Cell Membrane and Transport

Components of Membrane


  • allows lipid soluble molecules enter and leave cell but prevents water soluble molecules
  • form bilayers - hydrophillic heads pointing in interacting with water inside / hydrophobic tails pointing out interacting with water outside cell
  • Hydrophobic tails of both point towards centre of membrane (non-polar - move towards centre)


  • Extrinisic - surface of bilayer or partly embedded - recognition sites
  • Intrinsic - extends across both layers - carrier (water soluble products) or channel (ions)


Attached to proteins (glycooproetins) or phospholipids (glycolipids) for cell to cell recognition


Increases ridigity and stability of membrane

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Membrane as a barrier

  • Small uncharged molecules freely diffuse
  • Non-polar molecules (glycerol,vitamin A) dissolve in lipids
  • Polar molecules (glucose,amino acids,ions) pass through intrinisic proteins

Permeability of membrane is affected by:

  • Temperature
  • Lipids
  • Organic solvents
  • Cholesterol
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Random passive movement of ions from a region of high concentration to lower concentration, until equillibrium is reached

Rate is affected by:

  • Concentration gradient
  • Distance over diffusion takes place
  • Surface area
  • Temperature
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Facilitated Diffusion

Passive diffusion through a channel/carrier protein down concentration gradient 

Quicker than diffusion

Affected by: temperature, concentration gradient, number of transport proteins

Two types of proteins


  • water filled pores with polar groups allowing ions to pass through
  • each one is specific for one type of ion
  • open and close depending on need of cells


  • allows large polar molecules (glucose, amino acids) to diffuse
  • molecule attaches to receptor site causes protein to change shape releasing molecule
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Active Transport

  • Requires energy in the form of ATP
  • Against concentration gradient
  • Requires carrier proteins - acts as pump
  • Transports charged molecules or ions

When a respiratory inhibitor is added, ATP production is inhibited 

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Bulk Transport


Englulfing the material by plasma mebrane bringing it into cell

  • Phagocytosis - solid materials
  • Pinocytosis - liquid material


Substances leaving cell after being transported through cytoplasm in a vesicle

e.g. digestive enzymes

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Osmosis : passage of water from a high water potenital to low water potential through a partially permeable membrane

Water Potential : pressure created by water molecues measured in kPa

Pure water has a water potential of 0 kPa (highest potential energy)

Addition of solute will lower water potential of pure water


Solute potential : concentration of dissolved substances inside cell vacuole

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Osmosis in Solutions


  • Higher water potential outside cell than inside
  • Water enters cell by osmosis


  • Lower water potential outside cell than inside
  • Water leaves cell by osmosis


  • Water potential same inside and outside cell
  • No net movement of water
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Osmosis in Solutions


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