/strong> is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of the cell.<\/p>\r\n\r\n<figure id=\"fig-ch03_01_03\"><figcaption><\/figcaption>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"495\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0303_Lipid_Bilayer_With_Various_Components-3.jpg\" alt=\"This image shows a lipid bilayer with different types of proteins, lipids and cholesterol embedded in it.\" width=\"495\" height=\"400\" \/> Figure 3. Cell Membrane. The cell membrane of the cell is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.[\/caption]<\/figure>\r\n<p id=\"fs-id1417839\">Another important group of integral proteins are cell recognition proteins, which serve to mark a cell\u2019s identity so that it can be recognized by other cells. A <strong>receptor<\/strong> is a type of recognition protein that can selectively bind a specific molecule outside the cell, and this binding induces a chemical reaction within the cell. A <strong>ligand<\/strong> is the specific molecule that binds to and activates a receptor. Some integral proteins serve dual roles as both a receptor and an ion channel. One example of a receptor-ligand interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to flow into the cell.<\/p>\r\n<p id=\"fs-id1542775\">Some integral membrane proteins are glycoproteins. A <strong>glycoprotein<\/strong> is a protein that has carbohydrate molecules attached, which extend into the extracellular matrix. The attached carbohydrate tags on glycoproteins aid in cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx. The <strong>glycocalyx<\/strong> is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx can have various roles. For example, it may have molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients. The glycocalyces found in a person\u2019s body are products of that person\u2019s genetic makeup. They give each of the individual\u2019s trillions of cells the \u201cidentity\u201d of belonging in the person\u2019s body. This identity is the primary way that a person\u2019s immune defense cells \u201cknow\u201d not to attack the person\u2019s own body cells, but it also is the reason organs donated by another person might be rejected.<\/p>\r\n<p id=\"fs-id1955600\"><strong>Peripheral proteins<\/strong> are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein. These proteins typically perform a specific function for the cell. Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream.<\/p>\r\n\r\n<\/section><section id=\"fs-id1516545\">\r\n<h1>Transport across the Cell Membrane<\/h1>\r\n<p id=\"fs-id1332770\">One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca<sup>++<\/sup>, Na<sup>+<\/sup>, K<sup>+<\/sup>, and Cl<sup>\u2013<\/sup>; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO<sub>2<\/sub>), which must leave the cell.<\/p>\r\n<p id=\"fs-id1667456\">The membrane\u2019s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has <strong>selective permeability<\/strong> allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials\u2014like glucose, amino acids, and electrolytes\u2014need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. <strong>Passive transport<\/strong> is the movement of substances across the membrane using their own kinetic energy, without the expenditure of chemical energy. In contrast, <strong>active transport<\/strong> is the movement of substances across the membrane using energy from the hydrolysis of adenosine triphosphate (ATP).<\/p>\r\n\r\n<section id=\"fs-id1497364\">\r\n<h2>Passive Transport<\/h2>\r\n<p id=\"fs-id1435471\">In order to understand <em>how <\/em>substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A <strong>concentration gradient<\/strong> is the difference in concentration of a substance across a space. Molecules (or ions) will spread\/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move<em> down<\/em> their concentration gradient.) <strong>Diffusion<\/strong> is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept. Imagine being inside a closed bathroom. If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the bathroom, and this diffusion would go on until no more concentration gradient remains. Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures. Having an internal body temperature around 98.6<sup>\u00b0 <\/sup>F thus also aids in diffusion of particles within the body.<\/p>\r\n<p id=\"fs-id1502280\">Whenever a substance exists in greater concentration on one side of a semipermeable membrane than on the other side, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O<sub>2<\/sub>) and CO<sub>2<\/sub>. O<sub>2<\/sub> generally diffuses into cells because it is more concentrated outside of them, and CO<sub>2<\/sub> typically diffuses out of cells because it is more concentrated inside of them.\u00a0 In both\u00a0these examples the molecules rely on\u00a0their own kinetic energy to move, so neither of these examples requires any chemical energy output from\u00a0the cell. \u00a0The\u00a0movement of molecules across a cell membrane without the expenditure\u00a0of cellular energy\u00a0is referred to as <strong>passive transport, <\/strong>or<strong> diffusion<\/strong>.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"380\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0305_Simple_Diffusion_Across_Plasma_Membrane-3.jpg\" alt=\"This figure shows the simple diffusion of small non-polar molecules across the plasma membrane. A red horizontal arrow pointing towards the right indicates the progress of time. The nonpolar molecules are shown in blue and are present in higher numbers in the extracellular fluid. There are a few nonpolar molecules in the cytoplasm and their number increases with time.\" width=\"380\" height=\"339\" \/> Figure 4. Simple Diffusion across the Cell (Plasma) Membrane. The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.[\/caption]\r\n\r\nBefore moving on, you need to review the gases that can diffuse across a cell membrane. Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O<sub>2<\/sub> inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid into the cytoplasm within the cell. On the other hand, because cells produce CO<sub>2<\/sub> as a byproduct of metabolism, CO<sub>2<\/sub> concentrations rise within the cytoplasm; therefore, CO<sub>2<\/sub> will move from the cell into the interstitial fluid, where its concentration is lower. Both these molecules are small and nonpolar, which means they can easily interact with the hydrophobic core of a\u00a0lipid bilayer and move between the molecules to get from one side to the other. \u00a0This mechanism of small, nonpolar molecules slipping between the lipid tails of a cell\u00a0membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called <strong>simple diffusion<\/strong> (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_04\">Figure 4<\/a>).\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"300\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0306_Facilitated_Diffusion-3.jpg\" alt=\"This diagram shows the different means of facilitated diffusion across the plasma membrane. In the top panel, a channel protein is shown to allow the transport of solutes across the membrane. In the bottom panel, the membrane contains carrier proteins in addition to channel proteins.\" width=\"300\" height=\"987\" \/> Figure 5. Facilitated Diffusion. (a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.[\/caption]\r\n\r\nLarge polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. \u00a0Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane. <strong>Facilitated diffusion<\/strong> is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size, charge, and\/or polarity (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_05\">Figure 5<\/a>). A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.\u00a0 Glucose and other relatively large polar molecules typically bind to transport proteins that change shape to allow the molecules into the cell by a process known as <strong>carrier-mediated facilitated diffusion<\/strong>.\r\n<figure id=\"fig-ch03_01_05\"><figcaption><\/figcaption><\/figure>\r\n<p id=\"fs-id1535725\">As an example, even though sodium ions (Na<sup>+<\/sup>) are highly concentrated outside of cells, these electrolytes are charged and cannot pass through the nonpolar lipid bilayer of the membrane. Their diffusion is facilitated by membrane proteins that form sodium channels (or \u201cpores\u201d), so that Na<sup>+<\/sup> ions can move down their concentration gradient from outside the cells to inside the cells.\u00a0 The use of a protein that acts as a channel through which an ion or small polar molecule can move down its concentration gradient is referred to as\u00a0<strong>channel-mediated facilitated diffusion.<\/strong><\/p>\r\nThere are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require chemical energy expenditure by the cell.\r\n\r\n[caption id=\"\" align=\"alignleft\" width=\"280\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0307_Osmosis-3.jpg\" alt=\"This figure shows the diffusion of water through osmosis. The left panel shows a beaker with water and different solute concentrations. A semipermeable membrane is present in the middle of the beaker. In the right panel, the water concentration is higher to the right of the semipermeable membrane.\" width=\"280\" height=\"308\" \/> Figure 6. Osmosis. Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic relative to the solution\u00a0on the left side of the membrane.[\/caption]\r\n\r\nVery small polar molecules, including water, can cross a phospholipid bilayer via simple diffusion due to their small size. \u00a0The rate\u00a0at which water can move across cell membranes is increased by the presence of membrane\u00a0proteins called aquaporins that form channels through which water molecules (but not solutes) can pass. \u00a0<strong>Osmosis<\/strong>\u00a0refers to the passive movement\u00a0of water across\u00a0a semipermeable membrane (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_06\">Figure 6<\/a>). Osmosis across a cell membrane\u00a0therefore includes the movement of water molecules by either\u00a0simple diffusion or\u00a0facilitated diffusion or both.\r\n<figure id=\"fig-ch03_01_06\"><figcaption><\/figcaption><\/figure>\r\n<p id=\"fs-id1211584\">The movement of water across a cell membrane cannot be\u00a0directly determined by cells, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes inside the cells (in the cytoplasm). Two solutions that have the same concentration of solutes are said to be <strong>isotonic<\/strong> (equal tension). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"280\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0346_Concentration_of_Solutions-3.jpg\" alt=\"This image shows how a red blood cell responds to the tonicity of solution. The left panel shows the hypertonic case, the middle panel shows the isotonic case and the right panel shows the hypotonic case.\" width=\"280\" height=\"238\" \/> Figure 7. Concentration of Solutions. A hypertonic solution has a solute concentration higher than another solution. An isotonic solution has a solute concentration equal to another solution. A hypotonic solution has a solute concentration lower than another solution.[\/caption]\r\n\r\nOsmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. A solution that has a higher concentration of solutes than another solution is said to be <strong>hypertonic<\/strong>, and water molecules tend to diffuse into a hypertonic solution (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_07\">Figure 7<\/a>). Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be <strong>hypotonic<\/strong>, and water molecules tend to diffuse out of a hypotonic solution. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. A critical aspect of homeostasis in living things is to create an internal environment in which all of the body\u2019s cells are in an isotonic solution. Various organ systems, particularly the kidneys, work to maintain this homeostasis.\r\n<figure id=\"fig-ch03_01_07\"><figcaption><\/figcaption><\/figure>\r\n<p id=\"fs-id1307738\">Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluid\u2014and the solutes within it\u2014from a higher pressure area to a lower pressure area. Filtration is an extremely important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. Filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream.<\/p>\r\n\r\n<\/section><section id=\"fs-id850990\">\r\n<h2>Active Transport<\/h2>\r\n<p id=\"fs-id1666810\">For all of the transport methods described above, the cell does not need to use chemical energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of carrier proteins, and usually <em>against<\/em>\u00a0the concentration gradient of the substance being moved.<\/p>\r\n<p id=\"fs-id1465738\">One of the most common types of active transport involves proteins that serve as pumps. The word \u201cpump\u201d probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, chemical energy from ATP is required for these membrane proteins to transport substances\u2014molecules or ions\u2014across the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration).<\/p>\r\n<p id=\"fs-id2056316\">The <strong>sodium-potassium pump<\/strong>, which is also called Na<sup>+<\/sup>\/K<sup>+<\/sup> ATPase, transports sodium out of a cell while moving potassium into the cell. The Na<sup>+<\/sup>\/K<sup>+<\/sup> pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes.\u00a0 An <strong>electrical gradient<\/strong> is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na<sup>+<\/sup>\/K<sup>+<\/sup> pump moves three Na<sup>+<\/sup> ions out of the cell and two K<sup>+<\/sup> ions into the cell for each ATP molecule that is used (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_08\">Figure 8<\/a>). This process is so important for nerve cells that it accounts for the majority of their ATP usage.<\/p>\r\n\r\n<figure id=\"fig-ch03_01_08\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"550\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0308_Sodium_Potassium_Pump-3.jpg\" alt=\"This diagram shows many sodium potassium pumps embedded in the membrane. Potassium is pumped into the cytoplasm and sodium is pumped out of the cytoplasm.\" width=\"550\" height=\"500\" \/> Figure 8. Sodium-Potassium Pump. The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.[\/caption]<\/figure>\r\n<p id=\"eip-152\">Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell. In this way, the action of an active transport pump (the sodium-potassium pump) powers the passive transport of sodium ions by creating a concentration gradient. When active transport of one substance is used to power the transport of another substance in this way, it is called <strong>secondary active transport<\/strong>, to distinguish it from\u00a0<strong>primary active transport<\/strong>\u00a0mechanisms that use the chemical energy in ATP to directly drive the movement of an ion or molecule.<\/p>\r\n<p id=\"eip-214\"><strong>Symporters<\/strong> are <strong>secondary active transporters<\/strong> that move two substances in the same direction. For example, the sodium-glucose symporter uses sodium ions to \u201cpull\u201d glucose molecules into the cell. Because cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside. However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient.<\/p>\r\n<p id=\"eip-677\">Conversely, <strong>antiporters<\/strong> are secondary active transport systems that transport substances in opposite directions. For example, the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H+) out of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell's interior.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignleft\" width=\"380\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0309_Three_Forms_of_Endocytosis-3.jpg\" alt=\"This image shows the three different types of endocytosis. The left panel shows phagocytosis, where a large particle is seen to be engulfed by the membrane into a vacuole. In the middle panel, pinocytosis is shown, where a small particle is engulfed into a vesicle. In the right panel, receptor-mediated endocytosis is shown; the ligand binds to the receptor and is then engulfed into a coated vesicle.\" width=\"380\" height=\"513\" \/> Figure 9. Three Forms of Endocytosis. Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.[\/caption]\r\n\r\nOther forms of active transport do not involve membrane carriers. <strong>Endocytosis<\/strong> (bringing \u201cinto the cell\u201d) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_09\">Figure 9<\/a>). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A <strong>vesicle<\/strong> is a membranous sac\u2014a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested. <strong>Phagocytosis<\/strong> (\u201ccell eating\u201d) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. In contrast to phagocytosis, <strong>pinocytosis<\/strong> (\u201ccell drinking\u201d) brings fluid containing dissolved substances into a cell through membrane vesicles.\r\n<p id=\"fs-id1097508\">Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. <strong>Receptor-mediated endocytosis<\/strong> is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor\u2019s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"242\"]<img class=\"\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0310_Exocytosis-3.jpg\" alt=\"This figure shows the process of exocytosis. A vesicle is shown fusing with the membrane and then releasing its contents into the extracellular fluid.\" width=\"242\" height=\"272\" \/> Figure 10. Exocytosis. Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.[\/caption]\r\n<p id=\"fs-id1171152\">In contrast with endocytosis, <strong>exocytosis<\/strong> (taking \u201cout of the cell\u201d) is the process of a cell exporting material using vesicular transport (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_10\">Figure 10<\/a>). Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_11\">Figure 11<\/a>). Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.<\/p>\r\n\r\n<\/section><section>\r\n<figure id=\"fig-ch03_01_10\"><\/figure>\r\n<figure id=\"fig-ch03_01_11\">\r\n<div class=\"title\"><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"370\"]<img class=\"\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0311_Pancreatic_Cells_Micrograph-3.jpg\" alt=\"This micrograph shows the structure of a pancreatic acinar cell and the location of secretory vesicles.\" width=\"370\" height=\"175\" \/> Figure 11. Pancreatic Cells' Enzyme Products. The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will be exported from the cells via exocytosis. LM \u00d7 2900. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]<\/figure>\r\n<div id=\"fs-id1128792\" class=\"note anatomy interactive um\">\r\n\r\n[caption id=\"attachment_3050\" align=\"aligncenter\" width=\"150\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/3.1-amoeba-cell-transport-150x150.png\" alt=\"\" width=\"150\" height=\"150\" class=\"size-thumbnail wp-image-3050\" \/> Watch this <a href=\"https:\/\/www.youtube.com\/watch?v=Ptmlvtei8hw\">amoeba sisters video<\/a> to learn more about cell transport![\/caption]\r\n\r\n<section>\r\n\r\n[caption id=\"attachment_2954\" align=\"aligncenter\" width=\"150\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/3.1-crashcourse-150x150.png\" alt=\"\" width=\"150\" height=\"150\" class=\"size-thumbnail wp-image-2954\" \/> Watch this <a href=\"https:\/\/www.youtube.com\/watch?v=dPKvHrD1eS4\">CrashCourse video<\/a> on membranes and transport![\/caption]\r\n\r\n[caption id=\"attachment_2955\" align=\"aligncenter\" width=\"150\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/3.1-khan-150x150.png\" alt=\"\" width=\"150\" height=\"150\" class=\"size-thumbnail wp-image-2955\" \/> Check out the <a href=\"https:\/\/www.khanacademy.org\/science\/biology\/membranes-and-transport\">Khan Academy<\/a> membranes and transport section to find out more[\/caption]\r\n\r\n<\/section><\/div>\r\n<\/section><\/section><section id=\"fs-id1495386\" class=\"multiple-choice\">\r\n<dl id=\"fs-id1944092\" class=\"definition\">\r\n \t<dt><\/dt>\r\n<\/dl>\r\n<\/section>","rendered":"<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Describe the structure and components of the cell membrane<\/li>\n<li>Describe the function of the plasma membrane<\/li>\n<li>Describe the &#8220;fluid mosaic&#8221; model of membrane structure<\/li>\n<li>Describe how the structure of the cell membrane affects membrane permeability<\/li>\n<li>Describe and explain the effects of placing erythrocytes in hypertonic, hypotonic, and isotonic solutions<\/li>\n<li>Describe and clearly distinguish between the processes of diffusion and active transport<\/li>\n<li>Describe and clearly distinguish between the processes of simple diffusion and facilitated diffusion<\/li>\n<li>Describe the process of osmosis and explain how it differs from diffusion<\/li>\n<li>Describe and clearly distinguish between the processes of phagocytosis and pinocytosis<\/li>\n<li>Explain the importance of each of the above transport processes to the functioning of a cell<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-id1455946\">Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.<\/p>\n<section id=\"fs-id2259833\">\n<h1>Structure and Composition of the Cell Membrane<\/h1>\n<p id=\"fs-id1859725\">The <strong>cell membrane<\/strong> is an extremely pliable structure composed primarily of back-to-back <strong>phospholipids<\/strong> (a \u201cbilayer\u201d). <strong>Cholesterol<\/strong> is also present, which contributes to the fluidity of the membrane, and there are various <strong>proteins<\/strong> embedded within the membrane that have a variety of functions.<\/p>\n<figure style=\"width: 280px\" class=\"wp-caption alignleft\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0301_Phospholipid_Structure-3.jpg\" alt=\"This diagram shows the structure of a phospholipid. The hydrophilic head group is shown as a pink sphere and the two tails are shown as yellow rectangles.\" width=\"280\" height=\"609\" \/><figcaption class=\"wp-caption-text\">Figure 1. Phospholipid Structure. A phospholipid molecule consists of a polar phosphate \u201chead,\u201d which is hydrophilic and a non-polar lipid \u201ctail,\u201d which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails.<\/figcaption><\/figure>\n<p>A single phospholipid molecule has a phosphate group on one end, called the \u201chead,\u201d and two side-by-side chains of fatty acids that make up the lipid tails (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_01\">Figure 1<\/a>). The phosphate group is negatively charged, making the head polar and hydrophilic\u2014or \u201cwater loving.\u201d A <strong>hydrophilic<\/strong> molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic\u2014or \u201cwater fearing.\u201d A <strong>hydrophobic<\/strong> molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules. An <strong>amphipathic<\/strong> molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away.<\/p>\n<figure id=\"fig-ch03_01_01\"><figcaption><\/figcaption><\/figure>\n<figure style=\"width: 280px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0302_Phospholipid_Bilayer-3.jpg\" alt=\"This diagram shows a phospholipid bilayer. Two sets of phospholipids are arranged such that the hydrophobic tails are facing each other and the hydrophilic heads are facing the extracellular environment.\" width=\"280\" height=\"308\" \/><figcaption class=\"wp-caption-text\">Figure 2. Phospolipid Bilayer. The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads contact the fluid inside and outside of the cell.<\/figcaption><\/figure>\n<p>The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_02\">Figure 2<\/a>). Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid.<strong> Intracellular fluid (ICF)<\/strong> is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. <strong>Extracellular fluid (ECF)<\/strong> is the fluid environment outside the enclosure of the cell membrane. <strong>Interstitial fluid (IF)<\/strong> is the term given to extracellular fluid not contained within blood vessels. Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place.<\/p>\n<\/section>\n<section id=\"fs-id1476432\">\n<h1>Membrane Proteins<\/h1>\n<p id=\"fs-id1858979\">The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins. Two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral protein (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_03\">Figure 3<\/a>). As its name suggests, an <strong>integral protein<\/strong> is a protein that is embedded in the membrane. A <strong>channel protein<\/strong> is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of the cell.<\/p>\n<figure id=\"fig-ch03_01_03\"><figcaption><\/figcaption><figure style=\"width: 495px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0303_Lipid_Bilayer_With_Various_Components-3.jpg\" alt=\"This image shows a lipid bilayer with different types of proteins, lipids and cholesterol embedded in it.\" width=\"495\" height=\"400\" \/><figcaption class=\"wp-caption-text\">Figure 3. Cell Membrane. The cell membrane of the cell is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1417839\">Another important group of integral proteins are cell recognition proteins, which serve to mark a cell\u2019s identity so that it can be recognized by other cells. A <strong>receptor<\/strong> is a type of recognition protein that can selectively bind a specific molecule outside the cell, and this binding induces a chemical reaction within the cell. A <strong>ligand<\/strong> is the specific molecule that binds to and activates a receptor. Some integral proteins serve dual roles as both a receptor and an ion channel. One example of a receptor-ligand interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to flow into the cell.<\/p>\n<p id=\"fs-id1542775\">Some integral membrane proteins are glycoproteins. A <strong>glycoprotein<\/strong> is a protein that has carbohydrate molecules attached, which extend into the extracellular matrix. The attached carbohydrate tags on glycoproteins aid in cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx. The <strong>glycocalyx<\/strong> is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx can have various roles. For example, it may have molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients. The glycocalyces found in a person\u2019s body are products of that person\u2019s genetic makeup. They give each of the individual\u2019s trillions of cells the \u201cidentity\u201d of belonging in the person\u2019s body. This identity is the primary way that a person\u2019s immune defense cells \u201cknow\u201d not to attack the person\u2019s own body cells, but it also is the reason organs donated by another person might be rejected.<\/p>\n<p id=\"fs-id1955600\"><strong>Peripheral proteins<\/strong> are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein. These proteins typically perform a specific function for the cell. Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream.<\/p>\n<\/section>\n<section id=\"fs-id1516545\">\n<h1>Transport across the Cell Membrane<\/h1>\n<p id=\"fs-id1332770\">One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca<sup>++<\/sup>, Na<sup>+<\/sup>, K<sup>+<\/sup>, and Cl<sup>\u2013<\/sup>; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO<sub>2<\/sub>), which must leave the cell.<\/p>\n<p id=\"fs-id1667456\">The membrane\u2019s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has <strong>selective permeability<\/strong> allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials\u2014like glucose, amino acids, and electrolytes\u2014need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. <strong>Passive transport<\/strong> is the movement of substances across the membrane using their own kinetic energy, without the expenditure of chemical energy. In contrast, <strong>active transport<\/strong> is the movement of substances across the membrane using energy from the hydrolysis of adenosine triphosphate (ATP).<\/p>\n<section id=\"fs-id1497364\">\n<h2>Passive Transport<\/h2>\n<p id=\"fs-id1435471\">In order to understand <em>how <\/em>substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A <strong>concentration gradient<\/strong> is the difference in concentration of a substance across a space. Molecules (or ions) will spread\/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move<em> down<\/em> their concentration gradient.) <strong>Diffusion<\/strong> is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept. Imagine being inside a closed bathroom. If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the bathroom, and this diffusion would go on until no more concentration gradient remains. Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures. Having an internal body temperature around 98.6<sup>\u00b0 <\/sup>F thus also aids in diffusion of particles within the body.<\/p>\n<p id=\"fs-id1502280\">Whenever a substance exists in greater concentration on one side of a semipermeable membrane than on the other side, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O<sub>2<\/sub>) and CO<sub>2<\/sub>. O<sub>2<\/sub> generally diffuses into cells because it is more concentrated outside of them, and CO<sub>2<\/sub> typically diffuses out of cells because it is more concentrated inside of them.\u00a0 In both\u00a0these examples the molecules rely on\u00a0their own kinetic energy to move, so neither of these examples requires any chemical energy output from\u00a0the cell. \u00a0The\u00a0movement of molecules across a cell membrane without the expenditure\u00a0of cellular energy\u00a0is referred to as <strong>passive transport, <\/strong>or<strong> diffusion<\/strong>.<\/p>\n<figure style=\"width: 380px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0305_Simple_Diffusion_Across_Plasma_Membrane-3.jpg\" alt=\"This figure shows the simple diffusion of small non-polar molecules across the plasma membrane. A red horizontal arrow pointing towards the right indicates the progress of time. The nonpolar molecules are shown in blue and are present in higher numbers in the extracellular fluid. There are a few nonpolar molecules in the cytoplasm and their number increases with time.\" width=\"380\" height=\"339\" \/><figcaption class=\"wp-caption-text\">Figure 4. Simple Diffusion across the Cell (Plasma) Membrane. The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.<\/figcaption><\/figure>\n<p>Before moving on, you need to review the gases that can diffuse across a cell membrane. Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O<sub>2<\/sub> inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid into the cytoplasm within the cell. On the other hand, because cells produce CO<sub>2<\/sub> as a byproduct of metabolism, CO<sub>2<\/sub> concentrations rise within the cytoplasm; therefore, CO<sub>2<\/sub> will move from the cell into the interstitial fluid, where its concentration is lower. Both these molecules are small and nonpolar, which means they can easily interact with the hydrophobic core of a\u00a0lipid bilayer and move between the molecules to get from one side to the other. \u00a0This mechanism of small, nonpolar molecules slipping between the lipid tails of a cell\u00a0membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called <strong>simple diffusion<\/strong> (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_04\">Figure 4<\/a>).<\/p>\n<figure style=\"width: 300px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0306_Facilitated_Diffusion-3.jpg\" alt=\"This diagram shows the different means of facilitated diffusion across the plasma membrane. In the top panel, a channel protein is shown to allow the transport of solutes across the membrane. In the bottom panel, the membrane contains carrier proteins in addition to channel proteins.\" width=\"300\" height=\"987\" \/><figcaption class=\"wp-caption-text\">Figure 5. Facilitated Diffusion. (a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.<\/figcaption><\/figure>\n<p>Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. \u00a0Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane. <strong>Facilitated diffusion<\/strong> is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size, charge, and\/or polarity (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_05\">Figure 5<\/a>). A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.\u00a0 Glucose and other relatively large polar molecules typically bind to transport proteins that change shape to allow the molecules into the cell by a process known as <strong>carrier-mediated facilitated diffusion<\/strong>.<\/p>\n<figure id=\"fig-ch03_01_05\"><figcaption><\/figcaption><\/figure>\n<p id=\"fs-id1535725\">As an example, even though sodium ions (Na<sup>+<\/sup>) are highly concentrated outside of cells, these electrolytes are charged and cannot pass through the nonpolar lipid bilayer of the membrane. Their diffusion is facilitated by membrane proteins that form sodium channels (or \u201cpores\u201d), so that Na<sup>+<\/sup> ions can move down their concentration gradient from outside the cells to inside the cells.\u00a0 The use of a protein that acts as a channel through which an ion or small polar molecule can move down its concentration gradient is referred to as\u00a0<strong>channel-mediated facilitated diffusion.<\/strong><\/p>\n<p>There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require chemical energy expenditure by the cell.<\/p>\n<figure style=\"width: 280px\" class=\"wp-caption alignleft\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0307_Osmosis-3.jpg\" alt=\"This figure shows the diffusion of water through osmosis. The left panel shows a beaker with water and different solute concentrations. A semipermeable membrane is present in the middle of the beaker. In the right panel, the water concentration is higher to the right of the semipermeable membrane.\" width=\"280\" height=\"308\" \/><figcaption class=\"wp-caption-text\">Figure 6. Osmosis. Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic relative to the solution\u00a0on the left side of the membrane.<\/figcaption><\/figure>\n<p>Very small polar molecules, including water, can cross a phospholipid bilayer via simple diffusion due to their small size. \u00a0The rate\u00a0at which water can move across cell membranes is increased by the presence of membrane\u00a0proteins called aquaporins that form channels through which water molecules (but not solutes) can pass. \u00a0<strong>Osmosis<\/strong>\u00a0refers to the passive movement\u00a0of water across\u00a0a semipermeable membrane (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_06\">Figure 6<\/a>). Osmosis across a cell membrane\u00a0therefore includes the movement of water molecules by either\u00a0simple diffusion or\u00a0facilitated diffusion or both.<\/p>\n<figure id=\"fig-ch03_01_06\"><figcaption><\/figcaption><\/figure>\n<p id=\"fs-id1211584\">The movement of water across a cell membrane cannot be\u00a0directly determined by cells, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes inside the cells (in the cytoplasm). Two solutions that have the same concentration of solutes are said to be <strong>isotonic<\/strong> (equal tension). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function).<\/p>\n<figure style=\"width: 280px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0346_Concentration_of_Solutions-3.jpg\" alt=\"This image shows how a red blood cell responds to the tonicity of solution. The left panel shows the hypertonic case, the middle panel shows the isotonic case and the right panel shows the hypotonic case.\" width=\"280\" height=\"238\" \/><figcaption class=\"wp-caption-text\">Figure 7. Concentration of Solutions. A hypertonic solution has a solute concentration higher than another solution. An isotonic solution has a solute concentration equal to another solution. A hypotonic solution has a solute concentration lower than another solution.<\/figcaption><\/figure>\n<p>Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. A solution that has a higher concentration of solutes than another solution is said to be <strong>hypertonic<\/strong>, and water molecules tend to diffuse into a hypertonic solution (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_07\">Figure 7<\/a>). Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be <strong>hypotonic<\/strong>, and water molecules tend to diffuse out of a hypotonic solution. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. A critical aspect of homeostasis in living things is to create an internal environment in which all of the body\u2019s cells are in an isotonic solution. Various organ systems, particularly the kidneys, work to maintain this homeostasis.<\/p>\n<figure id=\"fig-ch03_01_07\"><figcaption><\/figcaption><\/figure>\n<p id=\"fs-id1307738\">Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluid\u2014and the solutes within it\u2014from a higher pressure area to a lower pressure area. Filtration is an extremely important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. Filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream.<\/p>\n<\/section>\n<section id=\"fs-id850990\">\n<h2>Active Transport<\/h2>\n<p id=\"fs-id1666810\">For all of the transport methods described above, the cell does not need to use chemical energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of carrier proteins, and usually <em>against<\/em>\u00a0the concentration gradient of the substance being moved.<\/p>\n<p id=\"fs-id1465738\">One of the most common types of active transport involves proteins that serve as pumps. The word \u201cpump\u201d probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, chemical energy from ATP is required for these membrane proteins to transport substances\u2014molecules or ions\u2014across the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration).<\/p>\n<p id=\"fs-id2056316\">The <strong>sodium-potassium pump<\/strong>, which is also called Na<sup>+<\/sup>\/K<sup>+<\/sup> ATPase, transports sodium out of a cell while moving potassium into the cell. The Na<sup>+<\/sup>\/K<sup>+<\/sup> pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes.\u00a0 An <strong>electrical gradient<\/strong> is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na<sup>+<\/sup>\/K<sup>+<\/sup> pump moves three Na<sup>+<\/sup> ions out of the cell and two K<sup>+<\/sup> ions into the cell for each ATP molecule that is used (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_08\">Figure 8<\/a>). This process is so important for nerve cells that it accounts for the majority of their ATP usage.<\/p>\n<figure id=\"fig-ch03_01_08\">\n<figure style=\"width: 550px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0308_Sodium_Potassium_Pump-3.jpg\" alt=\"This diagram shows many sodium potassium pumps embedded in the membrane. Potassium is pumped into the cytoplasm and sodium is pumped out of the cytoplasm.\" width=\"550\" height=\"500\" \/><figcaption class=\"wp-caption-text\">Figure 8. Sodium-Potassium Pump. The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"eip-152\">Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell. In this way, the action of an active transport pump (the sodium-potassium pump) powers the passive transport of sodium ions by creating a concentration gradient. When active transport of one substance is used to power the transport of another substance in this way, it is called <strong>secondary active transport<\/strong>, to distinguish it from\u00a0<strong>primary active transport<\/strong>\u00a0mechanisms that use the chemical energy in ATP to directly drive the movement of an ion or molecule.<\/p>\n<p id=\"eip-214\"><strong>Symporters<\/strong> are <strong>secondary active transporters<\/strong> that move two substances in the same direction. For example, the sodium-glucose symporter uses sodium ions to \u201cpull\u201d glucose molecules into the cell. Because cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside. However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient.<\/p>\n<p id=\"eip-677\">Conversely, <strong>antiporters<\/strong> are secondary active transport systems that transport substances in opposite directions. For example, the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H+) out of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell&#8217;s interior.<\/p>\n<figure style=\"width: 380px\" class=\"wp-caption alignleft\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0309_Three_Forms_of_Endocytosis-3.jpg\" alt=\"This image shows the three different types of endocytosis. The left panel shows phagocytosis, where a large particle is seen to be engulfed by the membrane into a vacuole. In the middle panel, pinocytosis is shown, where a small particle is engulfed into a vesicle. In the right panel, receptor-mediated endocytosis is shown; the ligand binds to the receptor and is then engulfed into a coated vesicle.\" width=\"380\" height=\"513\" \/><figcaption class=\"wp-caption-text\">Figure 9. Three Forms of Endocytosis. Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.<\/figcaption><\/figure>\n<p>Other forms of active transport do not involve membrane carriers. <strong>Endocytosis<\/strong> (bringing \u201cinto the cell\u201d) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_09\">Figure 9<\/a>). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A <strong>vesicle<\/strong> is a membranous sac\u2014a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested. <strong>Phagocytosis<\/strong> (\u201ccell eating\u201d) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. In contrast to phagocytosis, <strong>pinocytosis<\/strong> (\u201ccell drinking\u201d) brings fluid containing dissolved substances into a cell through membrane vesicles.<\/p>\n<p id=\"fs-id1097508\">Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. <strong>Receptor-mediated endocytosis<\/strong> is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor\u2019s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes.<\/p>\n<figure style=\"width: 242px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0310_Exocytosis-3.jpg\" alt=\"This figure shows the process of exocytosis. A vesicle is shown fusing with the membrane and then releasing its contents into the extracellular fluid.\" width=\"242\" height=\"272\" \/><figcaption class=\"wp-caption-text\">Figure 10. Exocytosis. Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.<\/figcaption><\/figure>\n<p id=\"fs-id1171152\">In contrast with endocytosis, <strong>exocytosis<\/strong> (taking \u201cout of the cell\u201d) is the process of a cell exporting material using vesicular transport (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_10\">Figure 10<\/a>). Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis (<a class=\"autogenerated-content\" href=\"#fig-ch03_01_11\">Figure 11<\/a>). Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.<\/p>\n<\/section>\n<section>\n<figure id=\"fig-ch03_01_10\"><\/figure>\n<figure id=\"fig-ch03_01_11\">\n<div class=\"title\"><\/div>\n<figure style=\"width: 370px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/0311_Pancreatic_Cells_Micrograph-3.jpg\" alt=\"This micrograph shows the structure of a pancreatic acinar cell and the location of secretory vesicles.\" width=\"370\" height=\"175\" \/><figcaption class=\"wp-caption-text\">Figure 11. Pancreatic Cells&#8217; Enzyme Products. The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will be exported from the cells via exocytosis. LM \u00d7 2900. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/figcaption><\/figure>\n<\/figure>\n<div id=\"fs-id1128792\" class=\"note anatomy interactive um\">\n<figure id=\"attachment_3050\" aria-describedby=\"caption-attachment-3050\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/3.1-amoeba-cell-transport-150x150.png\" alt=\"\" width=\"150\" height=\"150\" class=\"size-thumbnail wp-image-3050\" \/><figcaption id=\"caption-attachment-3050\" class=\"wp-caption-text\">Watch this <a href=\"https:\/\/www.youtube.com\/watch?v=Ptmlvtei8hw\">amoeba sisters video<\/a> to learn more about cell transport!<\/figcaption><\/figure>\n<section>\n<figure id=\"attachment_2954\" aria-describedby=\"caption-attachment-2954\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/3.1-crashcourse-150x150.png\" alt=\"\" width=\"150\" height=\"150\" class=\"size-thumbnail wp-image-2954\" \/><figcaption id=\"caption-attachment-2954\" class=\"wp-caption-text\">Watch this <a href=\"https:\/\/www.youtube.com\/watch?v=dPKvHrD1eS4\">CrashCourse video<\/a> on membranes and transport!<\/figcaption><\/figure>\n<figure id=\"attachment_2955\" aria-describedby=\"caption-attachment-2955\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-content\/uploads\/sites\/149\/2017\/08\/3.1-khan-150x150.png\" alt=\"\" width=\"150\" height=\"150\" class=\"size-thumbnail wp-image-2955\" \/><figcaption id=\"caption-attachment-2955\" class=\"wp-caption-text\">Check out the <a href=\"https:\/\/www.khanacademy.org\/science\/biology\/membranes-and-transport\">Khan Academy<\/a> membranes and transport section to find out more<\/figcaption><\/figure>\n<\/section>\n<\/div>\n<\/section>\n<\/section>\n<section id=\"fs-id1495386\" class=\"multiple-choice\">\n<dl id=\"fs-id1944092\" class=\"definition\">\n<dt><\/dt>\n<\/dl>\n<\/section>\n","protected":false},"author":10,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-781","chapter","type-chapter","status-publish","hentry"],"part":763,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/pressbooks\/v2\/chapters\/781","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/wp\/v2\/users\/10"}],"version-history":[{"count":10,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/pressbooks\/v2\/chapters\/781\/revisions"}],"predecessor-version":[{"id":1403,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/pressbooks\/v2\/chapters\/781\/revisions\/1403"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/pressbooks\/v2\/parts\/763"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/pressbooks\/v2\/chapters\/781\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/wp\/v2\/media?parent=781"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/pressbooks\/v2\/chapter-type?post=781"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/wp\/v2\/contributor?post=781"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol11031109\/wp-json\/wp\/v2\/license?post=781"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}