Biology Campbell 12 ed Membrane Structure and Function PDF

7 Membrane Structure and Function KEY CONCEPTS 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 Act...

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