Electrons Transport System (ETS) and Oxidative Phosphorylation

Terminal oxidation takes plae is two steps i.e., electron transport and oxidative phosphory-lation.

Electron transport system (ETS):

Through which the electron passes from one carrier to another and takes place in the inner mitochondrial membrane. ETS consists of flavins, FeS complex, quinones (Ubiquinone or CoQ) and cytochromes (cyt b, cyt c, cyt c] cyt a and cyt a). Cytochromes possess iron which can undergoes Fe²⁺ -> Fe³⁺ -> Fe^2- change during passage of electrons, cyt a, additionally contains copper which helps in transferring electrons to oxygen.

Complex I:

NADH dehydrogenase which oxidises electrons from NADH produced in the mitochondrial matrix, during Kreb’s cycle and electrons are then transferred to ubiquinone in the inner membrane.

Complex II:

Succinate dehydrogenase –

Ubiquinone receives electrons from NADH and also by reducing equivalents via FADH, which is generated during oxidation of succinate, through the activity of the enzyme succinate dehydrogenase.

Complex III:

Dihydroubiquinone, cytochrome oxidoreductase and two forms of cytochrome b and cytochrome c. The reduced ubiquinone (ubiquinol) is oxidised with the transfer of electrons to cytochrome c via this complex.

Complex IV:

Cytochrome c, Cytochrome oxidase, cytochrme a, cytochrome a, and two copper centres : Electrons are handed over to cyt C₁ –> cyt c –> cyt a cyt a₃. Both cyt a and cyt a fuction as cytochrome oxidase and handover electrons to oxygen. Reduced coenzyme NADH + H⁺ helps is pushing out 3 pairs of H⁺ to outer chamber while FADH, sends two pairs of H⁺ to outer chamber.

Cytochrome c acts as mobile carrier for transfer of electrons be’tween complex III and IV.

Complex V:

ATP synthase-Electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase for the production of ATP from ADP and inorganic phosphate.

3 pairs of protons (6H⁺) are transported from mitochondrial matrix to outer side of the inner mitochondrial membrane. The energy released during electron transfer is used for this work.

Oxidative phosphorylation:

The accumulation of protons outside the inner membrane results in their higher concentration outside the inner membrane than in the matrix creating a proton gradient (A.pH) and electric potential (∆Ψ) across the membrane.

Energy liberated during electron transport in used is building a proton gradient or proton motive force (PMF) in the outer membrane.

The process of synthesis of ATP at the expense of proton motive force is called chemiosmosis which is explained by Mitchell (1961, Nobel Prize 1978).

Protons cannot diffuse back into the matrix across the membrane but can enter the membrane via a proton channel established by the membrane bound adenosine triphosphatase (ATPase).

ATPase is a multienzyme complex containing two parts F and F₁ (acting as ATP sythethase) catalyses the synthesis of ATP from ADP For each pair of protons flowing back into the matrix, one molecule of ATP is synthesized.

F₀ is an integral membrane protein complex that forms the channel through which protons cross the inner membrane.

FADH, donates its electrons to CoQ and not to FMN, only 4 protons are transported outside the membrane. Therefore, the back flow of these 4 protons through F₀ – F complex results in the formation of only 2 ATP molecules.

Total 38 ATP molecules are yielded during aerobic respiration of glucose. In most eukaryotic cells, 2 ATP molecules are expended in transporting the NADH produced in glycolysis into the mitochondrion for further oxidation in such cells, the net gain of 36 ATP molecules instead of 38.

Pentose pathway (Pentose phosphate pathway (PPP) or Hexose monophosphate (HMP) shunt or Warburg Dickens pathway

i. Takes place mainly in liver, lactating mammary glands, adipose tissues, adrenal cortex, gonads and erythrocytes.

ii. Takes place in the cytoplasm like glycolysis.

iii. This starts with a 6-carbon sugar, glucose 6- phosphate formed by phosphorylation of simple glucose with the help of hexokinase and ATP, and produces 5-carbon sugar, ribose 5-phosphate.

iv. Six molecules of glucose produce via HMP shunt as much energy as produced by one glucose molecule via glycolysis and Kreb’s cycle.

v. Ribulose 5-phosphate formed in this pathway is of major importance in reaction involved in photosynthesis.

Compensation Point :

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.

CO₂ compensation point:

The atmospheric concentration of CO₂ at which photosynthesis just compensates for respiration

CO₂ compensation point is reached when the amount of CO₂ uptake is equal to that generated through respiration at a non limiting light intensity.

The CO₂ compensation point is usually much higher in C, plants (25-100 µ1.1^-1) as compared to C₄ plants (less than 5 µ1.1^-1).