The calculations of the net gain of ATP for every glucose molecule oxidised can be made on the following assumptions
(i) There is sequential pathway that follows, i.e., glycolysis, TCA cycle and ETS in cytoplasm, mitochondrial matrix and inner mitochondrial membrane respectively.
(ii) NADH, synthesised in glycolysis enters in to ETC for phosphorylation.
(iii) None of the intermediates in the pathway are utilised to synthesise any other compound.
(iv) Glucose forms respiratory substrate.
These assumptions are not valid for a living system because of following reasons
(i) These all pathways work simultaneously and do not take place one after the other.
(ii) ATP is utilised when needed.
(iii) Rate of enzyme actions is controlled by multiple means.
Comparisan between fermentation and aerobic respiration are as follows
| Fermentation | Aerobic Respiration |
|---|---|
| Fermentation is partial breakdown of glucose. Net gain of only 2 ATP. | It is complete breakdown of glucose. 38 ATP are produced. |
| Oxidation of NADH to NAD ${ }^{+}$is slow process. | It is a vigorous reaction in aerobic respiration. |
Glycolysis occurs in cytoplasm. One glucose molecule forms 2 pyruvic acid molecules.
In anaerobic conditions it forms 2 ATP and ethanol + water.
In aerobic conditions it form 36 ATP + water $+\mathrm{CO}_2$. The steps of glycolysis are as follows
