Membrane Structure and Function

Transcription

1 Membrane Structure and Function Chapter Concepts 4.1 Plasma Membrane Structure and Function The plasma membrane regulates the passage of molecules into and out of the cell. 68 The membrane contains lipids and proteins. Each protein has a specific function The Permeability of the Plasma Membrane Some substances, particularly small, noncharged molecules, pass freely across the plasma membrane. Ions and other types of molecules need assistance to cross the membrane Diffusion and Osmosis Molecules spontaneously diffuse (move from an area of higher concentration to an area of lower concentration), and some can diffuse across a plasma membrane. 73 Water diffuses across the plasma membrane, and this can affect cell size and shape Transport by Carrier Proteins Carrier proteins assist the transport of some ions and molecules across the plasma membrane Exocytosis and Endocytosis Vesicle formation takes other substances into the cell, and vesicle fusion with the plasma membrane discharges substances from the cell. 78 A single cell about to be pierced by a fine probe so that DNA can be removed by the suction tube on the bottom. An intact plasma membrane is necessary to the life of any cell and if it is ruptured the cell cannot continue to exist. 67

2 68 Part 1 Cell Biology 4-2 Banners flying on a castle wall mark off the community within from the surrounding countryside. Inside, residents go about their appointed tasks for the good of the community. Commands passed along from royalty to knights to workers are obeyed by all. The almost impenetrable wall prevents the enemy without from entering and disturbing the peace within. Only certain small creatures can pass through the open slitlike windows, and the drawbridge must be lowered for most needed supplies. The plasma membrane, which carries markers identifying it as belonging to the individual, can be likened to the castle wall. Under the command of the nucleus, the organelles carry out their specific functions and contribute to the working of the cell as a whole. Very few molecules can freely cross the membrane, and most nutrients must be transported across by special carriers. The cell uses these nutrients as a source of building blocks and energy to maintain the cell. The operations of the cell will continue only as long as the plasma membrane selectively permits specific materials to enter and leave and prevents the passage of others. 4.1 Plasma Membrane Structure and Function The plasma membrane is a phospholipid bilayer in which protein molecules are either partially or wholly embedded (Fig. 4.1). The phospholipid bilayer has a fluid consistency, comparable to that of light oil. The proteins are scattered throughout the membrane; therefore they form a mosaic pattern. This description of the plasma membrane is called the fluid-mosaic model of membrane structure. Phospholipids spontaneously arrange themselves into a bilayer. The hydrophilic (water loving) polar heads of the phospholipid molecules face the outside and inside of the cell where water is found, and the hydrophobic (water fearing) nonpolar tails face each other (Fig. 4.1). In addition to phospholipids, there are two other types of lipids in the plasma membrane. Glycolipids have a structure similar to phospholipids except that the hydrophilic head is a variety of sugars joined to form a straight or Outside cell glycolipid glycoprotein carbohydrate chain hydrophilic heads hydrophobic tails hydrophilic heads integral protein phospholipid bilayer cholesterol Inside cell peripheral protein filaments of the cytoskeleton plasma membrane Figure 4.1 Fluid-mosaic model of plasma membrane structure. The membrane is composed of a phospholipid bilayer in which proteins are embedded. The hydrophilic heads of phospholipids are a part of the outside surface and the inside surface of the membrane. The hydrophobic tails make up the interior of the membrane. Note the plasma membrane s asymmetry carbohydrate chains are attached to the outside surface and cytoskeleton filaments are attached to the inside surface.

3 4-3 Chapter 4 Membrane Structure and Function 69 branching carbohydrate chain. Cholesterol is a lipid that is found in animal plasma membranes; related steroids are found in the plasma membrane of plants. Cholesterol reduces the permeability of the membrane to most biological molecules. The proteins in a membrane may be peripheral proteins or integral proteins. Peripheral proteins occur either on the outside or the inside surface of the membrane. Some of these are anchored to the membrane by covalent bonding. Still others are held in place by noncovalent interactions that can be disrupted by gentle shaking or by change in the ph. Integral proteins are found within the membrane and have hydrophobic regions embedded within the membrane and hydrophilic regions that project from both surfaces of the bilayer: move through the hydrophobic center of the membrane.) The fluidity of a phospholipid bilayer means that cells are pliable. Imagine if they were not the long nerve fibers in your neck would crack whenever you nodded your head! Although some proteins are often held in place by cytoskeletal filaments, in general proteins are free to drift laterally in the fluid lipid bilayer. This has been demonstrated by fusing mouse and human cells, and watching the movement of tagged proteins (Fig. 4.2). Forty minutes after fusion, the proteins are completely mixed. The fluidity of the membrane is needed for the functioning of some proteins such as enzymes which become inactive when the membrane solidifies. The fluidity of the membrane, which is dependent on its lipid components, is critical to the proper functioning of the membrane s proteins. hydrophobic region hydrophilic regions mouse cell human cell Many integral proteins are glycoproteins, which have an attached carbohydrate chain. As with glycolipids, the carbohydrate chain of sugars projects externally. Therefore it can be said that the plasma membrane is sugarcoated. The plasma membrane is asymmetrical: the two halves are not identical. The carbohydrate chains of the glycolipids and proteins occur only on the outside surface and the cytoskeletal filaments attach to proteins only on the inside surface. cell fusion immediately after fusion The plasma membrane consists of a phospholipid bilayer. Peripheral proteins are found on the outside and inside surface of the membrane. Integral proteins span the lipid bilayer and often have attached carbohydrate chains. The Fluidity of the Plasma Membrane At body temperature, the phospholipid bilayer of the plasma membrane has the consistency of olive oil. The greater the concentration of unsaturated fatty acid residues, the more fluid is the bilayer. In each monolayer, the hydrocarbon tails wiggle, and the entire phospholipid molecule can move sideways at a rate averaging about 2 µm the length of a prokaryotic cell per second. (Phospholipid molecules rarely flip-flop from one layer to the other, because this would require the hydrophilic head to mixed membrane proteins Figure 4.2 Experiment to demonstrate lateral drifting of plasma membrane proteins. After human and mouse cells fuse, the plasma membrane proteins of the mouse (blue circles) and of the human cell (red circles) mix within a short time.

4 70 Part 1 Cell Biology 4-4 The Mosaic Quality of the Membrane The plasma membranes of various cells and the membranes of various organelles each have their own unique collections of proteins. The proteins form different patterns according to the particular membrane and also within the same membrane at different times. When you consider that the plasma membrane of a red blood cell contains over 50 different types of proteins, you can see why the membrane is said to be a mosaic. The integral proteins largely determine a membrane s specific functions. As we will discuss in more detail, certain plasma membrane proteins are involved in the passage of molecules through the membrane. Some of these are channel proteins through which a substance can simply move across the membrane; others are carrier proteins that combine with a substance and help it to move across the membrane. Still others are receptors; each type of receptor protein has a shape that allows a specific molecule to bind to it. The binding of a molecule, such as a hormone (or other signal molecule), can cause the protein to change its shape and bring about a cellular response. Some plasma membrane proteins are enzymatic proteins that carry out metabolic reactions directly. The peripheral proteins associated with the membrane often have a structural role in that they help stabilize and shape the plasma membrane. Figure 4.3 depicts the various functions of membrane proteins. Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl ) channel; a thick mucus collects in airways and in pancreatic and liver ducts. Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The inability of some persons to use energy for sodium-potassium (Na + K + ) transport has been suggested as the cause of their obesity. Cell Recognition Protein The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by blood cells responsible for immunity. The mosaic pattern of a membrane is dependent on the proteins, which vary in structure and function. Cell Cell Recognition The carbohydrate chains of glycolipids and glycoproteins serve as the fingerprints of the cell. The possible diversity of the chain is enormous; it can vary by the number of sugars (15 is usual, but there can be several hundred), by whether the chain is branched, and by the sequence of the particular sugars. Glycolipids and glycoproteins vary from species to species, from individual to individual of the same species, and even from cell to cell in the same individual. Therefore, they make cell cell recognition possible. Researchers working with mouse embryos have shown that as development proceeds, the different type cells of the embryo develop their own carbohydrate chains and that these chains allow the tissues and cells of the embryo to sort themselves out. As you probably know, transplanted tissues are often rejected by the body. This is because the immune system is able to recognize that the foreign tissue s cells do not have the same glycolipids and glycoproteins as the rest of the body s cells. We also now know that a person s particular blood type is due to the presence of particular glycoproteins in the membrane of red blood cells. Receptor Protein Is shaped in such a way that a specific molecule can bind to it. Pygmies are short, not because they do not produce enough growth hormone, but because their plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone. Enzymatic Protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase; sodium ions and water leave intestinal cells and the individual dies from severe diarrhea. Figure 4.3 Membrane protein diversity. These are some of the functions performed by proteins found in the plasma membrane.

5 The Growing Field of Tissue Engineering Tissue culture, the growing of animal cells in laboratory glassware, has been done for quite some time, but researchers always used cancer cells that divide without coaxing. Now researchers have learned how to grow all sorts of human cells in tissue culture and have hopes that they can even make the need for organ transplantation obsolete. Organ transplantation encounters two hurdles that are hard to overcome: (1) there is an overwhelming need, but few human organs are available to be transplanted; and (2) immunosuppressive drugs must be administered even if the organs are carefully matched to the recipient, because the body tends to reject foreign organs. To address these problems, some researchers have turned to pigs as a source of organs for humans. Through genetic engineering, they have crippled the enzymes that produce plasma membrane carbohydrate chains on pig cells; therefore, the human body is unable to recognize a pig organ as being foreign. Pigs carry viruses such as the one that causes swine flu, but pig viruses are not expected to cause infections in humans. Therefore, it is predicted that pig-to-human transplants will someday be safely done. Tissue engineering offers another possible solution to the transplant problem. Tissue engineering is an endeavor that produces manufactured bioproducts that can replace normal structures in the body. Integra is an artificial skin that consists of a porous matrix made of the protein collagen and a derivative of shark cartilage. This product, which is available in unlimited quantity, will not cause an immune reaction. Integra is used to cover extensive burns. Once the bottom layer of skin (the dermis) regrows, a graft of the patient s own outer layer of skin (the epidermis) replaces the artificial matrix. Researchers have also had success growing human cartilage for knee operations. In one study, 23 patients who were experiencing pain because of a lack of cartilage received a batch of their own chondrocytes (cartilage cells) grown in the laboratory. All patients reported that they were doing much better following the procedure. Other procedures have also been tried. It is possible to grow tissues to bolster weak ureters that take urine back to the kidneys instead of to the bladder where it belongs. And artificial tissue can be stitched into a bladder to increase its capacity. If research continues to be successful, nearly every human tissue is expected to undergo tissue engineering. Several groups are working on methods to reconstruct breast tissue after mastectomy so that one day women may have an alternative to silicone breast implants. Epithelial-lined plastic blood vessels are being developed because the walls of plastic blood vessels now used to replace weakened arteries sometimes cause the blood to clot. Certain organs produce chemicals that are needed by other cells. Diabetes mellitus occurs when the pancreas is no longer producing insulin, a molecule that causes all cells to take up glucose and the liver to store glucose as glycogen. Tissue engineering can possibly come to the rescue. Insulin-producing pancreatic cells from a pig can be grown in the laboratory. The cells are encased in plastic capsules called microreactors, because reactors are typically large vats where chemicals are produced (Fig. 4A). These capsules are so small they can be placed into the abdomen where they will float freely and produce insulin as needed. The membrane of the capsule contains pores that are large enough to allow oxygen and nutrients to flow in and wastes and insulin to flow out by diffusion. But the membrane of a microreactor will prevent immune cells from coming into contact with the enclosed pancreatic cells. Unless immune cells actually come in contact with transplanted cells, they cannot recognize them as foreign and destroy them. Researchers are even busily growing implantable liver tissue. They use a spongy material that can be seeded with the patient s own liver cells. Human embryonic cells are grown in tissue culture, and if differentiation can one day be achieved, it may be possible to supply Alzheimer patients with nerve cells, and cardiac patients with heart cells, and so forth. Figure 4A Microreactors. Microreactors filled with insulin-producing pancreatic cells from pigs flourished for 10 weeks in a diabetic mouse without immune systemsuppressing drugs. 71

6 72 Part 1 Cell Biology 4-6 Table 4.1 Passage of Molecules into and out of Cells Name Direction Requirement Examples Passive Transport Means Active Transport Means DIFFUSION Toward lower Concentration Lipid-soluble molecules, concentration gradient only water, and gases FACILITATED TRANSPORT Toward lower Carrier and Some sugars and concentration concentration gradient amino acids ACTIVE TRANSPORT Toward greater Carrier plus cellular Other sugars, amino acids, concentration energy and ions EXOCYTOSIS Toward outside Vesicle fuses with Macromolecules plasma membrane ENDOCYTOSIS Phagocytosis Toward inside Vacuole formation Cells and subcellular material Pinocytosis Toward inside Vesicle formation Macromolecules (includes receptormediated endocytosis) 4.2 The Permeability of the Plasma Membrane The plasma membrane is differentially (selectively) permeable. Some substances can move across the membrane and some cannot (Fig. 4.4). Macromolecules cannot diffuse across the membrane because they are too large. Ions and charged molecules cannot cross the membrane because they are unable to enter the hydrophobic phase of the lipid bilayer. Noncharged molecules such as alcohols and oxygen are lipid-soluble and therefore can cross the membrane with ease. They are able to slip between the hydrophilic heads of the phospholipids and pass through the hydrophobic tails of plasma membrane macromolecule noncharged molecule H 2 O + + charged molecules and ions the membrane. Small polar molecules such as carbon dioxide and water also have no difficulty crossing through the membrane. These molecules follow their concentration gradient which is a gradual decrease in concentration over distance. To take an example, oxygen is more concentrated outside the cell than inside the cell because a cell uses oxygen during aerobic cellular respiration. Therefore oxygen follows its concentration gradient as it enters a cell. Carbon dioxide, on the other hand, which is produced when a cell carries on cellular respiration, is more concentrated inside the cell than outside the cell, and therefore it moves down its concentration gradient as it exits a cell. Special means are sometimes used to get ions and charged molecules into and out of cells. Macromolecules can cross a membrane when they are taken in or out by vesicle formation (Table 4.1). Ions and molecules like amino acids and sugars are assisted across by one of two classes of transport proteins. Carrier proteins combine with an ion or molecule before transporting it across the membrane. Channel proteins form a channel that allows an ion or charged molecule to pass through. Our discussion in this chapter is largely restricted to carrier proteins. Carrier proteins are specific for the substances they transport across the plasma membrane. Ways of crossing a plasma membrane are classified as passive or active (Table 4.1). Passive ways, which do not use chemical energy, involve diffusion or facilitated transport. These passive ways depend on the motion energy of ions and molecules. Active ways, which do require chemical energy, include active transport, endocytosis, and exocytosis. Figure 4.4 How molecules cross the plasma membrane. The curved arrows indicate that these substances cannot cross the plasma membrane and the back and forth arrows indicate that these substances can cross the plasma membrane. The plasma membrane is differentially permeable. Certain substances can freely pass through the membrane and others must be transported across either by carrier proteins or by vacuole formation.

7 4-7 Chapter 4 Membrane Structure and Function 73 water molecules (solvent) dye molecules (solute) a. Crystal of dye is placed in water b. Diffusion of water and dye molecules c. Equal distribution of molecules results Figure 4.5 Process of diffusion. Diffusion is spontaneous, and no chemical energy is required to bring it about. a. When dye crystals are placed in water, they are concentrated in one area. b. The dye dissolves in the water, and there is a net movement of dye molecules from higher to lower concentration. There is also a net movement of water molecules from a higher to a lower concentration. c. Eventually, the water and the dye molecules are equally distributed throughout the container. 4.3 Diffusion and Osmosis M Diffusion is the movement of molecules from a higher to a lower concentration that is, down their concentration gradient until equilibrium is achieved and they are distributed equally. Diffusion is a physical process that can be observed with any type of molecule. For example, when a crystal of dye is placed in water (Fig. 4.5), the dye and water molecules move in various directions, but their net movement, which is the sum of their motion, is toward the region of lower concentration. Therefore, the dye is eventually dissolved in the water, resulting in a colored solution. A solution contains both a solute, usually a solid, and a solvent, usually a liquid. In this case, the solute is the dye and the solvent is the water molecules. Once the solute and solvent are evenly distributed, they continue to move about, but there is no net movement of either one in any direction. As discussed, the chemical and physical properties of the plasma membrane allow only a few types of molecules to enter and exit a cell simply by diffusion. Gases can diffuse through the lipid bilayer; this is the mechanism by which oxygen enters cells and carbon dioxide exits cells. Also, consider the movement of oxygen from the alveoli (air sacs) of the lungs to blood in the lung capillaries (Fig. 4.6). After inhalation (breathing in), the concentration of oxygen in the alveoli is higher than that in the blood; therefore, oxygen diffuses into the blood. The principle of diffusion can be employed in the treatment of certain human disorders, as is discussed in the Science Focus on page 71. oxygen capillary alveoli Figure 4.6 Gas exchange in lungs. Oxygen (O 2 ) diffuses into the capillaries of the lungs because there is a higher concentration of oxygen in the alveoli (air sacs) than in the capillaries. Molecules diffuse down their concentration gradients. A few types of small molecules can simply diffuse through the plasma membrane.

8 74 Part 1 Cell Biology 4-8 less water (higher percentage of solute) net movement of water to inside of thistle tube water solute solution rises due to movement of water toward lower percentage of solute more water (lower percentage of solute) 10% 5% < 10% > 5% a. c. differentially permeable membrane b. Figure 4.7 Osmosis demonstration. (Far left) A thistle tube, covered at the broad end by a differentially permeable membrane, contains a 10% sugar solution. The beaker contains a 5% sugar solution. (Middle) The solute (green circles) is unable to pass through the membrane, but the water (blue circles) passes through in both directions. There is a net movement of water toward the inside of the thistle tube, where there is a lower percentage of water molecules. (Far right) Due to the incoming water molecules, the level of the solution rises in the thistle tube. Osmosis Osmosis is the diffusion of water into and out of cells. To illustrate osmosis, a thistle tube containing a 10% sugar solution 1 is covered at one end by a differentially permeable membrane and is then placed in a beaker containing a 5% sugar solution (Fig. 4.7). The beaker contains more water molecules (lower percentage of solute) per volume, and the thistle tube contains fewer water molecules (higher percentage of solute) per volume. Under these conditions, there is a net movement of water from the beaker to the inside of the thistle tube across the membrane. The solute is unable to pass through the membrane; therefore, the level of the solution within the thistle tube rises (Fig. 4.7c). Notice the following in this illustration of osmosis: 1. A differentially permeable membrane separates two solutions. The membrane does not permit passage of the solute. 2. The beaker has more water (lower percentage of solute), and the thistle tube has less water (higher percentage of solute). 3. The membrane permits passage of water, and there is a net movement of water from the beaker to the inside of the thistle tube. 4. In the end, the concentration of solute in the thistle tube is less than 10%. Why? Because there is now less solute per volume. And the concentration of solute in the beaker is greater than 5%. Why? Because there is now more solute per volume. Water enters the thistle tube due to the osmotic pressure of the solution within the thistle tube. Osmotic pressure is the pressure that develops in a system due to osmosis 2. In other words, the greater the possible osmotic pressure the more likely water will diffuse in that direction. Due to osmotic pressure, water is absorbed from the human large intestine, is retained by the kidneys, and is taken up by capillaries from tissue fluid. Tonicity Tonicity refers to the strength of a solution in relationship to osmosis. In the laboratory, cells are normally placed in isotonic solutions; that is, the solute concentration is the same on both sides of the membrane, and therefore there is no net gain or loss of water (Fig. 4.8). The prefix iso means the same as, and the term tonicity refers to the strength of the solution. A 0.9% solution of the salt sodium chloride (NaCl) is known to be isotonic to red blood cells. Therefore, intravenous solutions medically administered usually have this tonicity. Solutions that cause cells to swell, or even to burst, due to an intake of water are said to be hypotonic solutions. The prefix hypo means less than, and refers to a solution with a lower percentage of solute (more water) than the cell. If a cell is placed in a hypotonic solution, water enters the cell; the net movement of water is from the outside to the inside of the cell. Any concentration of a salt solution lower than 0.9% is hypotonic to red blood cells. Animal cells placed in such a solution expand and sometimes burst due to the buildup of pressure. The term lysis is used to refer to disrupted cells; hemolysis, then, is disrupted red blood cells. 1 Percent solutions are grams of solute per 100 ml of solvent. Therefore, a 10% solution is 10 g of sugar with water added to make up 100 ml of solution. 2 Osmotic pressure is measured by placing a solution in an osmometer and then immersing the osmometer in pure water. The pressure that develops is the osmotic pressure of a solution.

9 4-9 Chapter 4 Membrane Structure and Function 75 plasma membrane Animal Cells Under isotonic conditions, there is no net movement of water. In a hypotonic environment, water enters the cell, which may burst (lysis). In a hypertonic environment, water leaves the cell, which shrivels (crenation). Plant Cells nucleus cell wall plasma membrane chloroplast Under isotonic conditions, there is no net movement of water. In a hypotonic environment, vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic environment, vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell. Figure 4.8 Osmosis in animal and plant cells. The arrows indicate the net movement of water. In an isotonic solution, a cell neither gains nor loses water; in a hypotonic solution, a cell gains water; and in a hypertonic solution, a cell loses water. The swelling of a plant cell in a hypotonic solution creates turgor pressure. When a plant cell is placed in a hypotonic solution, we observe expansion of the cytoplasm because the large central vacuole gains water and the plasma membrane pushes against the rigid cell wall. The plant cell does not burst because the cell wall does not give way. Turgor pressure in plant cells is extremely important to the maintenance of the plant s erect position. If you forget to water your plants they wilt due to decreased turgor pressure. Solutions that cause cells to shrink or to shrivel due to a loss of water are said to be hypertonic solutions. The prefix hyper means more than, and refers to a solution with a higher percentage of solute (less water) than the cell. If a cell is placed in a hypertonic solution, water leaves the cell; the net movement of water is from the inside to the outside of the cell. Any solution with a concentration higher than 0.9% sodium chloride is hypertonic to red blood cells. If animal cells are placed in this solution, they shrink. The term crenation refers to red blood cells in this condition. Meats are sometimes preserved by salting them. The bacteria are not killed by the salt but by the lack of water in the meat. When a plant cell is placed in a hypertonic solution, the plasma membrane pulls away from the cell wall as the large central vacuole loses water. This is an example of plasmolysis, a shrinking of the cytoplasm due to osmosis. Dead plants you see along a salted roadside after the winter died because they were exposed to a hypertonic solution. In an isotonic solution, a cell neither gains nor loses water. In a hypotonic solution, a cell gains water. In a hypertonic solution, a cell loses water and the cytoplasm shrinks.

10 76 Part 1 Cell Biology 4-10 Outside carrier Inside Outside carrier Inside 1 protein 1 protein 2 2 energy 3 3 Membrane Membrane Figure 4.9 Facilitated transport. A carrier protein speeds the rate at which a solute crosses a membrane from higher solute concentration to lower solute concentration. (1) Molecule enters carrier. (2) Molecule is transported across the membrane and exits on inside. (3) Carrier returns to its former state. Figure 4.10 Active transport. Active transport allows a solute to cross the membrane from lower solute concentration to higher solute concentration. (1) Molecule enters carrier. (2) Chemical energy of ATP is needed to transport the molecule which exits inside of cell. (3) Carrier returns to its former state. 4.4 Transport by Carrier Proteins M The plasma membrane impedes the passage of all but a few substances. Yet, biologically useful molecules do enter and exit the cell at a rapid rate because there are carrier proteins in the membrane. Carrier proteins are specific; each can combine with only a certain type of molecule, which is then transported through the membrane. It is not completely understood how carrier proteins function; but after a carrier combines with a molecule, the carrier is believed to undergo a change in shape that moves the molecule across the membrane. Carrier proteins are required for facilitated and active transport (see Table 4.1). Some of the proteins in the plasma membrane are carriers; they transport biologically useful molecules into and out of the cell. Facilitated Transport Facilitated transport explains the passage of such molecules as glucose and amino acids across the plasma membrane, even though they are not lipid soluble. The passage of glucose and amino acids is facilitated by their reversible combination with carrier proteins, which in some manner transport them through the plasma membrane. These carrier proteins are specific. For example, various sugar molecules of identical size might be present inside or outside the cell, but glucose can cross the membrane hundreds of times faster than the other sugars. This is a good example of the differential permeability of the membrane. The carrier for glucose has been isolated and a model has been developed to explain how it works (Fig. 4.9). It seems likely that the carrier has two conformations and that it switches back and forth between the two states. After glucose binds to the open end of a carrier, it closes behind the glucose molecule. As glucose moves along, the constricted end of the carrier opens in front of the molecule. After glucose is released into the cytoplasm of the cell, the carrier changes its conformation so that the binding site for glucose is again open. This process can occur as often as 100 times per second. Apparently, the cell has a pool of extra glucose carriers. When the hormone insulin binds to a plasma membrane receptor, more glucose carriers ordinarily appear in the plasma membrane. Some forms of diabetes are caused by insulin insensitivity; that is, the binding of insulin does not result in extra glucose carriers in the membrane. The model shows that after a carrier has assisted the movement of a molecule to the other side of the membrane, it is free to assist the passage of other similar molecules. Neither diffusion, explained previously, nor facilitated transport requires an expenditure of chemical energy because the molecules are moving down their concentration gradient in the same direction they tend to move anyway.

11 4-11 Chapter 4 Membrane Structure and Function 77 Active Transport During active transport, ions or molecules move through the plasma membrane, accumulating either inside or outside the cell. For example, iodine collects in the cells of the thyroid gland; nutrients are completely absorbed from the gut by the cells lining the digestive tract, and sodium ions (Na ) can be almost completely withdrawn from urine by cells lining the kidney tubules. In these instances, substances have moved to the region of higher concentration, exactly opposite to the process of diffusion. It has been estimated that up to 40% of a cell s energy supply may be used for active transport of solute across its membrane. Both carrier proteins and an expenditure of energy are needed to transport molecules against their concentration gradient (Fig. 4.10). In this case, energy (ATP molecules) is required for the carrier to combine with the substance to be transported. Therefore, it is not surprising that cells involved primarily in active transport, such as kidney cells, have a large number of mitochondria near the membrane through which active transport is occurring. Proteins involved in active transport often are called pumps, because just as a water pump uses energy to move water against the force of gravity, proteins use energy to move a substance against its concentration gradient. One type of pump that is active in all cells, but is especially associated with nerve and muscle cells, moves sodium ions (Na ) to the outside of the cell and potassium ions (K ) to the inside of the cell. These two events are presumed to be linked, and the carrier protein is called a sodiumpotassium pump. A change in carrier shape after the attachment, and again after the detachment, of a phosphate group allows the carrier to combine alternately with sodium ions and potassium ions (Fig. 4.11). The phosphate group is donated by ATP, which is broken down enzymatically by the carrier. The passage of salt (NaCl) across a plasma membrane is of primary importance in cells. The chloride ion (Cl ) usually crosses the plasma membrane because it is attracted by positively charged sodium ions (Na ). First, sodium ions are pumped across a membrane, and then chloride ions simply diffuse through channels that allow their passage. As noted in Figure 4.3, the chloride ion channels malfunction in persons with cystic fibrosis, and this leads to the symptoms of this inherited (genetic) disorder. Outside Carrier has a shape that allows it to take up three sodium ions (Na + ). ATP is split, and phosphate group is transferred to carrier. Change in shape results that 3 Na + causes carrier to release three sodium ions (Na + ) outside K + the cell. New shape allows carrier to take up potassium ions (K + ). Phosphate group is released from carrier. Change in shape results that causes carrier to release potassium ions (K + ) inside the cell. New shape is suitable to take up three sodium ions (Na + ) once again. 3 Na + K + 3 Na + P carrier P K + Inside P ATP ADP Figure 4.11 The sodium-potassium pump. A carrier protein actively moves three sodium ions (Na ) to the outside of the cell for every potassium ion (K ) pumped to the inside of the cell. Note that chemical energy of ATP is required. During facilitated transport, substances follow their concentration gradient. During active transport, substances are moved against their concentration gradient.

12 78 Part 1 Cell Biology Exocytosis and Endocytosis M What about the transport of macromolecules such as polypeptides, polysaccharides, or polynucleotides, which are too large to be transported by carrier proteins? They are transported in or out of the cell by vesicle formation, thereby keeping the macromolecules contained so that they do not mix with those in the cytoplasm. red blood cell plasma membrane Exocytosis During exocytosis, vesicles often formed by the Golgi apparatus and carrying a specific molecule, fuse with the plasma membrane as secretion occurs. This is the way that insulin leaves insulin-secreting cells, for instance. a. Phagocytosis solute vacuole plasma membrane b. Pinocytosis vesicle Inside Notice that the membrane of the vesicle becomes a part of the plasma membrane. During cell growth, exocytosis is probably used as a means to enlarge the plasma membrane, whether or not secretion is also taking place. solute receptor protein Endocytosis During endocytosis, cells take in substances by vesicle formation (Fig. 4.12). A portion of the plasma membrane invaginates to envelop the substance, and then the membrane pinches off to form an intracellular vesicle. When the material taken in by endocytosis is large, such as a food particle or another cell, the process is called phagocytosis. Phagocytosis is common in unicellular organisms like amoebas and in amoeboid cells like macrophages, which are large cells that engulf bacteria and worn-out red blood cells in mammals. When the endocytic vesicle fuses with a lysosome, digestion occurs. Pinocytosis occurs when vesicles form around a liquid or very small particles. Blood cells, cells that line the kidney tubules or intestinal wall, and plant root cells all use this method of ingesting substances. Whereas phagocytosis can be seen with the light microscope, the electron microscope must be used to observe pinocytic vesicles, which are no larger than 1 2 µm. c. Receptor-mediated endocytosis vesicle Figure 4.12 Three methods of endocytosis. a. Phagocytosis occurs when the substance to be transported into the cell is large; certain specialized cells in the body can engulf wornout red blood cells by phagocytosis. Digestion occurs when the resulting vacuole fuses with a lysosome. b. Pinocytosis occurs when a macromolecule such as a polypeptide is to be transported into the cell. The result is a small vacuole or vesicle. c. Receptor-mediated endocytosis is a form of pinocytosis. The substance to be taken in (a ligand) first binds to a specific receptor protein which migrates to a pit or is already in a pit. The vesicle that forms contains the ligand and its receptor. Sometimes the receptor is recycled, as shown in Figure 4.13.

13 4-13 Chapter 4 Membrane Structure and Function 79 receptor protein 1 solute endocytosis 2 receptor protein 3 solutes removed 4 exocytosis a. b. Figure 4.13 Receptor-mediated endocytosis. a. (1) The receptors in the coated pits combine only with a solute. (2) The vesicle that forms is at first coated with a fibrous protein (blue squares), but soon the vesicle loses its coat. (3) Solutes leave the vesicle. (4) When exocytosis occurs, membrane and therefore receptors are returned to the plasma membrane. b. Electron micrographs of a coated pit in the process of forming a vesicle. Receptor-mediated endocytosis is a form of pinocytosis that is quite specific because it involves the use of a receptor protein shaped in such a way that a specific molecule such as vitamins, peptide hormones, and lipoproteins can bind to it. The binding of a solute to the receptors causes the receptors to gather at one location. This location is called a coated pit because there is a layer of fibrous protein on the cytoplasmic side (see step 1, Fig. 4.13). Once the vesicle is formed, the fibrous coat is released and the vesicle appears uncoated (see step 2). The fate of the vesicle and its contents depends on the kind of solute it contains. Sometimes the solute simply enters the cytoplasm (step 3). A spent hormone, on the other hand, may be digested when the vesicle fuses with a lysosome. The membrane of the vesicle and, therefore, the receptors are returned to the plasma membrane (step 4), or the vesicle can go to other membranous locations. Aside from simply allowing substances to enter cells selectively from an extracellular fluid, coated pits are also involved in the transfer and exchange of substances between cells. Such exchanges take place when the substances move from maternal blood into fetal blood at the placenta, for example. The importance of receptor-mediated endocytosis is demonstrated by a genetic disorder called familial hypercholesterolemia. Cholesterol is transported in blood by a complex of lipids and proteins called low-density lipoprotein (LDL). These individuals have inherited a gene that causes them to have a reduced number and/or defective receptors for LDL in their plasma membranes. Instead of cholesterol entering cells, it accumulates in the walls of arterial blood vessels, leading to high blood pressure, occluded (blocked) arteries, and heart attacks. Substances are secreted from a cell by exocytosis. Substances enter a cell by endocytosis. Receptormediated endocytosis allows cells to take up specific kinds of molecules and then sort them within the cell.

14 80 Part 1 Cell Biology 4-14 Such celebrities as Mohammad Ali, a former heavyweight boxing champion, Janet Reno, the attorney general of the United States, and Michael J. Fox, a favorite movie actor, have Parkinson disease. By age 65, Parkinson disease (PD) affects roughly one of every 100 Americans. Due to the death of brain cells that produce a substance called dopamine, motor control is not as smooth as it should be. The three obvious signs of Parkinson are slowness of movement, tremor, and rigidity. As the disease worsens, patients become unable to carry out even the simplest activities. You might think that the condition could be cured by simply giving a patient dopamine, but dopamine, like many other chemical substances, cannot cross the blood-brain barrier. The blood-brain barrier is simply due to the impermeability of the capillaries serving the brain. Nutrients, such as glucose, and essential amino acids can only pass through due to facilitated transport. Most drugs can t get through at all. Luckily a precursor of dopamine called L-dopa can get through the bloodbrain barrier, and when L-dopa is given as a medication, it will be changed into dopamine until there are few cells left to do the job. Along the way, physicians and patients are faced with a wide assortment of adjunct remedies. Some of these are surgical procedures. Michael J. Fox opted for pallidotomy, a procedure that kills off cells that go out of control when the dopamineproducing cells die off. An experimental surgical procedure, however, involves the transplantation of dopamine-producing fetal tissue into the brains of people with PD. People who have received such transplants report a lessening of symptoms. Is it ethical to use tissue from aborted fetuses for transplants? Is it possible that women would have abortions just to make fetal tissue available to loved ones, or for payment? Should there be governmental safeguards to prevent such a possibility? Questions 1. Is it ethical to use fetal tissue to prevent older people from having a debilitating disorder? Why or why not? 2. Suppose you had a choice between using fetal tissue (no payment required) and adult cells bioengineered to produce dopamine (payment required), which would you choose and why? 3. Do you favor banning all research using fetal tissue, or doing such research under certain circumstances? Explain. Summarizing the Concepts 4.1 Plasma Membrane Structure and Function There are two components of the plasma membrane: lipids and proteins. In the lipid bilayer, phospholipids are arranged with their hydrophilic heads at the surfaces and their hydrophobic tails in the interior. The lipid bilayer has the consistency of oil, and therefore proteins can move laterally in the membrane. Glycolipids and glycoproteins are involved in marking the cell as belonging to a particular individual and tissue. The hydrophobic portion of an integral protein lies in the lipid bilayer of the plasma membrane, and the hydrophilic portion lies at the surface. Proteins act as receptors, carry on enzymatic reactions, join cells together, form channels, or act as carriers to move substances across the membrane. 4.2 The Permeability of the Plasma Membrane Some substances like gases and water are free to cross a plasma membrane, and others, particularly ions, charged molecules, and macromolecules, have to be assisted across. Passive ways of crossing a plasma membrane (diffusion and facilitated transport) do not require an expenditure of chemical energy. Active ways of crossing a plasma membrane (active transport and vesicle formation) do require an expenditure of chemical energy. 4.3 Diffusion and Osmosis Lipid-soluble compounds, water, and gases simply diffuse across the membrane from the area of higher concentration to the area of lower concentration. The diffusion of water across a differentially permeable membrane is called osmosis. Water moves across the membrane into the area of lower water (higher solute) content. When cells are in an isotonic solution, they neither gain nor lose water; when they are in a hypotonic solution, they gain water; and when they are in a hypertonic solution, they lose water. 4.4 Transport by Carrier Proteins Some molecules are transported across the membrane by carrier proteins that span the membrane. During facilitated transport, a carrier protein assists the movement of a molecule down its concentration gradient. No energy is required. During active transport, a carrier protein acts as a pump that causes a substance to move against its concentration gradient. The sodium-potassium pump carries Na to the outside of the cell and K to the inside of the cell. Energy in the form of ATP molecules is required for active transport to occur. 4.5 Exocytosis and Endocytosis Larger substances can enter and exit a membrane by endocytosis and exocytosis. Exocytosis involves secretion. Endocytosis includes phagocytosis and pinocytosis which includes receptor-mediated endocytosis. Receptor-mediated endocytosis makes use of receptor molecules in the plasma membrane. Once specific solutes bind to their receptors, the coated pit becomes a coated vesicle. After losing the coat, the vesicle can join with the lysosome, or after freeing the solute, the receptor-containing vesicle can fuse with the plasma membrane.

15 4-15 Chapter 4 Membrane Structure and Function 81 Studying the Concepts 1. Describe the structure of the plasma membrane, including the phospholipid bilayer and the various types of proteins Why is a plasma membrane called differentially permeable? What are the mechanisms by which substances enter and exit cells? Which are passive ways, and which are active ways? Define diffusion, and give an example Define osmosis. Define isotonic, hypertonic, and hypotonic solutions, and give examples of how these concentrations affect red blood cells Draw a simplified diagram of a red blood cell before and after being placed in these solutions. What terms are used to refer to the condition of the red blood cell in a hypertonic solution and in a hypotonic solution? Draw a simplified diagram of a plant cell before and after being placed in these solutions. Describe the cell contents under these conditions How does facilitated transport differ from simple diffusion across the plasma membrane? How does active transport differ from facilitated transport? Give an example Diagram and define endocytosis and exocytosis. Describe and contrast three methods of endocytosis Testing Yourself Choose the best answer for each question. 1. Label this diagram of the plasma membrane. a. b. j. i. h. 2. The fluid-mosaic model of membrane structure refers to a. the fluidity of proteins in the membrane and the pattern of phospholipids in the membrane. b. the fluidity of phospholipids and the pattern of proteins in the membrane. c. the fluidity of cholesterol and the pattern of sugar chains outside the membrane. d. the lack of fluidity of internal membranes compared to the plasma membrane, and the ability of the proteins to move laterally in the membrane. e. the fluidity of hydrophobic regions, proteins and mosaic pattern of hydrophilic regions. c. g. f. d. e. 3. A phospholipid molecule has a head and two tails. The tails are found a. at the surfaces of the membrane. b. in the interior of the membrane. c. spanning the membrane. d. where the environment is hydrophilic. e. Both a and b are correct. 4. During diffusion, a. all molecules move only from the area of higher to lower concentration. b. solvents move from the area of higher to lower concentration. c. there is a net movement of molecules from the area of higher to lower concentration. d. a cell must be present for any movement of molecules to occur. e. molecules move against their concentration gradient if they are small and charged. 5. When a cell is placed in a hypotonic solution, a. solute exits the cell to equalize the concentration on both sides of the membrane. b. water exits the cell toward the area of lower solute concentration. c. water enters the cell toward the area of higher solute concentration. d. solute exits and water enters the cell. e. Both c and d are correct. 6. When a cell is placed in a hypertonic solution, a. solute exits the cell to equalize the concentration on both sides of the membrane. b. water exits the cell toward the area of lower solute concentration. c. water exits the cell toward the area of higher solute concentration. d. solute exits and water enters the cell. e. Both a and c are correct. 7. Active transport a. requires a carrier protein. b. moves a molecule against its concentration gradient. c. requires a supply of energy. d. does not occur during facilitated transport. e. All of these are correct. 8. The sodium-potassium pump a. helps establish an electrochemical gradient across the membrane. b. concentrates sodium on the outside of the membrane. c. utilizes a carrier protein and energy. d. is present in the plasma membrane. e. All of these are correct. 9. A scientist observing a protozoan notices a vacuole discharging its contents at the plasma membrane. This is an example of a. phagocytosis and vacuole formation. b. endocytosis and active transport. c. exocytosis and secretion. d. active transport and vacuole release. e. Both c and d are correct.

16 82 Part 1 Cell Biology Receptor-mediated endocytosis a. is no different from phagocytosis. b. brings specific substances into the cell. c. helps to concentrate proteins in vesicles. d. All of these are correct. 11. Write hypotonic solution or hypertonic solution beneath each cell. Justify your conclusions. a. b. Thinking Scientifically cell wall 1. Considering the movement of molecules across the plasma membrane: a. Contrast the manner in which alcohol and water enter a cell (page 72). b. Contrast the manner in which sodium ions (Na ) and chloride ions (Cl ) exit a cell (Fig. 4.3). c. Contrast the manner in which amino acids and proteins enter a cell (page 72). d. How might the proteins from question c be digested (chapter 3)? 2. Exocytotic vesicles add plasma membrane to the cell, and endocytotic vesicles remove plasma membrane. a. In a cell in which the amount of plasma membrane stays constant, how many exocytotic vesicles per endocytotic vesicles would you expect? b. Imagine a cell that is moving from left to right. If vesicle formation is facilitating movement, where would you expect exocytosis to be occurring? Where would you expect endocytosis to be occurring? c. Receptor-mediated endocytosis is a process by which a substance combines with a receptor before endocytosis brings the entire complex into the cell. Imagine a virus that enters a cell in this manner. If so, what additional step is needed for the virus to enter the cell proper? Understanding the Terms active transport 77 carrier protein 70 channel protein 70 cholesterol 68 concentration gradient 72 differentially permeable 72 diffusion 73 endocytosis 78 enzymatic protein 70 exocytosis 78 facilitated transport 76 fluid-mosaic model 68 glycolipid 68 glycoprotein 69 hypertonic solution 75 hypotonic solution 74 integral protein 69 isotonic solution 74 osmosis 74 osmotic pressure 74 peripheral protein 69 phagocytosis 78 pinocytosis 78 plasmolysis 75 receptor-mediated endocytosis 79 receptor protein 70 sodium-potassium pump 77 solute 73 solvent 73 tonicity 74 turgor pressure 75 Match the terms to these definitions: a. Movement of molecules from a region of higher concentration to a region of lower concentration. b. Internal pressure that adds to the strength of a cell and builds up when water moves by osmosis into a plant cell. c. Solution that contains the same concentration of solute and water as the cell. d. Passive transfer of a substance into and out of a cell along a concentration gradient by a process that requires a carrier. e. Process in which an intracellular vesicle fuses with the plasma membrane so that the vesicle s contents are released outside the cell. Using Technology Your study of membrane structure and function is supported by these available technologies: Essential Study Partner CD-ROM Cells Cell Membrane Visit the Mader web site for related ESP activities. Exploring the Internet The Mader Home Page provides resources and tools as you study this chapter. hhttp:// Virtual Physiology Laboratory CD-ROM Diffusion, Osmosis, & Tonicity Enzyme Characteristics Life Science Animations 3D Video 4 Diffusion 5 Osmosis