Sugar (a simple carbohydrate) metabolism (chemical reactions) is a classic example of the many cellular processes that use and produce energy. Living things consume sugar as a major energy source because sugar molecules have considerable energy stored within their bonds. Consumed carbohydrates have their origins in photosynthesizing organisms like plants. During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas into sugar molecules, like glucose. Because this process involves synthesizing a larger, energy-storing molecule, it requires an energy input to proceed.
Starch and glycogen are the storage forms of glucose in plants and animals, respectively. These long polysaccharide chains may be branched or unbranched. Such storage forms make glucose slightly less accessible for metabolism; however, this prevents the glucose from leaking out of the cell or creating a high osmotic pressure that could cause the cell to uptake excessive water. Both glycogen and starch are used as energy reserves, which can be broken down into glucose to supply ATP molecules.
The processes of making and breaking down sugar molecules illustrate two types of metabolic pathways. A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product or products. In the case of sugar metabolism, the first metabolic pathway synthesizes sugar from smaller molecules, and the other pathway breaks sugar down into smaller molecules. Scientists call these two opposite processes—the first requiring energy and the second producing energy—anabolic (building) and catabolic (breaking down) pathways, respectively. Consequently, building (anabolism) and degradation (catabolism) comprise metabolism.
This text is adapted from Openstax, Biology 2e, Section 6.1: Energy and Metabolism
