What happens when substrates collide in cells?

• A. Only a small proportion of collisions lead to a reaction
• B. All collisions lead to products
• C. Collisions always require ATP
• D. Only a small proportion of collisions lead to a reaction, and enzymes remain unchanged after catalysis

Answer: (D)

When substrates collide, they must hit with enough energy and the correct orientation to push the reaction forward. In cells, many collisions are not productive, so only a small fraction actually leads to a reaction. Enzymes help by providing an active site where substrates bind to form an enzyme–substrate complex, which lowers the activation energy needed for the reaction to proceed. After the reaction occurs, the products are released and the enzyme is free to catalyze more reactions; the enzyme is not consumed in the process. This combination—only productive collisions occur and the enzyme remains unchanged after catalysis—is why the statement that describes both aspects best fits what happens.