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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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Principles of SA:V

  • Surface area and volume are both very important factors in the exchange of materials in organisms

  • As the surface area and volume of an organism increase (and therefore the overall ‘size’ of the organism increases), the surface area : volume ratio decreases

  • This is because volume increases more rapidly than surface area as size increases

Importance of a high surface area to volume ratio

  • Having a high surface area to volume ratio increases the ability of a biological system to perform the following important functions

    • Obtaining necessary resources eg, oxygen, glucose, amino acids

    • Eliminating waste products eg. carbon dioxide, urea

    • Acquiring or dissipating thermal energy (heat)

    • Otherwise exchanging chemicals and energy with the surroundings eg. absorbing hormones at the cell surface in the hormone’s target organ

Three cubes with side lengths of 1 cm, 2 cm, and 3 cm; table of their surface areas, volumes, and surface area-to-volume ratios below.
As size increases, the surface area : volume ratio decreases

 

Cube

Cuboid

Cylinder

 Diagram
cube-all-sides-length-s
cuboid-with-length-width-height-showing
cylinder-with-radius-height-showing

Surface Area

6s2

2lh + 2lw + 2wh

1x rectangle = 2πrh

2x circular ends = 2πr2

SA = 2πrh + 2πr2

Volume

s3

l × w × h

πr2h

Example

If s = 1 cm then SA = (1×1)×6 SA = 6cm2

V = s3 = 13 =1cm3 ∴ SA:V ratio = 6:1

If l = 4cm, w = 2cm, h = 1cm, then SA = 2((4×1)+(4×2)+(2×1)) = 28cm2

V = 4 × 2 × 1 = 8cm3

∴ SA:V ratio = 28:8 = 3.5:1

If r = 2 cm and h = 6cm, then SA = 2πrh + 2πr2 = 8π +24π = 32π cm2

V = π(2)2 × 6 = 24π cm2

∴ SA : V ratio = 32 : 24 = 1.33:1

Worked Example

Calculate the surface area-to-volume ratios of the two following microorganisms:

  1. A bacterial cell from the species Staphylococcus aureus; you can assume that each cell is a cube with side length of 800 nm (8 × 10-9 m)

  2. A bacterial cell from the species Bacillus subtilis; these are rod-shaped cells which you can assume to be cylindrical in shape. They are 5 µm long and 1 µm in diameter

Comment on your calculated answers. 

Solution

1. For the Staphylococcus aureus cell: side length = 800 nm

Convert this value into µm by dividing by 1000

fraction numerator 800 space nm over denominator 1000 end fraction equals space 0.8 straight mu straight m

Surface area of cube = 6(s2)

= 6 × (0.8 × 0.8) = 3.84 µm2

Volume = 0.8 × 0.8 × 0.8

= 0.512 µm3

A ratio is one number divided by another with the larger number divided by the smaller number, so

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