This lecture is essentially a preview of what’s coming in the next few lectures: that the exergonic transfer of electrons through a series of protein complexes powers the endergonic synthesis of ATP. There are two major themes here:
- molecules need to donate electrons
- other molecules need to accept these electrons
On the donating side, fuel molecules pass their electrons to a small number of universal electron acceptors such as NAD+ and FAD. On the accepting side, the electrons then flow from these universal acceptors–as NADH and FADH–to a final acceptor. For these lectures, that final acceptor is oxygen.
Energetic Pathways for Aerobic Organisms
Glycolysis: the oxidation of glucose into pyruvate. Here, molecules with a high hydrolysis potential (i.e., the phosphorylated glucose molecules) transfer their phosphate to an ADP to from ATP. This is what is known as Substrate-Level Phosphorylation. The electrons produced by the oxidation of glucose are passed to NAD+ to form NADH.
Citric Acid Cycle (CAC): during glycolysis, glucose is not fully oxidized, so pyruvate reacts with coenzyme-A to form Acetyl Co-A. The acetyl group feeds the CAC to complete the oxidation of glucose.
The major role of CAC is to create a large amount of electron acceptors. One step is CAC is coupled with the synthesis of GTP by substrate-level phosphorylation. Later GTP is converted into ATP.
Electron Transport Chain (ETC): the electrons stored in NADH and FADH2 are passed through a series of protein complexes. This transfer o f electrons releases energy that leads to the synthesis of ATP via ATP Synthase. This type of synthesis is called Oxidative Phosphorylation.
Under some conditions, aerobic organisms will operate under anaerobic conditions. Here, NADH can’t be converted back to NAD+ via oxidative phosphorylation. They rely on fermentation to do this recycling.
Scenarios for anaerobic organisms: ATP production through glycolysis and sometimes fermentation. They can also use the ETC as well. Anaerobic organisms sue diverse mechanisms to harvest energy from fuel molecules. They can review electrons from many sources and donate them to final acceptors other than oxygen.
For example, denitrifying bacteria. The organisms use an electron acceptors in nitrate that is converted into dinitrogen, coupled with transfer of electrons that power the synthesis of ATP.
Glucose isn’t the only source of electron-donating fuel molecules. Cells are good at harvesting electrons from a variety of fuel molecules. In aerobic organisms, amino acids and fatty acids are converted into Acetyl CO-A that feed CAC and the synthesis of ATP through oxidative phosphorylation.