Which of the following processes occurs during the first stage of cellular respiration (glycolysis)?

• A. Oxygen is used to produce water.
• B. Glucose is broken down into two molecules of pyruvate.
• C. Carbon dioxide is released from the mitochondria.
• D. ATP is produced by the electron transport chain.

Answer: (B)

Verified step by step guidance

1. Understand that glycolysis is the first stage of cellular respiration, which occurs in the cytoplasm of the cell and does not require oxygen (anaerobic process).
2. Recognize that the main purpose of glycolysis is to break down one molecule of glucose (C₆H₁₂O₆) into two molecules of pyruvate (C₃H₄O₃), which are three-carbon compounds.
3. Note that during glycolysis, a small amount of ATP is produced directly through substrate-level phosphorylation, and NAD⁺ is reduced to NADH, which will be used later in cellular respiration.
4. Eliminate the incorrect options: Oxygen is not used in glycolysis, carbon dioxide is not released during this stage, and ATP production by the electron transport chain occurs in a later stage of cellular respiration (oxidative phosphorylation).
5. Conclude that the correct process occurring during glycolysis is the breakdown of glucose into two molecules of pyruvate.