Electron Transport System (ETS) & Oxidative Phosphorylation

i. All the reduced hydrogen acceptors like NADH + H⁺ and FADH₂ move to the ETS where they release their hydrogen and get reoxidised to NAD⁺ & FAD⁺, so that they can again enter into the respiration process.

ii. ETS is the chain of some hydrogen and electron carriers present in the inner mitochondrial membrane.

iii. The significance of ETS is to remove hydrogens from reduced hydrogen acceptors NADH + H⁺ & FADH₂. During this process, hydrogen acceptors get reoxidised and ATP are produced.

Mechanism  

i. Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidised by complex I, and electrons are then transferred to ubiquinone located within the inner membrane. 

ii. Ubiquinone also receives reducing equivalents via complex II that is generated during oxidation of succinate in the citric acid cycle. 

iii. The reduced ubiquinone (ubiquinol) is then oxidised with the transfer of electrons to cytochrome c via complex III. 

iv. 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

v. When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate. 

vi. The number of ATP molecules synthesised 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. 

vii. 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.

• O₂ is last H acceptor in oxidative phosphorylation & due to its reduction, water is formed. 

• During each step of mitochondrial ETS, redox reaction occurs and energy is released which is utilized in creation of proton gradient for the synthesis of ATP in presence of oxygen (oxidative phosphorylation).

Passage of 4H⁺ through F₀ particle or proton channel leads to synthesis of 1 ATP.

1.0Shuttle System

Cytosolic or extra mitochondrial or glycolytic NADH transported to ETS by two type of shuttles (Only in eukaryotes):

(a) Glycerol phosphate shuttle 2NADH+2H⁺ ⎯→ 2FADH₂

(b) Malate aspartate shuttle 2NADH+2H⁺ ⎯→ 2NADH₂

In prokaryotes, shuttle mechanism is absent. They always get 38 ATP from aerobic respiration of 1 glucose.

2.0Mechanism of Cellular Respiration

Glycolysis/EMP (Embden, Meyerhof, Parnas) Pathway

i. The term glycolysis has originated from the Greek words, glycos for sugar, and lysis for splitting. The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway. Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms. 

ii. In all living organisms; whether it is aerobic or anaerobic, the first step in cellular respiration is partial breakdown or oxidation of glucose into two molecules of pyruvic acid and it is called glycolysis. 

iii. The glycolysis is a common phase between aerobic & anaerobic respiration. In anaerobic organisms, it is the only process in respiration.

iv. In plants, the glucose is derived from sucrose (product of photosynthesis) or from starch (storage carbohydrate). 

v. Sucrose is converted into glucose and fructose by the enzyme invertase and these two monosaccharides enter the glycolytic pathway.

vi. Glycolysis occurs in the cytoplasm of the cell. This process is independent of oxygen that is it can occur in both conditions, it means in the presence of O₂ or absence of O₂. 

vii. It involves a series of ten biochemical enzymatic reactions in the cytoplasm.

viii. Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate and fructose-6-phosphate respectively by the activity of the enzyme hexokinase. This phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. Subsequent steps of metabolism of glucose and fructose are same. 

ix. In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose.

• In glycolysis, no consumption of oxygen & no liberation of CO₂ takes place.
• In glycolysis, 1 glucose (6C) break down into 2 molecule of pyruvic acid (3C) (Partial oxidation).
• Pyruvic acid is the key product of glycolysis.
• 2 NADH+2H⁺, produced during the process enter into ETS (in mitochondria) to produce 4ATP (if glycerol phosphate shuttle is present) or 6ATP (if malate aspartate shuttle is present), this ATP formation is called oxidative phosphorylation.
• Substrate level phosphorylation–When the substrate releases energy for phosphorylation of ADP (formation of ATP) without ETS then this method of ATP formation is called as substrate level phosphorylation. It forms 4 ATP, 2ATP are consumed, so 2ATP are gained by substrate level phosphorylation. (Direct gain)

Fate of Pyruvic Acid

The fate of Pyruvic acid depends on the cellular need and availability of oxygen. There are three major ways in which different cells handle pyruvic acid produced by glycolysis. These are lactic acid fermentation, alcoholic fermentation and aerobic respiration. Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to CO₂ and H₂O, however, organisms adopt Kreb's cycle which is also called as aerobic respiration. This requires O₂ supply.

3.0Also Read