How Cells Use Energy

Chemical Reactions in Living Things


In a series of steps, energy is released from food and used to drive the reactions that build the molecules cells need.

On a molecular level, metabolism is made up of the biological processes that build or break molecules; on an organismal level, metabolism is all of the chemical reactions within an organism that provide cells with the capacity to acquire energy and use that energy to carry out different functions. Metabolism includes two main components: catabolism and anabolism. First, cells obtain energy by breaking down food molecules in a process called catabolism, which is a sequence of chemical reactions that breaks down or decomposes molecules into smaller units while generating energy. The energy that is released comes from the carbohydrates, proteins, and fats that are found in food. These molecules all contain chemical potential energy, which is the energy released during a chemical reaction. Carbohydrates, proteins, and fat molecules are the first reactants in the initial steps of metabolism.

A reactant's chemical potential energy is released when the molecular bonds are broken by the cell in a chemical reaction. When molecules are broken down, the potential energy stored in their chemical bonds is released and can be used by the body to generate usable cellular energy. Anabolism, also called biosynthesis, is a sequence of chemical reactions that constructs or synthesizes molecules from smaller units—the reverse of catabolism. Anabolic reactions use energy obtained from catabolism to assemble the proteins, DNA, and RNA that can be used to build new cells. These molecules are the final products of metabolism reactions.

Organisms are divided into two groups depending on how they obtain energy. An autotroph is an organism that can make its own food—for example, plants create glucose through photosynthesis. Organisms such as humans that need to ingest food are called heterotrophs. A heterotroph is an organism that obtains energy and carbon from consuming other organisms. Ultimately, heterotrophs get their food from autotrophs.

Energy Cycle

Autotrophs use sunlight energy, water, and carbon dioxide (CO2) to perform photosynthesis and produce glucose and oxygen (O2). Heterotrophs consume glucose and breathe O2 to perform cellular respiration and produce adenosine triphosphate (ATP), CO2, and water (H2O).

Energy in Chemical Reactions

Free energy change can be used to predict whether a chemical reaction will occur spontaneously.

The use and production of energy is the driving force for all chemical reactions, including those that make up metabolism.

Free energy, often referred to as Gibbs free energy (G), is the capacity of a system to do work, or the total potential energy of a system.

The change in free energy that occurs during a chemical reaction is called ΔG\Delta {\rm{G}} is equal to the free energy of the products minus the free energy of the reactants, or
ΔG=GProductsGReactants\Delta {\rm{G = G}}_{\text{Products}}-{\rm{G}}_{\text{Reactants}}
The ΔG\Delta {\rm{G}} of a reaction depends, in part, upon the change in the enthalpy of the reaction system. Enthalpy (H) is a measure of the bond energy of a system, and the change in enthalpy, ΔH\Delta {\rm{H}}, is a measure of how much energy is released or absorbed when a chemical reaction occurs. For example, consider a system where reactants with high chemical potential energy react to form products at lower chemical potential energy. The first law of thermodynamics states that energy cannot be created or destroyed, only transformed from one type of energy to another type of energy. Therefore, some energy must be released to the surroundings. The released energy is often in the form of heat. Heat is a form of molecular kinetic energy, which is the energy that an object possesses when in motion, because molecules with a lot of heat vibrate, or move, very quickly. A chemical reaction that releases heat, therefore, converts some chemical potential energy into molecular kinetic energy. ΔG\Delta {\rm{G}} also depends on entropy. Entropy (S) is a measure of the disorder or predictability of a system. The more disordered the system, the higher the entropy. In fact, the change in free energy can be represented as:
ΔH\Delta {\rm{H}} is the enthalpy change, T is the absolute temperature in Kelvin, and ΔS\Delta {\rm{S}} is the change in entropy. In a closed system, the second law of thermodynamics states that the total entropy of an isolated system only increases over time, or, to put it another way, ΔS>0\Delta {\rm{S}} \gt 0.

Gibbs Free Energy

The sign ΔG\Delta {\rm{G}} determines whether a chemical reaction will occur spontaneously.
The ΔG\Delta {\rm{G}} of a reaction provides information as to whether or not the reaction will occur spontaneously. The tendency of all systems is to minimize the free energy; in other cvwords, systems prefer that the Gproducts be smaller than the Greactants. An exergonic reaction is a reaction that releases energy. So, for an exergonic reaction:
ΔG=GproductsGreactants<0\Delta{\rm{G} ={G}}_{\text{products}}-{\rm{G}}_{\text{reactants}}<0
Exergonic reactions are, therefore, energetically favorable and, thus, occur spontaneously. This means that the reaction will move forward to form more products. In contrast, an endergonic reaction is a reaction that takes in energy. In endergonic reactions, Gproducts>Greactants{\rm {G}}_{\text{products}}>{\rm {G}}_{\text{reactants}}. They, therefore, have a positive ΔG\Delta \text{G}, are energetically unfavorable, and do not occur spontaneously. In this case, the reverse reaction is energetically favorable, and the reaction will tend to occur in the reverse direction to generate more reactants. If ΔG\Delta {\rm{G}}=0, neither positive nor negative, the chemical reaction is at equilibrium. Equilibrium is a state in which there is no net change through time, a state when changes will not occur. In other words, at equilibrium, the rates of the forward and reverse reactions are equal to each other, and, therefore, the concentration of reactants and products remains unchanged.
A reaction reaches equilibrium when forward and reverse reaction rates are equal. It also means that the amounts of reactants and products no longer change over time.