The part of cellular respiration that must occur before any other step can occur is glycolysis.

Here's why:

* Glycolysis is the breakdown of glucose. This is the starting point for cellular respiration, and it provides the pyruvate molecules that are necessary for the subsequent steps.

* Glycolysis does not require oxygen. This makes it the only step that can occur in both aerobic and anaerobic conditions.

Let me know if you'd like more detail about the steps in glycolysis!