Starch, glycogen and cellulose: how their structures fit their functions

Starch, glycogen and cellulose are polysaccharides made by condensation of many glucose molecules. Starch and glycogen are made of α-glucose and are energy stores: starch in plants, glycogen in animals. Cellulose is made of β-glucose, which forms long straight chains that hydrogen bond together into strong microfibrils, giving plant cell walls their strength.

Alpha and beta glucose

Glucose has two isomers, and AQA expects you to know both structures. The only difference is on carbon 1 of the ring: in α-glucose the OH group is below the ring, and in β-glucose it is above. That one difference decides whether glucose units join into a coiled or branched store, or into straight structural chains.

Glucose units join by condensation reactions, which release a molecule of water and form a glycosidic bond. Hydrolysis, adding water, breaks the bond again.

Starch

Starch is the energy store in plant cells, made of α-glucose. It is a mixture of two polysaccharides:

• Amylose: long unbranched chains of α-glucose joined by 1,4 glycosidic bonds. The chain coils into a helix, which makes it compact.
• Amylopectin: α-glucose chains with 1,4 glycosidic bonds plus 1,6 glycosidic bonds that form side branches. The branches give many ends where enzymes can hydrolyse glucose off.

Starch is insoluble, so it doesn't affect the water potential of the cell or diffuse out of it, and it is compact, so a lot of glucose fits in a small space.

Glycogen

Glycogen is the energy store in animal cells, found especially in liver and muscle cells. Like amylopectin, it is α-glucose joined by 1,4 and 1,6 glycosidic bonds, but it is more branched, with shorter chains.

• Highly branched: many chain ends, so enzymes can hydrolyse it to glucose quickly when an animal needs energy.
• Compact: a lot of glucose stored in a small space.
• Insoluble: it doesn't affect water potential.

Cellulose

Cellulose is a structural polysaccharide in plant cell walls, made of β-glucose joined by 1,4 glycosidic bonds. To form these bonds, every other β-glucose unit is turned upside down, so the chain is long, straight and unbranched.

• Straight chains lie side by side and are linked by many hydrogen bonds. Each hydrogen bond is weak, but there are so many that together they are strong.
• Chains group into microfibrils, which bundle into fibres, giving cellulose great tensile strength.
• Function: cellulose cell walls stop plant cells bursting when water enters by osmosis, keeping cells turgid and supporting the plant.

Starch, glycogen and cellulose compared

Testing for starch

AQA lists the iodine/potassium iodide test for starch: add iodine solution and a positive result turns from orange-brown to blue-black. Benedict's solution tests for reducing sugars, and for non-reducing sugars after first boiling with dilute acid.

Exam questions with answers

Explain how the structure of cellulose is related to its function. (4 marks)

Cellulose is made of β-glucose, so it forms long, straight, unbranched chains. The chains run parallel and are joined by many hydrogen bonds, forming microfibrils. Together these provide strength, so the cell wall supports the cell and resists bursting when water enters.

Give two ways glycogen is suited to its function as an energy store. (2 marks)

It is highly branched, so it can be hydrolysed quickly to release glucose; it is insoluble, so it doesn't affect water potential; and it is compact, so it stores a lot in a small space. Any two.

Common mistakes

• Saying cellulose is made of α-glucose. Cellulose is β-glucose; starch and glycogen are α-glucose.
• Calling hydrogen bonds strong. Each is weak; it is the large number that gives strength.
• Mixing up amylose and amylopectin. Amylose is unbranched and helical; amylopectin is branched.
• Saying glycogen is found in plants. Glycogen is the animal store; plants store starch.

Practise with a short timed paper

Carbohydrates are in the first year of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day.

Read next

Biology · Biological moleculesAlpha glucose and beta glucose: structure and differencesThe structure of alpha glucose and beta glucose, the one difference between them, how glycosidic bonds form, and why it matters for starch and cellulose.