Membrane structure and function notes


Membrane Structure

  • The basis of membrane structure is the phospholipid bilayer with hydrophilic heads facing outwards and hydrophobic tails facing inwards
  • Proteins span the bilayer (transmembranal) or are embedded or lie on the surface of the bilayer
  • Cholesterol is found in the hydrophobic centre of animal cell membranes
  • The fluid mosiac model suggests a fluid phospholipid bilayer with scattered protein molecules forming a mosiac pattern
  • Carbohydrates bind to produce glycoproteins and glycolipids on the outer suface of the cell surface membrane where they form the glycocalyx and are involved in cell recognition as cell receptors
  • Some proteins act as enzymes
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Movement of molecules across membrane

  • Non-polar molecules and very small molecules (O2 and CO2) can move through the phospholipid bilayer 
  • Transmembrane proteins may be involved in the movement of ions and polar molecules
  • Ions may move by diffusion through the open pores of channel proteins, while polar (water soluble) molecules e.g. glucose may move by facilitated diffusion through carrier proteins 
  • Some polar molecules may also be moved against the concentration gradient by active transport, via carrier proteins that use ATP to pump the molecules across
  • Water can move through the phospholipid bilayer as the molecules are so small, though more often they move through aquaporins
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  • Osmosis is the diffusion of water from an area of high water potential to an area of lower water potential through a partially permeable membrane
  • Water potential has 2 components, solute potential and pressure potential
  • The presence of solutes in solution attracts water molecules and reduces the water potential (more negative) as water is less free to move
  • Plant cells placed in water take in water to develop a pressure potential (turgor pressure) resisted by the cell wall. Animal cells burst (lyse) when placed in water
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  • Exocytosis is the secretion of large molecules (e.g proteins) as a result of vesicles moving to, and fusing with, the cell-surface membrane
  • Endocytosis allows materials to be taken into the cell - large molecules into vesicles (pinocytosis) or larger molecules (e.g bacteria) into vacuoles (phagocytosis)
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