Chapter 7: Membrane Structure and Function PDF

Chapter 7: Membrane Structure and Function Campbell Biology, 12th Edition  Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings. The plasma membrane exhibits selective permeability, allowing some substance...

Chapter 7: Membrane Structure and Function Campbell Biology, 12th Edition  Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings. The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others. in p t +  Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Loading… Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it  WATER Hydrophilic head Hydrophobic tail WATER  The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer. Loading… Most of the lipids, and some proteins, drift laterally. Rarely does a molecule flip-flop transversely across the membrane.  Lateral movement Flip-flop (~107 times per second) (~ once per month) (a) Movement of phospholipids Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails (b) Membrane fluidity Cholesterol (c) Cholesterol within the animal cell membrane  As temperatures cool, membranes switch from a fluid state to a solid state. The temperature at which a membrane solidifies depends on the types of lipids. Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids. Membranes must be fluid to work properly; they are usually about as fluid as salad oil.  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  Membrane Proteins and Their Functions A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate protein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments Peripheral of cytoskeleton proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE  Peripheral proteins are bound to the surface - - of the membrane. Loading… Integral proteins penetrate the hydrophobic core. Inside the Cell membrane  A report of Six major functions of membrane proteins: it holds the molecule because Carrier proteinsCan't be channels – Transport but channels is within a carrier , – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) S  Signaling molecule Enzymes Receptor O ATP Signal transduction (a) Transport ATP btetsanas jua (b) Enzymatic activity (c) Signal transduction > Carbohydrates - Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular therefore it's mainly hydrophobic matrix (ECM) of Largest the is core of cell membrane part  The Role of Membrane Carbohydrates in 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.  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 © 2008 Pearson Education, Inc., publishing as Pearson 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  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  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.  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 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 Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes  Substances diffuse down their concentration gradient, (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  Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane. Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration. Lower Higher Same concentration concentration concentration of sugar of solute (sugar) of sugar H2O Selectively permeable membrane Osmosis  Water Balance of Cells Without Walls 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. Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water. The solute outside the cell is greater than inside the cell / less water Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water. The solute outside the cell is less than inside the cell / more water  Hypotonic solution Isotonic solution Hypertonic solution H2 H2 H2 H2 O O O O (a) Animal cell bodies our In Lysed Normal Shriveled wal Shrinkage H2 e membrane H2 H2 H2 O O O O I (b) Plant > - vacuole cell Turgid (normal) Flaccid Plasmolyzed  Water Balance of Cells with 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  Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include – Aquaporins, for facilitated diffusion of water – Ion channels that open or close in response to a stimulus (gated channels) EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Loading… Carrier protein Solute (b) A carrier protein  Concept 7.4: Active transport uses energy to move solutes against their gradients, consumes energy Some transport proteins, however, can move solutes against their concentration 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 Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system EXTRACELLULAR [Na+] high Na FLUID [K+] low Na + + Na Na Na + + + Na Na + + Na + 1 Na CYTOPLASM + [Na+] low [K+] high 2 ⑭ so P ADP ATPloses one it becomes ADP ATP phosphate 3 P 3 sodium out Ast Change Needs ATP in 2 potassium K + K + K K + K + & + P K P + 6 5 4 end change  Passive transport Active transport ATP Diffusion Facilitated diffusion  Cotransport: Coupled Transport by a Membrane Protein Active Transport Secolag Cotransport occurs when active transport of a solute indirectly drives transport of another solute. Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell.  – + D ATP H + H – + + Proton pump H H + + – + H – H + + + H Diffusion + of H+ Sucrose-H+ cotransporter H + Sucrose – + – + Sucrose  Huge Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy  & Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products  A Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis: Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis  In phagocytosis a cell engulfs a particle in a vacuole. The vacuole fuses with a lysosome to digest the particle. In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles.  PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM 1 µm FLUID Pseudopodium False Pseudopodium of amoeba “Food”or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit Coat and a coated protein vesicle formed during receptor- mediated endocytosis (TEMs) Plasma membrane 0.25 µm  In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule