Which of the following best defines a substrate in the context of metabolism?

In the context of metabolism, a substrate is best defined as a starting material for a biochemical reaction. This is crucial because substrates are the specific reactants that enzymes act upon during metabolic processes. Enzymes bind to substrates at their active sites, facilitating various chemical reactions necessary for metabolism.

Understanding the role of substrates is fundamental in biochemistry, as they undergo transformations and are converted into products through these enzymatic reactions. For example, in cellular respiration, glucose acts as a substrate that undergoes several metabolic pathways, ultimately leading to the production of energy.

The other options, while related to metabolic processes, do not accurately capture the definition of a substrate. A product refers to the end result of a reaction, waste products are byproducts that are expelled, and enzyme inhibitors are substances that decrease the activity of enzymes rather than serving as starting materials for reactions.