Starch, glycogen and cellulose Flashcards
What is starch?
A polysaccharide that is found in many parts of the plant in the form of small grains. Starch is made up of
chains of α-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions.
What may the starch chains be?
Branched or unbranched. The unbranched chain is wound into a tight coil that makes that molecule very compact.
What roles does starch have?
- Energy storage.
- It is insoluble and therefore doesn’t affect water potential, so water is not drawn into the cells by osmosis.
- It is large and insoluble, it doesn’t diffuse out of cells.
- It is compact, so a lot of it can be stored in a small space.
- When hydrolysed it forms α-glucose, which is 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.
Starch is not found in animal cells, what is?
A similar polysaccharide, called glycogen, serves the same role.
Where is glycogen found?
In animals and bacteria but never plants.
Describe the structure of glycogen.
The structure is very similar to starch but has shorter chains and is highly branched.
How is glycogen stored in animals?
As small granules mainly in muscles and the liver.
How is the structure of glycogen suited to its purpose?
Its suitability because of structure is identical to starch, but it is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. It is rapidly broken down to form glucose monomers.
How does cellulose differ from starch?
It is made of β-glucose monomers instead of α-glucose.
How is the structure of cellulose different from starch?
Rather than forming a coiled chain, cellulose has straight unbranched chains.These run parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent chains.
How much strength do the cross-links add to cellulose?
Each individual cross-link adds very little strength, however the sheer overall number of them adds a considerable contribution to strengthening cellulose.
What are cellulose molecules grouped together to form?
Microfibrils, which are arranged into parallel groups called fibres.
How is cellulose used in plants?
It’s a major component of cell walls, it provides rigidity to the plant cell. The cell wall also prevents the cell bursting as water enters it.
How does a cell wall stop plant cells bursting as water enters them?
By exerting an inward pressure that stops any further influx of water, as a result plan cells are turgid and push against each other, making the non-woody parts of the plant semi-rigid. Being turgid is important for the stem and leaves to maintain a large surface area for photosynthesis.
