Catabolism

Metabolism is the total set of chemical processes in a living cell. It consists of two main parts that work in opposition. One part is called anabolism, which is the constructive process of building up molecules. The other part is catabolism, which is the destructive process of breaking them down.

Catabolism is essential because it provides the chemical energy required for life. Without this breaking-down process, cells could not maintain themselves or grow. The word itself comes from Neo-Latin roots derived from the Greek words "kato," meaning downward, and "ballein," meaning to throw.

This accurately describes how the cell throws large structures down into smaller units.

The mechanism of catabolism involves a specific sequence of chemical changes. First, the cell takes large molecules, known as polymers, and breaks them into smaller units called monomers. For example, polysaccharides are broken into monosaccharides, and lipids are broken into fatty acids. Proteins are broken down into amino acids, while nucleic acids become nucleotides.

Once these monomers are released, the cell can use them in two ways. It can use them as building blocks for new polymers through anabolism. Alternatively, it can degrade the monomers further into simple waste products. This final degradation is usually an oxidation process that releases chemical free energy.

This released energy is managed by the cell through a specific transfer system. During oxidation, some energy is lost as heat to the environment. The rest of the energy is captured to drive the synthesis of adenosine triphosphate, or ATP.

ATP serves as the energy currency of the cell. It acts as a bridge by transferring the energy released during catabolic reactions to the energy-requiring reactions of anabolism. This cycle ensures that the energy harvested from breaking down food or storage molecules is available exactly where the cell needs to build something new.

There are many distinct types of catabolic pathways that the body uses depending on the need. One example is glycolysis, which is a process used to break down glucose. Another major pathway is the citric acid cycle. The body also performs catabolism in specialized tissues, such as the breakdown of fat in adipose tissue to produce fatty acids. In certain situations, the body even breaks down muscle protein. This provides amino acids to be used as substrates for gluconeogenesis, which is the creation of new glucose.

Even neurotransmitters undergo catabolism, such as through the oxidative deamination performed by monoamine oxidase.

Biological signals called hormones control when these catabolic pathways occur. Endocrinologists have categorized many hormones as catabolic based on whether they stimulate these breaking-down processes. Some of the classic catabolic hormones were identified in the early 20th century. These include cortisol, glucagon, and adrenaline. In recent decades, scientists have discovered even more molecules with catabolic effects. These include cytokines, orexin (also known as hypocretin), and melatonin.

Each of these hormones has a specific role in managing the body's energy levels. Cortisol is released from the adrenal gland in response to stress. Its primary role is to increase blood glucose levels through gluconeogenesis. Glucagon is released from the alpha cells in the pancreas when a person is starving or needs more energy. It stimulates the liver to break down glycogen to raise blood glucose. This works in a negative-feedback system with insulin to stabilize sugar levels.

Adrenaline is released by the sympathetic nervous system. It increases heart rate, constricts blood vessels, and dilates the bronchi in the lungs to increase oxygen supply. It also stimulates gluconeogenesis to provide quick energy.

When catabolism breaks molecules down completely, it produces several cellular wastes. These waste products include lactic acid, acetic acid, carbon dioxide, ammonia, and urea.

The balance between catabolism and anabolism is what allows for the maintenance of life. While catabolism is destructive, it is the necessary partner to the constructive nature of anabolism. Together, these two processes form the complete metabolic system that powers every living organism.