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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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Phloem sieve tube elements & companion cells: structure & function

  • The function of phloem tissue in a plant is to:

    • Transport organic compounds (assimilates), particularly sucrose, from the source (e.g. leaf) to the sink (e.g. roots)

      • The transport of these compounds in phloem tissue can occur up and down the plant

  • The organic compounds are dissolved in water to form sap

  • Phloem is a complex tissue made up of various cell types

    • Its bulk is made up of sieve tube elements which are the main conducting cells and companion cells

    • Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres

  • Mature phloem tissue contains living cells, unlike xylem tissue

Diagram showing plant cell structures: A. Spongy mesophyll; B. Phloem tissue with detailed labels for parenchyma cell, companion cell, sieve tube, and pore.
Structure of phloem tissue. (a) Microscope slide image and drawing of a sieve tube element and companion cell in transverse section (TS), (b) photomicrograph image and drawing of a sieve tube element and companion cell in longitudinal section (LS).
Diagram of a plant phloem showing sieve tube elements, companion cells, sieve pores, and sieve plates, labelled with arrows and boxed text.
Structure of phloem tissue. (c) Transmission electron micrograph image and drawing of a sieve tube element and companion cell in transverse section (TS)

Sieve tube elements

  • Sieve tube elements line up end to end to form a continuous tube

  • They transport sugars and nutrients up and down the plants in sieve cells

Structure

Function

Sieve plates with sieve pores

Allows for continuous movement of organic substances

Cellulose cell wall

Strengthens the wall to withstand the hydrostatic pressures that move assimilates

No nucleus, ribosomes or vacuole in mature cells

Maximises space for movement of assimilates

Thin cytoplasm

Reduces friction to facilitate movement of assimilates

Companion cells

  • Each sieve tube element has a companion cell associated with it as companion cells control the metabolism of their associated sieve tube member

  • They also play a role in loading and unloading of sugars into the phloem

Structure

Function

Nucleus and other organelles present

Provides metabolic support to sieve tube elements and helps with loading and unloading of assimilates

Transport proteins in plasma membranes

Moves assimilates into and out of sieve tube elements

Large numbers of mitochondria

Provides ATP for the active transport of assimilates

Plasmodesmata (channels in cell wall)

The link to sieve tube elements, allowing organic compounds to move from the companion cells into the sieve tube elements

Comparing xylem and phloem

  • Xylem and phloem are similar in some ways but there are key differences that ensure they can carry out their separate functions in transport around the plant

 

Xylem

Phloem

Living cells

No (hollow cells)

Yes (companion cells)

Substances transported

Water and mineral ions

Organic compounds/assimilates

Process of transportation

Transpiration

Active translocation

Direction of flow

One way / upwards (roots to leaves)

Two ways / up and down (source to sink)

Presence of end walls

No

Yes (sieve plates)

Cell wall material

Lignin and cellulose

Cellulose

Examiner Tips and Tricks

Understand the difference between sieve tube elements and companion cells, and how they are different to xylem tissue.

Remember that mature xylem tissue is dead, so there is no evidence of organelles, and they have lignified cell walls, whereas sieve tube elements have no lignin, do have sieve plates, and their companion cells contain nuclei and dense cytoplasm.