Which substance serves as the substrate for glycolysis?

Glycolysis is a metabolic pathway that is essential for converting glucose into pyruvate, resulting in the production of ATP and NADH. In this process, glucose, a six-carbon sugar, undergoes a series of enzymatic reactions that break it down into two molecules of pyruvate, which can then enter various metabolic pathways depending on the availability of oxygen. This pathway is crucial for both aerobic and anaerobic respiration.

While fructose can also be utilized in glycolysis, it needs to be converted into intermediates first. Lactate is a product of anaerobic respiration and not a substrate for glycolysis. Similarly, citric acid, which is part of the citric acid cycle, is not involved in the glycolytic pathway as a substrate. Glucose is the primary and direct substrate for glycolysis, making it the correct choice for this question.