Cellular respiration is the process by which animals convert food into a type of energy usable by their cells, known as ATP. The first step of cellular respiration is called glycolysis and results in the formation of pyruvate.

Aerobic cellular respiration occurs when oxygen is present, and pyruvate will enter the Krebs cycle allowing the electron transport chain to proceed. Anaerobic cellular respiration does not require the presence of oxygen and pyruvate will undergo lactic acid fermentation. Comparing the result of aerobic and anaerobic respiration highlights why oxygen is so important for cellular respiration.

Diagram showing cellular respiration steps. Within a cell, carbohydrates are broken down to glucose. Glucose undergoes glycolysis to produce pyruvate, which enters the mitochondria. In the presence of oxygen, aerobic respiration proceeds. The pyruvate becomes acetyl Co A and enters the Krebs cycle. Krebs cycle products carry over to the electron transport chain, where oxygen is the final electron receptor. NADH produced during glycolysis also enters the electron transport chain. In the absence of oxygen, pyruvate undergoes fermentation to produce lactic acid, which builds up in the cell.

Figure 1: Cellular respiration steps.

Cellular respiration is the process that converts the energy from chemical bonds in food to a form of energy that the cell can use, ATP.