In biochemistry, an enzyme speeds up a chemical reaction inside cells mainly by doing which of the following to the activation energy of that reaction?

Introduction / Context:
Enzymes are biological catalysts found in all living cells. They allow chemical reactions that would otherwise be too slow at normal body temperatures to proceed rapidly and efficiently. A central idea in understanding enzyme action is activation energy, the minimum energy required for reactant molecules to transform into products. This question tests whether learners know how enzymes change activation energy to speed up reactions.

Given Data / Assumptions:
- The question is about the effect of enzymes on activation energy.
- Activation energy is the energy barrier that must be overcome for a reaction to proceed.
- We assume normal cellular conditions where enzymes function without being permanently changed.
- Options include raising, absorbing, lowering, or not affecting activation energy.

Concept / Approach:
In chemical kinetics, the rate of a reaction depends on how many reactant molecules have enough energy to reach the transition state and form products. The activation energy is the height of this energy barrier. Enzymes speed up reactions by providing an alternative reaction pathway with a lower activation energy. By stabilising the transition state or bringing reactants together in the correct orientation, enzymes reduce the energy required to start the reaction. As a result, at a given temperature, more molecules can overcome the barrier and react per unit time. Enzymes do not raise activation energy; that would slow reactions. They also do not simply absorb energy and stop reactions. Therefore, the correct statement is that enzymes lower the activation energy so that more reactant molecules can react.

Step-by-Step Solution:
Step 1: Recall that activation energy is the minimum energy needed for reactant molecules to form products. Step 2: Understand that a high activation energy means fewer molecules have enough energy, so the reaction is slow. Step 3: Remember that enzymes are catalysts, and catalysts work by lowering the activation energy of a reaction. Step 4: Recognise that lowering activation energy allows more molecules to reach the transition state at the same temperature. Step 5: Choose the option that correctly states that enzymes lower the activation energy so that more reactant molecules can react.

Verification / Alternative check:
Biology and chemistry textbooks describe enzyme action using energy diagrams where the curve with an enzyme shows a lower peak (activation energy) compared with the uncatalysed reaction. The overall energy difference between reactants and products remains the same, but the barrier is reduced. Experiments show that enzyme catalysed reactions proceed much faster at body temperature than the same reactions without enzymes, supporting the idea that activation energy is lowered. This consistent description confirms that lowering activation energy is the key function of enzymes in speeding up reactions.

Why Other Options Are Wrong:
- Raising the activation energy required so that fewer molecules can react would slow the reaction instead of speeding it up.
- Absorbing energy so that the reaction stops completely does not describe catalysis; enzymes help reactions proceed, they do not block them.
- Having no effect at all on the activation energy of the reaction cannot explain the large increase in reaction rate observed in the presence of enzymes.

Common Pitfalls:
Some learners may mistakenly think that enzymes simply “add energy” to reactants, but enzymes do not supply energy like fuel. Instead, they rearrange the pathway of the reaction to require less energy to start. Others may confuse activation energy with the total energy released or absorbed by the reaction, but catalysts do not change the overall energy balance. Remembering that enzymes lower the activation energy while remaining unchanged themselves is the key to correctly answering questions about enzyme action.

Final Answer:
An enzyme speeds up a reaction by Lowering the activation energy so that more reactant molecules can react.