Final answer:
Lactose is not a direct substrate for glycolysis; it must be broken down into its monosaccharides, glucose, and galactose, to be utilized in the pathway. Other mentioned sugars such as galactose, mannose, and fructose can enter the glycolytic pathway after being converted into appropriate intermediates.
Step-by-step explanation:
In glycolysis, the substrate that is not used directly is C) lactose. Glycolysis is a metabolic pathway that involves the degradation of glucose and similar sugars to produce energy in the form of ATP. While glucose is the primary molecule used in glycolysis, other sugars such as galactose (A), mannose (B), and fructose (D) can be converted into intermediates that enter the glycolytic pathway. However, lactose itself is not a direct substrate for glycolysis; it must first be broken down into its constituent monosaccharides, glucose, and galactose, before it can participate in the energy-yielding pathway.
Lactose is a disaccharide consisting of glucose and galactose. Glycolysis starts with glucose or a molecule that can be transformed into an intermediate of the pathway. Galactose, after conversion to glucose-1-phosphate, can be funneled into the glycolytic pathway. Similarly, fructose can be phosphorylated to fructose-6-phosphate, an intermediate in glycolysis. In contrast, mannose can be converted to mannose-6-phosphate then to fructose-6-phosphate to enter the pathway.