Question

1. What is the difference between Oxidative and Substrate level phosphorylation?
2. What occurs in Fermentation pathways? What is the energy yield and where does it come from?

Answer 1

During respiration, glucose is first broken down into two pyruvate molecules in the cytosol by glycolysis. During aerobic respiration the pyruvate enters the mitochondria where is completes the kreb cycle and completely degrades to form carbon dioxide. However, in prokaryotes mitochondria is absent , so the next step occurs in cytosol itself. During these processes of glycolysis and kreb cycle, redox reactions occur catalysed by dehydrogenase enzymes which transfer electrons from substrate to NAD+ and form NADH. The next stage is electron transport chain (in inner mitochondrial membrane) which accepts electrons from products of first two stages like NADH And FADH2 and passes them through a series of electron carriers . At the end, the electrons combine with hydrogen and oxygen to form water. During this process electron transport chain as the electrons pass downhill and get terminally absorbed by O2 , an electro chemical gradient develops because the movement of electrons from NADH and FADH2 is spontaneous, and the free energy released is used to pump the protons H+ into the intermembrane space. Hence this space is proton rich than the matrix. Now, an enzyme complex called ATP synthase which uses this difference in proton concentration or proton gradient as power source to produce ATP , is present in the inner membrane of mitochondria(by chemiosmosis).The protons flow down the gradient through the ATP synthase H+ channels . Thus, the stored energy in the form of chemical potential gradient is used to made ATP from ADP and inorganic phosphate. ATP acts as an energy currency. This process is known as Oxidative Phosphorylation and it accounts for 90% of total ATP production.

The rest of 10% energy is produced by Substrate level phosphorylation which occurs by the help of an enzyme that can transfer phosphate group from a substrate molecule to ADP and no inorganic phosphate is used. This substrate is an organic intermediate produced during breakdown of glucose. Such ATP production occurs both during glycolysis and Kreb cycle.

Glycolysis is common in both aerobic respiration as well as fermentation. There is no electron transport chain used in fermentation, hence the only source of ATP is by substrate level phosphorylation. During glycolysis, 2 ATP is produced by substrate level phosphorylation, and 2 NADH is also produced.NAD+ is the oxidising agent for glycolysis,so it has to be degerated. In fermentation , the NADH doesn't enter an electron transport chain, instead an organic molecule like pyruvate or acetaldehyde acts as terminal electron acceptor and the end product lactic acid or ethanol is produced respectively. Therefore , fermentation produces only 2ATP by substrate level phosphorylation.