Glycolysis

ferric ion, sulfate, carbonate, and fumarate or other organic compounds. The final yield of ATP will be less than seen in aerobic respiration.

Fermentation is not the same thing as anaerobic respiration. It does involve an anerobic situation and does involve the catabolism of glucose. Oxygen is also not the final electron acceptor. It starts with glycolysis and ends with pyruvate, yielding two molecules of ATP and two molecules of NADH. Fermenting organisms, however, do not have an electron transport chain and do not use the Krebs cycle or tricarboxylic acid cycle. The re-oxidize NADH with pyruvate as the final electron acceptor to make ethanol and CO2, as well as acids like lactic acid. Fermentation is used by humans because we eat fermented things like cheese, tofu, bread, and alcohol.

GLYCOLYSIS

Glycolysis is the beginning point of all metabolism of glucose by most living cells. It is completely anaerobic and takes place in the cytoplasm of all cells. Glucose can enter through secondary active transport processes against a glucose gradient or can enter through GLUT proteins, which are specialized glucose transporter proteins in the cell membrane. The GLUT proteins involve facilitated diffusion of glucose rather than active transport.

Glycolysis starts with the typical six carbon ring and ends up with a three-carbon sugar, called pyruvate. As mentioned, there are two distinct phases, phase 1 and phase 2. Phase 1 requires energy and phase 2 creates energy.

The first enzyme is hexokinase, which causes the phosphorylation of all six-carbon sugars. Any enzyme that is a kinase will phosphorylate something. It uses ATP to provide the phosphate, making glucose-6-phosphate. Figure 41 shows the glycolysis pathway, beginning with this step:

Figure 41.

When glucose-6-phosphate is made, it can no longer leave the cell through the GLUT protein because it is too negatively charged to pass through its hydrophobic center. Instead, it gets treated with an isomerase to make fructose-6-phosphate. This helps the process of eventually making a three-carbon molecule.

In step 3, phosphofructokinase gets phosphorylated again with a second ATP molecule to make fructose-1,6-bisphosphate. This is called a rate limiting step because, when ADP