BIOL231 Ch 11 FA2024 Membrane Structure PDF

Chapter 11 Membrane Structure Membranes make up the plasma membrane and surround organelles: compartments Function of Membranes Physical barrier – defines boundaries Allow certain molecules in/out - selective...

Chapter 11 Membrane Structure Membranes make up the plasma membrane and surround organelles: compartments Function of Membranes Physical barrier – defines boundaries Allow certain molecules in/out - selective Also involved in communication (signals/receptors) Mediates adhesion between cells Flimsy but flexible – allows for movement and expansion; how flexible? Lazer tweezers demo https://www.youtube.com/watch? v=jM_xePC70Yo  Cell membranes are composed of: lipids proteins  Fluid Mosaic Model “Protein Icebergs Floating in a Sea of Lipids” Davson-Danielli (1935) vs. Singer-Nicolson Model (1972) Fluid lipid bilayer - sea Mosaic of associated proteins - iceberg  Three Major Classes of Membrane Lipids Cholesterol Glycolipids Phospholipids Three Major Types of Phospholipids Phosphatidyl Phosphatidyl Phosphatidyl choline Serine Inositol Serine Polar head group Polar head group Polar head group contains contains contains Phosphocholine Phosphoserine Phosphoinositol Phospholipids form bilayers in aqueous environment Polar heads Hydrophobic region Polar heads Amphipathic Nature of these Molecules fit into the bilayer All Membranes are not Composed of the Same Phospholipids!  Triacylglycerol vs Phospholipid Polar No Polar Head Head Group Group 3 2 hydrocarbon hydrocarbon chains chains Nonpolar Amphipathic TAGs are NOT found in Cell Membranes !!!!!!!!!!!  The Lipid Bilayer is Asymmetrical (Different Monolayers) Amt and type of phospholipid Amt of cholesterol Proteins Glycolipids more often found on extracellular side in plasma membrane  “Sea of Lipids” refers to the Fluidity of the Membrane Singer-Nicolson Model Both phospholipids and proteins can move!  Movement of Phospholipids in Bilayer Rotation Lateral Diffusion RARELY !  Yes to Lateral Diffusion! No to Flip Flop! How do we know? FRAP Exceptions: Flippase/floppase/scramblase (random) Fluidity of the Membrane is influenced by: 1. Number of double bonds in hydrocarbon tails of phospholipids 2. Length of hydrocarbon tails of phospholipids 3. Amount of cholesterol in membrane Fluidity of the Membrane is Affected by the Number of double bonds Saturated vs Unsaturated To increase Fluidity of the Membrane: ↑ unsaturated hydrocarbon chains Fluidity of the Membrane is Affected by the Length of the Hydrocarbon Chain To increase Fluidity of the Membrane: the hydrocarbon chain length  Cholesterol is Rigid and Stiffens Cell Membranes at Physiological Temperatures Most Organisms Can Regulate Membrane Fluidity How would a drop in temperature affect membrane fluidity? How would an increase in temperature affect membrane fluidity?  Lipid Rafts (aka lipid microdomains) Localized regions of membrane lipids (glycosphingolipids, cholesterol) and specific proteins Dynamic - changes composition Important in allowing cell to detect and respond to extracellular signals Lipids are just half of the Story! Four Major Functions of Membrane Proteins Transporters – allow molecules to pass thru membrane Anchors - assist in linking PM to extracellular matrix/cytoskeleton Receptors – receive signals/communication Enzymes – catalytic activity  Membrane proteins are important in other cell functions uptake and secretion of substances by endocytosis and exocytosis Structural roles to stabilize and shape membrane Proteins may be embedded or loosely associated with the membrane Three Main Classes of Membrane Proteins A. Integral Membrane Proteins B. Peripheral Membrane Proteins C. Lipid Anchored Membrane Proteins  A. Integral Membrane Proteins Four types: 1. Integral monotopic 2. Singlepass 3. Multipass 4. Multisubunit proteins Some regions are embedded into the membrane (aa comp?) Regions outside membrane usually rich in __________ amino acids Many have Transmembrane domains  1. Integral Monotopic Proteins A region of the protein is embedded into one monolayer (one leaflet) of the membrane No Transmembrane domains One region is outside of the membrane Found in some membrane-bound enzymes 2. Singlepass Proteins contain one Transmembrane Domain that is usually an alpha helix Amino acids in transmembrane domain? Amino acids outside membrane? Commonly found in Receptors and Anchors  3. Multipass Proteins Single polypeptide containing multiple transmembrane domains Transmembrane domains may be composed of alpha helix or beta sheets Can form pores/opening in membrane to transport polar molecules across Multipass Proteins composed of alpha helices Amino acid composition of alpha helices? Multipass Proteins composed of Beta Sheets are called Beta Barrels Amino acid composition of beta sheets?  4. Multi-subunit Integral Proteins Separate polypeptides, each with transmembrane domains Transmembrane domains aggregate together in the membrane through noncovalent bonding Found in membrane bound enzymes and receptors  Detergents are required to isolate membrane bound proteins SDS sodium dodecyl sulfate or Triton X-100 CMC=critical micelle concentration  B. Peripheral Membrane Proteins Mostly hydrophilic amino acids on outside of protein Bound to membrane surfaces through weak electrostatic forces and hydrogen bonds  C. Lipid-Anchored Membrane Proteins Protein on surfaces of membranes (aa comp?) Covalently bound to lipid molecules embedded in the bilayer Important Aspects of the Plasma Membrane The plasma membrane is supported by an underlying actin protein network called the cell cortex The cortex is the main determinant of cell shape and therefore plays a fundamental role in processes such as cell division, migration, and tissue morphogenesis. Plants, yeast, bacteria really don’t rely on this. Why?  Cell Cortex in Healthy Red Blood Cells  Spectrin is the major protein in the Cell Cortex of RBC Cytoplasmic side Fibrous protein on cytoplasmic side of membrane to help protect the thin 10nm membrane Linked to anchor proteins through attachment proteins (ankryins) Extracellular side What happens if mutation occurs Provides support, maintain shape in spectrin? Anemia, abnormally shaped RBCs Restriction of lateral movement of membrane proteins Glycocalyx found on Extracellular Side of the Plasma Membrane Composed of: Glycoproteins Glycolipids Proteoglycans (proteins with one or more long polysaccharide chains)  Purpose of Glycocalyx Cell-Cell Recognition Helps immune system recognize and attack foreign organisms Cell adhesion in tissues Protect from chemical injury Gives outside of cell a slimy coating