Respiration is an energy releasing adn enzymatically controlled catabolic process which involves a step-wise oxidative breakdown of organic process which involves a stet-wise oxidative breakdown of organic substances indside living . In this statament about respiration explain the meaing of
(a) Step-wise breakdown
(b) Organic substances (used as substrates).

Explanation

(a) Respiration is stepwise oxidation of organic molecules in a cell involving main three steps.
(i) Glycolysis (ii) Kreb's cycle (iii) Eelectrons transport chain
Glucose passes through series of enzymatically controlled reactions and is finally converted into `H_(2)O+ATP+CO_(2)`.
(b) Organic sunstances are the molecules normally found in living system. They are usually composed of carbon in rings or long chains to which other atoms such as hydrogen . Oxygen and nitrogen adn respiration e.g., glucose, fatty acids, amino etc.
These molecules burnt as substrate to produces energy. Respiration of glucose and fatty acids is called floating respiration and respiration of protein and amino acids are called protoplasmic respiration.