Question

In paragraph form, explain how an electrochemical gradient is generated and how it is used to synthesize ATP in the mitochondrion during cellular respiration. In your answer you should discuss (A) which structures/enzymes/proteins and major electron donors/acceptors are involved, (B) where it happens, and (C) what all of the inputs and outputs are. (D) Your answer should discuss potential and kinetic energy in this process. Finally, (E) what would happen to this process if oxygen became unavailable, and why?

Answer 1

Cellular respiration occurs due to the existence of an electrochemical proton gradient in the cell, specifically the inner mitochondrial membrane. This is owing to a difference in hydrogen ion concentration.

During cellular respiration, three main steps occur.

1. Glycolysis: Where a glucose molecule is broken down to release a net energy of 2 ATP molecules. This occurs in the cytoplasm. The pyruvate molecules (2 molecules) produced at the end of glycolysis move into the mitochondria.

2.The citric acid cycle or Krebs cycle takes place in the mitochondria. At the end of this process, due to a series of oxidation-reduction reactions, NADH and FADH2 are produced. These are molecules carrying hydrogen ions and electrons, acting as a set of high energy batteries that create an electric potential across the membrane.

High energy electrons are released during oxidative phosphorylation of NADH and FADH2, which is passed through the electron transport chain along the inner mitochondrial membrane. This is transferred to carriers embedded with the membrane which are called complexes (I-IV). Complex I and II pass on electrons to Ubiquinone which then passes electrons to Complex III. It then passes to Cytochrome c to pass electrons to Complex IV which reduces oxygen to make water (using two nearby H+ ions). Complex II and III transports electrons by pumping H+ ions from within the mitochondria to the intermembrane space. This pumping is possible due to the release of energy due to the redox reactions that the carrier proteins carry out. This movement of H+ ions creates a potential across the membrane, and a pH gradient is created that stores potential energy. This gradient causes ATP synthase to get activated to pump the H+ ions back into the mitochondria. This is known as chemiosmosis.

As this happens, ADP gets converted to ATP through phosphorylation brought about by the protons being pumped through the membrane, converting the proton potential energy into kinetic energy. This process of creating ATP through chemiosmosis is known as oxidative phosphorylation. This process generates the majority of ATP in the mitochondria.

In the absence of oxygen, glycolysis is paused due to the lack of NAD molecules as NADH continues to build up. In order to regenerate NAD, the energy stored in NADH is used to convert pyruvate gets converted to lactic acid in animals or ethanol+carbon dioxide in plants. This process produces NAD and is called fermentation. NOw NAD is available for participation in glycolysis. Therefore, in anaerobic conditions, glycolysis + fermentations continues to take place, using the energy stored in NADH molecules, which when broken down release 2ATP - lower energy compared to the 36-38 ATP in cellular respiration.