Introduction to Metabolism: Videos & Practice Problems

Metabolism refers to all the chemical reactions that occur within a living organism to maintain life. These reactions are organized into metabolic pathways, where substrates are converted into products through a series of enzymatic steps. Metabolism is crucial because it enables organisms to grow, reproduce, maintain their structures, and respond to environmental changes. It encompasses both catabolic pathways, which break down molecules to release energy, and anabolic pathways, which build larger molecules and require energy input. Understanding metabolism helps us grasp how organisms manage energy and resources for growth and maintenance.

Catabolic pathways, or catabolism, involve the breakdown of larger molecules into smaller ones, releasing energy in the process. These pathways are associated with exergonic reactions, which release energy into the environment. In contrast, anabolic pathways, or anabolism, involve the synthesis of larger molecules from smaller ones, requiring an input of energy. These pathways are associated with endergonic reactions, which consume energy. Essentially, catabolism breaks down molecules to release energy, while anabolism builds up molecules and requires energy input.

Enzymes are biological catalysts that speed up chemical reactions in metabolic pathways. They work by lowering the activation energy required for a reaction to occur. In a metabolic pathway, each enzyme is specific to a particular substrate and catalyzes a specific step in the pathway. For example, in a series of reactions converting substrate A to final product E, enzyme 1 converts A to B, enzyme 2 converts B to C, and so on. This sequential action ensures that substrates are efficiently converted into final products, facilitating the organism's metabolic processes.

Energy plays a crucial role in both anabolic and catabolic pathways. In catabolic pathways, energy is released as large molecules are broken down into smaller ones. This released energy can be harnessed for various cellular activities. In contrast, anabolic pathways require an input of energy to build larger molecules from smaller ones. This energy is often derived from ATP (adenosine triphosphate) generated during catabolic processes. Thus, the energy released in catabolism is used to drive the energy-consuming reactions in anabolism, maintaining a balance within the organism's metabolism.

An example of a metabolic pathway is glycolysis, which is the process of breaking down glucose into pyruvate, releasing energy in the form of ATP. Glycolysis is a catabolic pathway that occurs in the cytoplasm of cells and is the first step in cellular respiration. It is significant because it provides the cell with a quick source of energy and intermediates for other metabolic pathways. The ATP produced during glycolysis is essential for various cellular functions, and the pyruvate generated can be further processed in the mitochondria to produce even more ATP through the citric acid cycle and oxidative phosphorylation.