* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download membrane_structure_and_function
Survey
Document related concepts
Cell encapsulation wikipedia , lookup
G protein–coupled receptor wikipedia , lookup
Cell nucleus wikipedia , lookup
Cytoplasmic streaming wikipedia , lookup
Magnesium transporter wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Theories of general anaesthetic action wikipedia , lookup
Membrane potential wikipedia , lookup
Lipid bilayer wikipedia , lookup
Extracellular matrix wikipedia , lookup
Cytokinesis wikipedia , lookup
SNARE (protein) wikipedia , lookup
Model lipid bilayer wikipedia , lookup
Ethanol-induced non-lamellar phases in phospholipids wikipedia , lookup
Signal transduction wikipedia , lookup
Cell membrane wikipedia , lookup
Transcript
Chapter 7 Membrane Structure and Function • The plasma membrane is the boundary that separates the living cell from its nonliving surroundings. Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Overview: Life at the Edge Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Phospholipids, the most abundant lipid in membranes, are “amphipathic” molecules, containing hydrophobic and hydrophilic regions. WATER Hydrophilic head Hydrophobic tail WATER 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 Phospholipids in the plasma membrane can move within the bilayer. Most of the lipids, and some proteins, drift laterally. Rarely does a molecule flip-flop transversely across the membrane. Lateral movement (~107 times per second) Flip-flop (~ once per month) The Fluidity of Membranes Membranes must be fluid to work properly. Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids. Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydrocarbon tails without kinks The Fluidity of Membranes (continued) 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. At lower temperatures, membranes undergo a transition to a crystalline state in which fatty acid tails are fully extended, packing is highly ordered, and van der Waals interactions between adjacent chains are maximal. Normal Temperature Low Temperature Liquid Crystal Crystal The Fluidity of Membranes (continued) The steroid cholesterol has different effects on membrane fluidity at different temperatures. At warm temperatures, cholesterol restrains movement of phospholipids. At cool temperatures, cholesterol maintains fluidity by preventing tight packing. Cholesterol within the animal cell membrane The Fluidity of Membranes (continued) A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer. Proteins in the membrane can drift within the bilayer. Proteins are larger than lipids and move more slowly. Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell To investigate whether membrane proteins move, researchers fused a mouse cell and a human cell. The Fluidity of Membranes (continued) The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices. EXTRACELLULAR SIDE N-terminus C-terminus a Helix CYTOPLASMIC SIDE Six major functions of membrane proteins: 1. 2. 3. 4. 5. 6. Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix Membrane Proteins and Their Functions Enzymes Receptor ATP Transport Signal Enzymatic activity Signal transduction Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) Glycoprotein Cell-cell recognition Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane. Membrane carbohydrates may be bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins). • The Role of Membrane Carbohydrates A cell must exchange materials with its surroundings, a process controlled by the selectively permeable plasma membrane. Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane easily. Polar molecules and large macromolecules do not cross the membrane easily. Channel proteins called “aquaporins” facilitate the passage of water through the membrane. Plasma Membranes Are Selective Permeable The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others. Transport proteins allow passage of specific substances across the membrane. Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel. • Carrier proteins bind to molecules and change shape to shuttle them across the membrane. Channel Protein Carrier Protein Transport Proteins 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 Passive Transport Requires No Energy 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. Net diffusion Net diffusion Net diffusion Net diffusion Equilibrium Equilibrium Passive Transport Uses No Energy 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. Lower Higher concentration concentration of solute (sugar) of sugar Same concentration of sugar H2O Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Osmosis Effects of Osmosis on Water Balance Tonicity: 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 Hypotonic solution: solute concentration is less than that inside the cell; cell gains water Water Balance of Cells Without Walls The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump. Filling vacuole Contracting vacuole 50 µm 50 µm 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. Water Balance of Cells With Walls Hypotonic solution Isotonic solution Hypertonic solution Animal cell H2O H2O Turgid (normal) H2O Shriveled Normal Lysed Plant cell H2O H2O H2O H2O H2O Flaccid Plasmolyzed In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane. Channel Proteins Carrier Proteins Facilitated Diffusion Is Aided by Proteins Active transport uses energy, usually in the form of ATP, to move substances against their concentration gradient. Passive transport Active transport ATP Diffusion Facilitated diffusion Active Transport Uses Energy Cotransport occurs when active transport of a solute indirectly drives transport of another solute. – + H+ ATP H+ – + H+ Proton pump H+ – + H+ – + H+ Sucrose-H+ cotransporter Diffusion of H+ H+ – – + + Sucrose Cotransport: Coupled Transport by a Membrane Protein Large macromolecules cross the membrane via vesicles. In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents. In endocytosis, the cells take in molecules by forming vesicles from the plasma membrane. Exocytosis Endocytosis Bulk Transport: Exocytosis and Endocytosis