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Components of Cells
The Macromolecules

Storing excess foods

Food that is eaten and absorbed but is not used immediately for cellular growth or metabolism is stored. Excess carbohydrate is converted into the polymer glycogen (similar to starch in plants) and is stored in the liver and in the muscles. Amino acids, from broken down ingested proteins, are usually recycled into new polypeptides inside cells, but excess amino acids can be made into carbohydrate or fatty acids.

Once the limited storage capacity of cells, the liver and the muscles is exceeded, however, all energy containing molecules that are not needed immediately are transported around the body and stored in adipose tissue just under the skin.

Fatty acids

Free fatty acids are either taken in from the digestive system directly, or are made in the liver. If the diet is rich in triglycerides (from a fatty meal), these are transported in the form of chylomicrons (from the gut) or lipoproteins (from the liver) to regions near the adipose tissue. An enzyme, lipoproteinlipase (LPL), is secreted by the adipocytes into the fluid surrounding each cell where it breaks down the triglycerides into free fatty acids and glycerol.

The fatty acids are then transported across the adipocyte membrane into the cytoplasm of the cell. Here they are combined with a molecule called coenzyme A to form a thioester and then sequentially joined to a second and a third fatty acid to form mono-, di- and triglycerides. The final products are placed for longer-term storage into a large, central lipid vacuole.


Most of the glycerol used in the making of glycerides comes from glucose - also absorbed by the cells. Inside the cell the glucose is converted to glycerol phosphate before being joined to the first fatty acid.

© 2005, Professor John Blamire