* ATP production in respiration: Cellular respiration is the process that breaks down glucose (sugar) to generate energy in the form of ATP. There are two main types:

* Aerobic respiration: Requires oxygen and produces a much larger amount of ATP (around 36-38 ATP molecules per glucose).

* Anaerobic respiration: Occurs in the absence of oxygen and is less efficient, yielding only a small amount of ATP (2 ATP molecules per glucose).

Why only 2 ATP in anaerobic respiration?

* Glycolysis: The first stage of both aerobic and anaerobic respiration is glycolysis, which happens in the cytoplasm. Glycolysis itself produces 2 ATP.

* Electron transport chain: The second stage, the electron transport chain, occurs in the mitochondria and is where the majority of ATP is produced. This process *requires* oxygen as the final electron acceptor.

Yeast and bacteria can both perform both aerobic and anaerobic respiration. When oxygen is scarce, they switch to anaerobic respiration, producing only the 2 ATP from glycolysis.