Cellular Metabolism

Metabolism is the sum total of all anabolic (those that build molecules) and catabolic (those that break down molecules) reactions of the body; aerobic reactions require oxygen and anaerobic reactions do not.

When discussing the basics of metabolism, it is important to first understand that metabolism is the building of large molecules from small molecules and the breaking down of large molecules into small molecules. Living things must build the molecules that carry out cellular functions through anabolic processes, which take in energy. Organisms get the energy they need to carry out the functions of life through catabolic processes, which release energy. This energy often takes the form of a molecule called adenosine triphosphate, or ATP. Adenosine triphosphate (ATP) is a nucleotide consisting of the sugar ribose, the base adenine, and three phosphate groups. It contains chemical energy in the bonds of its phosphate groups used to fuel cellular processes. When a phosphate ion is released from ATP, energy is released along with it. The molecule that remains is adenosine diphosphate, or ADP. The formation and breaking of the bonds in these molecules provides the energy that drives the processes of life.

Other molecules act as electron carriers, which harness the energy released in redox reactions (oxidation-reduction reactions that involve the transfer of charge from one molecule to another) to improve the energy efficiency of metabolic reactions.

There are two types of metabolic reactions: anabolic and catabolic. An anabolic reaction builds large molecules from smaller molecules. In anabolic reactions, energy is added to the reactants (the substances that undergo changes during a reaction) in order to produce the products (the substances produced during a reaction). In the simplest terms, an anabolic reaction is one in which energy is added to reactants to form the products. An example of an anabolic reaction is synthesis (the reaction of simple materials with one another to produce chemical compounds), such as the synthesis of proteins in humans.

Anabolic Reactions

In anabolic reactions, larger molecules are built from smaller molecules. Energy is added to the reactants in order for the product to form.
Catabolic reactions are the opposite of anabolic reactions. A catabolic reaction is a reaction that breaks down large molecules into smaller molecules, releasing energy in the process. In simple terms, a catabolic reaction is one in which energy is released as reactants break down into products. Some examples of catabolic reactions include cellular respiration, fermentation, and glycolysis. Cellular respiration is the process by which biochemical energy from nutrients is converted into ATP and waste products. Fermentation is the production of pyruvate in the absence of oxygen. The breakdown of glucose by enzymes is completed through a process known as glycolysis. In addition to releasing energy, glycolysis produces pyruvic acid.

Catabolic Reactions

In catabolic reactions, larger molecules are broken down into smaller molecules. Energy is released in the process.