Scheme of Work Cambridge International as & a level

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bio scheme of work

Extension activity: Learners study a graph showing how the rate of transpiration varies during a 24-hour day and interpret the plot using a word list (include, for example, stomata, photosynthesis, gas exchange, etc.). Encourage learners to consider how, and why, their graph would be different for a xerophytic plant.

7.2.6 State that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes.

7.2.7 Explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins.

7.2.8 Explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink.

Show an image of trees that have been subject to ‘ring barking.’ Ask learners for suggestions as to why these trees die and identify the location of the phloem vessels. Through a class discussion, encourage ideas from learners in order to arrive at a consensus to explain what is happening. (F)

Learners prepare a numbered list to show the sequence of events that occur in the phloem sieve elements / companion cells. They should compare their work with an on-screen animation, for example:

www.saps.org.uk/animations/plant_biology/index.html?video=1 http://highered.mheducation.com/sites/9834092339/student_view0/chapter38/animation_-_phloem_loading.html

Write on the board: ‘Why is _____ an example of _____?’ Learners use this shell to pose questions about the role of the phloem. An example is ‘Why is the phloem an example of a tissue?’ (F)

Extension activity: Learners suggest how aphid stylets and radioactive markers are used to determine the movement of assimilates through phloem vessels.

Past and specimen papers

Past/specimen papers and mark schemes are available to download at www.cambridgeinternational.org/support (F)

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