Aerobic respiration :
- In aerobic respiration, glucose is oxidized in three steps.
- Glycolysis
- Tricarboxylic acid cycle (Krebs cycle)
- Electron transfer chain reaction
1. Glycolysis :(EMP pathway)
- Occurs in cytoplasm.
- A molecule of glucose is oxidized step by step in this process and two molecules of each i.e. pyruvic acid, ATP, NADH2 and water are formed.
- Molecules of pyruvic acid formed in this process are converted into molecules of Acetyl-Coenzyme-A.
- Two molecules of NADH2and two molecules of CO2 are released during this process.
2. Tricarboxylic acid cycle (Krebs cycle) :
- Both molecules of acetyl-CoA enter the mitochondria.
- Cyclic chain of reactions called as tricarboxylic acid cycle is operated on it in the mitochondria.
- Acetyl part of acetyl-CoA is completely oxidized through this cyclical process and molecules CO2, H2O, NADH2, FADH2 are derived.
3. Electron transfer chain reaction :
- Molecules of NADH2 and FADH2 participate in ETS.
- 3 molecules of ATP are obtained from each NADH2 molecule and 2 molecules of ATP from each FADH2 molecule.
- Water molecules are also formed.
- Operated in mitochondria only.
Thus, a molecule of glucose is completely oxidized in aerobic respiration and molecules of CO2 and H2O are produced along with energy.
- If there is insufficient amount of carbohydrates in body due to exceptional conditions like fasting and hunger, then lipids and proteins are used for energy production.
- In case of lipids, they are converted into fatty acids.
- Proteins are converted into amino acids.
- Fatty acids and amino acids are converted into acetyl-CoA.
- Energy is obtained through complete oxidation of acetyl-CoA by the process of Krebs cycle in mitochondria.
ATP: 
- Adenosine triphosphate is energy-rich molecule.
- Energy is stored in the bonds by which phosphate groups are attached to each other.
- These molecules are stored in the cells as per need.
- Chemically, ATP is triphosphate molecule formed from adenosine ribonucleoside.
- It contains
- A nitrogenous compound-adenine
- Pentose sugar- ribose
- Three phosphate groups.
- As per the need, energy is derived by breaking the phosphate bond of ATP; hence ATP is called as ‘energy currency’ of the cell.
+ Point
- NAD - Nicotinamide Adenine dinucleotide
- FAD - Flavin adenine dinucleotide
Both coenzymes are formed in the cells and used in cellular respiration
