Concept of Metabolism

Metabolism is the sum total of all the chemical reactions occurring within every cell of a living organism — reactions that both supply the energy needed for the organism's vital processes and provide the raw material for building new organic matter. It is a continuous process of breakdown and synthesis of biomolecules, and the great majority of these individual metabolic reactions do not occur in isolation — they are linked, one feeding into the next, as part of larger metabolic pathways.

Every cell is, in effect, a busy 'work centre', running two broad kinds of metabolic pathway:

• Catabolic pathways break a complex biomolecule down into simpler ones, typically releasing energy in the process. For example, when we eat starchy food such as bread or chapati, the gastrointestinal tract hydrolyses the starch (with the help of enzymes) into glucose units, releasing energy that is captured as ATP.
• Anabolic pathways (also called biosynthetic pathways) do the reverse — building a more complex biomolecule from simpler starting units, and consuming energy in the process, e.g. the synthesis of glycogen from glucose, or of protein from amino acids. …