Biology » Cell Metabolism » Energy and Metabolism

Glossary of Words

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Energy and Metabolism

Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the chemical reactions that take place within it. There are metabolic reactions that involve the breaking down of complex chemicals into simpler ones, such as the breakdown of large macromolecules. We refer to this process as catabolism, and such reactions are associated with a release of energy.

On the other end of the spectrum, anabolism refers to metabolic processes that build complex molecules out of simpler ones, such as the synthesis of macromolecules. Anabolic processes require energy. Glucose synthesis and glucose breakdown are examples of anabolic and catabolic pathways, respectively.

Types and forms of energy

Energy comes in many different forms. Objects in motion do physical work, and kinetic energy is the energy of objects in motion. Objects that are not in motion may have the potential to do work, and thus, have potential energy. Molecules also have potential energy because the breaking of molecular bonds has the potential to release energy.

Living cells depend on the harvesting of potential energy from molecular bonds to perform work. Free energy is a measure of energy that is available to do work. The free energy of a system changes during energy transfers such as chemical reactions, and this change is referred to as ∆G.

The ∆G of a reaction can be negative or positive, meaning that the reaction releases energy or consumes energy, respectively. We refer to a reaction with a negative ∆G that gives off energy as an exergonic reaction. We refer to one with a positive ∆G that requires energy input as an endergonic reaction.

Exergonic reactions are said to be spontaneous, because their products have less energy than their reactants. The products of endergonic reactions have a higher energy state than the reactants, and so these are nonspontaneous reactions. However, all reactions (including spontaneous -∆G reactions) require an initial input of energy in order to reach the transition state, at which they’ll proceed. This initial input of energy is called the activation energy.

Glossary of Words

Activation energy

energy necessary for reactions to occur


(also, anabolism) pathways that require an input of energy to synthesize complex molecules from simpler ones


study of energy flowing through living systems


(also, catabolism) pathways in which complex molecules are broken down into simpler ones

Chemical energy

potential energy in chemical bonds that is released when those bonds are broken


describes chemical reactions that require energy input


total energy of a system


describes chemical reactions that release free energy

Free energy

Gibbs free energy is the usable energy, or energy that is available to do work.

Heat energy

total bond energy of reactants or products in a chemical reaction

Kinetic energy

type of energy associated with objects or particles in motion


all the chemical reactions that take place inside cells, including anabolism and catabolism

Potential energy

type of energy that has the potential to do work; stored energy

Transition state

high-energy, unstable state (an intermediate form between the substrate and the product) occurring during a chemical reaction

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