Biology Revision

Unit three

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Unit 13.1

  • Active transport - this is when substances are absorbed against a constant gradient. It requires energy which comes from respiration. It allows cells to absorb ions from a very dilute soloution. Other substances such as sugar and ions can also pass through cell membranes.
  • Dissolved substances move by diffusion. Water moves by osmosis.
  • Many organ systems are speciallised to be able to exchange materials.
  • In humans - the surface area of the lungs is increased by the alveoli so the maximum amount of gaseous exchange can take place. In the gut there is lots of villi.
  • The alveoli provide a large surface area, moist surface and a rich supply of blood cappilaries so that gases can often diffuse in and out of the blood.
  • The villi also provide a large surface area and an extensive network of blood cappilaries to absorb the products of digestion by diffusion and active transport.
  • The lungs are situated in the thorax (upper part of the body) and is protected by the ribcage. It is seperated from the abdomen (lower part of the body) by the diaphram.
  • The breathing system takes air into and out of the body so that oxygen can diffuse from the air into the bloodstream and carbon dioxide can diffuse from the bloodstream to the air.
  • In plants - carbon dioxide enters the leaves by diffusion and water and mineral ions enter through root hair cells.
  • The surface area in leaves is increase by the flattened shape and internal air spaces. The surface area in the roots is increase by the root hair cells.
  • Plants have stomatas to contain the carbon dioxide.
  • Transpiration - the loss of water vapour from the leaves. It is very rapid in hot, windy and dry conditions.
  • Transpiration happens mostly in the stomata. The size of the stomata is controlled by guard cells which surrounds it. If plants loose water from the leaves quicker than it can be replaced by the roots then the stomata can close which prevents wilting. 
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