Transcript Chapter 5
The plasma membrane • The plasma membrane is the boundary that separates the internal contents of the cell from its external environment. • The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins • Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hydrophilic head Hydrophobic tails Membrane Model • The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 7-5b Fluid Unsaturated hydrocarbon tails with kinks Membrane fluidity Viscous Saturated hydrocarbon tails Cholesterol and Membranes’ Fluidity • The steroid cholesterol has different effects on membrane fluidity at different temperatures • At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids • At cool temperatures, it maintains fluidity by preventing tight packing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 7-5c Cholesterol Cholesterol within the animal cell membrane In-Text Art, Chapter 5, p. 84 (2) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 7-7 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE http://www.youtube.com/watch?v=S7-VFJHd0uA&list=PL13jH2BYL65_q5j1vBga32xNiYT57P5KC Transport Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Enzymatic Activity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Receptors for Signal Transduction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell-Cell Recognition • Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane • Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) • Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell-Cell Recognition Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Synthesis of Membrane Components and Their Orientation in the Membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 7.2: Membrane structure results in selective permeability • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane • Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Permeability of the Lipid Bilayer • Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly • Polar molecules, such as sugars, do not cross the membrane easily. • Hydrophilic substances require transport proteins to cross the membrane. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment • Diffusion is the tendency for molecules to spread out evenly into the available space • Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diffusion • Substances diffuse down their concentration gradient, which is the difference in concentration of a substance from one area to another • No work must be done to move substances down the concentration gradient • The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of Membrane Transport • Passive processes – No cellular energy (ATP) required – Substance moves down its concentration gradient (i.e from high to low concentration) • Active processes – Energy (ATP) required – Occurs only in living cell membranes – Substance moves against its concentration gradient (i.e from low to high concentration) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of Passive Processes • Simple diffusion • Facilitated diffusion: – Carrier-mediated facilitated diffusion – Channel-mediated facilitated diffusion • Osmosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Passive Processes: Simple Diffusion • Nonpolar lipid-soluble (hydrophobic, such as O2, CO2) substances diffuse directly through the phospholipid bilayer Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Passive Processes: Simple Diffusion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Small nonpolar solutes move down their concentration gradients. Interstitial fluid Oxygen Cytosol Carbon dioxide Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings •At dynamic equilibrium, as many molecules cross one way as cross in the other direction Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Simple Diffusion of one solute Equilibrium LE 7-11b Net diffusion Net diffusion Net diffusion Net diffusion Simple Diffusion of two solutes Equilibrium Equilibrium Passive Processes: Facilitated Diffusion • Certain hydrophilic molecules (e.g., glucose, amino acids, and ions) use carrier proteins OR channel proteins, both of which: – Exhibit specificity (selectivity) – Are saturable; rate is determined by number of carriers or channels Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transport Proteins • Transport proteins allow passage of hydrophilic substances across the membrane • Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel • Channel proteins called aquaporins facilitate the passage of water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Small lipidinsoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 3.7c Channel Transport Proteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane • A transport protein is specific for the substance it moves Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lipid-insoluble solutes (such as sugars or amino acids) (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 3.7b Carrier Transport Proteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Effects of Osmosis on Water Balance • Osmosis is the diffusion of water across a selectively permeable membrane • The direction of osmosis is determined only by a difference in total solute concentration • Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 7-12 Lower concentration of solute (sugar) Higher concentration of sugar H2O Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Osmosis Same concentration of sugar Water Balance of Cells • Tonicity is the ability of a solution to cause a cell to gain or lose water • Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water Balance of Cells Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water. Hypotonic solution: solute concentration is less than that inside the cell; cell gains water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water Balance of Cells Without Cell Walls • Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment • To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance • The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump to get rid of excess water. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 7-14 Filling vacuole Paramecium Contracting vacuole 50 µm 50 µm Figure 4.12 Isotonic solution Hypotonic solution Interstitial fluid is less concentrated than cytosol. Interstitial fluid is the same concentration as cytosol. Erythrocyte Water leaves cell. Erythrocyte Erythrocyte SEM 11,550x SEM 9030x SEM 6900x Interstitial fluid is more concentrated than cytosol. Water enters cell. No net movement of water. Normal erythrocytes (a) Hypertonic solution Erythrocytes nearing hemolysis (b) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Erythrocytes undergoing crenation (c) CRENATION HUMAN RED BLOOD CELL In 4% Salt Solution Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings HEMOLYSIS HUMAN RED BLOOD CELL In Distilled Water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Thinking Questions! • Solutions with a solute concentration greater than 1% are_________ compared to a RBC. • Solutions with a solute concentration less than 1% are _________compared to a RBC. • Solutions with a solute concentration___________1% are isotonic compared to a RBC. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water Balance of Cells with Cell Walls • Cell walls help maintain water balance • A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) • If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt • In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Water Balance of Plant Cells Plasmolyzed Flaccid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Turgid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 7.4: Active transport uses energy to move solutes against their gradients • Active transport moves substances against their concentration gradient • Active transport requires energy, usually in the form of ATP • Active transport is performed by specific proteins embedded in the membranes. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Active Transport • Active transport allows cells to maintain concentration gradients that differ from their surroundings • The sodium-potassium pump is one type of active transport system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 7-16 EXTRACELLULAR [Na+] high FLUID [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ CYTOPLASM [Na+] low [K+] high Na+ Cytoplasmic Na+ bonds to the sodium-potassium pump P ATP P ADP Na+ binding stimulates phosphorylation by ATP. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. Loss of the phosphate restores the protein’s original conformation. K+ is released and Na+ sites are receptive again; the cycle repeats. P P Extracellular K+ binds to the protein, triggering release of the phosphate group. Active Transport / The sodium-potassium pump Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 7-17 Passive transport Active transport ATP Diffusion Facilitated diffusion