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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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Estimating water potential in plant tissue

  • It is possible to investigate the effects of immersing plant tissue in solutions of different water potentials and then use the results to estimate the water potential of the plant tissue itself

  • The most common osmosis practical of this kind involves cutting cylinders of potato and placing them into solutions with a range of different water potentials

    • These are usually sucrose solutions of increasing concentration

    • At least five different concentrations are usually required

Method

  • The required number of potato cylinders are cut (one for each of the solutions you are testing—or more than one per solution if you require repeats; this is good experimental practice)

  • They are all cut to the same length and, once blotted dry to remove any excess moisture, their initial mass is measured and recorded before placing into the solutions

  • They are left in the solutions for a set length of time (e.g., 30 minutes), usually in a water bath (set at around 30o)

  • They are then removed and dried to remove excess liquid

  • The final length and mass of each potato cylinder are then measured and recorded

Diagram outlining the steps to measure osmosis in potatoes: cutting, measuring, immersing in solutions, and recording mass changes in boiling tubes.
You will need to use apparatus appropriately to measure out the volumes of your solutions and record your measurements

Analysis

  • The percentage change in mass for each potato cylinder is calculated

  • If there is a potato cylinder that has neither increased nor decreased in mass, it means there was no overall net movement of water into or out of the potato cells

  • This is because the solution that this particular potato cylinder was in had the same water potential as the solution found in the cytoplasm of the potato cells

    • So there was no concentration gradient and therefore no net movement of water into or out of the potato cells

  • The concentration of sucrose inside the potato cylinders can be found if a graph is plotted showing how the percentage change in mass changes with the concentration of sucrose solution

    • The point at which the line of best fit crosses the x-axis is the concentration of sucrose inside the potato cylinders

Flowchart with sugar concentration effects on potato mass, showing calculations, a table, graph, and conclusion on osmotic concentration.
A positive change in mass indicates that the potato tissue has gained water by osmosis, whereas a negative change would suggest the opposite

  • A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution had a higher water potential than the potato

    • This would make the potato cells turgid, as the water exerts turgor pressure (or hydrostatic pressure) on the cell walls—the potatoes will feel firm

  • A negative percentage change suggests the opposite, that is, the solution had a lower water potential than the potato

    • The potato cylinder in the strongest sucrose concentration will have decreased in mass the most as there is the greatest concentration gradient in this tube between the potato cells (higher water potential) and the sucrose solution (lower water potential)

    • This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the potato cylinder – the potato cylinders will feel floppy

    • If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall

Examiner Tips and Tricks

Questions involving osmosis experiments are common and you should be able to use your knowledge of osmosis to explain the results obtained.

If describing or analysing an experiment remember to include mention of variables that need to be controlled and mention repeats, as this helps improve the reliability of the results obtained.

Don’t worry if it is an experiment you haven’t done yourself in the lab – simply figure out where the higher concentration of water molecules is (the solution with the higher water potential) and explain which way the molecules move due to the differences in water potential.