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Tang 05 membrane structure and functions

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    Tang 05  membrane structure and functions Tang 05 membrane structure and functions Presentation Transcript

    • MEMBRANE STRUCTURE & FUNCTIONS
    • MEMBRANE STRUCTURE AND FUNCTION Recall: Membranes are made of phospholipids, which have a phosphate group attached to fatty acid (lipid) chains. Phospholipids have a polar and non-polar end, making them amphipathic
    • MEMBRANE STRUCTURE AND FUNCTION The amphipathic nature of phospholipids means that it forms a bilayer or a single-layer micelle spontaneously in water. Vesicle (bilayer) Micelle (unilayer) Phospholipid bilayer
    • MEMBRANE STRUCTURE AND FUNCTION A bilayer arrangement allows the membrane to have water on either side. Water Water Hence the cell membrane is called the phospholipid bilayer.
    • MEMBRANE STRUCTURE AND FUNCTION This bilayer is not static; it is dynamic. There is fluidity in the membrane.
    • MEMBRANE STRUCTURE AND FUNCTION Fluidity is assisted by sterols (i.e. Cholesterol in animal cells) Sterols stabilize the membrane. Hydrophilic end Hydrophobic end Hydrophobic tail
    • MEMBRANE STRUCTURE AND FUNCTION At high temperatures, sterols restrain movement of phospholipids At low temperatures, sterols occupy spaces between phospholipids to prevent the lipids from sticking together (enhances fluidity)
    • MEMBRANE STRUCTURE AND FUNCTION Fluid mosaic model: Membranes are fluid (not rigid); proteins embedded in the lipids float freely.
    • MEMBRANE STRUCTURE AND FUNCTION Two types of membrane proteins: 1) Integral membrane protein – protein embedded in the lipid bilayer 2) Peripheral membrane protein – protein on the surface of the membrane
    • MEMBRANE STRUCTURE AND FUNCTION Two types of membrane proteins:
    • MEMBRANE STRUCTURE AND FUNCTION Some integral proteins are transmembrane proteins because they cross the thickness of the lipid bilayer. Protein crosses the membrane
    • MEMBRANE STRUCTURE AND FUNCTION How are proteins embedded in the membrane? The amino acids of the protein that cross the cell membrane have hydrophobic R-groups. The parts that are exposed to the environment have hydrophilic R-groups. Gly Ala Val Leu Ile Phe Trp Pro Ala Leu Ile Phe Trp Pro Gly Ala Leu Ile Phe Trp Pro
    • MEMBRANE STRUCTURE AND FUNCTION How are proteins embedded in the membrane? These nonpolar (hydrophobic) amino acids are often found in the transmembrane sections of proteins.
    • MEMBRANE STRUCTURE AND FUNCTION How are proteins embedded in the membrane? hydrophobic hydrophilic hydrophilic
    • MEMBRANE STRUCTURE AND FUNCTION How are proteins embedded in the membrane? Hydrophobic section with nonpolar R-groups
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 1) Transport 2) Receptor 3) Enzyme 4) Anchor (attachment)/recognition Some membrane proteins have multiple functions
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 1) Transport proteins: Move molecules and ions across a membrane
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 1) Transport proteins: can be channel and carrier proteins Channel protein Carrier protein
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 1) Transport proteins: Some can transport two substances in opposite directions simultaneously (i.e. sodium potassium pump)
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 2) Receptor proteins: relays signals between cells internal and external environments
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 3) Enzyme proteins: catalyzes reactions at the membrane
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 4) Anchor (attachment)/recognition proteins: Attachment points for cytoskeleton and ECM components; also for cell- cell recognition (i.e. by immune system)
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 4) Anchor (attachment)/recognition proteins: Immune cells can identify which cells are foreign, and which cells are infected
    • MEMBRANE STRUCTURE AND FUNCTION Functions of membrane proteins: 4) Anchor (attachment)/recognition proteins: Often have carbohydrate groups on the extracellular side; these are called glycoproteins
    • MEMBRANE STRUCTURE AND FUNCTION Carbohydrates are often used as “markers” in recognition: Glycoproteins: Protein with a carbohydrate attached Glycolipid: Lipid with a carbohydrate attached