Carbohydrates: Structure, Types and Biological Roles
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Carbohydrates are made of carbon, hydrogen and oxygen. They include simple sugars (glucose, fructose), double sugars (sucrose, maltose, lactose) and complex molecules (starch, glycogen, cellulose). Their main roles are energy source, energy storage and structural support.
Carbohydrates are the body's primary energy source. For IGCSE Biology, you need to know the three levels of carbohydrate complexity, their roles in living organisms, and how to test for them using reagents. This guide covers all the exam-essential content.
Types of Carbohydrates
Type
Examples
Solubility
Role
Monosaccharides (single sugars)
Glucose, fructose, galactose
Soluble
Quick energy source; building blocks for larger carbohydrates
Energy storage (starch in plants, glycogen in animals); structural support (cellulose in cell walls)
Supplement: Large molecules (polymers) are built from smaller molecules (monomers) by condensation reactions, releasing water. They are broken down by hydrolysis reactions, which add water.
Roles of Key Polysaccharides
Starch: the energy storage molecule in plants. Stored in chloroplasts and storage organs (e.g. Potato tubers). Insoluble so it does not affect osmosis. Compact, so large quantities can be stored.
Glycogen: the energy storage molecule in animals. Stored mainly in the liver and muscles. Similar structure to starch but more branched for faster breakdown when energy is needed quickly.
Cellulose: the structural molecule in plant cell walls. Long, straight chains that form fibres, providing strength and rigidity to the cell wall. Humans cannot digest cellulose: it passes through the gut as dietary fibre.
Why are storage molecules insoluble? If they were soluble, they would dissolve in cell water and affect the water potential, drawing water in by osmosis and disrupting the cell.
Testing for Carbohydrates
Test for reducing sugars (glucose, maltose):
Add Benedict's reagent (blue) to the food solution.
Heat in a water bath at about 80°C for 2–3 minutes.
A colour change from blue → green → yellow → orange → brick-red indicates reducing sugar is present.
The more sugar present, the more the colour changes towards brick-red (semi-quantitative).
Test for starch:
Add a few drops of iodine solution (yellow-brown) to the food.
A colour change to blue-black indicates starch is present.
Try a focused question (3 marks)
Explain why plants store glucose as starch rather than keeping it as glucose. (3 marks)
Show the answer and marking guidance
Starch is insoluble, so it does not affect the water potential of the cell / does not draw water in by osmosis.
Starch is compact, allowing large amounts of energy to be stored in a small space.
Glucose is soluble and osmotically active: storing glucose would draw water into the cell by osmosis, which could damage the cell.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying Benedict's reagent turns "red": the correct term is "brick-red" or "orange-red".
Confusing starch (plant storage) with glycogen (animal storage): learn which organism stores which.
Saying cellulose is an energy source: humans cannot digest cellulose; it is structural in plants and dietary fibre in humans.
Practical connection
Food Tests: Reagents, Methods and Colour Changes
Use Benedict’s plus heat for reducing sugar, iodine for starch, biuret for protein, ethanol then water for fat, and DCPIP decolourisation for vitamin C. State both the reagent and the positive colour change.