Describe the respiratory balance sheet.

(N/A) It is possible to calculate the net gain of $ATP$ for every glucose molecule oxidized, but in reality, this remains a theoretical exercise. These calculations are based on the following assumptions:
$(1)$ There is a sequential, orderly pathway functioning, where one substrate forms the next, and glycolysis, $TCA$ cycle, and $ETS$ pathway follow one after another.
$(2)$ The $NADH$ synthesized in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
$(3)$ None of the intermediates in the pathway are utilized to synthesize any other compound.
$(4)$ Only glucose is being respired; no other alternative substrates enter the pathway at any intermediary stage.
However, these assumptions are not valid in a living system because:
- All pathways work simultaneously and not sequentially.
- Substrates enter and are withdrawn from pathways as and when necessary.
- $ATP$ is utilized as and when needed.
- Enzymatic rates are controlled by multiple mechanisms.
Despite this, the exercise is useful to appreciate the efficiency of living systems in extracting and storing energy. There is a net gain of $38$ $ATP$ molecules during the aerobic respiration of one molecule of glucose.
Comparison between Fermentation and Aerobic Respiration:

Fermentation	Aerobic Respiration
$(1)$ Glucose is partially decomposed; either ethanol or lactic acid is formed.	$(1)$ Complete decomposition occurs, forming $CO_{2}$ and $H_{2}O$.
$(2)$ Only two molecules of $ATP$ are produced per glucose molecule.	$(2)$ $A$ large amount of $ATP$ is produced.
$(3)$ Oxidation of $NADH$ to $NAD^{+}$ is a slow reaction.	$(3)$ Oxidation of $NADH$ to $NAD^{+}$ is a fast reaction.
$(4)$ Occurs in yeast, bacteria, and internal parasites.	$(4)$ Occurs in most higher organisms.