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Chapter 7 Membrane Structure and Function Figure 7.1 The plasma membrane Figure 7.2 Phospholipid bilayer (cross section) WATER Hydrophilic head Hydrophobic tail WATER Figure 7.3 The fluid mosaic model for membranes Hydrophillic region of protein Phospholipid bilayer Video Hydrophobic region of protein Figure 7.8 The structure of a transmembrane protein EXTRACELLULAR SIDE N-terminus C-terminus a Helix CYTOPLASMIC SIDE Figure 7.4 Research Method Freeze-Fracture APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior. TECHNIQUE A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Extracellular layer Knife Proteins Cytoplasmic Plasma membrane layer RESUTS These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. Extracellular layer Cytoplasmic layer • A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholid bilayer prior to electron microscopy. • This shows protein particles interspersed with a smooth matrix, supporting the fluid mosaic model. Fig. 8.3 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1. Membranes are fluid • Membrane molecules are held in place by relatively weak hydrophobic interactions. • Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other. Fig. 8.4a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Figure 7.5 The fluidity of membranes Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydroCarbon tails (b) Membrane fluidity Cholesterol (c) Cholesterol within the animal cell membrane • The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animals cells. • At warm temperatures, it restrains the movement of phospholipids and reduces fluidity. • At cool temperatures, it maintains fluidity by preventing tight packing. Fig. 8.4c Figure 7.6 Inquiry Do membrane proteins move? EXPERIMENT Researchers 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. 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. Glycocalyx Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in cross section Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid Microfilaments of cytoskeleton Cholesterol Peripheral protein EXTRACELLULAR SIDE OF MEMBRANE Integral CYTOPLASMIC SIDE protein OF MEMBRANE • Membranes have distinctive inside and outside faces. – The two layers may differ in lipid composition, and proteins in the membrane have a clear direction. – The outer surface also has carbohydrates. – This asymmetrical orientation begins during synthesis of new membrane in the endoplasmic reticulum. Fig. 8.8 • The carbohydrates attached to some of the proteins and lipids of the cell membrane are added as the membrane is refined in the Golgi apparatus; the new membrane then forms transport vesicles that travel to the cell surface. On which side of the vesicle membrane are the carbohydrates? – Interior surface of the vesicle membrane – Exterior (cytoplasmic) surface of the vesicle membrane • The proteins in the plasma membrane may provide a variety of major cell functions. Fig. 8.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings video • In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient. – This spontaneous process decreases free energy and increases entropy by creating a randomized mixture. • Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances. Fig. 8.10b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Figure 7.11 The diffusion of solutes across a membrane (a) Diffusion of one solute. The membrane Molecules of dye Membrane (cross section) has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side WATER with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic Net diffusion Net diffusion equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. 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. Net Osmosis diffusion Osmosis video Net diffusion Net diffusion Net diffusion Equilibrium Equilibrium Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Figure 7.12 Osmosis Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Water molecules cluster around sugar molecules More free water molecules (higher concentration) Fewer free water molecules (lower concentration) Osmosis Water moves from an area of higher free water concentration to an area of lower free water concentration • Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer. • This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis. • Osmosis continues until the solutions are isotonic. Fig. 8.11 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings HYPOTONIC Solute potential= Osmotic potential =0 HYPERTONIC Solute potential is less than zero Water pot.= Solute + Pres. Pot. =0 Pressure for water to diffuse in • A solution of 1 M glucose is separated by a selectively permeable membrane from a solution of 0.2 M fructose and 0.7 M sucrose. The membrane is not permeable to the sugar molecules. Which of the following statements is correct? – – – – – Side A is hypotonic relative to side B. The net movement of water will be from side B to side A. The net movement of water will be from side A to side B. Side B is hypertonic relative to side A. There will be no net movement of water. An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to the disaccharide sucrose. • Which solute(s) will exhibit a net diffusion into the cell? – – – sucrose glucose fructose • Which solute(s) will exhibit a net diffusion out of the cell? – – – sucrose glucose fructose • Which solution is hypertonic to the other? – the cell contents – the environment • In which direction will there be a net osmotic movement of water? – out of the cell – into the cell – neither • After the cell is placed in the beaker, which of the following changes will occur? – The artificial cell will become more flaccid. – The artificial cell will become more turgid. – The entropy of the system (cell plus surrounding solution) will decrease. – The overall free energy stored in the system will increase. – The membrane potential will decrease Figure 7.13 The water balance of living cells Hypotonic solution (a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. H2O Isotonic solution (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. Osmosis Animation H2O H2O Normal Lysed Turgid (normal) Turgid Leaf H2O Shriveled H2O H2O H2O Hypertonic solution Flaccid H2O Plasmolyzed Plasmolysis video Pearson Diffusion & Osmosis Lab • Print out Answers to Lab quiz tonight. Figure 7.14 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation Filling vacuole 50 µm Contractile Vacuole (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. 50 µm Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell. • You observe plant cells under a microscope that have just been placed in an unknown solution. First the cells plasmolyze; after a few minutes, the plasmolysis reverses and the cells appear normal. What would you conclude about the unknown solute? 1. It is hypertonic to the plant cells, and its solute can not cross the pant cell membranes. 2. It is hypotonic to the plant cells, and its solute can not cross the pant cell membranes. 3. It is isotonic to the plant cells, but its solute can cross the plant cell membranes. 4. It is hypertonic to the plant cells, but its solute can cross the plant cell membranes. 5. It is hypotonic to the plant cells, but its solute can cross the plant cell membranes. • For a cell living in an isotonic environment (for example, many marine invertebrates) osmosis is not a problem. – Similarly, the cells of most land animals are bathed in an extracellular fluid that is isotonic to the cells. • Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation to maintain their internal environment. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings video video EXTRACELLULAR FLUID Figure 7.15 Two types of transport proteins that carry out facilitated diffusion Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Solute Carrier protein (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute. Figure 7.17 Review: passive and active transport compared Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. ATP 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 or carrier proteins. Active transport animation Creates membrane potential in nerve cells Figure 7.18 An electrogenic pump EXTRACELLULAR FLUID – + – ATP + H+ H+ Proton pump H+ – H+ + H+ – + CYTOPLASM H+ – + Figure 7.19 Cotransport: active transport driven by a concentration gradient – + H+ ATP – H+ + H+ Proton pump H+ – + H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ – + – + Sucrose Cotransport • An experiment is designed to study the mechanism of sucrose uptake by plant cells. Cells are immersed in a sucrose solution, and the pH of the solution is monitored with a pH meter. Samples of the cells are taken at intervals, and the sucrose in the sampled cells is measured. The measurements show that sucrose uptake by the cells correlates with a rise in the pH of the surrounding solution. The magnitude of the pH change is proportional to the starting concentration of sucrose in the extracellular solution. A metabolic poison known to block the ability of cells to regenerate ATP is found to inhibit the pH changes in the extracellular solution. Based on this information which of the following statements would you predict is correct? * 1. Sucrose uptake is the result of simple diffusion 2. Hydrogen ion movement is the result of facilitated diffusion. 3. Sucrose moving through the membrane forces hydrogen ions in to the cell 4. Sucrose and Hydrogen ions are transported in opposite directions across the membrane 5. Sucrose transport is the result of a hydrogen ion cotransporter. Do Lab simulation • Go to this site: • http://midpac.edu/~biology/Intro%20Biolog y/PH%20Biology%20Lab%20Simulations/ biomembrane1/intro.html • Do the lab simulation on Membrane Structure and Transport, then take the practice test – print out the test to turn in. Figure 7.20 Exploring Endocytosis in Animal Cells video In phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membraneenclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. PHAGOCYTOSIS EXTRACELLULAR FLUID 1 µm CYTOPLASM Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, but other molecules (green) are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle. RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Ligand Coated pit A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). Coat protein Plasma membrane 0.25 µm • In pinocytosis, “cellular drinking”, a cell creates a vesicle around a droplet of extracellular fluid or small particles. – This is a non-specific process. Fig. 8.19b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Receptor-mediated endocytosis is very specific in what substances are being transported. • This process is triggered when extracellular substances bind to special receptors, ligands, on the membrane surface, especially near coated pits. • This triggers the formation of a vesicle Fig. 8.19c Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Receptor-mediated endocytosis enables a cell to acquire bulk quantities of specific materials that may be in low concentrations in the environment. – Human cells use this process to absorb cholesterol. – Cholesterol travels in the blood in low-density lipoproteins (LDL), complexes of protein and lipid. – These lipoproteins bind to LDL receptors and enter the cell by endocytosis. – In familial hypercholesterolemia, an inherited disease, the LDL receptors are defective, leading to an accumulation of LDL and cholesterol in the blood. – This contributes to early atherosclerosis. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Do Lab simulation • Go to this site: • http://midpac.edu/~biology/Intro%20Biolog y/PH%20Biology%20Lab%20Simulations/ biomembrane2/intro.html • Do the lab simulation on Membrane Transport and Communication, then take the practice test – print out the test to turn in. Chapter 7 Membrane Structure and Function