Chapter 5: Cell Membranes and Signaling

Transcription

1 Chapter Review 1. For the diagram below, explain what information you would use to determine which side of the membrane faces the inside of the cell and which side faces the extracellular environment. Label these items: phospholipid, cholesterol, cytoskeleton, cell interior (cytoplasm), integral protein, peripheral protein, and carbohydrate. Write your explanation below the figure. Evidence for inside versus outside: Carbohydrates attached to either lipids or protein molecules are typically found on the outside of the cell membrane. Therefore, the upper portion of the membrane shown in this figure is the outer layer, facing the extracellular environment, and the lower portion of the membrane faces the inside of the cell. 2. The current model of the plasma membrane is referred to as the fluid mosaic model. Provide evidence that the membrane is fluid and describe the mosaic of this model. The plasma membrane is considered fluid because the phospholipid bilayer forms a lipid lake in which a variety of proteins float. The membrane is a mosaic due to the wide array of proteins, some of which are non-covalently embedded in the phospholipid bilayer, that are held within the membrane by their hydrophobic regions. 3. Explain how the structure of a phospholipid molecule is amphipathic and can form a membrane layer that is nonpolar in the middle and polar on the outsides. A phospholipid molecule is amphipathic because it has two opposing chemical properties: its water-facing regions are anionic and have hydrophilic phosphates, whereas its fatty-acid tails are hydrophobic. A membrane is formed when phospholipid molecules arrange themselves in a bilayer sheet, two molecules thick. The nonpolar, hydrophobic tails face each other and are packed tightly together in the center of the membrane, and the hydrophilic heads face outward where they can interact with water.

2 4. What are the two primary factors that influence membrane fluidity? Lipid composition and temperature. 5. Molecules that are amphipathic have both polar and nonpolar regions. For a large, amphipathic protein embedded in the phospholipid membrane, describe how this characteristic facilitates its placement in membranes. Draw a diagram of such an amphipathic protein embedded in the membrane below and label the polar and nonpolar regions. Some amino acids have nonpolar, hydrophobic R groups, while others have polar, hydrophilic R groups. The arrangement of these amino acids in a membrane protein determines how the protein will associate with the membrane: the nonpolar region will associate with the lipid bilayer, whereas the polar ends of proteins will associate with the polar region of the lipid bilayer. The result is an asymmetrical distribution of proteins on the inner and outer surfaces of membranes. 6. Describe a biochemical change in membrane composition that helps organisms that endure hot summers and cold winters cope with their temperature extremes. In some organisms, the lipid composition of the membranes changes upon exposure to different temperatures, replacing saturated with unsaturated fatty acids and using fatty acids with shorter chains when it is cold and the opposite when it is hot. 7. Describe the two major structural components of glycoproteins and describe one function of glycoproteins. A glycoprotein consists of a carbohydrate of 15 or fewer monosaccharide units covalently bonded to a protein. The carbohydrates extending from the outer surface of the plasma membrane can bind to another complementary shaped molecule, providing the basis for cell-cell recognition and adhesion. 8. In this example, a drop of ink was placed into a bowl of gelatin. Explain how the ink diffused throughout the gel even though there were no currents to help move it around. The ink moved around in the bowl of gelatin by diffusion, which is the process of random movement toward a state of equilibrium. Initially the pigment molecules are very concentrated, but they will move about at random, slowly spreading until the intensity of color is exactly the same throughout the gel. 9. Describe two differences between active and passive transport. Active transport requires the input of metabolic energy from an outside source, whereas passive transport does not. Passive transport can occur by diffusion, but active transport cannot. All forms of active transport require membrane-bound proteins, but some forms of passive transport do not. 10. Briefly explain how each of the three factors below can impact the diffusion of solutes across membranes. size of the diffusing solute: Molecules or ions with smaller diameters diffuse faster than do large ones.

3 temperature: Higher temperatures lead to faster diffusion than cold because the heat provides more energy for molecular motion and solute movement. concentration gradient: The greater the concentration gradient for a solute across a membrane, the more rapidly that solute can diffuse. 11. Some topical anesthetics dissolve into the membranes of sensory neurons. Describe two structural properties of an anesthesia-inducing molecule that would make it a likely candidate for this route of anesthetic effect. The effectiveness of a topical anesthetic will be enhanced if its chemical composition includes i) small molecules that are ii) hydrophobic and soluble in lipids. The more lipid-soluble a molecule is, the more rapidly it diffuses through the lipid bilayer. 12. Even though water can readily move across many natural membranes, explain why it might be expected to move slowly or not at all through artificial membranes constructed without proteins. Diffusion can be facilitated by two types of proteins. Channel proteins are integral membrane proteins that form channels across the membrane through which substances such as water can pass; water channels (aquaporins) are a good example. Carrier proteins speed up diffusion through the phospholipid bilayer by binding substances to membrane proteins, changing shape so that the bound solute now faces the opposite side of the membrane, and then releasing the substance. 13. The three terms below are used when comparing solute concentration on either side of a cell membrane. Define each term and provide a description how that condition might affect a cell s shape. isotonic: Isotonic solutions have equal solute concentrations on each side of a membrane. This results in a cell with a characteristic shape, since there is no net movement of water into or out of the cell. hypotonic: Hypotonic solutions have a lower solute concentration than the cytoplasm of the cell. This results in enlargement of the cell due to the swelling that occurs as water enters the cell. hypertonic: Hypertonic solutions have higher solute concentration than the cytoplasm of the cell. This can cause cells to shrivel and exhibit an irregular shape, as water exits the cell. 14. Facilitated diffusion refers to a special type of transport: for example, the entry of glucose in to the muscles in your body. Is this type of trans-membrane movement considered to be an example of active or passive transport? Explain why. Facilitated diffusion is a process that allows substances to move across membranes according to their concentration gradients, but this diffusion is made faster by channel or carrier proteins. Particular channel or carrier proteins allow diffusion both into and out of a cell or organelle, so they are bidirectional. Facilitated diffusion can be described as an enhanced type of passive transport, as no additional metabolic energy is required, but the efficiency of the diffusion process is improved by these special proteins. 15. After several days without watering, plants tend to wilt. When the plant is watered, it will often return to its normal shape. Explain how cells are involved in the transition from wilted to normal. If a plant has wilted due to dehydration, its cells have become hypertonic, resulting in a flaccid, wilted appearance. When the dehydrated plant is watered, water moves into the shrunken cells, expanding them. The cells will become plump again due to the internal pressure build up against the cell wall, and the plant will regain its original shape, losing its wilted appearance. This pressure within the cell in called turgor pressure, and it keeps the green parts of plants upright and it is a driving force for enlargement of plant cells.

4 16. Explain how the carrier protein in the diagram below is facilitating the diffusion of a molecule. Include in your answer an explanation why the protein is needed. Diffusion is facilitated by the actual binding of the transported substance to a membrane protein called a carrier protein. These proteins transport polar molecules, such as sugars and amino acids, across the membrane, at faster rate than by simple diffusion. The molecules of the diffused substance and the carrier protein bind when they attach at a specific threedimensional site on each molecule; changes in the shape of the protein after binding or unbinding determine the affinity of the binding site for the solute and which side of the membrane it is released. 17. Describe the primary chemical process that drives active transport. The primary chemical process that drives active transport is the hydrolysis of ATP. The resulting transfer of energy is coupled to the transport mechanism. This energy released by ATP hydrolysis drives the movement of specific ions and other solutes against their concentration gradients. 18. Complete the table below: Simple Diffusion Cellular energy required? Driving force Membrane protein required? Osmosis Facilitated Diffusion Active Transport No No No Yes Concentration gradient No Concentration gradient Yes for some cells, No for others Concentration gradient ATP hydrolysis (against concen. gradient) Directional? No No No Yes Specificity? No No Yes Yes 19. The Na+-K+ ATPase is the most active and wide-spread active-transport system in the human body. Label five items on the diagram below detailing how this pump functions. Yes Yes

5 20. Explain how phagocytosis and pinocytosis are similar and different. Phagocytosis and pinocytosis, which are two types of endocytosis, are similar because they involve the invagination of the plasma membrane to form a small pocket around materials from the extracellular environment. The pocket deepens and forms a vesicle, which separates from the membrane and migrates with its contents into the cell s interior. In phagocytosis or cellular eating, large particles or even entire cells are engulfed. However, in pinocytosis or cellular drinking, the vesicles are smaller, bringing fluids and dissolved substances, such as proteins, into the cell. 21. Describe receptor-mediated endocytosis with enough detail to explain whether or not this process meets the criteria for active transport or passive transport. In receptor-mediated endocytosis, molecules at the cell surface recognize and trigger the uptake of specific materials. These receptor molecules, which are proteins, bind to macromolecules in the cell s environment. This initiates specific cellular responses involving clathrin-coated pits to allow the receptors with the ligands to be internalized in the cell. The receptors are integral membrane proteins located a particular regions on the extracellular surface of the plasma membrane, but the uptake process is similar to that in phagocytosis. This complex process requires the input of energy and is therefore a type of active transport. Concept 5.5 explains how organisms and cells respond to stimuli (signals) from their environment. The signal may be a physical stimulus such as light or heat, or a chemical such as a hormone. In order to respond, the cell must have a specific receptor that becomes modified by the stimulus. Once a receptor in the membrane is activated by the signal, it sets off a series of biochemical changes within the cell. These pathways are sequences of events and chemical reactions that lead to a cell s response to a signal. This ability to respond to the environment is critical to the organism s or cell s ability to maintain precision in its homeostatic mechanisms. 22. Describe each of the three major steps in cell signaling. a. Signal A signal molecule arrives at the target cell. The signal molecule may be a specific molecule or physical stimulus, and it can arrive from within a multicellular organism or from its external environment. b. Receptor - The signal molecule fits into a three-dimensional binding site of the receptor protein on the cell surface or in the cytoplasm. The molecule and receptor bind, creating a ligand-receptor (protein) complex. c. Response The creation of the ligand-receptor complex causes the receptor to change its threedimensional shape, and that conformational change initiates a cellular response. The ligand does not typically contribute further to the cellular response; it is usually released rather than being metabolized. 23. Different receptor proteins for different signals are found in the cytoplasm or on the membrane of the cell. Give an example of each and discuss the properties of the ligand (signal molecule) that activates this receptor. intracellular receptor protein: Small or nonpolar ligands, can diffuse across the phospholipid bilayer of the plasma membrane and bind to intracellular receptor cells in the cytoplasm or nucleus. Estrogen is an example of a lipid-soluble steroid hormone that can easily diffuse across the plasma membrane and bind to a receptor in the cytoplasm or nucleus. membrane-bound receptor protein: Large or polar ligands cannot readily cross the lipid bilayer; they instead bind to the extracellular binding domain of a transmembrane receptor. These receptor protein then changes shape upon ligand binding. There are three categories of plasma membrane receptors: ion channels, protein kinases, and G-protein linked receptors. An example of a membrane-bound receptor protein is the insulin receptor on skeletal muscles.

6 24. If a cell had no proteins in its membrane, will it be able to respond to any environmental stimuli? Explain your answer. A cell must have the appropriate receptor to respond to any environmental stimulus, and receptor cells are always composed of protein molecules. Therefore, a cell would not be capable of responding to an environmental stimulus the absence of protein in its membrane. Theoretically, exceptions could include membrane-soluble signals that interact with intracellular receptor-proteins, or perhaps a large-enough temperature change that could alter the phospholipid bilayer. 25. Complete the diagram to the right showing an example of a signal cascade. 26. Explain how the signal cascade in the diagram to the right achieves amplification. Signaling at the plasma membrane initiates a cascade or series of events in the cell, in which proteins interact with other proteins, until the final responses are achieved. The initial signal can be amplified, often by intermediary molecules. These intermediaries, or second messengers, are small molecules that distribute the initial signal within the cell. There may actually be multiple sequential steps, in which particular enzymes are either activated or inhibited by other enzymes in the pathway. This multi-step signal transduction cascade process produces amplification of the signal with each step. In this step, the effector protein is the enzyme glycogen phosphorylase. Thousands or even millions of glucose molecules can be released in consequence to binding just one epinephrine ligand at the beginning of this cascade. 27. Describe how the signal cascade above is terminated after the necessary response has been obtained. Signal transduction is a temporary event in a cell which gets turned off once the cell has responded. There are enzymes that convert each activated transducer back to its inactive precursor once the response is complete. The balance between the activities of the regulating enzymes and the signaling enzymes themselves is what determines the ultimate cellular response to a signal.

7 Science Practices & Inquiry 28. To the right is a diagram of caffeine. Caffeine acts by binding with a receptor on the cell surface. Why doesn t it enter the cell? Label the parts of the molecule that make it difficult for caffeine to enter the cell. The caffeine molecule is somewhat large and polar, so it is unlikely to diffuse through the nonpolar cell membrane. Instead, it likely binds to receptors on the surfaces of nerve cells in the brain. 29. The drug ouabain inhibits the activity of the Na+ K+ATPase. A nerve cell is incubated in ouabain. Make a table in which you predict what would happen to the concentrations of Na+ and K+ inside the cell, as a result of the action of ouabain. Explain why. Normal nerve cell Nerve cell incubated in ouabain Conc. Of Na + inside cell Low concentration Concentration increases Conc. Of K + inside cell High concentration Concentration decreases Ouabain inhibits the activity of the sodium-potassium pump. This pump normally moves Na + and K + against the concentration gradient of each solute: three Na + are moved out of the cell, while two K + are moved into the cell, per hydrolysis of ATP. The sodium-potassium pump maintains the cell membrane potential, and the export of sodium ions from the cell provides the driving force for the active transport of other nutrients into the cell, including glucose and amino acids. Ouabain effectively shuts down this process, preventing the movement of sodium ions out of the cell, which causes the concentration of intracellular sodium to increase. It also affects the flow of nutrients into the cell by inhibiting the ability of certain sugar and protein molecules to be actively transported into the cell. Since excitable cells in the heart and chest are necessary for moving blood and breathing, ouabain can be a lethal drug.