polysaccharides are polymers made up from single sugar monomers joined by glycosidic links into long chains.
There are 3 main types of polysaccharides found in food: starch and cellulose in plants and glycogen in animals.
although all 3 are polymers of glucose molecules, they are sparingly soluble ( do not disolve easily) and dont taste sweet.
starch and glycogen act as energy storage molecules within cells. these polysaccharides are suitable for storage because they are compact molecules with low solubilty in water. this means they do not affect the concentraction of water in the cytoplasm and so dont affect movement of water into or out of the cell by osmosis.
Starch the storage carbohydrate found in plants, is made up of 2 molecules: amylose and amylopectin.
- Amylose is made of a straight chain of between 200 and 5000 glucose molecules with 1,4 glycosidic links between adjacent glucose molecules. the position of the bonds causes the chain to coil into a spiral shape.
- Amylopection is also a polymer of glucose, but it has side branches. A 1,6 glycosidic link holds each side branch onto the main chain.
starch grains in plants are composed of 70-80% amylopectin, 20-30% amylose. the compact spiral structure of starch makes it an excellent storage molecule.
more starch info
starch is a major source of energy in our diet, and is common in many foods, occurring naturally in fruits, veg and cereals.
the sticky gel formed when starch is mixed with water makes it a good thickening agent and is also added to many foods as a replacement for fat,
Bacteria, fungi and animals store glycogen instead of starch.
glycogen is also composed of glucose molecules. its numerous side branches mean that it can be rapidly hydrolysed, giving easy access to stored energy.
in human glycogen is stored in the liver and muscles.
cellulose in the diet is known as dietary fibre and it is also referred to as a non-starch polysaccharide .
Up to 10,000 glucose molecules are joined to form a straight chain with no branches.
cellulose is indigestible in the human gut and has an important role in the movement of material through the digestive tract.
dietary fibre is thought to help prevent 'western diseases' such a CHD, diabetes and bowel cancer.
lipids enhance the flavor and palatability of food.
they supply over twice the amount of energy as carbohydrates, which can be useful if a large amount of energy need to be consumed in a small mass of food. it also means a large amount of energy can be stored in a small mass, e.g. seeds.
lipids are organic molecules founds in every type of cell. and are insoluble in water , but soluble in organic solvents like ethanol.
the most common lipids that we eat are called triglycerides, used as energy stores in plants and animals.
triglycerides are made of 3 fatty acids and 1 glycerol molecule linked by condensation reactions.
the bonds that form between each fatty acid and the glycerol is called an ester bond.
3 ester bonds are formed in a triglyceride.