Fermentation follows glycolysis in some cells when oxygen is not available.

Answer

**Step-by-Step Solution:**

1. **Understanding Glycolysis**: Glycolysis is the first step in cellular respiration, where glucose is broken down into pyruvate, producing a small amount of ATP (energy) in the process. This occurs in the cytoplasm and does not require oxygen.

2. **Oxygen Availability**: Under aerobic conditions (when oxygen is present), pyruvate enters the mitochondria and undergoes further oxidation to produce more ATP through the Krebs cycle and oxidative phosphorylation.

3. **Anaerobic Conditions**: When oxygen is not available, cells cannot perform aerobic respiration. In such cases, they must rely on anaerobic processes to generate energy.

4. **Fermentation Process**: Fermentation is the process that follows glycolysis in the absence of oxygen. It allows for the regeneration of NAD+, which is essential for glycolysis to continue.

5. **Types of Fermentation**: There are two main types of fermentation:
- **Lactic Acid Fermentation**: In some cells (like muscle cells), pyruvate is converted into lactic acid. This process produces a small amount of ATP.
- **Alcoholic Fermentation**: In yeast and some bacteria, pyruvate is converted into ethanol and carbon dioxide.

6. **Energy Yield**: The energy yield from fermentation is significantly lower than that from aerobic respiration. While glycolysis produces 2 ATP molecules, fermentation only allows for a net gain of 2 ATP per glucose molecule.

7. **Conclusion**: Therefore, the statement "Fermentation follows glycolysis in some cells when oxygen is not available" is true, as fermentation is a necessary process to continue producing energy in the absence of oxygen.

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