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Unformatted text preview: 1 Biol 113 AU2011 Lecture 6 Membrane Structure & Function A. Overview: Life at the edge B. Cellular membranes are fluid mosaics of lipids and proteins C. Membrane structure results in selective permeability D. Membrane proteins and their functions E. Passive transport is diffusion of a substance across a membrane with no energy investment F. Facilitated diffusion: passive transport aided by proteins G. Active transport uses energy to move solutes against their gradients H. Ion pumps maintain membrane potential I. Co-transport: coupled transport by a membrane protein J. Bulk transport across the plasma membrane occurs by endocytosis and exocytosis Modified from Plant Physiology (2nd Ed): L. Taiz and E. Zeiger Open stomate Closed stomate Stomata CO 2 O 2 Mesophyll Vein Leaf cross section A convenient truth : Potassium pumps and channels in the plasma membrane of leaf guard cells and osmosis make many lives on earth possible. K + H 2 O H 2 O K + National Kidney Foundation W.J. Nelson (2004) Nature Thinkquest A. 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 Fig. 7.1 Hydrophilic head WATER Hydrophobic tail WATER B. 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 Fig. 7.2 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein Fig. 7.3 The fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in it 2 The Fluidity of Membranes 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 (a) Movement of phospholipids Lateral movement ( 10 7 times per second) Flip-flop ( once per month) Fig. 7.6 Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour Fig. 7.7 (b) Membrane fluidity Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydro- carbon tails Fig. 7.8 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 Cholesterol (c) Cholesterol within the animal cell membrane The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (such as 37C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing Fig. 7.8 C. Membrane Structure Results in Selective Permeability C....
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