Here's a breakdown:

* Aerobic Respiration:

* When oxygen is present, pyruvate is transported into the mitochondria and enters the Krebs Cycle (Citric Acid Cycle). This cycle produces 1 ATP, 3 NADH, and 1 FADH2 per pyruvate molecule.

* The NADH and FADH2 then enter the Electron Transport Chain, which is the primary ATP producer. The energy from these molecules is used to pump protons across the mitochondrial membrane, generating a proton gradient that drives ATP synthesis.

* The total theoretical ATP yield from one pyruvate molecule under aerobic conditions is approximately 12.5 ATP. This is based on the assumption that each NADH generates 2.5 ATP and each FADH2 generates 1.5 ATP in the electron transport chain.

* Anaerobic Respiration:

* In the absence of oxygen, pyruvate is converted to lactate in animals and ethanol in yeast. This process is called fermentation.

* Fermentation produces only 2 ATP molecules per glucose molecule, which is equivalent to 1 ATP per pyruvate molecule.

Important Note: These are theoretical maximum yields. The actual number of ATP molecules produced can vary depending on factors like efficiency of the electron transport chain, leakage of protons, and cellular conditions.

Let me know if you'd like more information about any of these processes!