5.1 Cell Membranes


Membrane function

  • Membanes at the cell surface
    • a barrier between the cell and its environment 
    • controls what enters and leaves 
    • partially permeable- let in some but not all molecules
    • molecules move across by diffusion, osmosis or active transport
    • cell signalling 


  • Membranes within cells
    • compartmentalisation- act as a barrier between organelle and the cytoplasm [e.g. enzymes are kept together inside a mitochondion by its outer membrane 
    • membranes for vesicles to transport substances between different areas of the cell [e.g. proteins from the RER to the golgi during protein synthesis] 
    • partially permeable- controls what enters and leaves organelle [e.g. RNA leaves the nucleus- DNA is too big]
    • membranes inside organelles- barriers between its contents and the rest of the organelle [e.g. thylakoid membranes in chloropasts keeps the components for photosynthesis together] 
    • site of chemical reactions- some are folded to increase surface area making the reactions more efficent [e.g. inner membrane of mitochondrion contains enzymes needed for respiration]

Membrane Structure

  • composed of lipids, proteins and carbohydrates 


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