Metabolism

Metabolism is the complete set of chemical reactions that occur within living cells and organisms. These reactions transform nutrients and other molecules, provide energy for biological processes, and produce materials needed to build and maintain cells and tissues. Metabolic reactions are organized into interconnected sequences called metabolic pathways, many of which are controlled by enzymes .^1,2

Metabolism is commonly described in terms of catabolism and anabolism. Catabolic pathways break larger molecules into smaller ones and can release usable energy. Anabolic pathways use smaller molecules to build more complex molecules, such as proteins and other cellular components, and generally require an input of energy. The two processes are interconnected because products and energy derived from catabolic pathways can support anabolic reactions .^1,2

Cells use molecules such as adenosine triphosphate, or ATP, to transfer energy between metabolic reactions and energy-requiring cellular processes. Nutrients including carbohydrates, fats, and proteins can enter metabolic pathways that contribute to ATP production or provide building blocks for biosynthesis. Metabolism therefore includes both energy-producing reactions and reactions involved in constructing, modifying, and breaking down biological molecules .^1,2

Metabolism should not be confused with metabolic rate. Metabolism refers to the network of chemical reactions themselves, whereas metabolic rate describes the rate of energy expenditure associated with metabolic processes. Terms such as basal metabolic rate and resting metabolic rate therefore describe aspects of energy expenditure rather than metabolism as a whole .¹