Question

What is substrate level and respiration-linked phosphorylation,

• What is oxidative phosphorylation, how is it different from photophosphorylation.

• What is the chemiosmotic theory

• describe the electron transport chain (ETC) with different complexes and components, what are the electron carries, what forms are electron moved etc.

• explain the Q cycle •

Describe the different cytochromes and iron-sulfur proteins of the ETC

• What is proton motive force, chemical potential and electrical potential

• describe the structure of ATP synthase and how it functions

• explain how 30 and 32 ATPs are synthesized from one glucose. What mechanism are used

• explain the P/O ratio

• How can the ETC be uncoupled to generate heat

Answer 1

Phosphorylation involves transfer of phosphate group from one compound to another which ultimately leads to formation of Adenosine triphosphate (ATP) which is used by organisms in form of energy.

• Substrate Level Phosphorylation is a direct phosphorylation of ADP (Adenosine di phosphate) with a phosphate group to give ATP and this generally occurs during Glycolysis process (in cytoplasm). Oxidative Phosphorylation (or respiratory linked phosphorylation) is the production of ATP from the oxidized NADH and FADH which takes place during the process of aerobic cellular respiration (inside mitochondria).
• Photophosphorylation, is another type of phosphorylation which occur during Photosynthesis inside thylakoids in chloroplasts present in plants. The energy source of Oxidative Phosphorylation is glucose while the energy source of photophosphorylation is sunlight.
• The chemiosmotic theory explains generation of ATP via the electron transfer chain (ETC) in mitochondria. This theory suggests that ATP synthesis in respiring cells occurs as a result of concentration gradient across the membranes of mitochondria. ETC is an oxidative phosphorylation reaction takes place in the inner membrane of the mitochondria. Glucose is metabolised to give acetyl coenzyme A (acetyl-CoA) intermidiate whose oxidation in the mitochondrial matrix is coupled to the reduction of a carrier molecules. Carrier molecules invovled are nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide FAD. NADH and FADH₂ looses electrons which are transferred along the chains and a certain amount of energy is released which is used to pump the protons to move from the matrix of the mitochondria to the inner membrane space against concentration gradient. Eventually, there concentration of protons in the membrane space becomes high and the protons flow back into the matrix of the mitochondrion via ATP synthase. This provides enough energy for ADP to combine with inorganic phosphate to form ATP. At the last pump in the ETC, electrons and protons are taken up by oxygen to form water.

The components of ETC are contained in four large protien complexes embedded in the inner mitochondrial membrane:

Complex I ( NADH-Q oxidoreductase): where electrons transfer from NADH to coenzyme Q

Complex II ( Succinate-Q reductase): some substrates with more positive redox potential than NAD⁺/NADH pass electron to Q via this complex

Complex III (Q-cytochrome c oxidoreductase): passes eectrons to cytochrome c

Complex IV (cytochrome c oxidase): this completes the chain by passing electrons to molecular oxygen which is then reduced to form water.