Membrane Structure PDF

Membrane Structure This animation shows how the diffusion rate of a GFP-tagged membrane protein...

Membrane Structure This animation shows how the diffusion rate of a GFP-tagged membrane protein can be determined by FRAP (Fluorescence Recovery After Photobleaching) Learning Objectives: Understand the properties of membranes that make them important in cell biology Understand the chemical properties of lipids that allows them to assemble into a bilayer membrane Understand the permeability of lipid bilayers to different classes of molecules Todays topics 1. Membrane lipids 2. Membrane proteins  Eukaryotic cells have membrane-bound organelles plasma membrane What are membranes made of and how do they work? Figure 1-24A Essential Cell Biology  Membrane structure (simplified) water hydrophilic surface hydrophobic membrane 5 nm core hydrophilic surface water  Membranes act as selective barriers Prokaryote Eukaryote otes: the plasma membrane is their only membrane. otes: in addition to the plasma membrane, they have internal membrane itional compartmentalization Figure 11-1 Essential Cell Biology  Lipid bilayers block the passage of most water-soluble molecules synthetic membrane Steroid hormones Figure 12-1 and 12-2 Essential Cell Biology The plasma membrane is more than a barrier… s the interface the cell has with its environme 4 send signals 5 permit cell division without rupture Figure 11-2 Essential Cell Biology Membranes are composed of lipids and protein Electron micrograph of a cell membrane erms of mass, the plasma membrane is about 50% lipids and 50% prote ids and membrane proteins can diffuse relatively freely within the bilay Figure 11-4 Essential Cell Biology  Membrane lipids are “amphipathic” Figure 11-5 Essential Cell Biology ost abundant lipids in membranes are phospho This is phosphatidylcholine (PC), depicted three ways: (fatty acids) Figure 11-6 Essential Cell Biology erent phospholipids have different ‘head grou “PS” “PC” head group Most lipids are either net neutral or have a net negative charge. As a result the surfaces of most membranes in cells have a net negative charge. Figure 10-3 Molecular Biology of the Cell  Membrane lipids form bilayers in water Figure 11-11 Essential Cell Biology Membrane lipids form bilayers in water water monolayer membrane (phospholipid bilayer) monolayer water Each “monolayer” of the bilayer is also called a “leaflet”  Phospholipid bilayers spontaneously close to form sealed compartments Liposome Figure 11-12 and 11-13 Essential Cell Biology spholipids move within the plane of the membr (essentially never occurs unless catalyzed by a protein) Fluidity’ of a membrane refers to the lateral movement of lipids and membrane proteins) Figure 11-14 Essential Cell Biology  Membranes with saturated lipids are less fluid than ones containing unsaturated lipids less fluid “unsaturated” “saturated” (fewer H atoms) more fluid  Membrane fluidity depends on several factors Cells adjust membrane compositions to temperature to keep membranes fluid (if organisms cannot control body temperature) Temperature higher temperature = more fluidity less fluid (temperature is constant in human cells) Fatty acid tail length Shorter chains reduce tendency of chains to interact with one another, increasing fluidity Ratio of saturated to unsaturated lipids more fluid higher unsaturation, higher fluidity Cholesterol content cholesterol stiffens the membrane of animal cells  Membranes also contain cholesterol Figure 11-7 Essential Cell Biology  Cholesterol alters membrane fluidity The plasma membrane of animal cells is about 20% cholesterol Cholesterol makes membranes stiffer and less permeable (but at low temperature cholesterol helps to increase membrane fluidity) Figure 11-15 Essential Cell Biology he plasma membrane lipid bilayer is asymmet PC Glycolipids Cholesterol - - - - - - - - PS How do cells generate asymmetric membranes Figure 11-19 Essential Cell Biology  Synthesis and redistribution of phospholipids Endoplasmic reticulum (ER) Scramblase Phospholipids are synthesized in the ER (and some in mitochondria) Figure 11-16 and 11-17 ECB6  Generation of asymmetry of phospholipids in membranes Golgi apparatus Flippase Figure 11-16 and 11-17 ECB6 Todays topics 1.Membrane lipids 2.Membrane proteins ma membrane proteins have a variety of funct EXTRACELLULAR SPACE Cell-to-cell Cell-to-matrix Cytoskeletal attachment CYTOPLASM Figure 11-20 Essential Cell Biology embrane proteins associate with the lipid bilay in several different ways embrane protein has a specific orientation with respect to the lip “Integral” “Peripheral” xtracellular space cytoplasm There are also peripheral membrane proteins that bind directly to the hydrophilic surface Figure 11-21 Essential Cell Biology of the membrane eins often cross membrane bilayers with an a- Backbone side view top view Side chains extend outward  A hydrophilic pore can be formed by multiple transmembrane a-helices With polar side-chains Nonpolar side chains interact with hydrophobic lipid tails Figure 11-24 Essential Cell Biology w to measure protein diffusion within membran Figure 11-4 Essential Cell Biology RAP” experiments assess mobility in membran A GFP-membrane protein fusion is expressed in cells Plasma membrane Fast with GFP-tagged protein Slow FRAP: Fluorescence Recovery After Photobleaching Invented in the lab of Watt Webb at Cornell! Figure 11-34 Essential Cell Biology; see movie 11-5 Lateral mobility of plasma membrane proteins can be restricted in several ways binding to the extracellular binding to proteins matrix of the cell cortex binding to proteins on the surface of another cell diffusion barrier Figure 11-31 Essential Cell Biology (a dense network of