WEILAI Online Academic assistant
WEILAI Online Academic assistant
Back to 课程

Biology_A-level_Cie

0% Complete
0/0 Steps
  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
课 Progress
0% Complete

Phloem: mass flow

  • The mass flow hypothesis was the model initially used to explain the movement of assimilates in the phloem tissue

  • The simple model consisted of:

    • Two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated)

    • These two membranes were placed into two chambers containing water and were connected via a passageway

    • The two membranes were joined via a tube

    • As the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution

    • This forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)

  • Scientists now support a modified version of this hypothesis – the pressure flow gradient

Diagram of water flow between two sections, A and B, showing osmosis, hydrostatic pressure, phloem, and xylem processes in partially permeable membranes.
An illustration of Münch’s model for mass flow in phloem tissue

Pressure (hydrostatic) flow gradient

  • Phloem sap (containing sucrose and other organic solutes) moves by mass flow up and down the plant

    • The advantage of mass flow is that it moves the organic solutes faster than diffusion

  • In xylem tissue the pressure difference that causes mass flow occurs because of a water potential gradient between the soil and leaf (this requires no energy input by the plant)

  • However in phloem tissue energy is required to create pressure differences for the mass flow of the organic solutes

  • The pressure difference is generated by actively loading sucrose into the sieve elements at the source (usually a photosynthesising leaf or storage organ) which lowers the water potential in the sap

  • This results in water moving into the sieve elements as it travels down the water potential gradient by osmosis

  • The presence of water within the sieve elements increases the hydrostatic or turgor pressure at the source and as solutes (e.g. sucrose) are removed / unloaded from the sieve elements causing water to follow by osmosis at the sink (creating a low hydrostatic pressure), a hydrostatic pressure gradient occurs

  • The pressure difference between the source and the sink results in the mass flow of water (containing the dissolved organic solutes) from the high hydrostatic pressure area to the low hydrostatic pressure area

  • The mass flow of organic solutes within the phloem tissue occurs above and below the sources (which is typically photosynthesising leaves). Therefore sap flows upwards and downwards within a plant

Diagram illustrating phloem translocation of sucrose from leaves to roots. Shows movement of water, solutes, and pressure changes in plant vessels.
The translocation of phloem sap (sucrose and other organic solutes) due to a hydrostatic pressure gradient from the source to the sink

Examiner Tips and Tricks

Remember that the source is not necessarily the leaves and the sink is not necessarily the roots.

Phloem sap moves up and down the plant (although it will only move in one direction per sieve tube).

The hydrostatic pressure gradient is dependent on water moving in and out of the xylem vessels by osmosis.