Explain how sucrose is formed from simple sugars and describe how sucrose is suited to its role as the main transport carbohydrate in plants.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants produce glucose by photosynthesis. Much of this glucose is converted into sucrose before being transported around the plant in the phloem.
Model answer (5 marks)
Sucrose is formed by the condensation of a glucose and a fructose molecule. The hydroxyl group of the anomeric carbon of glucose reacts with the hydroxyl group of the anomeric carbon of fructose, forming a glycosidic bond and releasing a molecule of water.
Sucrose is highly soluble in water, which allows it to be carried in the aqueous phloem sap.
Because sucrose is chemically stable and is not directly used in respiration, it can travel long distances without being metabolised.
At the sink tissues it is hydrolysed by invertase into glucose and fructose, which can then enter metabolic pathways.
Sucrose is highly soluble in water, which allows it to be carried in the aqueous phloem sap.
Because sucrose is chemically stable and is not directly used in respiration, it can travel long distances without being metabolised.
At the sink tissues it is hydrolysed by invertase into glucose and fructose, which can then enter metabolic pathways.
Examiner tips
- Show the condensation reaction and mention water loss
- Highlight sucrose’s solubility and stability
- Explain transport and hydrolysis at sinks
Common mistakes
- Confusing sucrose with glucose or starch
- Forgetting to mention the release of water
- Claiming sucrose is used directly in respiration
Mark scheme (5 marks)
- Sucrose is formed by joining glucose and fructose together
- A chemical bond forms between the two simple sugars / a condensation reaction occurs / water is released when they join
- Sucrose is soluble / dissolves in water
- Sucrose does not react / is chemically stable / is not used in respiration directly, so it can be transported without being broken down en route
- Sucrose can be broken down / hydrolysed into glucose and fructose at the destination / sink cells where it is needed
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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