Enzymes & Bioenergetics Basics

Q1. [3 marks — 1 each]

• (a) Metabolism: the sum of all chemical reactions occurring within a living organism/cell. (1) (Accept mention of anabolism + catabolism.)
• (b) Energy: the capacity to do work / bring about change. (1)
• (c) ATP (adenosine triphosphate): the universal energy currency of the cell; releases usable energy when its terminal phosphate bond is hydrolysed to ADP + Pi. (1)

Q2. [4 marks]

• First law (conservation): energy cannot be created or destroyed, only transformed/transferred. (1) In cells, chemical energy in glucose is converted to ATP and heat — total energy conserved. (1)
• Second law (entropy): every energy transfer increases the entropy (disorder) of the universe. (1) Cells maintain internal order by releasing heat/entropy to surroundings; some energy is always lost as heat (never 100% efficient). (1)

Q3. [4 marks]

• Exergonic: releases energy; $\Delta G < 0$ (negative); spontaneous. (1) Example: cellular respiration / ATP hydrolysis. (1)
• Endergonic: absorbs/requires energy; $\Delta G > 0$ (positive); non-spontaneous. (1) Example: photosynthesis / protein synthesis. (1)

Q4. [4 marks]

• (a) Activation energy ($E_a$): the minimum energy required to start a reaction / to reach the transition state so reactants can be converted to products. (2) (1 mark if only "energy to start a reaction".)
• (b) Enzymes lower the activation energy by stabilising the transition state, allowing the reaction to proceed faster at lower temperatures. (1) They do not change the overall $\Delta G$ (initial and final energy states unchanged). (1)

Q5. [4 marks]

• Active site: a specific 3-D region/pocket on the enzyme where the substrate binds (1) and where catalysis occurs; its shape is complementary to the substrate. (1)
• Lock-and-key model: the substrate (key) has a shape that exactly fits the rigid active site (lock) (1); only a specific substrate fits, explaining enzyme specificity. (1)

Q6. [3 marks]

• Induced-fit model: the active site is not rigid; when the substrate binds, the active site changes shape slightly to mould around/fit the substrate more closely. (1) This strains the substrate bonds and lowers activation energy. (1)
• Difference: lock-and-key assumes a rigid, pre-formed complementary site, whereas induced-fit assumes a flexible site that adjusts on binding. (1)

Q7. [4 marks]

• Rate increases with temperature (more kinetic energy → more effective collisions) up to a point. (1)
• Optimum temperature: the temperature at which enzyme activity is maximal (≈37 °C in humans). (1)
• Above the optimum, rate falls sharply (1) because high temperature denatures the enzyme — hydrogen/ionic bonds break, active site loses its shape, substrate can no longer bind. (1)
• (Sketch: rise to a peak then steep drop.)

Q8. [4 marks]

• At low substrate concentration, rate increases (roughly linearly) as substrate concentration increases, because more substrate molecules are available to bind. (1)(1)
• At high substrate concentration the rate levels off / plateaus at $V_{max}$. (1)
• This is because all enzyme active sites are saturated (occupied); enzyme concentration now limits the rate. (1)

Q9. [6 marks — 1 each]

Q10. [6 marks]

• (a) Cofactor: a non-protein inorganic ion (e.g. Zn²⁺, Mg²⁺) required for enzyme activity. (1) Coenzyme: an organic non-protein molecule (often vitamin-derived, e.g. NAD⁺, FAD) that assists the enzyme. (1)
• (b) Feedback inhibition: the end product of a metabolic pathway inhibits an enzyme (usually the first) earlier in the pathway, switching off its own production when levels are high. (2)
• The product binds to an allosteric site on the enzyme, changing the shape of the active site (allosteric regulation), reducing activity — a self-regulating mechanism. (2)

[
  {"claim":"Exergonic reactions have negative delta G (spontaneous)","code":"dG = -30; result = (dG < 0)"},
  {"claim":"Endergonic reactions have positive delta G (non-spontaneous)","code":"dG = 20; result = (dG > 0)"},
  {"claim":"Enzymes lower activation energy but leave delta G unchanged: Ea decreases, dG constant","code":"Ea_uncat=50; Ea_cat=25; dG_before=-30; dG_after=-30; result = (Ea_cat < Ea_uncat) and (dG_before == dG_after)"},
  {"claim":"Total marks sum to 40","code":"marks=[3,4,4,4,4,3,4,4,6,6]; result = (sum(marks)==40)"}
]