The Cell Membrane • AP Chapter 7 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Composition of the cell membrane: lipids (phospholipids and cholesterol) and proteins • Phospholipids are the most abundant lipid in the plasma membrane - they are amphipathic molecules, contain 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Development of the Fluid Mosaic Model • In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins • In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Sandwich model of the cell membrane What is wrong with this model? Proteins are mostly hydrophobic. All phospholipids are not alike. Fluid Mosaic Model Fig. 7-2 This slide shows how the hydrophobic and hydrophilic regions are set up. Hydrophilic head WATER Hydrophobic tail WATER Fig. 7-3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein Double phospholipid layer Different types of phospholipids Different types of phospholipids making up the membrane Fig. 7-4 TECHNIQUE RESULTS Extracellular layer Knife Plasma membrane Proteins Inside of extracellular layer Cytoplasmic layer Inside of cytoplasmic layer Freeze-fracture studies of the plasma membrane supported the fluid mosaic model The Fluidity of Membranes • Phospholipids in the plasma membrane can move within the bilayer • Most of the lipids, and some proteins, drift laterally (very rarely transverse flip-flops) fluidity of cell membrane fluidity of membrane Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-5 Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Fluid Biology Animations Unsaturated hydrocarbon tails with kinks Viscous Saturated hydrocarbon tails (b) Membrane fluidity Cholesterol (c) Cholesterol within the animal cell membrane This experiment shows how the proteins can move about the membrane. Fig. 7-6 RESULTS Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell The fluidity of the membrane depends on temperature and types of lipids making up the membrane. • Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids • Membranes must be fluid to work properly; they are usually about as fluid as salad oil. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-5b Fluid Unsaturated hydrocarbon tails with kinks (b) Membrane fluidity Viscous Saturated hydrocarbon tails • Would you expect an amoeba that lives in a pond in a cold northern climate to have a higher or lower percentage of saturated fatty acids in its membranes during the summer as compared to the winter? • 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 © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-5c Cholesterol (c) Cholesterol within the animal cell membrane 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 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 • Peripheral proteins are bound to the surface of the membrane • Integral proteins penetrate the hydrophobic core, are called transmembrane proteins • The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-8 N-terminus C-terminus Helix EXTRACELLULAR SIDE CYTOPLASMIC SIDE • Six major functions of membrane proteins: – Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-9 Signaling molecule Enzymes ATP (a) Transport Receptor Signal transduction (b) Enzymatic activity (c) Signal transduction (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Glycoprotein (d) Cell-cell recognition Intercellular joining E-selectin is a transmembrane protein expressed by endothelial cells that binds to an oligosaccharide expressed on the surface of leukocytes Construct a cell membrane Try this at home! constructing a cell membrane The Role of Membrane Carbohydrates in Cell-Cell Recognition • Cells recognize each other by binding to surface molecules, usually carbohydrates • 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 © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Membranes are bifacial • Carbohydrates (making glycoproteins and glycolipids) on outer surface • Peripheral proteins generally on cytoplasmic surface • Proteins have a distinct orientation, ie…receptor proteins oriented at surface, enzyme proteins oriented toward cytoplasm Synthesis and Sidedness of Membranes • Membranes have distinct inside and outside faces – determined when the membrane is built by the ER and Golgi. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-10 ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 apparatus Vesicle 3 4 Secreted protein Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Membrane glycolipid Can you guess where you would find this cell? This cell? Notice how thin-walled they are. Need a hint? These cells?