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Cell membrane (structures contained in cells)

  • Cell membranes are structures which separate the cell from the outside (extracellular) space.
  • It is formed by a double membrane of phospholipids.
    • The part facing the cytoplasm (cytosol) and the extracellular space is soluble in water (hydrophilic), whereas the inner part is hydrophobic.
  • Even though it is an impermeable membrane, there are numerous proteins interspersed which form:
    • pores through which proteins can pass through
    • receptors which bind to signalling molecules which communicate with inner proteins affecting genetic production,
    • anchors to structural components defining the shape of the cells (cytoskeleton) etc…
  • The membrane is semi-permeable, so it allows diffusion of small molecules, but not electrolytes: this ultimately allows the cell to remain stable (concentration in harmony with its surroundings), and not to explode or shrink (water balance, a process called Osmosis).
  • It also allows the internalization of external components (endocytosis, phagocytosis) and externalization of cellular components and products (exocytosis, secretion)