The breakdown of carbohydrates in the absence of oxygen is called anaerobic respiration. This process is less efficient than aerobic respiration, which uses oxygen, but it allows cells to generate a small amount of energy (ATP) even when oxygen is limited.

Here's a breakdown of the process:

1. Glycolysis: This is the first stage of both aerobic and anaerobic respiration, where glucose is broken down into pyruvate. This process produces a small amount of ATP (2 molecules) and NADH, a reducing agent.

2. Fermentation: This is the specific pathway used in anaerobic respiration to regenerate NAD+ from NADH. This is crucial because NAD+ is essential for glycolysis to continue. There are two main types of fermentation:

* Lactic Acid Fermentation: Pyruvate is converted into lactate. This occurs in muscle cells during intense exercise when oxygen supply is insufficient.

* Alcoholic Fermentation: Pyruvate is converted into ethanol and carbon dioxide. This is used by some microorganisms like yeast.

Key Points about Anaerobic Respiration:

* Low ATP yield: Only 2 ATP molecules are produced per glucose molecule, compared to 38 ATP in aerobic respiration.

* Incomplete breakdown of glucose: Glucose is not fully broken down, resulting in byproducts like lactate or ethanol.

* Occurs in oxygen-limited environments: This process is essential for organisms living in environments with low oxygen levels, such as muscle cells during exercise or in the intestines of humans.

* Can be harmful in excess: Lactic acid buildup in muscles can cause soreness and fatigue.

In summary, anaerobic respiration is a less efficient energy production process than aerobic respiration, but it is crucial for cells to survive in oxygen-limited environments.