What occurs during glycolysis?

• A. A series of enzyme-controlled chemical reactions that occur in the mitochondria to produce acetyl-CoA.
• B. A process that converts glucose to ethanol.
• C. The net yield of ATP from glycolysis is four.
• D. A series of enzyme-controlled chemical reactions in the cell's cytoplasm; each glucose molecule is split in half to produce two pyruvic acid molecules, four ATP molecules, and two NADH molecules (2 ATP used to split glucose, so the net ATP yield is 2 ATP).

Answer: (D)

Glycolysis is a cytoplasmic, enzyme-driven sequence that breaks one glucose molecule into two pyruvate molecules. It starts by investing a couple of ATP, then later steps generate energy as ATP and NADH. The net result per glucose is two ATP and two NADH, while two pyruvate molecules are formed for use in further energy-producing processes. Since it happens in the cytoplasm, not the mitochondria, and it provides the initial inputs for either aerobic respiration or fermentation depending on oxygen availability, this description fits glycolysis precisely. The two pyruvate molecules produced, along with the net two ATP and two NADH, are the key outcomes.