transport in cells ✩

• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration.
• The rate of diffusion depends on the concentration gradient (the difference between the concentrations at either end), temperature, surface area available for diffusion, and distance travelled
• diffusion examples

  • oxygen must diffuse into cells for aerobic respiration
  • carbon dioxide must diffuse into photosynthesising plant cells to be available for photosynthesis
• examples of substances that leave cells by diffusion

  • Liver cells break down excess amino acids into the waste product urea which is highly toxic and must diffuse out of cells to be excreted by the kidneys
  • Carbon dioxide produced by aerobic respiration must diffuse out of cells
• substance leaving cell by diffusion - waste products such as urea
• substance leaving cell by diffusion - water vapour lost through evaporation
• substance leaving cell by diffusion - carbon dioxide produced during respiration
• surface area : volume ratio

  • The surface area to volume ratio of an organism affects how easily substances can be exchanged between it, and its environment
• As the size of an organism increases, surface area: volume ratio decreases
• • Osmosis in biology is the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane
• required practical for osmosis

  A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution is more dilute, a negative percentage change suggests the opposite
• Diffusion and osmosis rely upon the passive transport of substances down concentration gradients, relying on the random movement of particles.
• Active transport moves substances from a more dilute solution to a more concentrated solution. This requires energy from respiration.
• active transport examples (plants)


  • Root hair cells lining the surface of plant roots need to move minerals such as magnesium ions from a region of lower concentration (the very dilute solution of minerals in the soil surrounding the roots) to a region of higher concentration (inside the cytoplasm of the cell)
• active transport examples (animals)


  • Food molecules (such as the sugar glucose) can be absorbed across the wall of the small intestine by diffusion, but this is dependent on a concentration gradient existing between the lumen of the intestine and the bloodstream
  • Active transport allows sugar such as glucose to be transported into the bloodstream from the lumen of the small intestine (the gut) when the concentration of sugar molecules in the blood is higher
  • Sugar molecules are used in respiration to release energy for cells to function