Which of the following statements correctly describes how enzymes affect chemical reactions in living cells?

Introduction / Context:
Enzymes are biological catalysts that play a central role in almost every chemical reaction inside living cells. They make reactions happen fast enough to sustain life. Exam questions often test your understanding of two key features: their effect on activation energy and their catalytic efficiency. This question asks you to judge whether each of two common statements about enzymes is correct and to choose the best combined answer.

Given Data / Assumptions:

• Statement A says enzymes lower activation energy.
• Statement B says enzymes speed up reaction rate and are not used up.
• We assume a basic understanding of energy diagrams and enzyme catalysis.

Concept / Approach:
Activation energy is the minimum energy barrier that reactant molecules must overcome for a reaction to proceed. Enzymes bind substrates at their active site and stabilise the transition state, effectively lowering the activation energy. By lowering this barrier, enzymes increase the rate at which reactions reach completion under physiological conditions. Importantly, enzymes emerge unchanged after each reaction cycle and can be reused many times. They do not alter the overall energy change of the reaction or the final equilibrium; they only speed up how quickly equilibrium is reached.

Step-by-Step Solution:
Step 1: Evaluate statement A: enzymes lower the activation energy. This is correct because enzymes stabilise the transition state and provide an alternative pathway with a lower energy barrier. Step 2: Evaluate statement B: enzymes speed up the rate of reaction without being used up. This is also correct; as catalysts, enzymes increase reaction rate and are regenerated at the end of each reaction cycle. Step 3: Since both statements are accurate descriptions of enzyme action, the best combined answer is that both A and B are correct.

Verification / Alternative check:
Energy diagrams in biochemistry show a high activation energy peak for uncatalysed reactions and a lower peak when an enzyme is present, while the starting and ending energy levels remain the same. Experimental measurements confirm that reaction rates increase dramatically when specific enzymes are added, even at low concentrations, and that the enzyme molecules can be recovered intact after the reaction. This evidence supports both statements about lowering activation energy and increasing rate without being consumed.

Why Other Options Are Wrong:

• Only statement A correct: This ignores the equally important fact that enzymes speed up reactions and are not consumed.
• Only statement B correct: This fails to recognise that lowering activation energy is the mechanism by which enzymes speed up reactions.
• Neither statement correct: This is clearly wrong because both statements are well established properties of enzymes.

Common Pitfalls:
Some learners think that enzymes change the overall energy released or absorbed in a reaction or that they are used up like reactants. It is important to remember that enzymes change only the speed of the reaction, not the total energy change or equilibrium position, and that they are regenerated at the end of each reaction cycle. Visualising enzymes as reusable tools that make it easier for molecules to react can help anchor these concepts.

Final Answer:
Both key statements are correct: enzymes lower the activation energy required for a reaction and speed up the rate of that reaction without being used up.