Cells produce a small amount of ATP and lactic acid in the absence of oxygen through the process of respiration.

The process you're referring to is called anaerobic respiration. Here's how it works:

1. Glycolysis: This is the first step of both aerobic and anaerobic respiration. It occurs in the cytoplasm of the cell. One glucose molecule (6 carbons) is broken down into two molecules of pyruvate (3 carbons each). This process produces 2 ATP molecules and also generates 2 NADH molecules.

2. In the presence of oxygen, these pyruvate molecules would go on to the Krebs cycle for further breakdown. But in the absence of oxygen, the cell undergoes anaerobic respiration.

3. Lactic Acid Fermentation: In human cells, the pyruvate from glycolysis is converted into lactic acid, and the NADH is oxidized back into NAD+. This process does not produce any ATP, but it allows glycolysis (which does produce ATP) to continue by recycling NAD+.

4. The lactic acid can build up in the muscle tissue causing muscle fatigue. However, it can be transported to the liver and converted back into glucose when more oxygen is available.

So, in the absence of oxygen, cells can still produce a small amount of ATP through glycolysis and maintain this production by converting the resulting pyruvate into lactic acid.