AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The basic structure and functions of glycogen, starch and cellulose.
Full specification wording
Carbohydrates: The basic structure and functions of glycogen, starch and cellulose.
Three polysaccharides, three jobs.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 6 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Three polysaccharides, three jobs. Starch is the storage carbohydrate of plants, found as grains in chloroplasts and storage organs; it is a mixture of helical amylose and branched amylopectin, both built from alpha-glucose joined by glycosidic bonds. Glycogen is the storage carbohydrate of animals and fungi, held as granules in liver and muscle cells; it is alpha-glucose with 1,4 bonds and many 1,6 branch points, making it more highly branched than amylopectin. Cellulose is not a store at all - it is the structural polysaccharide of the plant cell wall, made of straight beta-glucose chains that hydrogen bond to each other in parallel to form microfibrils and then fibres. Starch and glycogen store glucose, which releases energy when respired; exam answers should make clear that the energy is released on respiration, not stored directly in the polysaccharide.
What earns the marks
- Identifying starch as the storage polysaccharide of plants and glycogen as that of animals.
- Describing starch as amylose, which is helical and unbranched, and amylopectin, which is branched.
- Describing glycogen as alpha-glucose with 1,4 and 1,6 bonds and more branching than amylopectin.
- Describing cellulose as straight, unbranched beta-glucose chains held together by hydrogen bonds in microfibrils.
- Identifying cellulose as structural, giving the cell wall strength, rather than as a store.
Common mistakes and how to put them right
Writing that starch and glycogen store energy, when they store glucose which releases energy on respiration.
Describing cellulose as an energy store in plants.
Saying starch is one molecule and never naming amylose and amylopectin.
Claiming hydrogen bonds form within a single cellulose chain rather than between neighbouring chains.
Giving glycogen as the plant store and starch as the animal store.
Exam tips
- Write 'store of glucose' rather than 'store of energy' - the phrasing is marked.
- Remember that cellulose is the only one of the three with a structural role, so never offer it as a store.
- Learn the location as well as the molecule: chloroplasts and storage organs for starch, liver and muscle for glycogen, cell wall for cellulose.
Try the lesson
The start of the Carbohydrates: The basic structure and functions of glycogen, starch and cellulose. lesson, made from checked teaching notes for this specification point.
Have a go first
Which polysaccharide is the main storage carbohydrate in animal cells?
Listen
Three polysaccharides, three jobs. Starch is the storage carbohydrate of plants, found as grains in chloroplasts and storage organs; it is a mixture of helical amylose and branched amylopectin, both built from alpha-glucose joined by glycosidic bonds. Glycogen is the storage carbohydrate of animals and fungi, held as granules in liver and muscle cells; it is alpha-glucose with 1,4 bonds and many 1,6 branch points, making it more highly branched than amylopectin. Cellulose is not a store at all - it is the structural polysaccharide of the plant cell wall, made of straight beta-glucose chains that hydrogen bond to each other in parallel to form microfibrils and then fibres. Starch and glycogen store glucose, which releases energy when respired; exam answers should make clear that the energy is released on respiration, not stored directly in the polysaccharide. A strong answer earns marks for these points. identifying starch as the storage polysaccharide of plants and glycogen as that of animals. describing starch as amylose, which is helical and unbranched, and amylopectin, which is branched. describing glycogen as alpha-glucose with 1,4 and 1,6 bonds and more branching than amylopectin. describing cellulose as straight, unbranched beta-glucose chains held together by hydrogen bonds in microfibrils. Exam tip: Write 'store of glucose' rather than 'store of energy' - the phrasing is marked.
See it
Key words
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storage polysaccharides
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structural polysaccharide
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amylose and amylopectin
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microfibrils
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See every definition
- storage polysaccharides
- Starch is the storage carbohydrate of plants and glycogen is the storage carbohydrate of animals and fungi, storing glucose which releases energy when respired.
- structural polysaccharide
- Cellulose is the structural polysaccharide of the plant cell wall, giving the cell wall strength rather than acting as a store.
- amylose and amylopectin
- Starch is a mixture of helical amylose and branched amylopectin, both built from alpha-glucose joined by glycosidic bonds.
- microfibrils
- Straight beta-glucose chains of cellulose hydrogen bond to each other in parallel to form microfibrils and then fibres.
Label the diagram
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1Box 1
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3Box 3
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