**Glycolysis:**
Glycolysis is the most common pathway for glucose oxidation in various organisms. It occurs in the cytoplasm and does not require oxygen. In this process, glucose is broken down into two molecules of pyruvate, resulting in the production of a small amount of ATP and NADH.

**Aerobic Respiration:**
Aerobic respiration takes place in the presence of oxygen and is a more efficient way of extracting energy from glucose. It occurs in the mitochondria and involves three main stages: pyruvate decarboxylation, the Krebs cycle, and the electron transport chain.

1. Pyruvate Decarboxylation: The pyruvate molecules produced in glycolysis are transported into the mitochondria. Each pyruvate undergoes decarboxylation, resulting in the release of carbon dioxide and the formation of acetyl-CoA. This step generates NADH.

2. Krebs Cycle: Acetyl-CoA enters the Krebs cycle, also known as the citric acid cycle. In this cycle, acetyl-CoA combines with oxaloacetate to form citrate. Through a series of reactions, carbon dioxide is released, and high-energy molecules such as NADH and FADH2 are produced.

3. Electron Transport Chain: The NADH and FADH2 generated in the previous steps donate their electrons to the electron transport chain located in the inner mitochondrial membrane. As electrons move through the chain, energy is released, which is used to pump protons across the membrane, creating an electrochemical gradient. This gradient drives the synthesis of ATP through ATP synthase.

**Anaerobic Respiration:**
In the absence of oxygen, some organisms can still generate energy through anaerobic respiration. The most common form is fermentation.

1. Alcoholic Fermentation: This occurs in yeast and some bacteria. Pyruvate is converted into ethanol, releasing carbon dioxide in the process. The NADH generated in glycolysis is oxidized back to NAD+, allowing glycolysis to continue and produce a small amount of ATP.

2. Lactic Acid Fermentation: This occurs in certain bacteria and animal cells, including muscle cells during intense exercise. Pyruvate is converted into lactic acid, regenerating NAD+ and allowing glycolysis to proceed.

Both forms of fermentation are less efficient than aerobic respiration, as they only produce a limited amount of ATP compared to the complete oxidation of glucose in the presence of oxygen.

Following are the different ways in which the breakdown of glucose takes place

Anaerobic Respiration

The anaerobic respiration process takes place in the absence of oxygen. For example: In yeast during fermentation. In this process, the pyruvate is converted into ethanol and carbon dioxide.

Aerobic Respiration

In aerobic respiration, breakdown of pyruvate takes place in the presence of oxygen to give rise to 3 molecules of carbon dioxide and water. The release of energy in aerobic respiration is much more than anaerobic respiration.

Lack of Oxygen

At times when there is a lack of oxygen in our body, mainly during vigorous activity, in our muscles, pyruvate is converted into lactic acid which is a 3 carbon molecule compound. The formation of lactic acid in muscles causes cramp.