Even a reaction that gives out energy needs a shove to get going. That shove is the activation energy — and lowering it is the single thing every enzyme does.
📚 What you need to know
Activation energy is the energy the substrate needs to become unstable enough for the reaction to happen.
On an energy profile it is the height of the hump, measured from the substrate level up to the peak.
Enzymes lower the activation energy by straining and weakening bonds in the substrate.
The energy released is unchanged. Only the size of the hump changes, not the start and finish levels.
A lower hump means more molecules have enough energy, so the reaction is faster.
While bound in the enzyme–substrate complex, the substrate is in a transitional state.
Exergonic and endergonic are about total energy. Exothermic and endothermic are about heat only.
What activation energy is
Think about a match. A match burning gives out plenty of energy — but it does not light itself while it is sitting in the box. You have to strike it first. That initial input is the activation energy: the energy needed to get the reaction started, even when the reaction itself will release energy overall.
At the molecular level, the reason is that bonds have to be broken before new ones can form. The substrate must be pushed into an unstable, high-energy arrangement before it can rearrange into products.
Definition
activation energy = the energy needed to make the substrate unstable enough for the reaction to proceed
In a beaker you supply that energy with heat. A cell cannot, so it uses enzymes instead. That is the whole point of this section.
How an enzyme lowers the hump
When the substrate binds to the active site, the induced fit pulls it slightly out of shape. Its bonds are strained, which makes them less stable and easier to break. Holding the substrate in the correct orientation helps too — nothing has to be pushed into position by chance.
The result is that the substrate needs far less energy to reach the unstable state. The hump on the energy profile becomes smaller.
Measure activation energy from the substrate level up to the peak — not from the bottom of the axis, and not from the product level.
Why a smaller hump means a faster reaction: molecules have a range of energies. Lower the barrier and a much larger fraction of collisions now have enough energy to get over it, so far more of them are successful.
Watch what the enzyme does not change. It does not make the products more stable, it does not release extra energy, and it does not change whether the reaction is exergonic or endergonic. It only shortens the climb.
The equation you should be able to write
The whole of enzyme action can be squeezed into one line of shorthand.
If you can write this line and say what each letter means, you can answer most short questions on enzyme action.
Two pairs of words that get confused
This is the trap in this section, and it is easy to avoid once you see the pattern. One pair is about all the energy; the other is about heat only.
Word
What it measures
Direction
Exergonic
Net energy overall
Energy is released
Endergonic
Net energy overall
Energy is taken in
Exothermic
Thermal energy only
Heat is released
Endothermic
Thermal energy only
Heat is taken in
A memory hook: the -ergonic words share a root with energy, and the -thermic words share a root with thermometer. Energy in general, or heat in particular.
Linking back to the first page of this topic: catabolic reactions are exergonic and anabolic reactions are endergonic. Both still need enzymes, and both still have an activation energy that has to be got over.
Worked examples
WORKED EXAMPLE
Using an energy profile, explain how an enzyme increases the rate of a reaction. [3]
Step 1: name what changes on the graph
The enzyme lowers the activation energy, so the hump is smaller.
Step 2: say why that speeds things up
More substrate molecules now have enough energy to react, so more collisions are successful.
Step 3: say what does not change
The substrate and product energy levels are the same, so the energy released is unchanged.
Lower activation energy, more successful collisions, same overall energy changethe third point is the one students leave out — and it is often the third mark
WORKED EXAMPLE
On an energy profile, the substrate sits at 250 kJ, the peak without an enzyme is at 410 kJ, the peak with an enzyme is at 320 kJ, and the products sit at 180 kJ. Find both activation energies and the energy released.
Step 1: activation energy = peak − substratewithout enzyme: 410 − 250 = 160 kJwith enzyme: 320 − 250 = 70 kJStep 2: how much the enzyme saved160 − 70 = 90 kJ lowerStep 3: energy released = substrate − products250 − 180 = 70 kJ, and the enzyme does not affect thisEₐ = 160 kJ and 70 kJ; 70 kJ released either waymeasure the hump from the substrate line, never from zero
WORKED EXAMPLE
A student writes: “the enzyme speeds up the reaction because it gives the substrate extra energy.” Explain why this is wrong. [2]
Step 1: what the enzyme actually does
It binds the substrate and strains its bonds, which lowers the activation energy.
Step 2: correct the misunderstanding
No energy is added. The barrier is lowered so the energy already present is enough.
The enzyme lowers the energy needed; it does not supply energyan enzyme is a catalyst, not a battery
💡 Exam tip
Measure activation energy from the substrate level to the peak. Measuring from the x-axis is the classic error.
In any answer about enzymes and energy, include the phrase “the energy released is unchanged”. It is very often a mark.
If asked to draw the enzyme-catalysed curve, keep the start and end levels identical and only lower the hump.
Learn the pair test: -ergonic means total energy, -thermic means heat.
Write the equation E + S → ES → E + P and label each term. It answers a lot of short questions on its own.
Use “transitional state” for the substrate while it is held in the complex.
⚠ Common mix-up
Saying the enzyme provides energy. It lowers the energy required. Completely different claim.
Drawing the product level lower when an enzyme is added. The product level never moves.
Measuring the hump from zero instead of from the substrate level.
Using exothermic when you mean exergonic. Exothermic is only about heat.
Thinking exergonic reactions start on their own. They still need activation energy — that is why the match has to be struck.
Forgetting the enzyme appears on both sides of E + S → ES → E + P. That is exactly what shows it is a catalyst.
That completes Enzymes & Metabolism. Up next: Adenosine Triphosphate (ATP) — the molecule that carries the energy those catabolic reactions release, and spends it on the anabolic ones.
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