AS Biology: Carbohydrates in Plant and Animal Cells

Notes on glycogen, starch and cellulose.

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  • Created by: Lexi
  • Created on: 07-05-12 19:31

Starch

Starch

- Found in plant cells only (glycogen serves the same function in animal cells)

- Form of small grains found in large amounts in seeds and storage organs

- A polysaccharide: made up of chains of a-glucose connected by glycosidic bonds formed as a result of condensation reactions

- These unbranched chains are tightly coiled to create a compact molecule

- Main use: source of energy. It is adapted for this function in the following ways:

  • Insoluble: doesn't draw water into the cells by osmosis, doesn't easily diffuse out of cells
  • Compact: a lot can be stored in a smaller space
  • When hydrolysed it forms alpha glucose which can be easily transported and readily used up in respiration
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Glycogen

Glycogen

- Found only in animal cells (sometimes called 'animal starch')

- Located mainly in the muscles and liver in the form of small granules

- Similar in structure to starch but it has shorter chains and it is highly branched

- Main use: source of energy. It is adapted in the same ways as starch:

  • Insoluble: doesn't draw water in to cells by osmosis and doesn't easily diffuse out of cells
  • Compact: a lot can be stored in a small space
  • When hydrolysed it forms alpha glucose which is easily transported and readily used in respiration
  • HOWEVER the shorter chains results in glycogen being even more readily hydrolysed into a-glucose
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Cellulose

Cellulose

- Unlike starch and glycogen, cellulose is made of beta glucose as opposed to alpha glucose

- This causes a major structural and hence functional difference in cellulose

- The -OH and -H groups on a single carbon molecule are in the opposite positions than where they are in alpha glucoses

- In order to form glycosidic bonds, the glucose molecules are rotated 180degrees. This causes the -CH2OH groups alternate being above then below the chain

- The straight, unbranched chains of cellulose allow hydrogen bonds to form between the parallel chains. The number of these hydrogen bonds collectively contribute to the strength of cellulose

- Cellulose molecules are grouped together to form microfibrils which are arranged in parallel groups called fibres

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Cellulose

- Cellulose are major components of cell walls giving them ridgity

- Cellulose also prevents the cells from bursting because when water enters the cells by osmosis it exterts an inwards pressure which prevents a further influx of water

- This results in plant cells being turgid and pushing against eachother which is important in maintaining stems and leaves in a turgid state in order to provide the maximum surface area for photosynthesis

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