GCSEBiology B3 Higher Tier Revision Cards

AQA Revision Notes

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Diffusion, Osmosis and Active Transport


  • The movement of water from a dilute solution to a more concentrated solution.
  • The water must pass through a partially permeable membrane.
  • Osmosis is passive.


  • Substances move from a high concentration to a low concentration.
  • Substances must be either soluble or a gas.
  • This is how oxygen leaves the leaf.

Active Transport

  • Substances move against the concentration gradient.
  • Particles move from a low concentration to a high concentration.
  • Requires energy to take place.
  • This is how plants take in minerals through their roots.
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Gas Exchange

Exchange surfaces in humans

  • The surface area of the lungs is increased by the alveoli.
  • The surface area of the small intestine is increased by the villi.

Exchange in the lungs

  • The epithelium of the alveoli is very thin, which shortens the diffusion distance.
  • There are capillaries running over the surface of the alveoli, which removes carbon dioxide and takes oxygen to the cells for respiration.
  • The alveoli aid gaseous exchange by providing a large surface area, having a moist surface and having a copious blood supply.

Exchange in the gut

  • Villi and microvilli increase the surface area of the small intestine. This increase in surface area aids the absorbtion of digested products.
  • The villi have a rich blood supply which produces a steep concentration gradient for efficient diffusion.
  • Villi also have a thin wall (around 1 cell thick) which shortens the diffusion distance.
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Gas Exchange in Plants

  • Gasses diffuse in and out of the stomata, found on the underside of the leaves.
  • Leaves have a large surface area.
  • Air spaces within the leaves increases the surface area inside.
  • Guard cells control the size of the stomata.

Inside a leaf

  • Upper epidermis
  • Cuticle
  • Chloroplast - palisade layer
  • Xylem, phloem and air spaces - spongey layer
  • Stomata and guard cells - lower epidermis
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Different Adaptions for Gas Exchange


  • Worms breathe through their skin (no lungs)
  • Insects breathe through spiracles on the sides of their bodies (no lungs)
  • Dolphins breathe through their blowholes (lungs)
  • Land snails breathe through their mantle, a thin layer of tissue (lungs)
  • Water snails breathe through their gills (lungs)
  • Gasses diffuse across the surface of amoeba.
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Gas Exchange in Plants 2


  • Transpiration is the loss of water from the leaves through the stomata.
  • Plants lose water in the form of water vapour.
  • Capillary action is when water moves up the plant via fibres in the xylem.
  • Water molecules are cohesive, they stick together.

Speeding it up

  • Flatter, thinner leaves
  • Hot, dry, windy weather
  • Larger stomata
  • More stomata

Slowing it down

  • Humid climate
  • Small stomata
  • Fewer stomata
  • Waxy leaves
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Heart and Circulation

Blood flow

  • (Through the veins) From lungs --> left atrium --> left ventricle --> (through arteries) body tissues --> (through veins) right atrium --> right ventricle --> (through arteries) lungs.
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