MISA Biology Practice Exam 2025 - Free Biology Practice Questions and Study Guide

In glycolysis, glucose is the primary reactant. This metabolic process takes place in the cytoplasm of cells, where one molecule of glucose—a six-carbon sugar—undergoes a series of enzymatic reactions and is eventually broken down into two molecules of pyruvate, which are three-carbon compounds.

During these reactions, glucose is phosphorylated and modified through various steps, which require energy investment initially—hence, ATP is also involved, but it is not a reactant in the way that glucose is.

Carbon dioxide and oxygen do not play a direct role in glycolysis. Carbon dioxide is produced in later stages of cellular respiration, such as in the citric acid cycle during aerobic respiration, but it is not a starting material for glycolysis. Similarly, oxygen is not utilized directly in glycolysis, although it is crucial in later stages of cellular respiration when pyruvate is further processed in the presence of oxygen. Thus, glucose stands out as the essential reactant that kicks off the glycolytic pathway.