Biology B3 - Exchange of substances

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  • Created by: Roxy
  • Created on: 29-03-11 11:54

General notes

  • Diffusion is an essential process that is going on inside your body constantly and keeping you alive. It is the net movement of gas or dissolved molecules.
  • Your body's survival depends on oxygen and dissolved food molecules getting into your cells.
  • Oxygen and dissolved food molecules must diffuse in and out of the blood by transportation around the body.
  • The lungs have a large surface area. This maximises the exchange of oxygen and carbon dioxide with each breath.
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  • Carbohydrates, proteins and fats are made up of large molecules that cannot be used directly by the body.
  • Digestion breaks up larger molecules into smaller molecules, so they are more easily used by the body.
  • Dissolved fod molecules then need to be transported from the small intestine into the bloodstream.
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Inside the small intestine

  • The inner wall of the small intestine is lined with thousands of villi. Each villi has a network of capillaries to carry digested food away from the intestine.
  • The concentration of dissolved food molecules is higher in the small intestine than in the blood entering the villus.
  • This means that the dissolved food molecules dissolve from the small intestine into the blood, moving from higher to lower concentration.
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Adaptions of the villi

  • Good blood supply: this maintains a steep concentration gradient
  • Large surface area: so there is a higher chance of diffusion taking place.
  • Thin cell walls: so there is less distance for the gases to travel.
  • Moist linings: so the gases can dissolve easier.
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