Cell membranes

Cell membranes

The basics
1. Membranes control what passes through them
  1. Cell surface membranes
    1. Control what substance enter and leave the cell, they are partially permeable
    2. Allow recognition by other cells
    3. Allow cell communication
  2. Membranes within cells
    1. Around organelles, divide the cell into different compartmens
    2. Membranes of some organelles are folded to increase surface area to make chemical reactions more efficiant
    3. Form vesicles for transport
    4. Control what passes through the organelle and they are partially permeable
2. Fluid mosaic structure
  1. Phospholipid bilayer constantly moves. It contains cholesterol, protein molecules (glycoproteins) and (glycolipids)
  2. Around 7nm thick
3. Components
  1. Phospholipids
    1. Hydrophilic head and a hydrophobic tail
  2. Cholesterol
    1. Fits between the phospholipids molecules and makes they membrane more rigid and less fluid
  3. Proteins
    1. Some form channel proteins that allow small or charged particles through
    2. Carrier proteins transport ions across
    3. Proteins also act as receptors in cell signalling
  4. Glyco(proteins/lipids)
    1. Stabilise the membrane by forming hydrogen bonds with surrounding water molecules
    2. Act as receptors and antigens
Transport across cell membranes
1. Diffusion
  1. Passive, from high to low concentration
2. Osmosis
  1. Diffusion of water from high to low water potential across partially permeable membrane
3. Facilitated Diffusion
  1. From high to low concentration and it a passive process.
4. Active transport
  1. High to low concentration, active process. (ATP) Using carrier proteins to change the shape
5. Endo/exocytosis
  1. Bulk transport of materials fusing to the membrane
Cell signalling and temperature
1. Cells communicate using messenger molecules
  1. One cells releases MM, it travels to another cell, it is detected by the cell and binds to its receptor
2. The receptor proteins have a specific shape, a cell that responds to the MM is called a target cell
3. Drugs binds to receptors to trigger a response
4. Below 0C: Membrane doesn't move. Channel and carrier proteins denature, increasing permeability. Ice crystals peirce the membrane
  1. 0C-45C: Membrane is partially permeable, as temperature increases the membrane moves more - kinetic energy
  2. 45C+: Bilayer melts and the membrane is more permeable, channel and carrier proteins denature and increases permeability

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