The krebs cycle does NOT occur if

Krebs cycle does NOT occur in the absence of oxygen.

Explanation:

This is a cycle which occurs in mitochondrial matrix and generates chemical energy in the form of ATP, NADH. It produces carbon dioxide which we breathe out. This involves a series of reactions that reduces the "acetyl portion" of "acetyl coenzyme".

Kreb's cycle cannot produce sufficient carbon dioxide also the chemical energy (ATP) directly. It is the respiratory system which is responsible for production of large quantities of ATP and consumption of oxygen.

Respiratory cycle doesnt works in the absence of oxygen. Thus shutdowns the kreb cycle. This is the reason, this cycle is known as aerobic pathway for energy production.

Fermentation Occurs