Overview

Organisms harvest energy from food, but this energy cannot be directly used by cells. Cells convert the energy stored in nutrients into a more usable form: adenosine triphosphate (ATP).

ATP stores energy in chemical bonds that can be quickly released when needed. Cells produce energy in the form of ATP through the process of cellular respiration. Although much of the energy from cellular respiration is released as heat, some of it is used to make ATP.

During cellular respiration, several oxidation-reduction (redox) reactions transfer electrons from organic molecules to other molecules. Here, oxidation refers to electron loss and reduction to electron gain. The electron carriers NAD⁺ and FAD and their reduced forms, NADH and FADH₂, respectively, are essential for several steps of cellular respiration.

Aerobic respiration requires oxygen and generates ATP by breaking down glucose and oxygen into carbon dioxide and water. Some prokaryotes use anaerobic respiration, which does not require oxygen. Both aerobic and anaerobic respiration begins with glycolysis, which can occur without oxygen. Glycolysis breaks down glucose into pyruvate, yielding ATP. In the absence of oxygen, pyruvate ferments, producing NAD⁺ for continued glycolysis. Importantly, several types of yeast carry out alcoholic fermentation. Human muscle cells can use lactic acid fermentation when oxygen is depleted. Anaerobic respiration ends with fermentation.

Aerobic respiration, however, continues with pyruvate oxidation. Pyruvate oxidation generates acetyl-CoA, which enters the citric acid cycle. The citric acid cycle consists of several redox reactions that release the bond energy of acetyl-CoA, producing ATP and the reduced electron carriers NADH and FADH₂.

The final stage of cellular respiration, oxidative phosphorylation, generates most of the ATP. NADH and FADH₂ pass their electrons through the electron transport chain. The electron transport chain releases energy that is used to expel protons, creating a proton gradient that enables ATP synthesis.

Most organisms use aerobic (oxygen-requiring) respiration, which produces much more ATP than anaerobic respiration.

Procedure

Cellular respiration is a catabolic process in which organic molecules are broken down to create usable energy via the electron transport chain. This process requires oxygen in humans and most other organisms, and produces carbon dioxide, water, heat, and usable energy in the form of ATP.

Although different organic molecules, such as sugars, amino acids, and lipids, can be used as substrates in cellular respiration, glucose acts as the main substrate.

For glucose, the equation for cellular respiration is C₆H₁₂O₆  plus six O₂, leads to six CO₂   plus six H₂O plus energy, the reverse of photosynthesis.

This reaction occurs in multiple steps, each at a different location within a cell. While glycolysis takes place in the cytoplasm, pyruvate oxidation and the citric acid cycle occur in the mitochondrial matrix, and oxidative phosphorylation happens in the inner mitochondrial membrane.

Together, these processes power cellular activities such as flagellar movement, and muscle contraction, through the breakdown of organic molecules to produce ATP.