The part of cellular respiration in which glucose is broken down is called

Glycolysis

Explanation:

In cellular respiration Glycolysis is an anaerobic process in which one molecule of glucose is broken down into two molecules of pyruvate and two net ATP and 2 NADH are produced.

For complete oxidation of glucose in an aerobic condition pyruvate is converted to a two carbon acetyl CoA which enters the TCA or Krebs Cycle.

In the TCA cycle during cellular respiration acetyl CoA is completely oxidized to a carbon molecule CO2, and produces energy rich molecules that are used in the electron transport chain.