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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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Guard cells

Structure of guard cells

  • Each stoma is surrounded by two guard cells

  • Guard cells have the following features:

    • Thick cell walls facing the air outside the leaf and the stoma

    • Thin cell walls facing adjacent epidermal cells

    • Cellulose microfibrils arranged in bands around the cell

    • Cell walls have no plasmodesmata

    • Cell surface membrane is often folded and contains many channel and carrier proteins

    • Cytoplasm has a high density of chloroplasts and mitochondria

    • Chloroplasts have thylakoids but with few grana (unlike those in mesophyll cell chloroplasts)

    • Mitochondria have many cristae

    • Several small vacuoles rather than one large vacuole

Diagram showing open and closed stomata with labels for epidermal cells, chloroplasts, nucleus, vacuoles, guard cells, and cellulose microfibrils.
The structure of guard cells when the stoma is open and closed

Mechanism to open stomata

  • Guard cells open when they gain water and become turgid

  • Guard cells gain water by osmosis

  • A decrease in water potential in the guard cells is required for water to enter the cells by osmosis

  • In response to light, ATP-powered proton pumps in the guard cell surface membranes actively transport hydrogen (H+) ions out of the guard cell

  • This leaves the inside of the guard cells negatively charged compared to the outside

  • This causes channel proteins in the guard cell surface membranes to open, allowing potassium (K+) ions to move down the electrical gradient and enter the guard cells

  • The potassium (K+) ions also diffuse into the guard cells down a concentration gradient

    • The combination of the electrical gradient and concentration gradient is known as an electrochemical gradient

  • The influx of potassium (K+) ions increases the solute concentration inside the guard cells, lowering the water potential inside the cells

  • Water now enters the guard cells by osmosis through aquaporins in the guard cell surface membranes

    • Most of the water enters the vacuoles, causing them to increase in size

  • This increases the turgor pressure of the guard cells, causing the stoma to open

    • The bands of cellulose microfibrils only allow the guard cells to increase in length (not diameter)

    • The thin outer walls of the guard cells bend more easily than thick inner walls

    • This causes the guard cells to become curved, opening up the stoma

Diagram of guard cell function; ATP proton pump expels H⁺, K⁺ enters lowering water potential, water enters by osmosis, cell becomes turgid, stoma opens.
An outline of the mechanism to open stomata

Mechanism to close stomata

  • When certain environmental stimuli are detected (that lead to the closing of the stomata), the proton pumps in the guard cell surface membranes stop actively transporting hydrogen (H+) ions out of the guard cell

  • The potassium (K+) ions leave the guard cells

  • The water potential gradient is now reversed and water leaves the guard cells by osmosis

  • This causes the guard cells to become flaccid, closing the stoma