All living organisms contain biomolecules in a certain concentration. This concentration is expressed as moles per cell or moles per litre. Interestingly, biomolecules change into other biomolecules or are made from other biomolecules due to chemical reactions in living organisms. The entire set of biochemical reactions that take place in living organisms is known as metabolism.
A series of linked chemical reactions that transform the biomolecules is called a metabolic pathway. They can be either linear, as in the process of glycolysis or circular as in the Krebs cycle. Metabolic pathways can be divided into two categories—anabolic pathways and catabolic pathways. Anabolic pathways convert simple structures to complex ones. For example, in glycogenesis, glucose molecules are added in chains to form glycogen for storage.
On the other hand, catabolic pathways convert complex structures to simpler ones. They involve degradation and also lead to release of energy. For example, in glyogenolysis, glycogen is broken down to glucose. The energy released during degradation is trapped by living organisms and stored as chemical bonds. The most important biomolecule that contains energy in its chemical bonds is ATP. When the bonds of an ATP molecule break, a lot of energy is released.