AHL Content Statements

• C3.1.17

  Observations of tropic responses in seedlings
  

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  AOS: Students should gather qualitative data, using diagrams to record their observations of seedlings illustrating tropic responses. They could also collect quantitative data by measuring the angle of curvature of seedlings.
  NOS: Students should be able to distinguish between qualitative and quantitative observations and understand factors that limit the precision of measurements and their accuracy. Strategies for increasing the precision, accuracy and reliability of measurements in tropism experiments could be considered.
  

• C3.1.18

  Positive phototropism as a directional growth response to lateral light in plant shoots
  

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  Students are not required to know specific examples of other tropisms.
  

• C3.1.19

  Phytohormones as signalling chemicals controlling growth, development and response to stimuli in plants
  

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  Students should appreciate that a variety of chemicals are used as phytohormones in plants.
  

• C3.1.20

  Auxin efflux carriers as an example of maintaining concentration gradients of phytohormones

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  Auxin can diffuse freely into plant cells but not out of them. Auxin efflux carriers can be positioned in a cell membrane on one side of the cell. If all cells coordinate to concentrate these carriers on the same side, auxin is actively transported from cell to cell through the plant tissue and becomes concentrated in part of the plant.
  

• C3.1.21

  Promotion of cell growth by auxin

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  Include auxin’s promotion of hydrogen ion secretion into the apoplast, acidifying the cell wall and thus loosening cross links between cellulose molecules and facilitating cell elongation. Concentration gradients of auxin cause the differences in growth rate needed for phototropism.
  

• C3.1.22

  Interactions between auxin and cytokinin as a means of regulating root and shoot growth

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  Students should understand that root tips produce cytokinin, which is transported to shoots, and shoot tips produce auxin, which is transported to roots. Interactions between these phytohormones help to ensure that root and shoot growth are integrated.
  

• C3.1.23

  Positive feedback in fruit ripening and ethylene production

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  Ethylene (IUPAC name: ethene) stimulates the changes in fruits that occur during ripening, and ripening also stimulates increased production of ethylene. Students should understand the benefit of this positive feedback mechanism in ensuring that fruit ripening is rapid and synchronized.