Learn About Cellular Respiration

ATP production
The three processes of ATP production or celluar respiration include glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation. Encyclopaedia Britannica/UIG/Getty Images

Cellular Respiration

We all need energy to function and we get this energy from the foods we eat. The most efficient way for cells to harvest energy stored in food is through cellular respiration, a catabolic pathway (break down of molecules into smaller units) for the production of adenosine triphosphate (ATP). ATP, a high energy molecule, is expended by working cells in the performance of normal cellular operations. Cellular respiration occurs in both eukaryotic and prokaryotic cells, with most reactions taking place in the cytoplasm of prokaryotes and in the mitochondria of eukaryotes. 

In aerobic respiration, oxygen is essential for ATP production. In this process, sugar (in the form of glucose) is oxidized (chemically combined with oxygen) to yield carbon dioxide, water, and ATP. The chemical equation for aerobic cellular respiration is C6H12O6 + 6O2 → 6CO2 + 6H2O + ~38 ATP. There are three main stages of cellular respiration: glycolysis, the citric acid cycle, and electron transport/oxidative phosphorylation.


Glycolysis literally means "splitting sugars." Glucose, a six carbon sugar, is split into two molecules of a three carbon sugar. Glycolysis takes place in the cell's cytoplasm. Glucose and oxygen are supplied to cells by the bloodstream. In the process of glyoclysis, 2 molecules of ATP, 2 molecules of pyruvic acid and 2 "high energy" electron carrying molecules of NADH are produced. Glycolysis can occur with or without oxygen. In the presence of oxygen, glycolysis is the first stage of aerobic cellular respiration. Without oxygen, glycolysis allows cells to make small amounts of ATP. This process is called anaerobic respiration or fermentation. Fermentation also produces lactic acid, which can build up in muscle tissue causing soreness and a burning sensation.

The Citric Acid Cycle

The Citric Acid Cycle, also known as the tricarboxylic acid cycle or the Krebs Cycle, begins after the two molecules of the three carbon sugar produced in glycolysis are converted to a slightly different compound (acetyl CoA). This cycle takes place in the matrix of cell mitochondria. Through a series of intermediate steps, several compounds capable of storing "high energy" electrons are produced along with 2 ATP molecules. These compounds, known as nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), are reduced in the process. The reduced forms (NADH and FADH2) carry the "high energy" electrons to the next stage. The citric acid cycle occurs only when oxygen is present but doesn't use oxygen directly.

Electron Transport and Oxidative Phosphorylation

Electron transport in aerobic respiration requires oxygen directly. The electron transport chain is a series of protein complexes and electron carrier molecules found within the mitochondrial membrane in eukaryotic cells. Through a series of reactions, the "high energy" electrons generated in the citric acid cycle are passed to oxygen. In the process, a chemical and electrical gradient is formed across the inner mitochondrial membrane as hydrogen ions (H+) are pumped out of the mitochondrial matrix and into the inner membrane space. ATP is ultimately produced by oxidative phosphorylation as the protein ATP synthase uses the energy produced by the electron transport chain for the phosphorylation (adding a phosphate group to a molecule) of ADP to ATP. Most ATP generation occurs during the electron transport chain and oxidative phosphorylation stage of cellular respiration. 

Maximum ATP Yields

In summary, prokaryotic cells may yield a maximum of 38 ATP molecules, while eukaryotic cells have a net yield of 36 ATP molecules. In eukaryotic cells, the NADH molecules produced in glycolysis pass through the mitochondrial membrane, which "costs" two ATP molecules. Therefore, the total yield of 38 ATP is reduced by 2 in eukaryotes.