Final answer:
The substrate in the simulation is most likely a triglyceride, which is a type of lipid composed of three fatty acid units attached to glycerol.
Step-by-step explanation:
The substrate used in the simulation described is likely to be a triglyceride, which is a type of lipid. Lipids, such as animal fats and vegetable oils, are made up of triglycerides. These are esters composed of three fatty acid units joined to glycerol, a trihydroxy alcohol. Given the context of micelles facilitating absorption by microvilli, where fatty acids and monoglycerides diffuse out to form triglycerides, we can identify that fatty acids and glycerol are the relevant substances in the simulation.
Therefore, the simulation could be demonstrating the enzyme catalyzed breakdown (hydrolysis) or synthesis of triglycerides which would involve fatty acids and glycerol as substrates or products.