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Biology_A-level_Cie

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  1. 1-1-the-microscope-in-cell-studies
    5 主题
  2. 1-2-cells-as-the-basic-units-of-living-organisms
    5 主题
  3. 2-1-testing-for-biological-molecules
    3 主题
  4. 2-2-carbohydrates-and-lipids
    8 主题
  5. 2-3-proteins
    6 主题
  6. 2-4-water
    2 主题
  7. 3-1-mode-of-action-of-enzymes
    5 主题
  8. 3-2-factors-that-affect-enzyme-action
    8 主题
  9. 4-1-fluid-mosaic-membranes
    4 主题
  10. 4-2-movement-into-and-out-of-cells
    12 主题
  11. 5-1-replication-and-division-of-nuclei-and-cells
    6 主题
  12. 5-2-chromosome-behaviour-in-mitosis
    2 主题
  13. 6-1-structure-of-nucleic-acids-and-replication-of-dna
    4 主题
  14. 6-2-protein-synthesis
    5 主题
  15. 7-1-structure-of-transport-tissues
    4 主题
  16. 7-2-transport-mechanisms
    7 主题
  17. 8-1-the-circulatory-system
    7 主题
  18. 8-2-transport-of-oxygen-and-carbon-dioxide
    5 主题
  19. 8-3-the-heart
    4 主题
  20. 9-1-the-gas-exchange-system
    6 主题
  21. 10-1-infectious-diseases
    3 主题
  22. 10-2-antibiotics
    3 主题
  23. 11-1-the-immune-system
    4 主题
  24. 11-2-antibodies-and-vaccination
    6 主题
  25. 12-1-energy
    5 主题
  26. 12-2-respiration
    11 主题
  27. 13-1-photosynthesis-as-an-energy-transfer-process
    8 主题
  28. 13-2-investigation-of-limiting-factors
    2 主题
  29. 14-1-homeostasis-in-mammals
    8 主题
  30. 14-2-homeostasis-in-plants
    3 主题
  31. 15-1-control-and-coordination-in-mammals
    12 主题
  32. 15-2-control-and-coordination-in-plants
    3 主题
  33. 16-1-passage-of-information-from-parents-to-offspring
    5 主题
  34. 16-2-the-roles-of-genes-in-determining-the-phenotype
    7 主题
  35. 16-3-gene-control
    3 主题
  36. 17-1-variation
    4 主题
  37. 17-2-natural-and-artificial-selection
    7 主题
  38. 17-3-evolution
    2 主题
  39. 18-1-classification
    5 主题
  40. 18-2-biodiversity
    7 主题
  41. 18-3-conservation
    6 主题
  42. 19-1-principles-of-genetic-technology
    11 主题
  43. 19-2-genetic-technology-applied-to-medicine
    4 主题
  44. 19-3-genetically-modified-organisms-in-agriculture
    2 主题
  45. 1-1-the-microscope-in-cell-studies
  46. 1-2-cells-as-the-basic-units-of-living-organisms
  47. 2-1-testing-for-biological-molecules
  48. 2-2-carbohydrates-and-lipids
  49. 2-3-proteins
  50. 2-4-water
  51. 3-1-mode-of-action-of-enzymes
  52. 3-2-factors-that-affect-enzyme-action
  53. 4-1-fluid-mosaic-membranes
  54. 4-2-movement-into-and-out-of-cells
  55. 5-1-replication-and-division-of-nuclei-and-cells
  56. 5-2-chromosome-behaviour-in-mitosis
  57. 6-1-structure-of-nucleic-acids-and-replication-of-dna
  58. 6-2-protein-synthesis
  59. 7-1-structure-of-transport-tissues
  60. 7-2-transport-mechanisms
  61. 8-1-the-circulatory-system
  62. 8-2-transport-of-oxygen-and-carbon-dioxide
  63. 8-3-the-heart
  64. 9-1-the-gas-exchange-system
  65. 10-1-infectious-diseases
  66. 10-2-antibiotics
  67. 11-1-the-immune-system
  68. 11-2-antibodies-and-vaccination
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Osmoregulation

  • The control of the water potential of body fluids is known as osmoregulation

    • Osmoregulation is a key part of homeostasis

  • Specialised sensory neurones, known as osmoreceptors, monitor the water potential of the blood (these osmoreceptors are found in an area of the brain known as the hypothalamus)

  • If the osmoreceptors detect a decrease in the water potential of the blood, nerve impulses are sent along these sensory neurones to the posterior pituitary gland (another part of the brain just below the hypothalamus)

  • These nerve impulses stimulate the posterior pituitary gland to release antidiuretic hormone (ADH)

  • ADH molecules enter the blood and travel throughout the body

  • ADH causes the kidneys to reabsorb more water

  • This reduces the loss of water in the urine

Diagram showing the hypothalamus stimulating the posterior pituitary gland to release ADH into the blood, involving osmoreceptor neurones.
When osmoreceptors detect a decrease in blood water potential, nerve impulses stimulate the release of ADH at the posterior pituitary gland. This ADH then travels in the blood to the kidneys, causing them to increase water reabsorption

The effect of ADH on the kidneys

  • Water is reabsorbed by osmosis from the filtrate in the nephron

  • This reabsorption occurs as the filtrate passes through structures known as collecting ducts

  • ADH causes the luminal membranes of the collecting duct cells to become more permeable to water

  • ADH does this by causing an increase in the number of aquaporins (water-permeable channels) in the luminal membranes of the collecting duct cells. This occurs in the following way:

    • Collecting duct cells contain vesicles, the membranes of which contain many aquaporins

    • ADH molecules bind to receptor proteins, activating a signalling cascade that leads to the phosphorylation of the aquaporin molecules

    • This activates the aquaporins, causing the vesicles to fuse with the luminal membranes of the collecting duct cells

    • This increases the permeability of the membrane to water

  • As the filtrate in the nephron travels along the collecting duct, water molecules move from the collecting duct (high water potential), through the aquaporins, and into the tissue fluid and blood plasma in the medulla (low water potential)

    • As the filtrate in the collecting duct loses water it becomes more concentrated

    • As a result, a small volume of concentrated urine is produced. This flows from the kidneys, through the ureters and into the bladder

Diagram of water reabsorption in kidney collecting duct, showing ADH action, aquaporin movement, and water passage from tubule lumen to medulla.
How ADH affects water reabsorption in the collecting duct of the nephron

Examiner Tips and Tricks

If the water potential of the blood is too high, the exact opposite happens:

  • Osmoreceptors in the hypothalamus are not stimulated

  • No nerve impulses are sent to the posterior pituitary gland

  • No ADH released

  • Aquaporins are moved out of the luminal membranes of the collecting duct cells

  • Collecting duct cells are no longer permeable to water

  • The filtrate flows along collecting duct but loses no water and is very dilute

  • A large volume of dilute urine is produced

  • This flows from the kidneys, through the ureters and into the bladder