Electron Transport System (ETS) and Oxidative Phosphorylation
The glucose molecule is completely oxidized by the end of the citric acid cycle. But the energy is not released unless NADH and FADH2 are oxidized through the electron transport system. At this stage, it is better to explain the meaning of oxidation in terms of electrons. Here, oxidation of a compound means removal of electrons from it. This is usually accompanied by the removal of hydrogen. Reduction means addition of electrons to a compound, usually accompanied by addition to hydrogen. The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS) and it is operative in the inner mitochondrial membrane. Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidized by an NADH dehydrogenase (complex I) and electrons are then transferred to ubiquionone located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH2 that is generated during oxidation of succinate, through the activity of the enzyme, succinate dehydrogenase (complex II) in the citric acid cycle. The reduced ubiquinone (ubiquinol) is then oxidized with the transfer of electrons to cytochrome c via cytochrome bc1 complex (complex III). Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3 and two copper centers.
When the electrons pass from one carrier to another via complex I and IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate. The number of ATP molecules synthesized depends on the nature of the electron donor. Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP. During electron transfer, the hydrogen atoms split into protons and electrons. The electrons are carried by the cytochromes. They recombine with their protons before the final stage, when hydrogen atom is accepted by oxygen to form water. Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor. The whole process by which oxygen effectively allows the production of ATP by phosphorylation of ADP, is called oxidative phosphorylation.
As mentioned earlier, the energy released during the electron transport system is utilized in synthesizing ATP with the help of ATP synthase (complex V). This complex consists of two major components, F1 and F0 . The F1 headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. F0 is an integral membrane protein complex that forms the channel through which protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic site of the F1 component for the production of ATP. For each ATP produced, 3H+ pass through F0 from the intermembrane space to the matrix down the electrochemical proton gradient.
Oxidation of glucose takes place by another pathway, which is called pentose phosphate pathway(PPP). In pentose pathway, glucose-6-phosphate(6C) produced during the early stages of glycolysis, or photosynthates produced during photosynthesis, are oxidised to give rise to 6-phosphogluconate. This reaction takes place in the presence of the enzyme,glucose-6-phosphate dehydrogenase, and generates NADPH. The 6-phosphogluconate molecule is further oxidised by the enzyme glucose-6-phosphate dehydrogenase. As a result of this, one molecule each of ribulose-5-phosphate, carbon dioxide and NADPH is produced. Ribulose-5-phosphate undergoes many changes to produce glycolytic intermediate such as glyceraldehyde-3-phosphate and fructose-6-phosphate. The reactions take place in the cell cytoplasm.
At given low concentration of CO2 and non-limiting light intensity, the photosynthetic rate of a given plant will be equal to the total amount of respiration. The atmospheric concentration of CO2 at which photosynthesis compensates for respiration, is referred to as CO2 compensation point. The carbon dioxide compensation point is reached when the amount of CO2 uptake is equal to that generated through respiration at a non-limiting light intensity. Net photosynthesis under these conditions is zero. In C3 plants, the CO2 compensation point is usually much higher than in C4 plants.