IB Biology · Theme C: Interaction and interdependence · SL and HL
C1.1 Enzymes and metabolism
A one-page summary of C1.1 Enzymes and metabolism, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
In what ways do enzymes interact with other molecules?
What are the interdependent components of metabolism?
What C1.1 covers
Enzymes as catalysts
- C1.1.1Speeding up life's essential chemistry
- C1.1.2One cell, thousands of enzymes
- C1.1.3Building up, breaking down
- C1.1.4A pocket built from folding
- C1.1.5Substrate and enzyme both change shape
- C1.1.6Catalysis starts with a collision
- C1.1.6When something can't move
- C1.1.7Why one enzyme won't touch another substrate
- C1.1.7Denaturation: same sequence, wrong shape
Enzyme kinetics
- C1.1.8Rate rises, then collapses
- C1.1.8Every enzyme has its own optimum pH
- C1.1.8Why rate plateaus at Vmax
- C1.1.9Turning a curve into a number
- C1.1.10Lowering the barrier, not the drop
Metabolism as an interconnected network
- C1.1.11 · HLInside the cell, or outside it
- C1.1.12 · HLMetabolism is never perfectly efficient
- C1.1.13 · HLSome pathways loop back to the start
Regulating enzyme activity
- C1.1.14 · HLA second site, far from the active site
- C1.1.15 · HLCompetitive vs non-competitive
- C1.1.16 · HLThe end product shuts off its own pathway
- C1.1.17 · HLAn inhibitor that never lets go
C1.1 Enzymes and metabolism: summary
Enzymes as catalysts
- Enzymes lower activation energy without being consumed; ΔG unchanged.
- Active site shape, from tertiary structure, drives specificity.
Induced fit & denaturation
- Substrate binding induces a small conformational change in both molecules.
- Denaturation distorts the active site, usually irreversibly; primary structure stays intact.
Rates & metabolism
- Rate rises then collapses with temperature (collision + denaturation).
- Rate plateaus at Vmax once every active site is saturated.
- Anabolism (condensation, energy in) vs catabolism (hydrolysis in digestion, oxidation in respiration).
HL · Where enzymes act
- Intracellular (glycolysis, Krebs) vs extracellular (gut digestion).
HL · Cycles & heat
- Krebs/Calvin cycles regenerate their starting molecule; glycolysis doesn't.
- Heat is an inevitable, non-100%-efficient byproduct of metabolism.
HL · Inhibition
- Competitive (active site, reversible, statins) vs non-competitive (allosteric, reversible).
- Feedback (isoleucine) reversible; mechanism-based (penicillin) irreversible.
Key terms
- Active site
- The region of an enzyme, formed by its 3D folding, where a substrate binds and catalysis occurs.
- Induced fit
- Model in which substrate binding triggers a small conformational change that improves the fit between enzyme and substrate.
- Denaturation
- A usually irreversible change to a protein's tertiary structure that destroys its function without breaking its primary sequence.
- Metabolite
- Any molecule taking part in the reactions of metabolism, including substrates, intermediates and products.
- Activation energy
- The minimum energy needed to reach a reaction's transition state; enzymes lower it without changing ΔG.
- Vmax
- The maximum rate an enzyme population can sustain, reached once every active site is continuously occupied.
- Allosteric site HL
- A regulatory binding site distinct from the active site, where only specific effector molecules bind.
- Feedback inhibition HL
- Regulation in which a pathway's end product inhibits an enzyme early in that same pathway.
- Mechanism-based inhibition HL
- Irreversible inactivation caused by an inhibitor chemically modifying the active site.
Sample exam questions
Three of the 52 multiple-choice questions for C1.1. Try each one before opening the answer.
Question 1. Enzymes are described as biological catalysts. This means they:
- Change the equilibrium concentrations of products and reactants
- Lower the activation energy of reactions without being consumed or permanently changed
- Provide the energy required for endergonic reactions to proceed
- Increase the free energy change (ΔG) of reactions
Show the answer
Answer: B. Enzymes lower the activation energy (Ea) required to reach the transition state. They do this by binding substrates at the active site, orienting them optimally, and stabilising the transition state. Enzymes are not consumed — they are released unchanged.
Question 2. The induced-fit model of enzyme action proposes that:
- The active site undergoes a conformational change upon substrate binding, achieving a tighter, more complementary fit around the transition state
- The active site is a rigid, pre-shaped pocket that fits the substrate like a key in a lock
- The substrate permanently changes shape without the enzyme changing
- Multiple different substrates can bind simultaneously to the same active site
Show the answer
Answer: A. Induced fit: substrate binding induces a conformational change in the enzyme that optimises the fit and positions catalytic residues correctly. This model (Koshland, 1958) replaced the earlier rigid lock-and-key model.
Question 3. Penicillin inhibits the transpeptidase enzymes that bacteria use to cross-link peptidoglycan during cell wall synthesis. It binds irreversibly to the active site, causing a permanent chemical change so the enzyme can never catalyse another reaction. This is best described as:
- Competitive inhibition, since penicillin closely resembles the enzyme's normal substrate
- Non-competitive inhibition, since penicillin binds reversibly at an allosteric site distant from the active site
- Mechanism-based inhibition, since the inhibitor's binding causes an irreversible chemical change to the active site
- Feedback inhibition, since penicillin is the end product of the pathway it inhibits
Show the answer
Answer: C. Mechanism-based inhibitors bind at or near the active site and cause an irreversible chemical modification to it, permanently inactivating the enzyme. This is distinct from competitive inhibition (reversible, direct competition with substrate for the active site, overcome by excess substrate) and non-competitive inhibition (reversible binding at an allosteric site). Penicillin's action on bacterial transpeptidases is the guide's example of mechanism-based inhibition.
Linking questions
Questions that connect C1.1 to other parts of the course, the kind that come up in Paper 2.
- The tertiary structure of an enzyme (B1.2) creates its active site. Using a named enzyme as an example, explain how the specific R groups in the active site contribute to substrate binding and catalysis. (see B1.2)
- Lysosomes (B2.2) contain hydrolytic enzymes that function optimally at pH ~5. Explain why these enzymes must be compartmentalised, and what would happen if they leaked into the cytoplasm (pH ~7.2). (see B2.2)
- The energy for anabolic reactions is provided by ATP hydrolysis. Explain how the ATP produced during cell respiration (C1.2) is coupled to endergonic reactions via enzyme-catalysed phosphorylation. (see C1.2)
- During germination, starch in seeds is hydrolysed by amylase into glucose, which is then respired. Explain why amylase activity increases during germination, linking this to the metabolic demands of the growing seedling. (see C1.2)
Practise C1.1
Study notes, every question and full markschemes for C1.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.