Questions: What are substrates?

The answer is the first one: the reactants (ingredients) of enzyme-catalyzed reactions.

Explanation for each option:

1. The reactants (ingredients) of enzyme-catalyzed reactions: This is correct. In biochemistry, substrates are the specific reactants that an enzyme acts upon. Enzymes bind to substrates to catalyze chemical reactions, converting them into products.

2. A part of the lungs that helps with oxygen exchange: This is incorrect. Substrates are not related to lung anatomy or function. The parts of the lungs involved in oxygen exchange are alveoli and capillaries.

3. The products of chemical reactions: This is incorrect. Substrates are the starting materials or reactants in enzyme-catalyzed reactions, not the products. The products are the substances formed as a result of the reaction.

In summary, substrates are the reactants in enzyme-catalyzed reactions.