Catabolism and anabolism

Metabolism is usually divided into two categories.

Catabolism is the set of metabolic processes that break down large molecules. These include breaking down and oxidising food molecules. The purpose of the catabolic reactions is to provide the energy and components needed by anabolic reactions.

Macromolecules such as starch, cellulose or proteins cannot be rapidly taken up by cells and need to be broken into their smaller units before they can be used in cell metabolism. Several common classes of enzymes digest these polymers. These digestive enzymes include proteases that digest proteins into amino acids, as well as glycoside hydrolases that digest polysaccharides into monosaccharides.

Carbohydrate catabolism is the breakdown of carbohydrates into smaller units. Carbohydrates are usually taken into cells once they have been digested into monosaccharides.

The following picture shows hoe proteins, polysacharides and fats are broken down into smaller particles so they can be absorb or transform.

Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules. The complex molecules that make up cellular structures are constructed step-by-step from small and simple precursors.

Anabolism involves three basic stages. Firstly, the production of precursors such as amino acids, monosaccharides, isoprenoids and nucleotides, secondly, their activation into reactive forms using energy from ATP, and thirdly, the assembly of these precursors into complex molecules such as proteins, polysaccharides, lipids and nucleic acids.

Organisms differ in how many of the molecules in their cells they can construct for themselves.

In carbohydrate anabolism, simple organic acids can be converted into monosaccharides such as glucose and then used to assemble polysaccharides such as starch.

Fatty acids are made by fatty acid synthases that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the actyl group, reduce it to an alcohol, dehydrate it to an alkene group and then reduce it again to an alkane group.

Organisms vary in their ability to synthesize the 20 common amino acids, mammals can synthesize only eleven nonessential amino acids, nine essential amino acids must be obtained from food. All amino acids are synthesized from intermediates in glycolysis, the citric acid cycle, or the pentose phosphate pathway.
Amino acids are made into proteins by being joined together in a chain by peptide bonds.

Nucleotides are made from amino acids, carbon dioxide and formic acid in pathways that require large amounts of metabolic energy.