Krebs’ cycle was discovered by Krebs in pigeon muscles in 1940. Which step is called gateway step/link reaction/transition reaction in respiration?
a)	Glycolysis	b)	Formation of acetyl Co-A
c)	Citric acid formation	d)	ETS terminal oxidation

(b)

If oxygen is not available, pyruvic acid undergoes anaerobic respiration/fermentation, but under aerobic condition, the pyruvic acid enters into mitochondria and converted to Acetyl Co-A. Acetyl Co-A functions as substrate entrant for Krebs’ cycle so, a connecting link between glycolysis and Krebs’ cycle.

Glycolysis is the process of breakdown of glucose (hexose sugar) to two molecules of pyruvic acid through a series of enzyme mediated reactions. It occurs in cytoplasm and is common both to aerobic and anaerobic respiration. Last product is pyruvic acid.