Carbohydrate catabolism

Carbohydrate catabolism is the breakdown of carbohydrates into smaller units. Carbohydrates literally undergo combustion to retrieve the large amounts of energy in their bonds. Energy is secured by mitochondria in the form of ATP.

There are several different types of carbohydrates: polysaccharides (e.g., starch, amylopectin, glycogen, cellulose), monosaccharides (e.g., glucose, galactose, fructose, ribose) and the disaccharides (e.g., sucrose, maltose, lactose).

Glucose reacts with oxygen in the following redox reaction, C6H12O6 + 6O2 → 6CO2 + 6H2O, the carbon dioxide and water is a waste product and the chemical reaction is exothermic.

The breakdown of glucose into energy in the form of molecules of ATP is therefore one of the most important biochemical pathways found in living organisms.

Glycolysis
The six-carbon glucose molecule is broken down into two three-carbon pyruvate molecules yielding two ATP molecules and two high-energy NADH molecules.

Fermentation
Without oxidative phosphorylation, the NADH molecules cannot be used to produce ATP. When all the NAD+ molecules have been converted to NADH, glycolysis will stop unless the NAD+ is regenerated by fermentation.

Pyruvate oxidation
The three carbon pyruvate molecule loses a carbon atom and is shepherded into the citric acid cycle by coenzyme A.

The Citric acid cycle (also known as the Krebs cycle)
The acetyl group that came from pyruvate enters this biochemical cycle, releasing carbon dioxide, water, and the high energy molecules ATP, NADH, and FADH2.

Oxidative phosphorylation
The high energy molecules NADH and FADH 2 are converted into usable ATP molecules in the mitochondria by the mitochondrial electron transport chain.