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 for the production of adenosine triphosphate (ATP). ATP, a high energy molecule, is expended by working cells. Cellular respiration occurs in both eukaryotic and prokaryotic cells. 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, two molecules of ATP, two molecules of pyruvic acid and two "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 and 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).

Through a series of intermediate steps, several compounds capable of storing "high energy" electrons are produced along with two ATP molecules. These compounds, known as nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), are reduced in the process. These reduced forms carry the "high energy" electrons to the next stage.

The citric acid cycle occurs only when oxygen is present but it doesn't use oxygen directly. This cycle takes place in the matrix of cell mitochondria.

Electron Transport and Oxidative Phosphorylation

Electron transport requires oxygen directly. The electron transport chain is a series of electron carriers in the membrane of the mitochondria in eukaryotic cells. Through a series of reactions, the "high energy" electrons are passed to oxygen. In the process, a gradient is formed, and ultimately ATP is produced by oxidative phosphorylation. The enzyme ATP synthase uses the energy produced by the electron transport chain for the phosphorylation of ADP to ATP.

Maximum ATP Yields

In summary, prokaryotic cells can yield a maximum of 38 ATP molecules while eukaryotic cells can yield a maximum of 36. In eukaryotic cells, the NADH molecules produced in glycolysis pass through the mitochondrial membrane, which "costs" two ATP molecules.