Cells need energy to do work. This energy can be in the form of chemical, mechanical, or electrochemical energy. For example, chemical energy is used by cells to help build, rearrange, or break down substances. Mechanical energy is used during cell motility, and electrochemical energy is used to help move charged substances across membranes. Cells acquire energy by breaking down high-energy molecules obtained from food. Specifically, cells participate in metabolism. At a molecular level, metabolism includes the biological processes that build or break molecules. Metabolism provides cells with the capacity to acquire energy and then use that energy to carry out many different functions. These functions range from growth, reproduction, and movement to maintenance, such as pumping ions and replacing depleted molecules.
Cells use cellular respiration to acquire energy in the form of ATP (adenosine triphosphate, the energy currency of life) by breaking down nutrient molecules. Specifically, this process involves extracting ATP from the food consumed, which is usually in the form of glucose.
Chemical Reaction of Cellular Respiration
Most of the chemical processes involved in cellular respiration include oxidation reactions and reduction reactions. An oxidation reaction is a reaction that involves the removal of an electron from a compound. (An electron is a negatively charged subatomic particle that moves in orbitals around the atomic nucleus and a compound is a substance made of atoms of two or more elements bonded together in a certain ratio.) A reduction reaction involves the addition of this electron to a compound. It is helpful to remember the function of these reaction pairs using the acronym LEO GER ("LEO the lion goes GER"); LEO stands for "loses electrons oxidized," and GER stands for "gains electrons reduced." This reaction involving the transfer of electrons between two atoms or molecules is called a redox reaction. Oxidation and reduction reactions always occur together.
The Electron Carriers: NAD and FAD
Cellular respiration is a series of redox reactions, which involve the transfer of negatively charged particles, called electrons, by molecules called electron carriers. The principle electron carriers include nicotinamide adenine dinucleotide (NAD), which is derived from a vitamin B molecule called niacin, and flavin adenine dinucleotide (FAD), which is derived from a vitamin B molecule called riboflavin.Enzymes are used in cellular respiration to catalyze (cause or accelerate) reactions. A coenzyme is a non-protein molecule, such as NAD and FAD, that helps an enzyme function. When NADH is oxidized, it is converted into NAD+. The reduced form of NAD+ is NADH. This is because NAD+ has accepted two electrons and a positively charged particle called a proton (hydrogen ion), where the proton is the equivalent of a hydrogen atom attached to NAD+. Thus, NAD+ can oxidize a metabolite (a substance that is formed during metabolism, or required for metabolic processes) by accepting electrons and converting to NADH. NADH can reduce a metabolite by giving up those electrons and converting from NADH back to NAD+. Since NAD+ is present in the cell, this molecule can participate in redox reactions over and over again. FAD is another coenzyme that can be used by the cell in place of NAD under some conditions. FAD drives oxidation-reduction reactions by accepting two protons to become FADH2.