Functions of the cell membrane components

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The phospholipid bilayer

  • backbone of the cell-surface membrane
  • gives membrane much of its selectively or differentially permeable properties
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Cholesterol

  • Increases membrane stability (temperature stability buffer)
  • At high temperatures, it restricts the sideways movement of phospholipid molecules and decreases membrane fluidity
  • At low temperatures, the cholesterol helps to maintain membrane fluidity by acting as a wedge between adjacent phospholipid molecules and preventing them from sticking together

Makes membrane less fluid at high temperatures and more fluid at low temperatures

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Proteins

  • provide stability and support as they help to 'anchor' phospholipid molecules
  • act as enzymes:

-ideal place for enzymes in terms of substrate availability and pH

-need replaced less than other enzymes

  • act as adhesion sites - areas where adjacent cells are held together
  • involved in cell recognition and receptors or antigens
  • transport of substances across the bilayer
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Types of Proteins

Channel Proteins

  • span the membrane
  • create a hydrophilic channel that allows polar molecules to bypass the hydrophobic centre of the bilayer
  • may be permanently open or controlled opening (gated)

Carrier Proteins

  • carry specific ions and molecules across the membrane
  • the molecules may have charged groups or the molecules/ions have to be moved against the concentration gradient
  • can change shape to carry the substance from one side of membrane to the other
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Glycocalyx

The glycocalyx layer is the layer on the outside of the cell-surface membrane of polysaccharides that bind to the protein or lipid (glycoproteins or glycolipids)

  • They are involved in cell-to-cell recognition, allowing cells of similar type to recognise each other and group together to form tissues
  • Some glycoproteins act as antigens and receptor sites - provide sites on the cell-surface membrane that particular molecules fit (can occur because the receptor site and specific molecule concerned are complementary in shape to each other)
  • Receptor sites are important in hormone action and in the passage of neurotransmitters between neurones
  • Glycoproteins/lipids can form hydrogen bonds with water molecules outside the membrane so can help stabilise the membrane
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