Zadanie 8, Paper 2, listopad 2025, TZ1, Biologia IB HL

(a) Maximum 4:

A competitive inhibitor binds at the active site OR has a complementary shape for that site. ✔

It reduces substrate binding OR reduces the number of enzyme–substrate complexes. ✔

Its structure resembles the substrate. ✔

Example point: malonate inhibits succinate dehydrogenase OR a statin inhibits HMG-CoA reductase OR oxygen competes at Rubisco OR penicillin inhibits bacterial transpeptidase. ✔

Matching effect point: malonate reduces succinate-to-fumarate conversion OR statins reduce HMG-CoA-to-mevalonate conversion OR oxygen competition diverts Rubisco from carboxylation, reducing its carboxylation product yield. Penicillin earns the example point but not this separate conversion-effect point. ✔

(b) Maximum 7:

An arriving action potential triggers neurotransmitter release from the presynaptic ending. ✔

Release occurs by exocytosis OR vesicles fuse with the presynaptic membrane. ✔

Transmitter diffuses across the cleft. ✔

It binds receptors on the postsynaptic membrane. ✔

Ligand-gated ion channels open. ✔

An excitatory response depolarizes the membrane and may reach the threshold for an action potential. ✔

Transmitter is degraded or taken up again OR an external agonist may persist because it is not rapidly removed. ✔

External chemicals may suppress or increase transmission. ✔

Neonicotinoids bind insect nicotinic acetylcholine receptors. ✔

Excess sodium enters the postsynaptic neuron OR sodium-permeable channels remain open. ✔

Cocaine blocks dopamine reuptake transporters. ✔

Dopamine consequently remains in the cleft and continues receptor stimulation. ✔

(c) Maximum 4:

An infected person can transmit HIV to others. ✔

HIV infects OR damages or destroys CD4 helper T cells. ✔

Without effective treatment, progressive depletion of helper T cells can become severe. ✔

Fewer functioning helper T cells impair B-cell activation. ✔

Antibody production can become inadequate. ✔

Effective specific responses to new pathogens may fail. ✔

Opportunistic infections may become difficult to control. ✔

Advanced untreated infection may progress to AIDS. ✔

Rapid viral evolution can enable immune escape or drug resistance. ✔

Construction:1 mark for clear, concise, well-organized communication across the response, without substantial repetition or irrelevant material.

Parts(a),(b),(c):any4,7,4 distinct points respectively, max15 content marks. Add1 independent construction mark; total16. All printed Section B alternatives are retained as separate tasks. Accept other scientifically correct named competitive-inhibitor examples and matching effects. Penicillin is accepted for the named-enzyme example point only.