ALAIN VIEL: In the next set of videos, forming unit three of your course, our goal is to discuss how cells harvest energy from fuel molecules, and also to show that, at the most fundamental level, the exergonic transfer of electrons, through a series of protein complexes, powers the endergonic synthesis of ATP. In these videos, I would like to give you a quick overview of key pathways, leading to the synthesis of ATP. So don’t worry about the specifics for now. We have plenty of time for that later. Based on my introduction, two concepts emerge. First, molecules have to give electrons, and second, some molecules have to receive these electrons. Let’s identify what are the electron donating and accepting processes. First, in the electron donating process, fuel molecules will pass the electrons to a smaller number of universal electron acceptance, such as NAD+ or FAD. The electron accepting process is when you have a flow of electrons from these few universal electron acceptors to a final electron acceptor, which is, on this diagram, oxygen. Let’s take a look at the major parts of the energetic pathways in aerobic organisms. First, we start with glycolysis, that converts glucose into pyruvate. This oxidation of glucose is done by a series of steps, some of which coupled with the synthesis of ATP. In these steps, molecules with a high hydrolysis potential will transfer their phosphate to a ADP to form ATP. This type of ATP synthesis is called substrate level phosphorylation. In addition to producing pyruvate and ATP, the electrons that are produced by the oxidation of glucose are passed to NAD to form NADH. The second pathway is the citric acid cycle. During glycolysis, glucose is not fully oxidized. So pyruvate reacts with coenzyme A to former acetyl coenzyme A. And the acetyl group will feed the citric acid cycle to complete the oxidation of glucose. The major role of the citric acid cycle is to form a large quantity of these universal electron acceptors, NADH and FADH2. One step in the citric acid cycle is also coupled with the synthesis of GTP by substrate level phosphorylation. Subsequently, GTP will be converted into ATP. These two pathways are part of the electron donation process. Now, what about the electron accepting process? It is the third part of the biochemical pathway that interests us. The electrons stored in NADH and FADH2 are passed through a series of protein complex, forming the electron transport chain. This transfer of electrons releases energy that will lead to the synthesis disease of a ATP by the ATP synthase. In this case, this type of ATP synthesis is called oxidative phosphorylation. Under some conditions, aerobic organisms, or tissues from aerobic organisms, will operate under anaerobic conditions. So NADH cannot be converted back into NAD, because oxidative phosphorylation is not working. The necessary conversion of NADH to NAD+ is made possible by a series of pathways that are collectively called fermentation. So that is for organisms that have to live in the presence of oxygen– aeroboc organisms. So what is the situation for anaerobic organisms? They can produce a ATP through glycolysis. And, in some cases, through fermentation. But ATP synthesis, powered by the electron transport chain, is possible, as well. Anaerobic organisms use diverse mechanisms to harvest energy from fuel molecules. They can transfer electrons from a variety of electron donors to a variety of electron acceptors, other than oxygen. For example, denitrifying bacteria use an electron acceptor, nitrate, that is converted into dinitrogen. This conversion is coupled with the transfer of electrons that will power the synthesis of ATP. Don’t think that glucose is the only electron donating fuel molecule. In fact, cells are remarkably good at harvesting electrons from diverse fuel molecules. For example, in aerobic organisms, amino acids and fatty acids are converted into acetyl coenzyme A that will feed the citric acid cycle and lead to the synthesis of a large number of molecules of ATP, through oxidative phosphorylation. In conclusion, all the pathways leading to the ATP synthesis are collectively referred to as cellular respiration. If the final electron acceptor is oxygen, this process will be called aerobic respiration. If the electron acceptor is something else then oxygen– for example, nitrate, in the case of denitrifying bacteria– this type of respiration is called anaerobic respiration. The rest of the videos from unit three will focus on a detailed analysis of the biochemical pathways leading to the synthesis of ATP.