Catabolism

Lore & Background

Catabolism is the set of metabolic pathways responsible for breaking down large biomolecules—including polysaccharides, lipids, nucleic acids, and proteins—into smaller units such as monosaccharides, fatty acids, nucleotides, and amino acids. This destructive aspect of metabolism contrasts with anabolism, the constructive building-up phase. The monomers released from polymer breakdown are either used to construct new polymer molecules or degraded further into simple waste products, a process that releases chemical free energy. Cellular wastes generated include lactic acid, acetic acid, carbon dioxide, ammonia, and urea. The formation of these wastes typically involves oxidation, with some of the released energy lost as heat, while the remainder drives the synthesis of adenosine triphosphate (ATP), the cell’s energy currency that powers anabolic reactions. Notable catabolic processes include glycolysis, the citric acid cycle, the breakdown of muscle protein to supply amino acids for gluconeogenesis, the breakdown of fat in adipose tissue into fatty acids, and the oxidative deamination of neurotransmitters by monoamine oxidase. Catabolism is controlled by various signals, primarily hormones and metabolic molecules. Classic catabolic hormones known since the early 20th century include cortisol, glucagon, and adrenaline (along with other catecholamines). More recently discovered hormones with catabolic effects include cytokines, orexin (also called hypocretin), and melatonin. The term derives from Neo-Latin, with roots in Greek: *kato* meaning “downward” and *ballein* meaning “to throw.”

Reader's Guide

Catabolism is the set of metabolic pathways that break down large molecules into smaller units, which are then either oxidized to release energy or used in anabolic reactions. It is the destructive, breaking-down aspect of metabolism, in contrast to anabolism, the constructive, building-up aspect. Large biomolecules such as polysaccharides, lipids, nucleic acids, and proteins are broken down into their respective monomers: monosaccharides, fatty acids, nucleotides, and amino acids. Cells use these monomers either to construct new polymer molecules or to degrade them further into simple waste products like lactic acid, acetic acid, carbon dioxide, ammonia, and urea. This degradation is typically an oxidation process that releases chemical free energy; some of this energy is lost as heat, but the rest drives the synthesis of adenosine triphosphate (ATP). ATP acts as the cell's energy currency, transferring the energy released by catabolism to the energy-requiring reactions of anabolism. Catabolism thus provides the chemical energy necessary for cell maintenance and growth. Notable catabolic processes include glycolysis, the citric acid cycle, the breakdown of muscle protein to supply amino acids for gluconeogenesis, the breakdown of fat in adipose tissue into fatty acids, and the oxidative deamination of neurotransmitters by monoamine oxidase. Catabolism is controlled by many signals, primarily hormones. Classic catabolic hormones known since the early 20th century include cortisol, glucagon, and adrenaline (and other catecholamines). More recently discovered hormones with catabolic effects include cytokines, orexin (hypocretin), and melatonin.

Did You Know?

• Catabolism breaks down polysaccharides, lipids, nucleic acids, and proteins into monosaccharides, fatty acids, nucleotides, and amino acids.
• Cellular wastes from catabolism include lactic acid, acetic acid, carbon dioxide, ammonia, and urea.
• Classic catabolic hormones include cortisol, glucagon, and adrenaline.
• The word catabolism derives from Greek roots meaning 'downward' and 'to throw'.