Aerobic respiration occurs in normal conditions in human beings. Under intense conditions such as excercises heavy, muscles demand too much energy (ATP) and consume much more oxygen to produce that energy. This high consumption leads to oxygen scarcity and the muscle cells begin to make lactic acid by anaerobic respiration to fulfill their energetic needs. Similarly, yeast cells under deficient conditions of oxygen carry out anaerobic respiration, forming ethyl alcohol and $\mathrm{CO}_2$.

Aerobic respiration requires oxygen in order to generate ATP. Oxygen is strongly electronegative element and acts as final acceptor in respiratory process.

It pullse ${ }^{-}$(electrons) that energy from the electron transport chain ETC and take up protons from medium to form water.

$\mathrm{O}_2$ enters in the respiratory process at the end, though it's presence is vital. It drives the process of aerobic respiration by removing hydrogen from the system. Thus, acting as final hydrogen acceptor.

The energy is produced by the process of oxidative phosphorylation, utilising the energy of exidation reduction reactions.

(a) Respiration is a stepwise oxidation of organic molecules in a cell involving main three steps.

(i) Glycolysis (ii) Krebs' cycle (iii) Electron transport chain

Glucose passes through series of enzymatically controlled reactions and is finally converted into $\mathrm{H}_2 \mathrm{O}+$ ATP $+\mathrm{CO}_2$.

(b) Organic substances are the molecules normally found in living systems. They are usually composed of carbon atoms in rings or long chains to which other atoms such as hydrogen, oxygen and nitrogen are attached. e.g., glucose, fatty acids, amino acids etc. These molecules burnt as substrate to produce energy. Respiration of glucose and fatty acids is called floating respiration and respiration of protein and amino acids are called protoplasmic respiration.

Process of glycolysis is summarised as follow