Aerobic Respiration

Aerobic respiration consists of four distinct stages, glycolysis, link reaction, Krebs cycle and the electron transport chain.


Glycolysis is the first stage in aerobic respiration and occurs within the cells cytoplasm. Oxygen is not required therefore glycolysis can occur in both aerobic and anaerobic conditions. At the beginning of glycolysis, glucose is phosphorylated to create an unstable compound. This means that two ATP’s are used to create the unstable compound. The unstable compound is called hexose phosphate. Due to being unstable the 6-carbon compound, hexose phosphate, breaks down into two smaller 3-carbon sugar molecules called triose phosphate. Each 3-carbon compound is then converted in to pyruvic acid. This last conversion transfers enough energy for the synthesis of four ATP molecules and two reduced NAD molecules. Two ATP molecules are used and four ATP molecules are formed in glycolysis leaving a net gain of 2 two ATP molecules and two reduced NAD molecules and two molecules of pyruvic acid.

This occurs with the presence of oxygen in the mitochondrial matrix. Two of the pyruvate (pyruvic acid) molecules produced form glycolysis is converted in to 2-carbon compound, acetate and two molecules of reduced NAD. The conversion of a three-carbon molecule to a two-carbon molecule is known as decarboxylation, the removal of Carbon Dioxide. The acetate produced then combines to co-enzyme A to produce a molecule of Acetyl Co-Enzyme.

Krebs Cycle

Only occurs within the matrix of the mitochondrion under aerobic conditions. The Krebs cycle serves as a function to liberate the remaining energy from the carbon bonds. Also acts as an activator for the electron transport chain by allowing NAD and FAD to act as hydrogen carriers to the electron transport system.

Krebs cycle occurs once per acetyl co-enzyme A. Since there are two Acetyl Co-Enzyme A’s produces back in the link reaction, there will be two cycles of Krebs cycle per glucose molecule.

The acetyl co-enzyme A enters the Krebs cycle and combines with a four carbon compound called oxaloacetate. This forms a six carbon compound called citrate. The citrate will break down by further reactions to regenerate the four carbon compound, oxaloacetate. As the oxaloacetate is regenerated, the citrate is decarboxylated (removal of carbon dioxide) and is dehydrogenated (removal of hydrogen ions). The removed carbon dioxide is fed into the blood stream to be exhaled out of the body while the removed hydrogen ions pass onto different carriers called NAD and FAD. Each cycle produces three molecules of reduced NAD and two molecules of reduced FAD and one molecule of ATP. That is for one molecule of Acetyl Co-Enzyme A. Two molecules of such molecule would yield double the amount.

Electron Transport System

Much energy has been released and had been used to synthesise ATP, but most of the energy still remains in the hydrogen ions combined to its carriers.

Hydrogen split into their protons and electrons. Within the matrix allowing the protons to enter a pump to get pumped into the inter mitochondrial space and electrons pass down electron carriers located in the inner mitochondrion membrane. Protons combine with the electrons then flow back into the matrix through stalked particles called ATPsynthetase, from a high proton concentration within the inter membrane space down to the low proton concentration in the matrix, down its electro-chemical gradient. This enables the synthesis of ATP from ADP by using the energy within the electrons. The hydrogen atoms then combines with oxygen to form water.


QR Code
QR Code aerobic_respiration (generated for current page)