Carbohydrates — A-Level Biology Revision
Revise Carbohydrates for A-Level Biology. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to ProteinsWhat is Carbohydrates?
Carbohydrates are biological molecules made from carbon, hydrogen, and oxygen, typically with a hydrogen-oxygen atom ratio of 2:1. They are a key source of energy in living organisms and also serve as structural components. Monosaccharides like glucose are the simplest form, which can join to form disaccharides (e.g., sucrose) and polysaccharides (e.g., starch and cellulose).
Board notes: All major A-Level Biology boards (AQA, Edexcel, OCR) cover the structure and function of carbohydrates, including monosaccharides, disaccharides, and polysaccharides like starch, glycogen, and cellulose. Specific details of the required practicals may vary slightly.
Step-by-step explanationWorked example
To calculate the energy content of a carbohydrate, you can use a calorimeter. If burning 0.5g of a carbohydrate raises the temperature of 100g of water by 20°C, the energy released is calculated as: Energy (Joules) = mass of water × specific heat capacity of water (4.2 J/g°C) × temperature change. So, Energy = 100g × 4.2 J/g°C × 20°C = 8400 Joules or 8.4 kJ. The energy content is 8.4 kJ / 0.5g = 16.8 kJ/g.
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Common mistakes
- 1Confusing the structures of alpha and beta glucose. The hydroxyl (-OH) group on carbon 1 is below the ring in alpha glucose and above the ring in beta glucose, which has major implications for polysaccharide structure.
- 2Forgetting that the glycosidic bond in polysaccharides is formed through a condensation reaction, which releases a water molecule.
- 3Mixing up the functions of starch and cellulose. Starch (amylose and amylopectin) is the main energy storage polysaccharide in plants, while cellulose provides structural support to the cell wall.
Carbohydrates exam questions
Exam-style questions for Carbohydrates with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Carbohydrates
Core concept
Carbohydrates are biological molecules made from carbon, hydrogen, and oxygen, typically with a hydrogen-oxygen atom ratio of 2:1. They are a key source of energy in living organisms and also serve as…
Frequently asked questions
What is the difference between starch and glycogen?
Starch is the energy storage polysaccharide in plants, while glycogen is the equivalent in animals and fungi. Glycogen is more highly branched than starch, allowing for more rapid release of glucose when energy is needed.
How are disaccharides formed?
Disaccharides are formed from two monosaccharides joined by a glycosidic bond in a condensation reaction. For example, glucose and fructose combine to form sucrose.