Tuesday, September 15, 2020

Respiration in Plants - Notes | Class 11 | Part 4: Aerobic Respiration

14. RESPIRATION IN PLANTS 

AEROBIC RESPIRATION


It is a complete oxidation of organic substances in the presence of oxygen releasing CO2, water & energy.It occurs in mitochondria.

For this, the pyruvate (final product of glycolysis) is transported from the cytoplasm into the mitochondria.

The crucial events in aerobic respiration are:
  • Complete oxidation of pyruvate by stepwise removal of all the hydrogen atoms, leaving 3 CO2 molecules. It takes place in the matrix of mitochondria.
  • Passing on of electrons removed as part of H-atoms to molecular O2 with simultaneous synthesis of ATP. It occurs on the inner membrane of mitochondria.
Pyruvate (pyruvic acid) enters mitochondrial matrix and undergoes oxidative decarboxylation in presence pyruvic dehydrogenase. It needs coenzymes, NAD+ & Coenzyme A.

During this process, 2 NADH molecules are produced from 2 pyruvic acid molecules.


Acetyl CoA then enters tricarboxylic acid (TCA) cycle.

Tricarboxylic Acid Cycle 
(Krebs’ cycle or Citric acid cycle) 

TCA cycle was first elucidated by Hans Krebs.

Steps:
  1. Condensation of acetyl group with oxaloacetic acid (OAA) & water to form citric acid in presence of citrate synthase enzyme. A CoA molecule is released.
  2. Citrate is isomerised to isocitrate.
  3. Decarboxylation of isocitrate to a-ketoglutaric acid.
  4. Decarboxylation of a-ketoglutaric acid to succinyl-CoA.
  5. Succinyl-CoA is converted to succinic acid and a GTP molecule is synthesised (substrate level phosphorylation). In a coupled reaction, GTP is converted to GDP with simultaneous synthesis of ATP from ADP.
  6. Oxidation of succinate to Fumarate and then to Malate.
  7. Oxidation of malate to OAA.

At 3 points of TCA cycle, NAD+ is reduced to NADH + H+. At one point, FAD+ is reduced to FADH2. 

Continued oxidation of acetyl CoA via TCA cycle requires continued replenishment of OAA. It also requires regeneration of NAD+ & FAD+ from NADH & FADH2.

Summary equation of Krebs’ cycle:


Thus, a glucose is broken down to give 6 CO2, 8 NADH+H+, 2 FADH2 and 2 ATP.

Electron Transport System (ETS) & Oxidative Phosphorylation 

Electron transport system (ETS) is the metabolic pathway present in the inner mitochondrial membrane through which electron passes from one carrier to another.

This is to release and utilize energy stored in NADH+H+ and FADH2 (formed during TCA cycle) by oxidation.

The electrons are passed on to O2 to form H2O.

Electrons from NADH are oxidised by an NADH dehydrogenase (complex I).

Electrons are then transferred to ubiquinone (UQ) located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH2 (complex II) that is generated during oxidation of succinate in citric acid cycle.

The reduced ubiquinone (ubiquinol or UQH2) is then oxidised 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. It acts as a mobile carrier of electrons between complex III and IV.

Complex IV (cytochrome c oxidase) contains cytochromes a & a3, and 2 copper centres.

When the electrons pass from one carrier to another via complex I to IV, they are coupled to ATP synthase (complex V) for the ATP production.


Number of ATP molecules produced depends on nature of electron donor.

Oxidation of 1 NADH → 3 ATP 

Oxidation of 1 FADH2 → 2 ATP 

In aerobic respiration, the role of oxygen is limited to the terminal stage. Yet, oxygen is vital since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.

In respiration, energy of oxidation-reduction is utilised for the phosphorylation. So this process is called oxidative phosphorylation. It is not as photophosphorylation (Here, light energy is utilised to produce proton gradient for phosphorylation).

The energy released during the ETS is utilized to synthesize ATP by ATP synthase (complex V).

ATP synthase has two major components: F1 & F0.
  • F1 headpiece (peripheral membrane protein complex): Site for ATP synthesis from ADP & inorganic phosphate.
  • F0 (integral membrane protein complex): It forms a channel through which protons cross the inner membrane. The passage of protons is coupled to the catalytic site of the F1 component for ATP production.
Diagrammatic presentation of ATP synthesis in mitochondria 

For each ATP produced, 2H+ passes through F0 from the inter-membrane space to the matrix down the electrochemical proton gradient.
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