Chapter 7: Membrane Structure and Function PDF

7 KEY CONCEPTS Membrane Structure and Function 7.1 Cellular membranes are fluid mosaics of lipids and proteins p. 127 7.2 Membrane structure results in selective permeability p. 131 7.3 Passive transport is diffus...

7 KEY CONCEPTS Membrane Structure and Function 7.1 Cellular membranes are fluid mosaics of lipids and proteins p. 127 7.2 Membrane structure results in selective permeability p. 131 7.3 Passive transport is diffusion of a substance across a membrane with no energy investment p. 132 7.4 Active transport uses energy to move solutes against their gradients p. 136 7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis p. 139 Figure 7.1 Successful learning relies on communication between brain cells. Study Tip Here, the vesicles fusing with the plasma membrane of the top cell release molecules (yellow) that bind to membrane proteins (light green) on the surface of the bottom Make a visual study guide: Draw a cell, triggering the proteins to change shape. The plasma membrane that surrounds plasma membrane (two lines) all the each cell regulates its exchanges with its environment and surrounding cells. way down a piece of paper (or digitally). Label the cytoplasm and the extracellular fluid. At the top, draw the phospholipid bilayer in detail. As you read the chapter, How does the plasma membrane regulate draw and label membrane proteins that inbound and outbound traffic? you encounter and diagram the different Plasma membrane ways materials can enter or leave a cell. Vesicle Exocytosis: Large molecules are secreted when Cytoplasm Plasma Extracellular a vesicle fuses membrane fluid with the plasma membrane. Phospholipid bilayer Passive (draw in detail) transport Bulk Glycoprotein of small molecules transport (carbohydrate doesn’t require moves large + protein) energy; it may molecules. involve transport proteins. ADP ATP Endocytosis: Large molecules are P taken in when the plasma Go to Mastering Biology Transport membrane pinches protein inward, forming a vesicle. For Students (in eText and Study Area) Get Ready for Chapter 7 Figure 7.12 Walkthrough: Osmosis BioFlix® Animation: Membrane Transport Active transport For Instructors (in Item Library) of small molecules requires energy and a transport protein. Tutorial: Membrane Transport: Diffusion and Passive Transport Tutorial: Membrane Transport: Bulk Transport 126. Figure 7.2 Phospholipid bilayer (cross section). CONCEPT 7.1 Cellular membranes are fluid mosaics of lipids and proteins Two phospholipids Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. The most abun- WATER dant lipids in most membranes are phospholipids. Their Hydrophilic head ability to form membranes is inherent in their molecular struc- ture. A phospholipid is an amphipathic molecule, meaning Hydrophobic it has both a hydrophilic (“water-loving”) region and a hydro- tail phobic (“water-fearing”) region (see Figure 5.11). A phos- pholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement WATER shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water (Figure 7.2). VISUAL SKILLS Consulting Figure 5.11, circle and label Like membrane lipids, most membrane proteins are the hydrophilic and hydrophobic portions of the enlarged amphipathic. Such proteins can reside in the phospholipid phospholipids on the right. Explain what each portion contacts when the phospholipids are in the plasma membrane. bilayer with their hydrophilic regions protruding. This molec- ular orientation maximizes contact of hydrophilic regions of a Figure 7.3 shows the currently accepted model of the arrange- protein with water in the cytosol and extracellular ment of molecules in the plasma membrane. In this fluid fluid, while providing their hydrophobic parts mosaic model, the membrane is a mosaic of protein mol- with a nonaqueous environment. ecules bobbing in a fluid bilayer of phospholipids. Fibers of extra- cellular matrix (ECM) Glyco- Carbohydrates protein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Phospholipid Cholesterol Microfilaments of cytoskeleton Peripheral proteins m Figure 7.3 Current model of an animal cell’s plasma Integral membrane (cutaway view). Lipids are colored gray and protein gold, proteins purple, and carbohydrates green. CYTOPLASMIC SIDE Mastering Biology OF MEMBRANE Animation: Structure of the Plasma Membrane 127  The proteins are not randomly distributed in the mem- Some membrane proteins seem to move in a highly directed brane, however. Groups of proteins are often associated in manner, perhaps driven along cytoskeletal fibers by motor long-lasting, specialized patches, where they carry out com- proteins. And other proteins simply drift in the membrane, as mon functions. Researchers have found specific lipids in these shown the classic experiment described in Figure 7.4. patches as well and have proposed naming them lipid rafts; A membrane remains fluid as temperature decreases until however, the debate continues about whether such structures the phospholipids settle into a closely packed arrangement exist in living cells or are an artifact of biochemical tech- and the membrane solidifies, much as bacon grease forms niques. In some regions, the membrane may be much more lard when it cools. The temperature at which a membrane tightly packed with proteins than shown in Figure 7.3. Like all solidifies depends on the types of lipids it is made of. As the models, the fluid mosaic model is continually being refined as temperature decreases, the membrane remains fluid to a new research reveals more about membrane structure. lower temperature if it is rich in phospholipids with unsatu- rated hydrocarbon tails (see Figures 5.10 and 5.11). Because of kinks in the tails where double bonds are located, unsaturated The Fluidity of Membranes hydrocarbon tails cannot pack together as closely as satu- Membranes are not static sheets of molecules locked rigidly in rated hydrocarbon tails, making the membrane more fluid place. A membrane is held together mainly by hydrophobic (Figure 7.5a). interactions, which are much weaker than covalent bonds The steroid cholesterol, which is wedged between phos- (see Figure 5.18). Most of the lipids and some proteins can pholipid molecules in the plasma membranes of animal shift about sideways—that is, in the plane of the membrane, cells, has different effects on membrane fluidity at different like partygoers elbowing their way through a crowded room. temperatures (Figure 7.5b). At relatively high temperatures— Very rarely, also, a lipid may flip-flop across the membrane, at 37°C, the body temperature of humans, for example— switching from one phospholipid layer to the other. cholesterol makes the membrane less fluid by restraining The sideways movement of phospholipids within the phospholipid movement. However, because cholesterol also membrane is rapid. Adjacent phospholipids switch positions hinders the close packing of phospholipids, it lowers the about 107 times per second, which means that a phospho- temperature required for the membrane to solidify. Thus, lipid can travel about 2 mm—the length of a typical bacterial cholesterol can be thought of as a “fluidity buffer” for the cell—in 1 second. Proteins are much larger than lipids and membrane, resisting changes in membrane fluidity that can move more slowly, when they do move. Many membrane be caused by changes in temperature. Compared to animals, proteins seem to be held immobile by their attachment to the plants have very low levels of cholesterol; rather, related ste- cytoskeleton or to the extracellular matrix (see Figure 7.3). roid lipids buffer membrane fluidity in plant cells. ▼ Figure 7.4 Inquiry. Figure 7.5 Factors that affect membrane fluidity. Do membrane proteins move? Experiment Larry Frye and Michael Edidin, at Johns Hopkins (a) Unsaturated versus saturated hydrocarbon tails. University, labeled the plasma membrane proteins of a mouse cell Fluid Viscous and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. Results Membrane proteins + Unsaturated hydrocarbon Saturated hydrocarbon tails tails (kinked) prevent packing, pack together, increasing Mouse cell Mixed proteins enhancing membrane fluidity. membrane viscosity. Human cell after 1 hour Hybrid cell Conclusion The mixing of the mouse and human membrane (b) Cholesterol within the animal cell membrane. proteins indicates that at least some membrane proteins move Cholesterol Cholesterol reduces membrane sideways within the plane of the plasma membrane. fluidity at moderate tempera- Data from L. D. Frye and M. Edidin, The rapid intermixing of cell surface antigens tures by reducing phospholipid after formation of mouse-human heterokaryons, Journal of Cell Science 7:319 (1970). movement, but at low tempera- WHAT IF? Suppose the proteins did not mix in the hybrid cell, even tures it hinders solidification by disrupting the regular packing of many hours after fusion. Could you conclude that proteins don’t move phospholipids. within the membrane? What other explanation could there be? 128 UNIT TWO The Cell  Membranes must be fluid to work properly; the fluidity c Figure 7.6 The structure of a transmembrane protein. EXTRACELLULAR SIDE of a membrane affects both its permeability and the ability N-terminus Bacteriorhodopsin (a bacterial of membrane proteins to move to where their function is transport protein) has a distinct needed. Usually, membranes are about as fluid as olive oil. orientation in the membrane, with When a membrane solidifies, its permeability changes, and its N-terminus outside the cell and enzymatic proteins in the membrane may become inactive its C-terminus inside. This ribbon model highlights the secondary if their activity requires movement within the membrane. structure of the hydrophobic parts, However, membranes that are too fluid cannot support pro- including seven transmembrane tein function either. Therefore, extreme environments (for a helices, which lie mostly within example, those with extreme temperatures) pose a challenge the hydrophobic interior of c helix the membrane. The nonhelical for life, resulting in evolutionary adaptations that include dif- hydrophilic segments are in contact ferences in membrane lipid composition. with the aqueous solutions on the extracellular and cytoplasmic sides C-terminus CYTOPLASMIC SIDE of the membrane. Evolution of Differences in Membrane Lipid Composition EVOLUTION Variations in the cell membrane lipid compo- functions. Different types of cells contain different sets of sitions of many species appear to be evolutionary adaptations membrane proteins, and the various membranes within a cell that maintain the appropriate membrane fluidity under spe- each have a unique collection of proteins. cific environmental conditions. For instance, fishes that live Notice in Figure 7.3 that there are two major populations of in extreme cold have membranes with a high proportion of membrane proteins: integral proteins and peripheral proteins. unsaturated hydrocarbon tails, enabling their membranes to Integral proteins penetrate the hydrophobic interior of the remain fluid in spite of the low temperature (see Figure 7.5a). lipid bilayer. The majority are transmembrane proteins, At the other extreme, some bacteria and archaea thrive at which span the membrane; other integral proteins extend temperatures greater than 90°C (194°F) in thermal hot springs only partway into the hydrophobic interior. The hydrophobic and geysers. Their membranes include unusual lipids that regions of an integral protein consist of one or more stretches of may prevent excessive fluidity at such high temperatures. nonpolar amino acids (see Figure 5.14), typically 20–30 amino The ability to change the lipid composition of cell mem- acids in length, usually coiled into a helices (Figure 7.6). The branes in response to changing temperatures has evolved in hydrophilic parts of the molecule are exposed to the aqueous organisms that live where temperatures vary. In many plants solutions on either side of the membrane. Some proteins also that tolerate extreme cold, such as winter wheat, the per- have one or more hydrophilic channels that allow passage centage of unsaturated phospholipids increases in autumn, through the membrane of hydrophilic substances (even of water an adjustment that keeps the membranes from solidifying itself). Peripheral proteins are not embedded in the lipid during winter. Some bacteria and archaea also exhibit differ- bilayer at all; they are loosely bound to the surface of the mem- ent proportions of unsaturated phospholipids in their cell brane, often to exposed parts of integral proteins (see Figure 7.3). membranes, depending on the temperature at which they are On the cytoplasmic side of the plasma membrane, growing. Overall, natural selection has apparently favored some membrane proteins are held in place by attachment organisms whose mix of membrane lipids ensures an appro- to the cytoskeleton. And on the extracellular side, certain priate level of membrane fluidity for their environment. membrane proteins may attach to materials outside the cell. For example, in animal cells, membrane proteins may be attached to fibers of the extracellular matrix (see Figure 6.28; Membrane Proteins and Their Functions integrins are one type of integral, transmembrane protein). Now we come to the mosaic aspect of the fluid mosaic model. These attachments combine to give animal cells a stronger Somewhat like a tile mosaic (shown here), a membrane is a framework than the plasma membrane alone could provide. collage of different proteins, often clustered Figure 7.7 illustrates six major functions performed by together in groups, embedded in the proteins of the plasma membrane. A single cell may have fluid matrix of the lipid bilayer (see different membrane proteins that carry out various functions, Figure 7.3). More than 50 kinds of and one protein may itself carry out multiple functions. Thus, the proteins have been found in the membrane is a functional mosaic as well as a structural one. plasma membrane of red blood cells, for example. Phospholipids Mastering Biology Animation: Functions of the Plasma Membrane form the main fabric of the membrane, but proteins deter- Proteins on a cell’s surface are important in the medi- mine most of the membrane’s cal field. For example, a protein called CD4 on the surface b Tile mosaic. CHAPTER 7 Membrane Structure and Function 129. Figure 7.7 Some functions of membrane proteins. In many of immune cells helps the human immunodeficiency virus cases, a single protein performs multiple tasks. (HIV) infect these cells, leading to acquired immune defi- (a) Transport. Left: A protein that spans ciency syndrome (AIDS). Despite multiple exposures to HIV, the membrane may provide a hydrophilic however, a small number of people do not develop AIDS channel across the membrane that is and show no evidence of HIV-infected cells. Comparing selective for a particular solute (see Figures 6.32a and 7.15a). Right: Other their genes with the genes of infected individuals, research- transport proteins shuttle a substance ers learned that resistant people have an unusual form of a from one side to the other by changing gene that codes for an immune cell-surface protein called shape (see Figure 7.15b). Some of these proteins hydrolyze ATP as an energy CCR5. Further work showed that although CD4 is the main source to actively pump substances ATP HIV receptor, HIV must also bind to CCR5 as a “co-receptor” across the membrane. to infect most cells (Figure 7.8a). An absence of CCR5 on (b) Enzymatic activity. A protein built into Enzymes the cells of resistant individuals, due to the gene alteration, the membrane may be an enzyme with prevents the virus from entering the cells (Figure 7.8b). its active site (where the reactant binds) exposed to substances in the adjacent This information has been key to developing a treatment solution. In some cases, several enzymes for HIV infection. Interfering with CD4 causes dangerous in a membrane are organized as a team side effects because of its many important functions in cells. that carries out sequential steps of a metabolic pathway. Discovery of the CCR5 co-receptor provided a safer target for development of drugs that mask this protein and block HIV Signaling molecule entry. One such drug, maraviroc (brand name Selzentry), (c) Signal transduction. A membrane protein (receptor) may have a binding Receptor was approved for treatment of HIV in 2007; ongoing trials to site with a specific shape that fits the determine its ability to prevent HIV infection in uninfected, shape of a chemical messenger, such as a hormone. The external messenger at-risk patients have been disappointing. (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic The Role of Membrane Carbohydrates protein (see Figures 6.32a and 11.6). Signal transduction in Cell-Cell Recognition Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. It is important, for example, in the sorting (d) Cell-cell recognition. Some glyco- of cells into tissues and organs in an animal embryo. It is also proteins serve as identification tags that are specifically recognized by membrane the basis for the rejection of foreign cells by the immune sys- proteins of other cells. This type of tem, an important line of defense in vertebrate animals (see cell-cell binding is usually short-lived Glyco- compared with that shown in (e). protein Concept 43.1). Cells recognize other cells by binding to mol- ecules, often containing carbohydrates, on the extracellular surface of the plasma membrane (see Figure 7.7d). (e) Intercellular joining. Membrane proteins of adjacent cells may hook. Figure 7.8 The genetic basis for HIV resistance. together in various kinds of junctions, such as gap junctions or tight junctions HIV (see Figure 6.30). This type of binding is more long-lasting than that shown in (d). (f) Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the Receptor Receptor (CD4) cytoskeleton may be noncovalently (CD4) Co-receptor but no CCR5 Plasma bound to membrane proteins, a function that helps maintain cell shape and (CCR5) membrane stabilizes the location of certain membrane proteins. Proteins that can (a) HIV can infect a cell with (b) HIV cannot infect a cell lacking bind to ECM molecules can coordinate CCR5 on its surface, as in CCR5 on its surface, as in extracellular and intracellular changes most people. resistant individuals. (see Figure 6.28). MAKE CONNECTIONS Study Figures 2.16 and 5.17; each shows pairs VISUAL SKILLS Some transmembrane proteins can bind to a of molecules binding to each other. What would you predict about CCR5 particular ECM molecule and, when bound, transmit a signal into the cell. that would allow HIV to bind to it? How could a drug molecule interfere Use the proteins shown in (c) and (f) to explain how this might occur. with this binding? 130 UNIT TWO The Cell  Membrane carbohydrates are usually short, branched chains CONCEPT CHECK 7.1 of fewer than 15 sugar units. Some are covalently bonded to 1. VISUAL SKILLS Carbohydrates are attached to plasma lipids, forming molecules called glycolipids. (Recall that glyco membrane proteins in the ER (see Figure 7.9). On which refers to carbohydrate.) However, most are covalently bonded side of the vesicle membrane are the carbohydrates during transport to the cell surface? to proteins, which are thereby glycoproteins (see Figure 7.3). 2. WHAT IF? How might the membrane lipid composition of a The carbohydrates on the extracellular side of the plasma native grass found in very warm soil around hot springs differ membrane vary from species to species, among individuals of from that of a native grass found in cooler soil? Explain. the same species, and even from one cell type to another in For suggested answers, see Appendix A. a single individual. The diversity of the molecules and their location on the cell’s surface enable membrane carbohydrates CONCEPT 7.2 to function as markers that distinguish one cell from another. For example, the four human blood types designated A, B, AB, Membrane structure results and O reflect variation in the carbohydrate part of glycopro- in selective permeability teins on the surface of red blood cells. The biological membrane has emergent properties beyond those of the many individual molecules that make it up. The Synthesis and Sidedness of Membranes remainder of this chapter focuses on one of those proper- Membranes have distinct inside and outside faces. The two ties: A membrane exhibits selective permeability; that is, lipid layers may differ in lipid composition, and each protein it allows some substances to cross more easily than others. has directional orientation in the membrane (see Figure 7.6). The ability to regulate transport across cellular boundaries is Figure 7.9 shows how membrane sidedness arises: The asym- essential to the cell’s existence. We will see once again that metrical arrangement of proteins, lipids, and their associated form fits function: The fluid mosaic model helps explain how carbohydrates in the plasma membrane is determined as the membranes regulate the cell’s molecular traffic. membrane is being built. A steady traffic of small molecules and ions moves across the plasma membrane in both directions. Consider the chemical. Figure 7.9 Synthesis of membrane components and their orientation in the membrane. The cytoplasmic (orange) face of the plasma membrane differs from the extracellular (aqua) face. The latter arises from the inside face of ER, Golgi, and vesicle membranes. Lipid bilayer Transmembrane glycoprotein Glycolipid 11 Secretory proteins, membrane proteins, and lipids are Secretory synthesized in the endoplasmic reticulum (ER). In the ER, Attached protein carbohydrates (green) are added to the transmembrane carbohydrate proteins (purple dumbbells), making them glycoproteins. The carbohydrate portions may then be modified. Materials are transported in vesicles to the Golgi apparatus. 21 Inside the Golgi apparatus, the glycoproteins Golgi undergo further carbohydrate modification, and apparatus lipids acquire carbohydrates, becoming glycolipids. Vesicle 31 The glycoproteins, glycolipids, and secretory proteins (purple spheres) are transported in vesicles to the plasma membrane. ER 41 As vesicles fuse with the plasma membrane, the outside face of the vesicle becomes continuous with the inside (cytoplasmic) face of Glycolipid the plasma membrane. This releases the secretory proteins from the cell, a process called exocytosis, ER and positions the carbohydrates of membrane lumen glycoproteins and glycolipids on the outside (extracellular) face of the plasma membrane. Plasma membrane: DRAW IT Draw an integral membrane protein Cytoplasmic face Transmembrane extending from partway through the ER membrane glycoprotein Secreted Extracellular face into the ER lumen. Next, draw the protein where it protein would be located in a series of numbered steps ending Membrane at the plasma membrane. Would the protein contact glycolipid the cytoplasm or the extracellular fluid? Explain. CHAPTER 7 Membrane Structure and Function 131 exchanges between a muscle cell and the extracellular fluid This “glucose transporter” is so selective that it even rejects that bathes it. Sugars, amino acids, and other nutrients enter fructose, a structural isomer of glucose (see Figure 5.3). Thus, the cell, and metabolic waste products leave it. The cell takes in the selective permeability of a membrane depends on both O2 for use in cellular respiration and expels CO2. Also, the cell the discriminating barrier of the lipid bilayer and the specific regulates its concentrations of inorganic ions, such as Na+, K +, transport proteins built into the membrane. Ca2+, and Cl -, by shuttling them one way or the other across Mastering Biology Animation: Selective Permeability the plasma membrane. In spite of heavy traffic through them, of Membranes cell membranes are selective in their permeability: Substances What establishes the direction of traffic across a membrane? do not cross the barrier indiscriminately. The cell is able to take And what mechanisms drive molecules across membranes? up some small molecules and ions and exclude others. We will address these questions next as we explore two modes of membrane traffic: passive transport and active transport. The Permeability of the Lipid Bilayer CONCEPT CHECK 7.2 Nonpolar molecules, such as hydrocarbons, CO2, and O2, are hydrophobic, as are lipids. They can all therefore dissolve in 1. What property allows O2 and CO2 to cross a lipid bilayer without the aid of membrane proteins? the lipid bilayer of the membrane and cross it easily, without 2. VISUAL SKILLS Examine Figure 7.2. Why is a transport the aid of membrane proteins. However, the hydrophobic protein needed to move many water molecules rapidly interior of the membrane impedes direct passage through the across a membrane? membrane of ions and polar molecules, which are hydrophilic. 3. MAKE CONNECTIONS Aquaporins exclude passage of Polar molecules such as glucose and other sugars pass only hydronium ions (H3O+), but some aquaporins allow passage of slowly through a lipid bilayer, and even water, a very small glycerol, a three-carbon alcohol (see Figure 5.9), as well as H2O. Since H3O+ is closer in size to water than glycerol is, yet cannot polar molecule, does not cross rapidly relative to nonpolar pass through, what might be the basis of this selectivity? molecules. A charged atom or molecule and its surrounding For suggested answers, see Appendix A. shell of water (see Figure 3.8) are even less likely to penetrate the hydrophobic interior of the membrane. Furthermore, the lipid bilayer is only one aspect of the gatekeeper system CONCEPT 7.3 responsible for a cell’s selective permeability. Proteins built into the membrane play key roles in regulating transport. Passive transport is diffusion of a substance across a membrane Transport Proteins Specific ions and a variety of polar molecules can’t move with no energy investment through cell membranes on their own. However, these hydro- Molecules have a type of energy called thermal energy, due philic substances can avoid contact with the lipid bilayer by to their constant motion (see Concept 3.2). One result of this passing through transport proteins that span the membrane. motion is diffusion, the movement of particles of any sub- Some transport proteins, called channel proteins, function by stance so that they spread out into the available space. Each having a hydrophilic channel that certain molecules or ions molecule moves randomly, yet diffusion of a population of use as a tunnel through the membrane (see Figure 7.7a, left). molecules may be directional. Imagine a synthetic membrane For example, the passage of water molecules through the mem- separating pure water from a solution of a dye in water. Study Figure 7.11a carefully to see how diffusion would result in brane in certain cells is greatly facilitated by channel proteins known as aquaporins (Figure 7.10). Most aquaporin proteins equal concentrations of dye molecules in both solutions. At consist of four identical polypeptide subunits. Each polypep- that point, a dynamic equilibrium will exist, with as many dye tide forms a channel that water molecules pass through, single- molecules crossing per second in one direction as in the other. file, overall allowing entry of up to 3 billion water molecules Here is a simple rule of diffusion: In the absence of any other per second. Without aquaporins, only a tiny fraction of these forces, a substance will diffuse from where it is more concentrated water molecules would pass through the same area of the cell membrane in a second. Other transport proteins, called carrier c Figure 7.10 An proteins, hold on to their passengers and change shape in a way aquaporin. This computer model shows water molecules that shuttles them across the membrane (see Figure 7.7a, right). (red and gray) passing A transport protein is specific for the substance it translo- through an aquaporin (blue cates (moves), allowing only a certain substance (or a small ribbons), in a lipid bilayer group of related substances) to cross the membrane. For (yellow, hydrophilic heads; green, hydrophobic tails). example, a glucose carrier protein in the plasma membrane of red blood cells transports glucose across the membrane 50,000 times faster than glucose can pass through on its own. Mastering Biology Interview with Peter Agre: Discovering aquaporins Animation: Aquaporin 132 UNIT TWO The Cell to where it is less concentrated. Put another way, a substance (see Concept 2.2 and Figure 8.5b) and drives diffusion. diffuses down its concentration gradient, the region along Remember, though, that membranes are selectively permeable which the density of a chemical substance increases or decreases and therefore have different effects on the rates of diffusion of (in this case, decreases). Diffusion is a spontaneous process, various molecules. Water can diffuse very rapidly across the needing no input of energy. Each substance diffuses down its membranes of cells with aquaporins, compared with diffusion own concentration gradient, unaffected by the concentration in the absence of aquaporins. The movement of water across gradients of other substances (Figure 7.11b). the plasma membrane has important consequences for cells. Much of the traffic across cell membranes occurs by diffu- sion. When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for it to Effects of Osmosis on Water Balance diffuse across, down its concentration gradient (assuming that To see how two solutions with different solute concentra- the membrane is permeable to that substance). One important tions interact, picture a U-shaped glass tube with a selectively example is the uptake of oxygen by a cell performing cellular permeable artificial membrane separating two sugar solu- respiration. Dissolved oxygen diffuses into the cell across the tions (Figure 7.12). Pores in this synthetic membrane are too plasma membrane. As long as cellular respiration consumes small for sugar molecules to pass through but large enough the O2 as it enters, diffusion into the cell will continue because for water molecules. However, tight clustering of water mol- the concentration gradient favors movement in that direction. ecules around the hydrophilic solute molecules makes some The diffusion of a substance across a biological membrane is called passive transport because it requires no energy.. Figure 7.12 Osmosis. Two sugar solutions of different The concentration gradient itself represents potential energy concentrations are separated by a membrane that the solvent (water) can pass through but the solute (sugar) cannot. Water molecules move randomly and may cross in either direction, but overall, water. Figure 7.11 Diffusion of solutes across a synthetic diffuses from the solution with less concentrated solute to that with membrane. Each large arrow shows net diffusion of more concentrated solute. This passive transport of water, or osmosis, dye molecules of that color. makes the sugar concentrations on both sides roughly equal. Molecules of dye Membrane (cross section) Lower Higher More similar concentration concentration concentrations of solute of solute (sugar) of solute WATER Sugar molecule Net diffusion Net diffusion Equilibrium H 2O (a) Diffusion of one solute. Molecules of dye can pass through membrane pores. Random movement of dye molecules will cause some Selectively to pass through the pores; this happens more often on the side with permeable more dye molecules. The dye diffuses from the more concentrated side membrane to the less concentrated side (called diffusing down a concentration gradient). A dynamic equilibrium results: Solute molecules still cross, but Water molecules at roughly equal rates in both directions. can pass through Water molecules pores, but sugar cluster around molecules cannot. sugar molecules. This side has This side has more fewer solute solute molecules molecules and and fewer free more free water water molecules. molecules. Osmosis Net diffusion Net diffusion Equilibrium Water moves from an area of Net diffusion Net diffusion Equilibrium higher to lower free water concentration (lower to higher solute concentration). (b) Diffusion of two solutes. Solutions of two different dyes are sepa- VISUAL SKILLS If an orange dye capable of passing through the rated by a membrane that is permeable to both. Each dye diffuses membrane was added to the left side of the tube above, how would down its own concentration gradient. There will be a net diffusion it be distributed at the end of the experiment? (See Figure 7.11.) of the purple dye toward the left, even though the total solute Would the final solution levels in the tube be affected? What cellular concentration was initially greater on the left side. component does the membrane represent in this experiment? Mastering Biology Animation: Diffusion Mastering Biology Figure Walkthrough CHAPTER 7 Membrane Structure and Function 133 of the water unavailable to cross the membrane. As a result, are bathed in an extracellular fluid that is isotonic to the cells. the solution with a higher solute concentration has a lower In hypertonic or hypotonic environments, however, organ- free water concentration. Water diffuses across the membrane isms that lack rigid cell walls must have other adaptations for from the region of higher free water concentration (lower sol- osmoregulation, the control of solute concentrations and ute concentration) to that of lower free water concentration water balance. For example, the unicellular protist Paramecium (higher solute concentration) until the solute concentrations caudatum lives in pond water, which is hypotonic to the cell. on both sides of the membrane are more nearly equal. The Paramecium has a plasma membrane that is much less perme- diffusion of free water across a selectively permeable mem- able to water than the membranes of most other cells, but this brane, whether artificial or cellular, is called osmosis. The only slows the uptake of water, which continually enters the movement of water across cell membranes and the balance cell. The reason the Paramecium cell doesn’t burst is that it has of water between the cell and its environment are crucial to a contractile vacuole, an organelle that functions as a pump to organisms. Let’s now apply what we’ve learned about osmosis force water out of the cell as fast as it enters by osmosis (Figure in this system to living cells. 7.14). In contrast, the bacteria and archaea that live in hypersa- line (excessively salty) environments (see Figure 27.1) have cel- Water Balance of Cells Without Cell Walls lular mechanisms that balance the internal and external solute To explain the behavior of a cell in a solution, we must consider concentrations to ensure that water does not move out of the both solute concentration and membrane permeability. Both cell. We’ll examine other evolutionary adaptations for osmo- factors are taken into account in the concept of tonicity, the regulation by animals in Concept 44.1. ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concen- Water Balance of Cells with Cell Walls tration of solutes that cannot cross the membrane (nonpene- The cells of plants, prokaryotes, fungi, and some protists are trating solutes) relative to that inside the cell. If there is a higher surrounded by cell walls (see Figure 6.27). When such a cell concentration of nonpenetrating solutes in the surrounding is immersed in a hypotonic solution—bathed in rainwater, solution, water will tend to leave the cell, and vice versa. for example—the cell wall helps maintain the cell’s water bal- If a cell without a cell wall, such as an animal cell, is immersed ance. Consider a plant cell. Like an animal cell, the plant cell in an environment that is isotonic to the cell (iso means “same”), swells as water enters by osmosis (Figure 7.13b). However, the there will be no net movement of water across the plasma mem- relatively inelastic cell wall will expand only so much before brane. Water diffuses across the membrane, but at the same rate it exerts a back pressure on the cell, called turgor pressure, that in both directions. In an isotonic environ- ment, the volume of an animal cell is stable. Figure 7.13 The water balance of living cells. How living cells react to changes in the (Figure 7.13a). solute concentration of their environment depends on whether or not they have cell walls. (a) Animal cells, such as this red blood cell, do not have cell walls. (b) Plant cells do have cell Let’s transfer the cell to a solution that walls. (Arrows indicate net water movement after the cells were first placed in these solutions.) is hypertonic to the cell (hyper means Hypotonic solution Isotonic solution Hypertonic solution “more,” in this case referring to nonpenetrat- (a) Animal cell. An ing solutes). The cell will lose water, shrivel, animal cell, such as H2O H 2O H2O H2O and probably die. This is why an increase this red blood cell, in the salinity (saltiness) of a lake can kill does not have a cell wall. Animal cells fare the animals there; if the lake water becomes best in an isotonic hypertonic to the animals’ cells, they might environment unless they have special Lysed Normal Shriveled shrivel and die. However, taking up too adaptations that offset much water can be just as hazardous to a the osmotic uptake or loss of water. Plasma Cell wall cell as losing water. If we place the cell in a membrane H2O Plasma H2O solution that is hypotonic to the cell (hypo membrane H2O H 2O means “less”), water will enter the cell faster (b) Plant cell. Plant cells are turgid (firm) than it leaves, and the cell will swell and lyse and generally healthiest (burst) like an overfilled water balloon. in a hypotonic environ- A cell without rigid cell walls can’t toler- ment, where the uptake of water is eventually ate either excessive uptake or excessive loss balanced by the wall of water. This problem of water balance is pushing back on Turgid (normal) Flaccid Plasmolyzed automatically solved if such a cell lives in the cell. isotonic surroundings. Seawater is isotonic ? Why do limp celery stalks become crisp Mastering Biology Animation: Osmosis to many marine invertebrates. The cells of when placed in a glass of water? and Water Balance in Cells Video: Turgid Elodea most terrestrial (land-dwelling) animals Video: Plasmolysis in Elodea 134 UNIT TWO The Cell. Figure 7.14 The contractile vacuole of Paramecium. The. Figure 7.15 Two types of transport proteins that carry vacuole collects fluid from canals in the cytoplasm. When full, the out facilitated diffusion. In both cases, the protein can transport vacuole and canals contract, expelling fluid from the cell (LM). the solute in either direction, but the net movement is down the concentration gradient of the solute. 50 om Contractile vacuole (a) A channel EXTRACELLULAR protein has a FLUID channel through which water molecules or a specific solute can pass. Channel protein Solute CYTOPLASM Mastering Biology Video: Paramecium Vacuole (b) A carrier protein alternates between two opposes further water uptake. At this point, the cell is turgid shapes, moving (very firm), which is the healthy state for most plant cells. a solute across Plants that are not woody, such as most houseplants, depend the membrane during the for mechanical support on cells kept turgid by a surrounding shape change. hypotonic solution. If a plant’s cells and surroundings are iso- Carrier Solute protein tonic, there is no net tendency for water to enter and the cells become flaccid (limp); the plant wilts. Mastering Biology Animation: Facilitated Diffusion However, a cell wall is of no advantage if the cell is immersed in a hypertonic environment. In this case, a plant Channel proteins that transport ions are called ion cell, like an animal cell, will lose water to its surroundings and channels. Many ion channels function as gated channels, shrink. As the plant cell shrivels, its plasma membrane pulls which open or close in response to a stimulus (see Figure 11.8). away from the cell wall at multiple places. This phenomenon, For some gated channels, the stimulus is electrical. In a nerve called plasmolysis, causes the plant to wilt and can lead to cell, for example, a potassium ion channel protein (see com- plant death. The walled cells of bacteria and fungi also plas- puter model) opens in response. Potassium ion channel molyze in hypertonic environments. to an electrical stimulus, allow- protein (See also the side view ing a stream of potassium ions of a calcium channel protein in Facilitated Diffusion: Passive Transport to leave the cell. This restores Figure 6.32a.) Aided by Proteins the cell’s ability to fire again. Potassium Other gated channels have a ion Let’s look more closely at how water and certain hydrophilic chemical stimulus: They open solutes cross a membrane. As mentioned earlier, many polar or close when a specific sub- molecules and ions blocked by the lipid bilayer of the membrane stance (not the one to be trans- diffuse passively with the help of transport proteins that span the ported) binds to the channel. membrane. This phenomenon is called facilitated diffusion. Ion channels are important Cell biologists are still trying to learn exactly how various trans- in the functioning of the ner- port proteins facilitate diffusion. Most transport proteins are very vous system, as you’ll learn specific: They transport some substances but not others. in Chapter 48. As mentioned earlier, the two types of transport proteins are channel proteins and carrier proteins. Channel proteins simply provide corridors that allow specific molecules or ions to cross Mastering Biology the membrane (Figure 7.15a). The hydrophilic passageways pro- Interview with Elba Serrano: Investigating how ion channels enable you to hear vided by these proteins can allow water molecules or small ions to diffuse very quickly from one side of the membrane to the other. Aquaporins, the water channel proteins, facilitate the massive Carrier proteins, such as the glucose transporter men- levels of diffusion of water (osmosis) that occur in plant cells and tioned earlier, seem to undergo a subtle change in shape in animal cells such as red blood cells (see Figure 7.13). Certain that somehow translocates the solute-binding site across the kidney cells also have a high number of aquaporins, allowing membrane (Figure 7.15b). Such a change in shape may be them to reclaim water from urine before it is excreted. If the kid- triggered by the binding and release of the transported mole- neys didn’t perform this function, you would excrete about 180 L cule. Like ion channels, carrier proteins involved in facilitated of urine per day—and have to drink an equal volume of water! diffusion result in the net movement of a substance down CHAPTER 7 Membrane Structure and Function 135 Scientific Skills Exercise Interpreting a Scatter Plot with Two Sets. Glucose Uptake over Time in Guinea Pig Red Blood Cells. of Data 100 Is Glucose Uptake into Cells Affected by Age? Glucose, an important energy source for animals, is transported into cells by facilitated diffusion using protein carriers. In this exercise, you 80 radioactive glucose (mM) will interpret a graph with two sets of data from an experiment Concentration of that examined glucose uptake over time in red blood cells from 15-day-old guinea pigs of different ages. You will determine if the cells’ rate 60 of glucose uptake depended on the age of the guinea pig. guinea pig How the Experiment Was Done Researchers incubated guinea 40 pig red blood cells in a 300 mM (millimolar) radioactive glucose 1-month-old solution at pH 7.4 at 25°C. Every 10 or 15 minutes, they removed guinea pig a sample of cells and measured the concentration of radioactive 20 glucose inside those cells. The cells came from either a 15-day-old or a 1-month-old guinea pig. 0 0 10 20 30 40 50 60 Data from the Experiment When you have multiple sets of Incubation time (min) data, it can be useful to plot them on the same graph for com- parison. In the graph here, each set of dots (of the same color) Data from T. Kondo and E. Beutler, Developmental changes in glucose transport forms a scatter plot, in which every data point represents two of guinea pig erythrocytes, Journal of Clinical Investigation 65:1–4 (1980). numerical values, one for each variable. For each data set, a curve that best fits the points has been drawn to make it easier to see 2. From the data points on the graph, construct a table of the data. the trends. (For additional information about graphs, see the Put “Incubation Time (min)” in the left column of the table. Scientific Skills Review in Appendix D.) 3. What does the graph show? Compare and contrast glucose uptake in red blood cells from 15-day-old and 1-month-old guinea pigs. INTERPRET THE DATA 4. Develop a hypothesis to explain the difference between glucose 1. First make sure you understand the parts of the graph. uptake in red blood cells from 15-day-old and 1-month-old (a) Which variable is the independent variable—the variable guinea pigs. (Think about how glucose gets into cells.) controlled by the researchers? (b) Which variable is the depen- 5. Design an experiment to test your hypothesis. dent variable—the variable that depended on the treatment and was measured by the researchers? (c) What do the red Instructors: A version of this Scientific Skills Exercise can be dots represent? (d) The blue dots? assigned in Mastering Biology. its concentration gradient. No energy input is thus required: energy. Facilitated diffusion speeds transport of a solute by This is passive transport. The Scientific Skills Exercise gives you providing efficient passage through the membrane, but it does an opportunity to work with data from an experiment related not alter the direction of transport. Some other transport pro- to glucose transport. teins, however, can use energy to move solutes against their Mastering Biology BioFlix® Animation: Passive Transport concentration gradients, across the plasma membrane from the side where they are less concentrated (whether inside or CONCEPT CHECK 7.3 outside) to the side where they are more concentrated. 1. Speculate about how a cell performing cellular respiration might rid itself of the resulting CO2. 2. WHAT IF? If a Paramecium swims from a hypotonic to an The Need for Energy in Active Transport isotonic environment, will its contractile vacuole become To pump a solute across a membrane against its gradient more active or less? Why? requires work; the cell must expend energy. Therefore, this For suggested answers, see Appendix A. type of membrane traffic is called active transport. The transport proteins that move solutes against their concentra- tion gradients are all carrier proteins rather than channel pro- CONCEPT 7.4 teins. This makes sense because when channel proteins are Active transport uses energy to open, they merely allow solutes to diffuse down their concen- tration gradients rather than picking them up and transport- move solutes against their gradients ing them against their gradients. Despite the help of transport proteins, facilitated diffusion Active transport enables a cell to maintain internal concen- is considered passive transport because the solute is moving trations of small solutes that differ from concentrations in its down its concentration gradient, a process that requires no environment. For example, compared with its surroundings, 136 UNIT TWO The Cell c Figure 7.16 The sodium-potassium pump: a specific case EXTRACELLULAR [Na+] high Na+ of active transport. Na+ Na+ FLUID [K+] low This transport system Na+ pumps ions against steep Na+ Na+ concentration gradients. Na+ The pump oscillates Na+ between two shapes in a P ATP cycle that moves Na+ out ADP of the cell (steps 1 – 3 ) [Na+] low 2 Binding of 3 Na+ ions Na+ P and K + into the cell (steps CYTOPLASM [K+] high stimulates phosphorylation 4 – 6 ). The two shapes 1 Cytoplasmic Na+ binds to by a kinase, using ATP. 3 Phosphorylation leads to have different binding the sodium-potassium pump a change in protein shape, affinities for Na+ and K +. with high affinity when reducing its affinity for Na+; ATP hydrolysis powers the protein has this shape. 3 Na+ are released outside. the shape change by transferring a phosphate K+ group to the transport protein (phosphorylating the protein). K+ VISUAL SKILLS For each K+ ion (Na + and K + ), describe K+ K+ its concentration inside the P P cell relative to outside. How i K+ many Na + are moved out 4 The new shape has a of the cell and how many 6 2 K+ are released; high affinity for K+; 2 K+ K + moved in per cycle? affinity for Na+ is 5 Loss of the phosphate bind on the extracellular side, high again, and the group restores the protein’s triggering release of the Mastering Biology cycle repeats. original shape, which has a phosphate group. Animation: Active Transport lower affinity for K+. an animal cell has a much higher concentration of potassium. Figure 7.17 Review: passive Active transport. ions (K + ) and a much lower concentration of sodium ions and active transport. Some transport proteins expend energy and act (Na+ ). The plasma membrane helps maintain these steep gra- as pumps, moving dients by pumping Na+ out of the cell and K + into the cell. Passive transport. Substances diffuse substances across a As in other types of cellular work, ATP hydrolysis supplies spontaneously down their concentration membrane against their gradients, crossing a membrane with no concentration (or elec- the energy for most active transport. One way ATP can power expenditure of energy by the cell. The rate trochemical) gradients. active transport is when its terminal phosphate group is trans- of diffusion can be greatly increased Energy is usually supplied by transport proteins in the membrane. by ATP hydrolysis. ferred directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a sol- ute bound to the protein across the membrane. One transport system that works this way is the sodium-potassium pump, which exchanges Na+ for K + across the plasma membrane of animal cells (Figure 7.16). The distinction between passive transport and active transport is reviewed in Figure 7.17. How Ion Pumps Maintain Membrane Potential Diffusion. Facilitated diffusion. Many hydrophilic ATP All cells have voltages across their plasma membranes. Hydrophobic molecules and substances diffuse Voltage is electrical potential energy (see Concept 2.2)—a through membranes (at a slow rate) separation of opposite charges. The cytoplasmic side of the very small un- with the assistance of membrane is negative in charge relative to the extracellular charged polar transport proteins, molecules can either channel side because of an unequal distribution of anions and cations proteins (left) or carrier diffuse through on the two sides. The voltage across a membrane, called a the lipid bilayer. proteins (right). membrane potential, ranges from about - 50 to VISUAL SKILLS For each solute in the right panel, describe its - 200 millivolts (mV). (The minus sign indicates that the direction of movement, and state whether it is moving with or inside of the cell is negative relative to the outside.) against its concentration gradient. CHAPTER 7 Membrane Structure and Function 137  The membrane potential acts like a battery, an energy The sodium-potassium pump appears to be the major electro- source that affects the traffic of all charged substances across genic pump of animal cells. The main electrogenic pump of the membrane. Because the inside of the cell is negative com- plants, fungi, and bacteria is a proton pump, which actively pared with the outside, the membrane potential favors the transports protons (hydrogen ions, H +) out of the cell. The passive transport of cations into the cell and anions out of the pumping of H + transfers positive charge from the cytoplasm cell. Thus, two forces drive the diffusion of ions across a mem- to the extracellular solution (Figure 7.18). By generating volt- brane: a chemical force (the ion’s concentration gradient, age across membranes, electrogenic pumps help store energy which has been our sole consideration thus far in the chapter) that can be tapped for cellular work. One important use of and an electrical force (the effect of the membrane potential proton gradients in the cell is for ATP synthesis during cellular on the ion’s movement). This combination of forces acting respiration, as you will see in Concept 9.4. Another is a type of on an ion is called the electrochemical gradient. membrane traffic called cotransport. In the case of ions, then, we must refine our concept of pas- sive transport: An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient. Cotransport: Coupled Transport For example, the concentration of Na+ inside a resting nerve by a Membrane Protein cell is much lower than outside it. When the cell is stimulated, A solute that exists in different concentrations across a mem- gated channels open that facilitate Na+ diffusion. Sodium ions brane can do work as it moves across that membrane by dif- then “fall” down their electrochemical gradient, driven by the fusion down its concentration gradient. This is analogous concentration gradient of Na+ and by the attraction of these to water that has been pumped uphill and performs work as cations to the negative side (inside) of the membrane. In this it flows back down. In a mechanism called cotransport, a example, both electrical and chemical contributions to the transport protein (a cotransporter) can couple the “down- electrochemical gradient act in the same direction across the hill” diffusion of the solute to the “uphill” transport of a membrane, but this is not always so. In cases where electrical second substance against its own concentration gradient. forces due to the membrane potential oppose the simple diffu- For instance, a plant cell uses the gradient of H + generated by sion of an ion down its concentration gradient, active transport its ATP-powered proton pumps to drive the active transport may be necessary. In Concepts 48.2 and 48.3, you’ll learn about of amino acids, sugars, and several other nutrients into the the importance of electrochemical gradients and membrane cell. In the example shown in Figure 7.19, a cotransporter potentials in the transmission of nerve impulses. couples the return of H + to the transport of sucrose into the Some membrane proteins that actively transport ions con- cell. This protein can translocate sucrose into the cell against tribute to the membrane potential. Study Figure 7.16 to see if you can see why the sodium-potassium pump is a good exam-. Figure 7.19 Cotransport: active transport driven by a ple. Notice that the pump does not translocate Na+ and K + one concentration gradient. A carrier protein, such as this H+ /sucrose for one, but pumps three sodium ions out of the cell for every cotransporter in a plant cell (top), is able to use the diffusion of H+ two potassium ions it pumps into the cell. With each “crank” down its electrochemical gradient into the cell to drive the uptake of sucrose. (The cell wall is not shown.) Although not technically of the pump, there is a net transfer of one positive charge part of the cotransport process, an ATP-driven proton pump is from the cytoplasm to the extracellular fluid, a process that shown here (bottom), which concentrates H+ outside the cell. The stores energy as voltage. A transport protein that generates resulting H+ gradient represents potential energy that can be used voltage across a membrane is called an electrogenic pump. for active transport—of sucrose, in this case. Thus, ATP hydrolysis indirectly provides the energy necessary for cotransport. +. Figure 7.18 A proton pump. Proton pumps are electrogenic – pumps that store energy by generating voltage (charge separation) Sucrose across membranes. A proton pump translocates positive charge Sucrose H+/sucrose in the form of hydrogen ions (H+ , or protons). The voltage and H+ cotransporter Diffusion of H+ concentration gradient represent a dual energy source that can drive other processes, such as the uptake of nutrients. Most proton + H+ pumps are powered by ATP hydrolysis. (See Figure 6.32a.) – H+ H+ ATP – + EXTRACELLULAR – H+ + – FLUID H+ H+ + H+ H+ H+ Proton pump H+ H+ Proton pump H+ + H+ – + H+ ATP CYTOPLASM – + H+ 138 UNIT TWO The Cell its concentration gradient, but only if the sucrose molecule. Figure 7.20 Exocytosis. travels in the company of an H +. The H + uses the transport 1 A vesicle containing secretory proteins buds off from the Golgi. protein as an avenue to diffuse down its own electrochemical gradient, which is maintained by the proton pump. Plants use 2 The vesicle travels along a microtubule to H + /sucrose cotransport to load sucrose produced by photo- the plasma membrane. synthesis into cells in the veins of leaves. The vascular tissue of the plant can then distribute the sugar to roots and other Golgi 3 The outside of the nonphotosynthetic organs that do not make their own sugars. apparatus Micro- vesicle binds to the tubule inside of the plasma A similar cotransporter in animals transports Na+ into membrane. intestinal cells together with glucose, which is moving down its concentration gradient into the cell. (The Na+ is then 4 The two membranes pumped out of the cell into the blood on the other side by fuse, releasing (secreting) Na+ /K + pumps; see Figure 7.16.) Our understanding of Na+ / the proteins outside Plasma membrane the cell. glucose cotransporters has helped us find more effective treat- ments for diarrhea, a serious problem in developing countries. 5 The vesicle membrane becomes part of the plasma membrane. Normally, sodium in waste is reabsorbed in the colon, main- taining constant levels in the body, but diarrhea expels waste so rapidly that reabsorption is not possible, and sodium levels (Figure 7.20). A transport vesicle that has budded from the fall precipitously. To treat this life-threatening condition, Golgi apparatus moves along a microtubule of the cytoskel- patients are given a solution to drink containing high concen- eton to the plasma membrane. When the vesicle membrane trations of salt (NaCl) and glucose. The solutes are taken up by and plasma membrane come into contact, specific proteins Na+ /glucose cotransporters on the surface of intestinal cells in both membranes rearrange the lipid molecules of the two and passed through the cells into the blood. This simple treat- bilayers so that the two membranes fuse. The contents of ment has lowered infant mortality worldwide. the vesicle spill out of the cell, and the vesicle membrane becomes part of the plasma membrane. Mastering Biology BioFlix® Animation: Active Transport Many secretory cells use exocytosis to export products. For example, cells in the pancreas that make insulin secrete it into the CONCEPT CHECK 7.4 extracellular fluid by exocytosis. In another example, nerve cells 1. Na+>K + pumps help nerve cells establish a voltage across use exocytosis to release neurotransmitters that signal other neu- their plasma membranes. Do these pumps use ATP or pro- rons or muscle cells (see Figure 7.1). When plant cells are making duce ATP? Explain. cell walls, exocytosis delivers some of the necessary proteins and 2. VISUAL SKILLS Compare the Na+>K + pump in Figure 7.16 with the cotransporter in Figure 7.19. Explain why the carbohydrates from Golgi vesicles to the outside of the cell. Na+>K + pump would not be considered a cotransporter. 3. MAKE CONNECTIONS Review the characteristics of the lysosome in Concept 6.4. Given the internal environment of Endocytosis a lysosome, what transport protein might you expect to see In endocytosis, the cell takes in molecules and particulate in its membrane? matter by forming new vesicles from the plasma membrane. For suggested answers, see Appendix A. Although the proteins involved in the processes are different, the events of endocytosis look like the reverse of exocytosis. CONCEPT 7.5 First, a small area of the plasma membrane sinks inward to form a pocket. Then, as the pocket deepens, it pinches in, Bulk transport across the plasma forming a vesicle containing material that had been outside the cell. Study Figure 7.21 carefully to understand the three membrane occurs by exocytosis types of endocytosis: phagocytosis (“cellular eating”), pinocy- and endocytosis tosis (“cellular drinking”), and receptor-mediated endocytosis. Large molecules, such as proteins and polysaccharides, Human cells use receptor-mediated endocytosis to take in generally don’t cross the membrane by diffusion or transport cholesterol for membrane synthesis and the synthesis of other proteins. Instead, they usually enter and leave the cell in bulk, steroids. Cholesterol travels in the blood in particles called packaged in vesicles. low-density lipoproteins (LDLs), each a complex of lipids and a protein. LDLs bind to LDL receptors on plasma membranes Mastering Biology BioFlix® Animation: Exocytosis and Endocytosis and then enter the cells by endocytosis. In the inherited dis- ease familial hypercholesterolemia, characterized by a very Exocytosis high level of cholesterol in the blood, LDLs cannot enter cells The cell secretes certain molecules by the fusion of vesicles because the LDL receptor proteins are defective or missing. with the plasma membrane; this process is called exocytosis Cholesterol thus accumulates in the blood, contributing to CHAPTER 7 Membrane Structure and Function 139. Figure 7.21 Exploring Endocytosis in Animal Cells Phagocytosis Pinocytosis Receptor-Mediated Endocytosis EXTRACELLULAR FLUID Solutes Solutes Pseudopodium Receptor Plasma membrane Coat protein Food or Coated vesicle other particle Coated with specific pit solutes (purple) bound to receptors (red) Receptor-mediated endocytosis is a specialized type of pinocytosis that enables Coated the cell to acquire bulk quantities of specific vesicle substances, even though those substances may not be very concentrated in the Food extracellular fluid. Embedded in the plasma vacuole membrane are proteins with receptor sites In pinocytosis, a cell continually “gulps” exposed to the extracellular fluid. Specific droplets of extracellular fluid into tiny solutes bind to the receptors. The receptor vesicles, formed by infoldings of the plasma proteins then cluster in coated pits, and CYTOPLASM membrane. In this way, the cell obtains each coated pit forms a vesicle containing molecules dissolved in the droplets. Because the bound molecules. The diagram shows In phagocytosis, a cell engulfs a particle any a