1.4 Starch, glycogen and cellulose Flashcards
What is starch?
A polysaccharide found in many parts of a plant in the form of small grains. Especially large amounts occur in seeds and storage organs, such as potato tubers. It forms an important component of food and is the major energy source in most diets.
What is starch made up of?
Chains of a-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions. The chains may be branched or unbranched. The unbranched chain is wound into a tight coil that makes the molecule very compact.
What is the main role of starch?
Energy storage.
Why is the structure of starch especially suited for energy storage?
- It is insoluble and therefore doesn’t affect water potential, so water is drawn into the cells by osmosis.
- Being large and insoluble, it does not diffuse out of cells.
- It is compact, so a lot of it can be stored in a small space.
- When hydrolysed it forms a-glucose, which is both easily transported and readily used in respiration.
- The branched form has many ends, each of which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly.
Is starch found in animal cells?
No, instead a similar polysaccharide, called glycogen, serves the same role.
Where is glycogen found?
In animals and bacteria but never in plant cells.
What is the structure of glycogen?
Glycogen is very similar in structure to starch but has shorter chains and is more highly branched.
What is glycogen?
It is the major carbohydrate storage product of animals. In animals it is stored as small granules mainly in the muscles and in the liver. The mass of carbohydrate that is stored is relatively small because fat is the main storage molecule in animals.
Why is the structure of glycogen especially suited for storage?
- It is insoluble and therefore does not tend to draw water into the cell by osmosis.
- Being insoluble, it does not diffuse out of cells.
- It is compact, so a lot of it can be stored in a small space.
- It is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. It is therefore more rapidly broken down to form glucose monomers, which are used in respiration. This is important to animals which have a higher metabolic rate and therefore respiratory rate than plants because they are more active.
How does cellulose differ from starch and glycogen?
It is made of monomers of B-glucose rather than a-glucose.
What is the structure of cellulose?
Rather than forming a coiled chain like starch, cellulose has straight, unbranched chains. These run parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent chains. While each individual hydrogen bond adds very little to the strength of the molecule, the sheer overall number of them makes a considerable contribution to strengthening cellulose. The cellulose molecules are grouped together to form microfibrils which, in turn, are arranged in parallel groups called fibres.
What is cellulose?
Cellulose is a major component of plant cell walls and provides rigidity to the plant cell. This cellulose cell wall also prevents the cell from bursting as water enters it by osmosis. It does this by exerting an inward pressure that stops any further influx of water. As a result, living plant cells are turgid and push against one another. This is important in maintaining the stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis.
How is the structure of cellulose especially suited to its function of providing support and rigidity?
- Cellulose molecules are made up of B-glucose and so form long, straight, unbranched chains.
- These cellulose molecular chains run parallel to each other and are crossed linked by hydrogen bonds which add collective strength.
- These molecules are grouped to form microfibrils which in turn are grouped to form fibres all of which provides yet more strength.
