IB Biology HLEnzymes & MetabolismPaper 1 & 2~11 min read
Activation Energy (Skills)
Every reaction has a hill in front of it. Even a reaction that releases plenty of energy has to climb that hill before it can run downhill. An enzyme does not push the reaction — it makes the hill smaller.
📚 What you need to know
Virtually every reaction in a living organism is catalysed by an enzyme, so enzymes are essential for life.
Enzymes are biological catalysts: biological because they work in living systems, catalysts because they speed reactions up without being used up or permanently changed.
The reaction can be summarised as E + S → ES → P + E: enzyme plus substrate, then the enzyme–substrate complex, then product plus the free enzyme.
While bound, the substrate is in a transitional state.
Activation energy is the energy needed to make the substrate unstable enough for the reaction to occur and new products to form.
Enzymes work by reducing the stability of bonds in the substrate, which lowers the activation energy.
Lowering the barrier makes the reaction faster. The energy released is unchanged.
Exergonic reactions release energy to the surroundings; endergonic reactions absorb it.
The reaction in one line
Everything on the previous pages can be squeezed into a single equation, and it is worth being able to write it out and annotate it.
If you can write this line and label every part of it, you can answer most short questions on enzyme action.
What activation energy actually is
Reactants do not turn into products just because they are near each other. Bonds in the substrate have to be strained and broken first, and that costs energy. The substrate has to become unstable enough for the change to happen.
DefinitionActivation energy is the energy needed by the substrate to become unstable enough for a reaction to occur and for new products to be formed.
If a reaction has a high activation energy, only a tiny fraction of molecules will ever have enough energy at any moment, so the reaction is painfully slow. That is exactly the situation inside a cell, where nothing is hot.
How the enzyme helps
The enzyme binds the substrate and, through induced fit, closes around it. Doing this reduces the stability of the bonds in the substrate — they are pulled, twisted and weakened. A weakened bond needs less energy to break, so the activation energy is lowered.
Now a much larger share of molecules have enough energy to react, so the reaction runs quickly at body temperature.
The two curves start together and finish together. If you draw this in an exam, make sure your enzyme curve does not end at a different level.
Picture two towns with a mountain between them. A tunnel does not move either town and it does not change how far downhill you end up — it just means you no longer have to climb over the top. The enzyme is the tunnel.
The one sentence examiners want. “The enzyme lowers the activation energy, but the overall energy change of the reaction is unchanged.” Write that and you have secured the trickiest mark on this topic.
Exergonic and endergonic
Reactions either release energy to their surroundings or absorb it from them.
Exergonic: free energy is released. The products hold less energy than the reactants. Respiration is the obvious example.
Endergonic: energy is absorbed. The products hold more energy than the reactants. Protein synthesis is the obvious example.
Both kinds still need activation energy. An exergonic reaction gives energy back once it gets going, but it still has to get over the hill first — which is why enzymes are needed even for reactions that release plenty of energy.
⚠ Exergonic is not the same as exothermic
Exergonic and endergonic describe the net free energy released or taken in overall.
Exothermic and endothermic describe thermal energy only — heat.
They overlap a lot in biology, but they are not interchangeable, and using the wrong pair in an HL answer costs marks.
🧠
Reading the prefixes
Ex– means out: exergonic energy exits. En– means in: endergonic energy enters. Then -ergonic = all energy, -thermic = heat only.
Worked examples
WE 1
Explain how enzymes speed reactions up
Using the term activation energy, explain how an enzyme increases the rate of a reaction. (4 marks)
Point 1: define the barrier
Activation energy is the energy the substrate needs to become unstable enough for the reaction to occur and new products to form.
Point 2: what the enzyme does
The substrate binds to the active site, and binding reduces the stability of bonds in the substrate.
Point 3: the consequence
Less energy is now needed to break those bonds, so the activation energy is lowered.
Point 4: the outcome
More substrate molecules have enough energy to react, so the reaction proceeds more quickly — though the energy released overall is unchanged.
Lower barrier, same energy change, faster reactionthe last clause is the mark most students miss
WE 2
Interpret an energy profile
An energy profile shows two curves for the same reaction, one with an enzyme and one without. State two features that are the same and one that is different. (3 marks)
The same: starting point
Both begin at the same energy level, because the substrate is the same in each case.
The same: finishing point
Both end at the same energy level, so the overall energy released is identical.
Different: the peak
The curve with the enzyme has a lower peak, showing a lower activation energy.
Same start, same finish, lower hillif a graph question says “compare”, give both a similarity and a difference
WE 3
Classify a reaction by its energy
Amino acids are joined to form a protein. State whether this reaction is exergonic or endergonic, and explain your answer. (2 marks)
The classificationEndergonic — energy has to be absorbed for the reaction to take place.
The reason
The protein formed stores more energy than the separate amino acids did, so energy must be put in; this is an anabolic reaction and it is coupled to ATP.
Endergonic: the products store more energy than the reactants“endergonic”, not “endothermic” — nothing here is about heat specifically
💡 Exam tips
Learn the definition of activation energy word for word. It is a frequent one-mark question.
When drawing an energy profile, keep the start and end levels identical for both curves.
Label the activation energy as a vertical arrow from the reactant level to the peak, not from the axis.
Always add “the overall energy change is unchanged” to any answer about enzymes and energy.
Use E + S → ES → P + E if a question asks you to summarise enzyme action.
Keep exergonic/endergonic for free energy and exothermic/endothermic for heat.
⚠ Common mistakes
Saying the enzyme gives energy to the substrate. It does not supply energy; it lowers the amount needed.
Drawing the enzyme curve ending lower than the other one. That would mean more energy released, which is wrong.
Measuring activation energy from the x-axis. It is measured from the reactant energy level.
Claiming exergonic reactions need no activation energy. Every reaction has a barrier.
Writing “the enzyme is used up in the reaction”. The equation shows E on both sides.
Confusing the transitional state with the product. The transitional state is the substrate while it is bound and unstable.
Up next: Metabolic Pathways. Single reactions are rarely the whole story — the next page links them into chains and cycles, and shows where ATP fits in.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
