Biology - B3.1 - Exchange of Materials

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B3.1.1 - Osmosis

  • Osmosis - movement of water
  • Like diffusion, osmosis is random and requires no energy
  • Osmosis - diffusion of water across partially permeable membrane - cell membrance is partially permeable
  • Moves from area of high concentration (dilute solution) to lower concentration (concentrated solution)
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B3.1.2 - Active Transport

  • Cells may need to absorb substances in short supply: against concentration gradient - use active transport to absorb substances across partially permeable membrane against concentration gradient
  • Active transport requires energy released in respiration
  • Cells able to absorb substance from dilute solutions - eg. root cells absorb mineral ions from soil
  • Glucose reabsobed in kidney tubules by active transport
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B3.1.3 - The Sports Drink Dilemma

  • Exercise - muscles release energy through respiration, using glucose - body gets hot: sweat to cool down -> sweat a lot: dehydrated - need to replace sugar, mineral ions and water that have been used or lost after exercise
  • Sports drinks: sugar and mineral ion solutions - helps rehydrate cells
  • Drinks designed to help balance body fluid concentration and concentrations inside cells - if drink concentration = body fluid concentration: isotonic
  • Evidence of sports drinks benefits is varied - some think water is just as good, others think sports drinks are better as they replace sugar and minerals ions as well as water
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B3.1.4 - Exchanging Materials - The Lungs

  • Large, complex organisms have special exchange surfaces to obtain food and oxygen needed - intestines absorb soluable food materials (solutes), lungs absorb oxygen and CO2 is removed
  • Efficient exchange surfaces need: large surface area, thin wall/short diffusion path, efficient transport system - eg. blood supply
  • Lungs contain gaseos exchange surface - surface area is increased by alveoli
  • Alveoli have thin walls, large surface area and good blood supply
  • Lungs ventilated to maintain steep diffusion gradient
  • Oxygen diffuses into capillaries surrounding alveoli and CO2 diffuses back out into lungs to be breathed out
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B3.1.5 - Ventilating the Lungs

  • Lung - contain exchange surface of breathing system - situated in thorax, in ribcage, above diaphragm: separates lung from abdomen
  • When we breathe in:
    • intercoastal muscles between ribs and diaphragm contract
    • ribcage moves up and out and diaphragm flattens
    • volume of thorax increases
    • pressure in thorax decreases: air is drawn in
  • When we breathe out:
    • intercoastal muscles of ribcage and diaphragm relax
    • ribcage moves down and in and diaphragm becomes domed
    • volume of thorax decreases
    • pressure in thorax increases: air is forced out
  • Ventilation: movement of air in and out of lungs
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B3.1.6 - Artificial Breathing Aids

  • People can't get enough in their lungs because: alveoli are damaged: surface area decreased, tubes leading to lungs are narrow: less air can be moved through, paralysis: muscles don't work to pull ribcage up and out
  • Breathing aids have been developed:
  • Iron lung - used for people paralysed by polio - person lays with chest sealed in metal cylinder - air is drawn out: vacuum formed inside cylinder creates negative pressure: breathe in - air pumped back into cylinder: creates pressure on chest: breath out
  • Breathing aids force measured amounts of air into lungs use positive pressure - bags of air linked to masks force air down trachea
  • Positive pressure aids are smaller, easier to manage and can be linked to computers to control
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B3.1.7 - Exchange in the Gut

  • Food is digested in the gut into small, soluable molecules that are absorbed into blood in small intestine - villi line inner surface: exchange surface for food molecules
  • Villi - finger-like projections - increase surface area for absorbtion to take place
  • Walls of villi are thin with many capillaries close to them
  • Soluable products of digestion can be absorbed into villi by either diffusion or active transport
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B3.1.8 - Exchange in Plants

  • Gases diffuse in and out of plants through stomata - tiny holes, size controlled by guard cells
  • Gases are: oxygen and CO2 - movement of gases depends on which process is happening faster - respiration or photosynthesis
  • Plants lose water through stomata due to evaporation
  • Leaves are flat and thin - don't need to diffuse very far, also internal air spaces
  • Water and mineral ions taken up by roots - root hair cells increase root surface area
  • Stomata close to prevent wilting if water is being lost faster than being replaced
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B3.1.9 - Transpiration

  • Plants take up water through roots - water passes through plant to leaves - in leaves, water evaporates from leaf cells and water vapour diffuses through stomata
  • Transpiration stream: movement of water through plant
  • Plant can dehydrate if rate of evaporstion > root water uptake
  • Evaporation is faster in hot, dry, windy or bright conditions
  • Guard cells - close to prevent excess water loss
  • Wilting prevents water loss - leaves collapse and hang down: reduces surface area
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