Catabolism of Carbohydrates

CATABOLISM OF CARBOHYDRATES

The catabolism of carbohydrates is very important to the making of ATP energy. These catabolic processes happen sequentially involving a series of reactions and many intermediates that turn simple carbs like glucose into carbon dioxide and water.

Glycolysis is the first part of glucose metabolism. It occurs in eukaryotes, bacteria, and most archaea organisms. It provides some energy for the cell. The starting molecule for glycolysis will always be glucose. This process is anaerobic because it does not require oxygen. It can, however, be connected to reactions that do require oxygen at a later step in the process. The end product of glycolysis is called pyruvate, which is a three-carbon molecule. Pyruvate can be broken down further in subsequent reactions.

Glycolysis usually involves what’s called the EMP pathway, which is also referred to as the Embden-Meyerhof-Parnas pathway. There are two phases. The first phase is the investment stage, which actually requires energy. The end product is glyceraldehyde-3phosphate. The second phase involves an energy payoff stage, in which more energy is produced than was invested. Four ATP molecules are made, which offsets the two ATP molecules used in the investment phase, for a total of two ATP molecules made using one glucose molecule.

The ATP molecules mad are made through what’s called substrate level phosphorylation. This means that phosphate is removed from an organic molecule and is attached to an ADP molecule to make ATP. In the end of glycolysis, two ATO molecules and two NADH molecules are made. Pyruvate, as mentioned, is the end product. Two of these are made per glucose molecule. Figure 39 shows the glycolysis pathway:

Figure 39.

Most organisms use the EMP pathway. There are a few that use slightly different pathways. The Entner-Doudoroff pathway is used by certain bacteria, such as Pseudomonas aeruginosa and some E. coli organisms, is an alternative pathway. Still another alternative pathway is called the pentose phosphate pathway. All cells will use this pathway as an ancient version of glycolysis. It uses intermediates that help to make nucleic acids and amino acids. All cells can use the pentose phosphate pathway when nucleic acids or amino acids are necessary.

The pyruvate that is made can be further oxidized after glycolysis to make more energy. There is a transition reaction or bridge reaction that makes NADH from NAD+. A twocarbon molecule called acetyl is connected to a carrier molecule called coenzyme A. This compound then enters the Krebs cycle.

The Krebs cycle is also called the citric acid cycle or the tricarboxylic acid cycle. This is a closed loop system. The cycle makes two carbon dioxide molecules, one ATP molecule or one GTP molecule—made through substrate level phosphorylation. Three molecules of NADH and one molecule of FADH2 are made, which become important later on. Oxygen is not required. The coenzyme A molecule is completely recycled. Figure 40 shows the Krebs cycle:

Figure 40.