A food scientist is analysing the nutritional content of a new sports drink. The drink contains glucose, starch, and a small amount of fat. Describe the roles of carbohydrates and fats as biological molecules in the human body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sports drinks are designed to provide energy quickly and to support sustained activity. They often contain a mixture of biological molecules that serve different functions in the body.
Model answer (5 marks)
Glucose is the main fuel for cellular respiration, providing ATP for muscle activity.
Starch is a polysaccharide that is broken down to glucose and stored as glycogen in liver and muscle, giving a longer‑term energy reserve.
Fats are a dense energy store, containing about 9 kJ g⁻¹ compared with 17 kJ g⁻¹ for carbohydrates.
Fat molecules are also the main components of cell membranes (phospholipids) and act as insulation and organ protection.
Glucose can be converted to glycogen for storage in liver and muscles, which is then mobilised during sustained exercise.
Starch is a polysaccharide that is broken down to glucose and stored as glycogen in liver and muscle, giving a longer‑term energy reserve.
Fats are a dense energy store, containing about 9 kJ g⁻¹ compared with 17 kJ g⁻¹ for carbohydrates.
Fat molecules are also the main components of cell membranes (phospholipids) and act as insulation and organ protection.
Glucose can be converted to glycogen for storage in liver and muscles, which is then mobilised during sustained exercise.
Examiner tips
- Use the exact terms ‘glucose’, ‘starch’, ‘glycogen’, ‘fat’, ‘cell membranes’, ‘insulation’, ‘organ protection’. Show the energy comparison (9 kJ g⁻¹ vs 17 kJ g⁻¹) to hit the energy‑store point. Keep each point brief – one sentence per mark.
- common_mistakes
- :
- Mixing up starch with glycogen; they are different molecules. Forgetting to mention the energy density of fats. Using vague words like ‘energy source’ without specifying storage or membrane roles.
Mark scheme (5 marks)
- Glucose/carbohydrates are used as a source of energy (for respiration)
- Starch (a carbohydrate) is a storage molecule / provides a longer-term energy source
- Fats are used as an energy store / contain more energy per gram than carbohydrates
- Fats are used to make cell membranes / provide insulation / protect organs
- Carbohydrates/glucose can be converted to glycogen for storage in the liver and muscles
Key terms in this question
carbohydrate · glucose · starch · fat
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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