respiration during times of oxygen deficit, although it may not be sufficient to
sustain the organism's ATP needs. Fuel molecules oxidized without oxygen yield smaller amounts of ATP.
Fermentation involves the partial breakdown of glucose without using
oxygen. In aerobic cellular respiration, the final electron acceptor is oxygen, hence, the emphasis on oxygen in aerobic respiration.
The initial stage of cell respiration, is a process called glycolysis, which splits a glucose molecule into two molecules of pyruvate, a 3-carbon compound. Glycolysis occurs in the cytoplasm of the cell. What follows glycolysis depends on the presence or absence of oxygen.
Glucose uses 2 ATP molecules in the production of hexose phosphate and hexose bisphosphate, thus producing ADP. However, as soon as hexose bisphosphate is formed it breaks down to 2 molecules of glyceraldehyde phosphate. This is then converted into 2 molecules of pyruvate by being oxidised. The hydrogen atoms lost from it are taken up by NAD to produce NADH + H+. This process is exothermic and enough energy is released to produce 4 molecules of ATP. This is known as substrate level phosphorylation.
If oxygen is available to do aerobic respiration, the pyruvate molecules will be oxidized in the next stages of aerobic respiration. The reactions of aerobic respiration after glycolysis occur in the mitochondria and include the link reaction, the Kreb’s cycle and electron transport chain. If oxygen is not available, the pyruvate molecules will proceed with fermentation.
At this stage in anaerobic respiration no more ATP is used or produced in the rest of the process. Therefore, in total, anaerobic respiration produces only 4 molecules of ATP, but uses 2 in the creation of hexose bisphosphate. Therefore, the net gain is 2 ATP molecules.
ATP is not used or produced in the link reaction which occurs in aerobic respiration.
The Kreb’s cycle, which takes place in the matrix, occurs twice for every molecule of glucose used. At one point in the cycle, enough energy is released to phosphorylate one molecule of ADP, therefore producing ATP. Because the cycle happens twice per glucose, this means 2 molecules of ATP are produced as none are used.
The electron transport chain is the greatest ATP-yielding part of aerobic respiration. Whereas only 2 came from each of glycolysis and the Kreb’s cycle, 34 molecules of ATP are produced in oxidative phosphorylation. This is due to the energy released by the transfer of H+ ions between hydrogen carriers and e- between electron carriers. These are exothermic processes, containing enough energy to produce ATP from ADP + Pi. Each NADH + H+ molecule produces 3 molecules of ATP. 2 molecules of NADH + H+ are produced in both glycolysis and the link reaction and 6 molecules are produced in the Kreb’s cycle. Therefore, 30 molecules of ATP are produced from the oxidation of the 10 molecules of NADH + H+. FADH + H+ produces only 2 molecules of ATP. In total 2 molecules of FADH + H+ are produced, both in the Kreb’s cycle.
Therefore, with the 4 molecules of ATP produced by FADH + H+ in oxidative phosphorylation, and the 30 by NADH + H+ as well as the 4 molecules of ATP produced in earlier stages, a total of 38 molecules of ATP are produced by aerobic respiration. This is 36 more than that produced in anaerobic respiration. This shows why aerobic respiration is so more effective at producing energy than anaerobic respiration and, therefore, why it is used the majority of the time by eukaryotic organisms.
