1) During glycolysis, link reaction and Krebs cycle the respiratory substrates are oxidised at several : steps and as a result many reduced coenzymes : NADH⁺ H⁺ and FADH₂ are produced.
2) These reduced coenzymes are transported to inner membrane of mitochondria which are converted back to their oxidised forms produce electrons and protons.
3) In mitochondria, the inner membrane is folded in the form of finger projections towards the matrix called cristae. In cristae many oxysomes (F₁ particles) are present which have electron transport carriers are present. According to Peter Mitchell's Chemiosmotic theory this I electron transport is coupled to ATP synthesis. : Electron and hydrogentproton) transport takes I place across four multiprotein complexes(I-IV).They are
1. Complex-I (NADH dehydrogenase):
i) It contains a flavoprotein(FMN) and I associated with non-heme iron Sulphur protein (Fe-S). This complex is responsible: for passing electrons and protons from : mitochondrial NADH (Internal) to Ubiquinone(UQ).
NADH + H⁺ + UQ⇌NAD⁺ + UQH₂
ii) In plants, an additional NADH dehydrogenase (External) complex is present on the outer surface of inner membrane of mitochondria which can oxidise cytosolic NADH ⁺ H⁺.
2. Complex-Il (Succinic dehydrogenase) i) It contains FAD flavoprotein is associated with non-heme iron Sulphur (Fe-S) protein. ii) This complex receives electrons and protons from succinate in Krebs cycle and is converted into fumarate and passes to ubiquinone.
Succinate + UQ⟶ Fumarate + UQH₂
3. Complex-Ill (C) tochrome bCI complex) This complex oxidises reduced ubiquinone (ubiquinol) and transfers the electrons through Cytochrome bc, Complex (Iron Sulphur center bCI complex) to cytochrome c.
UQH₂+2Cyt c_oxidised⇌UQ+2Cyt c_reduced+2H⁺
4. Complex IV (Cytochrome c oxidase) i) This complex contains two copper centers (A and B) and cytochromes a and a3. Complex IV is the terminal oxidase and brings about the reduction of 1/2 O₂ to H₂O. Two protons are needed to form a molecule of H₂O (terminal oxidation).
2Cyt c_oxidised+2H⁺+1/2O₂⇌2Cyt c_reduced+H₂O
ii) The transfer of electrons from reduced coenzyme NADH to oxygen via complexes I to IV is coupled to the synthesis of ATP from ADP and inorganic phosphate (Pi) which is called Oxidative phosphorylation. This occurs during Aerobic respiration Fof, - ATP synthase (also called complex V) consists of F0 and F₁. F₁ converts ADP and Pi to ATP and is attached to the matrix side of the inner membrane. F₀ is present in inner membrane and acts as a channel through which protons come into matrix.
a) Oxidation of one molecule of NADH ⁺ H⁺ = 3 molecules of ATP
b) oxidation of one molecule of FADH₂ = 2 molecules of ATP
iii) But the Mechanism of mitochondrial ATP synthesis is based on Chemiosmotic hypothesis. According to this theory electron carriers present in the inner mitochondrial membrane allow for the transfer of protons (H+).
iv) For the production of single ATP, 3 protons (H⁺) are needed. The terminal oxidation of external NADH bypasses the first phosphorylation site and hence only two ATP molecules are produced per external NADH oxidised through mitochondrial electron transport chain. Complete oxidation of a glucose molecule in aerobic respiration results in the net gain of 36 ATP molecules in plants.

Explanation with
Substrates
Enzymes
ATP, NADH₂, FADH₂

Glycolysis / EMP Pathyway:

External Factors:
1) Temperature: Optimum temperature for respiration is 30°C. At low temperatures and very high temperatures rate of respiration decreases
2) Oxygen: When sufficient amount of O₂ is available the rate of aerobic respiration will be optimum and anaerobic respiration is completely stopped. This is called Extinction point.
3) Carbon di oxide: High concentration of CO₂ reduces the rate of respiration
4) Salt respiration: A plant or tissue transferred from water to salt solution will increase the rate of respiration. It is called salt respiration.
5) Light: Light is an indirect factor affecting the rate of respiration
6) Water: Rate of respiration decreases with decreasing amount of water. Proper hydration is essential for respiration.
7) Inhibitors: Some chemical substances act as inhibitors. Ex: Cyanides
8) Wounds/Tissue damage: Wounding of plant organs stimulates the rate of respiration in that region.
Internal Factors:
1) Protoplasm: The amount of protoplasm and its state of activity influence the rate of respiration
2) Respiratory substrate: Concentration of respiratory substrate is proportional to the rate of respiration.