Slides in this set
Starch is a polysaccharide that is 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. Starch is made up of chains of alpha-glucose monosaccharides linked
by glycosidic bonds that are formed by condensation reactions. The unbranched
chain is wound into a tight coil that makes the molecule very compact.
The main role of starch is energy storage, something it is especially suited for because:
It is insoluble and therefore does not tend to draw water into the cells by osmosis
Being insoluble, it does not easily diffuse out of cells
It is compact, so lots can be stored in a small place
When hydrolysed it forms alpha-glucose, which is both easily transported and
readily used in respiration.
Starch is never found in an animal cell. Instead a similar polysaccharide, called
glycogen serves the same role.…read more
Glycogen is similar in structure to starch but has
shorter chains and is more highly branched. It is
sometimes called `animal starch' because it is the
major carbohydrate storage product of animals. In
animals it is stored as small granules mainly in the
muscles and the liver. Its structure suits it for
storage for the same reasons as those given in
starch. However, because it is made up of smaller
chains, it is even more readily hydrolysed to
alpha-glucose. Glycogen is found in animal cells
but never in plant cells.…read more
Cellulose differs from both Starch and Glycogen in one major respect: it is made of monomers of beta
glucose rather than alpha glucose. This seemingly small variation produces fundamental differences
in the structure and function of this polysaccharide. The main reason for this is that, in the beta
glucose units, the positions of the H group and the OH group on a single carbon atom are
reversed. In beta-glucose the OH group is above, rather than below, the ring. This means that to
form glycosidic links, each beta glucose molecule must be rotated by 180° compared to its
neighbour. The result is that the CH2OH group on each beta glucose molecule alternates between
being above and below the chain.
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, making it the
valuable structural material that it is.
The cellulose molecules are grouped together to form microfibrils which in turn, are arranged in parallel
groups called fibres.
Cellulose is a major component of plant cell walls and provides rigidity to the plant cell. The cellulose
cell wall also prevents the cell from bursting as water enters 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, making herbaceous parts of the plant semi-ridged. This is especially
important in maintaining stems and leaves in a turbid state so that they can provide the maximum
surface area for photosynthesis.…read more