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Membranes

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Membranes Membranes Presentation Transcript

  • Cell Membranes
    • The cell membrane/plasma membrane represents the barrier that separates the cell’s contents from the surrounding environment and controls what moves in and out
    • In eukaryotic cells, membranes are also used to generate compartments within the cell, each with a specialised function e.g. golgi apparatus, endoplasmic reticulum, lysosomes etc
  • Membrane functions
        • Provides selectively permeable barriers
        • Compartmentalisation
        • Localises reactions in the cell
        • Transport of solutes often against the concentration gradient (active transport)
        • Signal transduction – receptor proteins on the membrane surface recognise and respond to different stimulating molecules, enabling specific responses to be generated within the cell
        • Cell to cell recognition – the external surface of the membrane is important as it represents the cell’s biochemical “personality”. In multicellular organisms this allows cells to recognise each other as similar or different, which is necessary for the correct association of cells during development.
  • Membrane Structure
    • The basic composition and structure of the plasma membrane is the same as that of the membranes that surround organelles and other subcellular compartments.
    • The foundation is a phospholipid bilayer – polar hydrophilic heads on the outer surface and hydrophobic non-polar fatty acid tails form the inner surface. The membrane as a whole is often described as a fluid mosaic – a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins which may function as channels or transporters across the membrane, or as receptors.
  • The Plasma Membrane
    • The idea that membranes were composed of phospholipids was first put forward in 1925. The currently accepted model for membrane structure was proposed by S.J. Singer (1971) as a lipid protein model and extended to include the fluid character in a publication with G.L. Nicolson in "Science" (1972)
    • The fluid mosaic model has 2 components, lipids and proteins. The lipids form the matrix bilayer of the membrane and the proteins carry out all of its functions
    • The membrane is not a static rigid structure, but a dynamic arrangement of lipids and proteins that drift laterally within it.
  • Types of Membrane Proteins
    • Proteins make up approximately 50% of the mass of the plasma membrane and can be classified into different groups depending on their arrangement in the membrane and/or their function
    • Proteins may be embedded in the lipid bilayer or attached to the surface
    • The embedded or INTRINSIC proteins may be transmembrane proteins (span the bilayer) or they may be linked to lipids on one side of the bilayer only
    • The peripheral or EXTRINSIC proteins are loosely attached to the membrane by ionic association with other proteins
    • Glycoproteins have carbohydrates attached to their extracellular domains.
  • Functions of Membrane Proteins
    • The main functions of these membrane proteins are as follows:
          • Transport
          • Cell recognition
          • Receptor sites
          • Enzymes
          • Intracellular Junctions
  • 1) TRANSPORT PROTEINS
    • Transport non-diffusable substances across the membrane. May be either:
      • (a) Channel proteins – provide a ‘pore’ across the membrane through which molecules (usually small and charged) can diffuse
      • (b) Carrier proteins – these are more specific with binding sites for only one solute
    • Both these proteins permit passive transport (with a concentration gradient this is called facilitated diffusion)
    • To transport molecules against the concentration gradient, special types of the carrier proteins are needed. These harness energy to drive the transport process during active transport e.g. sodium-potassium pump
  • 2) CELL RECOGNITION PROTEINS
    • Usually glycoproteins
    • The carbohydrate chain of the glycoprotein projects out of the cell enabling cell to cell recognition and serving as a cell “fingerprint”
    • Therefore, the immune system can recognise it’s own cells and organs e.g. ABO blood group antigens:
          • A = glycoprotein antigen A
          • O = no glycoprotein antigens
  • 3) RECEPTOR PROTEINS
    • These have a specific conformation (shape) that allows binding of a particular molecule (the ligand)
    • The binding of the ligand will then trigger a response in the cell
  • 4) ENZYMES
    • A protein that catalyses a specific reaction
    • Some receptor proteins have enzymatic activity; the cytoplasmic portion of the protein catalyses a reaction in response to binding a ligand
  • 5) INTRACELLULAR JUNCTIONS
    • Interactions between the plasma membranes of different cells is a frequent occurrence and takes place at cell junctions e.g.
    • -> In PLANTS
      • PLASMODESMATA – although each plant cell is encased in a boxlike cell wall, fine strands of cytoplasm, called plasmodesmata , extend through pores in the cell wall connecting the cytoplasm of each cell with that of its neighbors allowing direct exchange of materials
      • In ANIMALS , there are 3 types…
      • Spot desmosome – dense protein deposits that hold adjacent cells together by rivets. Mechanical strength is provided by the intracellular filaments passing from one desmosome to another
      • Tight junction – adjacent membrane proteins are bonded together preventing movement of materials in the space between the cells e.g. between epithelial cells lining the small intestine
      • Gap junction – doughnut shaped proteins from each cell joined together to form tiny channels allowing the passage of small molecules such as ions, amino acids and sugars