Key Idea: Almost every reaction inside a cell — and many just outside it — is controlled by an enzyme. The whole web of these reactions is the cell's metabolism. Enzymes are globular proteins that work by lowering the activation energy of a reaction, and each is specific because its active site is shaped to fit one substrate. This topic (C1.1) is one of the most heavily examined in the course. It shows up as a quick Paper 1A MCQ (classify a reaction, name the binding model, read an energy profile) and as a Paper 1B / Paper 2 data question — reading and explaining the enzyme-rate graphs is a perennial favourite.
🔄 Metabolism — anabolism vs catabolism
Metabolism is all of the enzyme-catalysed reactions in a cell. It splits into two halves. Anabolism builds larger molecules from smaller ones and uses energy (usually a condensation reaction). Catabolism breaks larger molecules down into smaller ones and releases energy (usually a hydrolysis reaction). To classify any process, ask one question: does the molecule get bigger or smaller? Bigger means anabolic; smaller means catabolic.
| Feature | Anabolism | Catabolism |
|---|---|---|
| Builds or breaks | Builds larger molecules from smaller ones | Breaks larger molecules into smaller ones |
| Energy | Uses (requires) energy | Releases energy |
| Usual reaction | Condensation (joins subunits) | Hydrolysis (adds water, splits bonds) |
| Examples | Making glycogen; protein synthesis; photosynthesis | Respiration; digestion; hydrolysis of macromolecules |
Anabolism = building up (an athlete builds muscle, absorbs energy). Catabolism = breaking down (a catastrophe tears things apart, creates free energy).
🔑 Active sites, induced fit & specificity
Every enzyme is a globular protein with a pocket called the active site. The molecule it acts on — the substrate — binds there, forming an enzyme-substrate complex. As the substrate binds, the active site changes shape slightly to mould around it — the induced fit model (the modern model, which replaced the rigid lock-and-key picture). Because the active site is complementary in shape to only one substrate, each enzyme is specific — it catalyses only one reaction. After the reaction the products leave and the enzyme is unchanged, so it can be reused.
An enzyme at work: the substrate binds the active site to form an enzyme-substrate complex (the site moulds around it — induced fit); the reaction happens, the products are released, and the enzyme is left unchanged and ready to be reused.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Lock-and-key = a stiff lock that never moves. Induced fit = a glove that adjusts around the hand once it goes in. If a diagram shows the active site changing shape as the substrate binds, the answer is induced fit.
⛰️ Activation energy & energy profiles
Even a reaction that releases energy needs a small starting 'push' — the activation energy (Eₐ), the minimum energy the reactants must have for the reaction to begin. On an energy profile, Eₐ is the height of the hill from the reactants up to the peak. An enzyme lowers this barrier, so the reaction goes much faster — more reactant particles have enough energy to react. Crucially, the enzyme changes only the barrier: the reactants and products levels — and so the energy released — stay exactly the same.
An energy profile: both routes start at the reactants level and finish at the same products level, but the enzyme route (green) climbs a far lower activation-energy barrier than the uncatalysed route (rose) — so the reaction goes faster without the start or end energy changing.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
An enzyme lowers only the activation energy (the hill in the middle). It does not change the energy of the reactants or products. On a with/without-enzyme graph, the curve with the lower barrier is always the enzyme-catalysed (faster) one.
🌡️ Temperature, pH & substrate concentration
Three factors set how fast an enzyme works, and each has its own graph shape. Temperature: rate rises (faster molecules → more collisions) to an optimum, then falls as heat denatures the enzyme. pH: a peak at the optimum pH; too acidic or alkaline denatures it. Substrate concentration: rate rises, then plateaus once the active sites are saturated. The single idea behind it all: anything that changes the active-site shape stops the enzyme working.
The three classic rate graphs: rate climbs to an optimum temperature then falls as the enzyme denatures; rate peaks at an optimum pH; and rate rises with substrate concentration then plateaus once every active site is saturated.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Factor | What the graph does | The key idea to explain |
|---|---|---|
| Temperature | Rises to an optimum, then falls steeply | Above the optimum the enzyme DENATURES — the active-site shape changes |
| pH | A single peak at the optimum pH | Too acidic or alkaline DENATURES the enzyme (distorts the active site) |
| Substrate concentration | Rises, then levels off (plateau) | SATURATION — every active site is full; enzyme number now limits the rate |
Denaturation (temperature & pH): Caused by **high temperature** or **extreme pH**. The active-site **shape is changed**. Substrate **no longer fits** → rate **falls**. Usually **permanent** (enzyme ruined).
Saturation (substrate concentration): Caused by **high substrate concentration**. **All active sites are occupied**. Rate **plateaus** (stays constant). Enzyme is **unharmed** — just fully busy.
Denatured = shape destroyed (heat or pH wrecks the active site → rate falls). Saturated = sites all seated (every active site is taken → rate plateaus, but the enzyme is fine).
🧪 Enzyme experiments & immobilized enzymes
To study an enzyme you change one factor (the independent variable), measure the rate (the dependent variable), and keep every other factor constant (the controlled variables — temperature, pH, substrate concentration, enzyme amount and time) so the test is fair. In industry the enzyme is often immobilized — fixed to a solid support such as gel beads. This lets it be reused (cheaper), keeps the product pure (no enzyme contamination) and makes it more stable. The classic application is immobilized lactase, used to make lactose-free milk.
| Feature | Free enzyme | Immobilized enzyme |
|---|---|---|
| Where it is | Free in solution with the substrate | Fixed to a support (e.g. beads) |
| Reuse | Used once, then lost | Reused many times (cheaper) |
| Product purity | Enzyme ends up in the product | Product stays pure |
| Stability | Denatures more easily | More stable over a wider range |
If two runs differ in two factors at once, you can't tell which one caused the change. Controlling every other variable is what makes the result trustworthy — the single most common thing examiners ask you to discuss in a data question.
🏠 Intracellular vs extracellular enzymes
Enzymes work in two places. Intracellular enzymes act inside the cell that made them, running its own metabolic pathways (e.g. respiration enzymes). Extracellular enzymes are secreted to act outside the cell (e.g. digestive enzymes — amylase, protease, lipase). A cell secretes a digestive enzyme because a large food molecule (starch, protein) is too big to cross the membrane — so it is hydrolysed into small soluble subunits outside the cell, which can then be absorbed. Both types are ordinary enzymes; only the location differs.
| Feature | Intracellular enzyme | Extracellular enzyme |
|---|---|---|
| Where it acts | Inside the cell that made it | Outside the cell — it is secreted |
| Typical job | Runs the cell's own metabolic pathways | Digests large molecules in the surroundings |
| Examples | Respiration enzymes; catalase | Amylase, protease, lipase; decomposers' enzymes |
| Still a true enzyme? | Yes | Yes — only the LOCATION differs |
Intracellular = the enzyme stays in. Extracellular = the enzyme exits (is secreted) to digest food too big to come in.
✍️ Worked examples — the whole topic
IB-style question — classify the reactions
For each process, state whether it is anabolic or catabolic: (i) amino acids being joined to make a protein, and (ii) glucose being broken down in respiration. [2]
Model answer:
(i) Protein synthesis joins small subunits (amino acids) into a larger molecule, so it is anabolic (and it uses energy).
(ii) Respiration breaks glucose down into smaller molecules and releases energy, so it is catabolic. (Mark 1: (i) anabolic. Mark 2: (ii) catabolic.)
(i) Anabolic — subunits joined into a larger molecule. (ii) Catabolic — a larger molecule broken down, releasing energy.
IB-style question — explain specificity
Explain how the structure of an enzyme makes it specific to its substrate. [3]
Model answer:
Each enzyme has an active site with a specific 3-D shape that is complementary to one particular substrate.
Only a substrate of the matching shape can fit and bind, forming an enzyme-substrate complex; a wrong-shaped substrate cannot bind.
As the substrate binds, the active site moulds around it (induced fit), so only the correct substrate produces the proper fit — the enzyme acts on only one reaction. (1 mark per distinct point, up to 3.)
The active site is complementary in shape to one substrate; only that substrate fits and binds (induced fit moulds the site around it), so the enzyme catalyses only one reaction.
IB-style question — read the energy profile
On an energy profile, region X is the height of the curve where it begins, and region Y is the height from the start of the curve up to its peak. State what X and Y represent, and explain what an enzyme changes. [3]
Model answer:
X is the energy of the reactants (the starting, left-hand level of the curve).
Y is the activation energy (Eₐ) — the height from the reactants up to the peak.
An enzyme lowers Y (the activation energy), so the reaction goes faster; it does not change the reactants or products levels. (Mark 1: X = reactants. Mark 2: Y = activation energy. Mark 3: enzyme lowers Eₐ only.)
X = energy of the reactants; Y = activation energy. An enzyme lowers the activation energy (Y) only, speeding the reaction up without changing the reactants or products levels.
IB-style question — explain a temperature graph
A graph of enzyme reaction rate against temperature rises to a peak at 40 degrees C and then falls steeply. Explain the falling part of the graph. [2]
Model answer:
Above the optimum the high temperature denatures the enzyme.
Denaturation changes the shape of the active site, so the substrate no longer fits and the rate falls. (Mark 1: denaturation. Mark 2: active-site shape changes so substrate cannot bind.)
Above 40 degrees C the enzyme is denatured: the active-site shape changes so the substrate no longer fits, and the rate falls.
IB-style question — variables to control
A student compares a free enzyme and an immobilized enzyme acting on the same substrate. Discuss the variables that must be controlled so the comparison is valid. [2]
Model answer:
Keep temperature, pH, substrate concentration, the amount of enzyme and the reaction time the same for both runs.
Each of these affects the rate on its own, so controlling them means any difference in rate is due to immobilization and not to another factor — making the comparison fair. (Mark 1: name two or more controlled variables. Mark 2: this keeps the test fair / valid.)
Control temperature, pH, substrate concentration, enzyme amount and time; otherwise a difference in rate might be caused by one of those rather than by immobilization, so the test would not be fair.
IB-style question — intracellular or extracellular?
A single-celled fungus is grown in liquid containing starch. Amylase is detected in the surrounding liquid, where the starch is being broken into maltose, even though the fungus has not absorbed the starch. Deduce whether the amylase is intracellular or extracellular, and justify your answer. [3]
Model answer:
The amylase is acting as an extracellular enzyme.
The amylase and the breakdown of starch are detected outside the cell, in the surrounding liquid — the reaction is happening outside the fungus.
Starch is too large to enter the cell, so the fungus secretes the enzyme to digest it externally into maltose, which is small enough to be absorbed. (Mark 1: extracellular. Mark 2: activity is outside the cell. Mark 3: starch too big, so digested externally first.)
Extracellular — the amylase and digestion of starch occur in the liquid outside the cell, because starch is too large to absorb and must be broken into maltose externally before the fungus can take it up.
✅ Quick self-check
Tap each card to check yourself.
What is metabolism, and its two halves? All the enzyme-catalysed reactions in a cell. Anabolism builds larger molecules and uses energy; catabolism breaks them down and releases energy.
Why is each enzyme specific? Its active site is complementary in shape to only one substrate, so only that substrate can fit and bind (induced fit moulds the site around it).
How does an enzyme speed a reaction up? It lowers the activation energy, so a smaller barrier must be crossed. More reactant particles then have enough energy to react. The reactants and products levels are unchanged.
Why does rate fall above the optimum temperature? The enzyme is denatured: the active site changes shape, so the substrate no longer fits and the rate falls. (pH does the same at the extremes.)
Why does the rate plateau at high substrate concentration? Every active site is occupied — the enzymes are saturated. Adding more substrate can't help; the number of enzymes now limits the rate. The enzyme is unharmed.
Why immobilize an enzyme, and a key example? It can be reused (cheaper), leaves a pure product and is more stable. Immobilized lactase is used to make lactose-free milk.
Intracellular vs extracellular? Intracellular enzymes act inside the cell (e.g. respiration); extracellular enzymes are secreted to act outside (e.g. digestive amylase/protease/lipase). Same molecule, different location.
Why secrete a digestive enzyme at all? Large food molecules are too big to cross the membrane, so they are hydrolysed outside the cell into small soluble subunits that CAN be absorbed.
Recap of the rate shapes: a rise-and-fall (temperature) and a peak (pH) both end in denaturation — the active-site shape is changed; a rise-and-plateau (substrate) is saturation — every active site is full, but the enzyme is unharmed.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Exam Tips
- To classify a reaction, ask if the molecule gets bigger (anabolic, uses energy) or smaller (catabolic, releases energy) — glycogen formation is anabolic, hydrolysis of macromolecules is catabolic.
- If a diagram shows the active site CHANGING SHAPE as the substrate binds, the model is induced fit, not lock-and-key.
- To explain specificity, link the complementary SHAPE of the active site and substrate — never just repeat the word 'specific'.
- Activation energy is the height from the reactants up to the PEAK; an enzyme lowers only that barrier, leaving the reactants and products levels unchanged.
- A rise-then-FALL graph (temperature or pH) ends in denaturation — say the active-site SHAPE changes so substrate no longer fits, not just 'the enzyme dies'.
- A rise-then-PLATEAU graph (substrate concentration) is saturation — all active sites full, so enzyme number limits the rate (the enzyme is unharmed).
- On a data question, read the TREND off the graph first, then give the biological REASON — examiners want both.
- For 'discuss controlled variables', NAME them (temperature, pH, substrate concentration, enzyme amount, time) AND say they keep the test fair.
- A safe application of immobilized enzymes: lactase used to make lactose-free milk.
- For 'distinguish intracellular and extracellular', make the contrast explicit (inside vs secreted/outside) AND give a named example of each.
Key Idea: Metabolism is the cell's web of enzyme-catalysed reactions — anabolism builds up (uses energy), catabolism breaks down (releases energy). Enzymes are globular proteins with a specific active site; the substrate binds by induced fit, and the enzyme works by lowering the activation energy. Temperature and pH give an optimum then denaturation; substrate concentration gives a plateau at saturation. Enzymes can be immobilized for reuse, and act either intracellularly or, when secreted, extracellularly — same molecule, different location.