The process by which cells break down molecules to release energy without using oxygen is called anaerobic respiration.

Here's a breakdown:

* Anaerobic means "without oxygen."

* Respiration refers to the process of breaking down food molecules to release energy.

In anaerobic respiration, cells use other molecules, such as glucose, as their energy source and do not require oxygen to do so. This process is less efficient than aerobic respiration, which uses oxygen, but it allows cells to survive in environments where oxygen is limited.

There are two main types of anaerobic respiration:

1. Lactic acid fermentation: This occurs in muscle cells during intense exercise when oxygen supply is insufficient. Glucose is broken down into lactic acid, producing a small amount of ATP (energy).

2. Alcoholic fermentation: This occurs in yeast and some bacteria. Glucose is broken down into ethanol and carbon dioxide, also producing a small amount of ATP.

Key takeaway: Anaerobic respiration is a vital process for cells to survive in environments lacking oxygen, though it is less efficient than aerobic respiration.