Chapter 7: Membrane Structure and Function

Transcription

1 Chapter 7: Membrane Structure and Function Concept 7.1 Cellular membranes are fluid mosaics of lipids and proteins 1. Phospholipids are amphipathic. Explain what this means. Name Period Amphipathic means that the phospholipid has both a hydrophilic region and a hydrophobic region. 2. In the 1960s, the Davson-Danielli model of membrane structure was widely accepted. Describe this model and then cite two lines of evidence that were inconsistent with it. The Davson-Danielli model of membrane structure suggested that the membrane might be coated on both sides with hydrophilic proteins, with a phospholipid bilayer between two layers of proteins. First, inspection of a variety of membranes revealed that membranes with different functions differ in structure and chemical composition. A second, more serious problem became apparent once membrane proteins were better characterized. Unlike proteins dissolved in the cytosol, membrane proteins are not very soluble in water because they are amphipathic. If such proteins were layered on the surface of the membrane, their hydrophobic parts would be in aqueous surroundings. 3. The currently accepted model of the membrane is the fluid mosaic model. Who proposed it? When? Describe this model. S. J. Singer and G. Nicolson proposed the fluid mosaic model in In this model, the membrane proteins reside in the phospholipid bilayer with their hydrophilic regions protruding. 4. What is meant by membrane fluidity? Describe the movements seen in the fluid membrane. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. Most of the lipids and some of the proteins can shift about laterally that is, in the plane of the membrane. The lateral movement of phospholipids within the membrane is rapid, occurring about 10 7 times per second. 5. Describe how each of the following can affect membrane fluidity: a. decreasing temperature: A membrane remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies. The temperature at which a membrane solidifies depends on the types of lipids it is made of. b. phospholipids with unsaturated hydrocarbon chains: The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails. Because of kinks in the tails where double bonds are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, and this makes the membrane more fluid. c. cholesterol: At relatively high temperatures at 37 C, the body temperature of humans, for example cholesterol makes the membrane less fluid by restraining phospholipid movement. Copyright 2011 Pearson Education, Inc

2 6. Membrane proteins are the mosaic part of the model. Describe each of the two main categories: integral proteins: penetrate the hydrophobic interior of the lipid bilayer peripheral proteins: appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins 7. Study Figure 7.10 in your text. Use it to briefly describe the following major functions of membrane proteins. Function Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to cytoskeleton and ECM Description A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. A protein built into the membrane may be an enzyme with its active site 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. 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. Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. 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. 8. Membrane carbohydrates are important in cell-cell recognition. What are two examples of this? Two examples include the sorting of cells into tissues and organs in an animal embryo, and the rejection of foreign cells by the immune system. 9. Distinguish between glycolipids and glycoproteins. Glycolipids: Membrane carbohydrates covalently bonded to lipids Glycoproteins: Membrane carbohydrates covalently bonded to proteins 10. Label the following components of an animal cell membrane on the figure that follows: See page 127 in your text for the labeled figure. Copyright 2011 Pearson Education, Inc

3 glycolipid glycoprotein integral protein peripheral protein cholesterol phospholipid ECM fibers cytoskeleton microfilaments integrins (go back to Chapter 6) Concept 7.2 Membrane structure results in selective permeability 11. Distinguish between channel proteins and carrier proteins. Channel proteins function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. Carrier proteins hold on to their passengers and change shape in a way that shuttles them across the membrane. 12. Are transport proteins specific? Cite an example that supports your response. Yes. A transport protein is specific for the substance it translocates, allowing only a certain substance to cross the membrane. For example, a specific water carrier protein in the plasma membrane of red blood cells transports glucose across the membrane. 13. Peter Agre received the Nobel Prize in 2003 for the discovery of aquaporins. What are they? Aquaporins are channel proteins that facilitate the passage of water molecules through the membrane of certain cells. 14. Consider the following materials that must cross the membrane. For each, tell how it is moved across. Material CO 2 Glucose H + O 2 H 2 O Method simple diffusion transport proteins transport proteins simple diffusion simple diffusion and protein channels (aquaporins) Copyright 2011 Pearson Education, Inc

4 Concept 7.3 Passive transport is diffusion of a substance across a membrane with no energy investment 15. Define the following terms: diffusion: The movement of molecules of any substance so that they spread out evenly into the available space concentration gradient: The region along which the density of a chemical substance increases or decreases passive transport: Diffusion of a substance across a biological membrane; cell does not have to expend energy to make it happen osmosis: The diffusion of water across a selectively permeable membrane isotonic: A solution with the same concentration of solutes as the cell it surrounds. There will be no net movement of water across the plasma membrane. In an isotonic environment, the volume of the animal cell is stable. hypertonic: A solution with more solutes than the cell it surrounds. The cell will lose water, shrivel, and probably die. hypotonic: A solution with less solutes than the cell it surrounds. The cell will swell and lyse (burst). turgid: Very firm flaccid: Limp plasmolysis: Phenomenon during which the plant cell shrivels, and its plasma membrane pulls away from the wall. 16. Use as many words from the list above to describe why a carrot left on the counter overnight would become limp. Underline or highlight each word you use. The cells of the carrot contain more water than the surrounding air, and therefore water will leave the carrot cells. The cells are hypotonic to the surrounding air. As water leaves the carrot cells, they will become flaccid as plasmolysis occurs. The water is leaving the cells by osmosis; the solutes remain in the cells. 17. What is facilitated diffusion? Is it active or passive? Cite two examples. Facilitated diffusion is the phenomenon during which polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. Facilitated diffusion is considered passive transport because the solute is moving down its concentration gradient, a process that requires no energy. Copyright 2011 Pearson Education, Inc

5 Examples include the movement of water through aquaporins, movement of sugars through protein channels, sodium ion gated channels in nerve cells, and any other example that requires a transport protein. 18. In the figure below, label the hypotonic solution, isotonic solution, and hypertonic solution. What is indicated by the blue arrows? Label them. Which cell is lysed? Turgid? Flaccid? Plasmolyzed? Apply all these labels. See page 134 in your text for the labeled figure. 19. Why does the red blood cell burst when placed in a hypotonic solution, but not the plant cell? Plant cells have a cell wall and animal cells do not. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the wall pushing back on the cell. Concept 7.4 Active transport uses energy to move solutes against their gradients 20. Describe active transport. What type of transport proteins are involved, and what is the role of ATP in the process? Active transport is a type of membrane traffic during which the cell must expend energy. The transport proteins involved are called carrier proteins. ATP supplies the energy for most active transport. 21. The sodium-potassium pump is an important system for you to know. Use the following diagram to understand how it works. Use these terms to label the figures, and briefly summarize what is occurring in each: extracellular fluid, cytoplasm, Na +, K +, ATP, ADP, P, and transport protein. See page 136 in your text for the labeled figure. Summary 1. Na + binds to the sodium-potassium pump. 2. This stimulates phosphorylation of the pump. 3. The pump changes shape and releases Na + to the outside. 4. The new pump shape now has an affinity for K +, and binds them. This triggers the dephosphorylation of the pump 5. This causes the pump to change shape again, and release K + to the inside. 6. In the new shape, the pump will now bind Na + again. Copyright 2011 Pearson Education, Inc

6 22. On the diagram below, add these labels: facilitated diffusion with a carrier protein, facilitated diffusion with a channel protein, active transport with a carrier protein, and simple diffusion. For each type of transport, give an example of a material that is moved in this manner. See page 136 in your text for the labeled figure. Examples facilitated diffusion with a carrier protein: glucose through glucose transporters facilitated diffusion with a channel protein: aquaporins transporting water active transport with a carrier protein: sodium-potassium pump simple diffusion: movement of oxygen 23. What is membrane potential? Which side of the membrane is positive? Membrane potential is the voltage across a membrane, which ranges from about 50 to 200 millivolts (mv). The minus sign indicates that the inside of the cell is negative relative to the outside. 24. What are the two forces that drive the diffusion of ions across the membrane? What is the combination of these forces called? Two forces drive the diffusion of ions across a membrane: a chemical force (the ion s concentration gradient) and an electrical force (the effect of the membrane potential on the ion s movement). This combination of forces acting on ions is called the electrochemical gradient. 25. What is cotransport? Explain how understanding it is used in our treatment of diarrhea. Cotransport is the coupling of the downhill transport of one substance to the uphill transport of another substance against its own concentration gradient. 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 a high concentration of salt (NaCl) and glucose. The solutes are taken up by sodium-glucose cotransporters on the surface of intestinal cells and passed through the cells into the blood. Concept 7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis 26. Define each of the following, and give a specific cellular example. Exocytosis: The cellular secretion of biological molecules by fusion of vesicles containing them with the plasma membrane Copyright 2011 Pearson Education, Inc

7 Endocytosis: Cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane receptor-mediated endocytosis: The movement of specific molecules into a cell by the inward budding of vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances phagocytosis: A type of endocytosis in which large particulate substances or small organisms are taken up by a cell. It is carried out by some protists and certain immune cells of animals. pinocytosis: A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes 27. What is a ligand? What do ligands have to do with receptor-mediated endocytosis? A ligand is a molecule that binds specifically to another molecule, usually a larger one. Human cells use receptor-mediated endocytosis to take in cholesterol for membrane synthesis and the synthesis of other steroids. Cholesterol travels in the blood in particles called low-density lipoproteins (LDLs), each a complex of lipids and a protein. LDLs bind to the LDL receptors on plasma membranes and then enter the cell by endocytosis. The LDLs thus act as ligands. 28. Are the processes you described in question 23 active or passive transport? Explain your response. Passive transport. With regard to membrane potential, 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. Testing Your Understanding Answers Now you should be ready to test your knowledge. Place your answers here: 1. b 2. c 3. a 4. d 5. b Reproduce the diagram for question 6, and draw arrows as instructed. 6b. The solution outside the cell is hypotonic to the cell. It has less sucrose, which is not able to move across the membrane. 6d. The artificial cell will become more turgid as it gains water. 6e. Eventually, the two solutions will have the same solute concentrations. Even though sucrose can t move through the membrane, water flow (osmosis) will lead to isotonic conditions. Copyright 2011 Pearson Education, Inc