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A-Level Biology May/June 2025 Q3(d): Explain, with reference to the life cycle of the carrot plant, when the carrot acts as…
A-Level Biology · Paper 9700/22 · May/June 2025 · Question 3(d) · [3 marks]
Explain, with reference to the life cycle of the carrot plant, when the carrot acts as a source and when the carrot acts as a sink.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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The carrot plant has a biennial life cycle, meaning it takes two years to complete.
In its first year of growth, the carrot root acts as a sink. During this period, the leaves are the source, producing assimilates (sugars) via photosynthesis. These sugars are transported in the phloem from the leaves to the taproot, where they are converted into starch for storage over the winter.
In its second year of growth, after a dormant period, the carrot root acts as a source. The stored starch in the root is hydrolysed into soluble sugars. These sugars are then loaded into the phloem and transported to the new growing areas, such as the flowering stem, to provide energy for growth and reproduction.
How the marks are awarded
- B1 — Correctly identifying that the carrot root is a sink in the first year of growth and a source in the second year, after a dormant period.
- B1 — Providing detail on the source function: explaining that stored starch is hydrolysed (mobilised) into sugars which are then loaded into the phloem for transport to growing areas.
- B1 — Providing detail on the sink function: explaining that the root receives assimilates (sugars) transported in the phloem from the leaves (the source) and converts them into starch for storage.
Common mistakes
- Reversing the roles, stating the carrot is a source in the first year and a sink in the second.
- Failing to link the source/sink roles to the specific stages of the biennial life cycle (first year vs second year).
- Using vague terms like 'food' or 'energy' instead of precise biological terms like 'assimilates', 'sugars', or 'starch'.
- Incorrectly stating that sugars are transported in the xylem instead of the phloem.
Examiner tip: Always consider the plant's life cycle and seasonal changes when determining the roles of different organs as sources or sinks, as these roles are dynamic and not fixed.
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