Starch accumulates as starch grains in many plant cells and is frequently their major carbohydrate store, e.g. potato tubers.
Glycogen is the animal store of carbohydrate. It is a branched structure of thousands of glucose residues. In vertebrates it is characteristically found as tiny granules in liver and muscle cells.
Cellulose is the structural component of plant wall cells. It consists of long chains of glucose residues. Hydroxyl groups project outwards to form hydrogen bonds with hydroxyl groups of adjacent chains; this gives the whole structure a high tensile strength. Cellulose is fully permeable to water and solutes and therefore dos not affect exchange of materials between the cell and environment.
Chitin is structurally very similar to cellulose, and forms bundles of long parallel chains which are an essential part of the arthropod skeleton.
I will now consider fatty acids and glycerol. Fatty acids have the general formula RCOOH. R is an H or alkyl group. Most fatty acids have an even number of carbon atoms, usually between 14 and 22. These form a long hydrocarbon tail which is hydrophobic. Unsaturated fatty acids contain one or more double bonds (C=C) while saturated fatty acids lack double bonds. .
Lipids are the esters of fatty acids and glycerol. Triglycerides are the most common lipids. They are formed when each of the three hydroxyl groups of glycerol condenses with a fatty acid. (See diagram). The fatty acids may be the same or different. Triglycerides are called fats if they are solid or oils if they are liquid at a room temperature of 20 degrees Celsius. They store twice the amount of energy as the equivalent mass of carbohydrate and act as important energy reserves in animals and plants. Extra fat is stored by hibernating animals and below the dermis of the skin in mammals where it helps insulate the body (e.g. blubber in whales). It also surrounds and protects various vital organs in the mammalian body e.
