How Cells Use Energy

Role of Enzymes in Metabolism

Enzymes degrade the carbohydrates, proteins, and lipids in food into simple subunits that will be further broken down either in glycolysis or in the citric acid cycle.
All of the energy that a cell uses is generated through a series of exothermic reactions that release energy. The first step in releasing this energy is to break down the large macromolecules that are consumed as food into smaller molecules that can be metabolized. Enzymes break down the carbohydrates, proteins, and lipids from food into simple subunits. These smaller molecules are further broken down either in glycolysis or in the citric acid cycle. An enzyme is a biological molecule that catalyzes a biological reaction. Like all catalysts, an enzyme causes the rate of a chemical reaction to increase without being consumed by the reaction.

Activation Energy with and without Enzymes

Enzymes lower the activation energy of a reaction.
An enzyme speeds up a reaction by lowering the activation energy of the reaction. Chemical reactions go through a transition state or states between the reactants and the products, and the ΔG\Delta {\rm{G}} between the reactants and the intermediate product is usually very large and positive, which means a lot of energy is needed to form the intermediate product. This makes the reaction proceed very slowly. In the presence of an enzyme, the activation energy is much smaller, and the reaction occurs much more quickly. This shifts the equilibrium of the reaction to the right, forming more products than reactants. In the body, enzymes break food down into subunits to prepare the food molecules to be broken down even further in glycolysis or the citric acid cycle during cellular respiration.