Zadanie 9, Paper 2, listopad 2025, TZ3, Biologia IB HL

(a) Maximum 4:

Signalling ligands bind receptors. ✔

A binding site specifically matches a ligand in shape or chemical properties. ✔

Receptors may lie in the plasma membrane OR within the cytoplasm or nucleus. ✔

Only cells with an appropriate receptor respond to a signal. ✔

Binding initiates a signal-transduction pathway. ✔

A suitable signalling example is a neurotransmitter (including acetylcholine), a named hormone such as insulin or adrenaline, a cytokine or calcium ions. ✔

(b) Maximum 7:

Enzymes are globular proteins with catalytic active sites. ✔

A substrate must bind for its reaction to be catalysed. ✔

Substrate binding can reshape the site through induced fit. ✔

Disruption of tertiary structure can prevent substrate binding or denature the enzyme. ✔

Denaturation reduces or abolishes activity. ✔

Excessive heat disrupts stabilizing interactions or changes active-site conformation. ✔

Low temperatures can inhibit activity OR rising temperature increases kinetic energy and collision frequency up to an optimum. ✔

pH changes alter charges, bonding or active-site shape and may denature the enzyme. ✔

An allosteric inhibitor binds outside the active site. ✔

Allosteric binding can rearrange stabilizing hydrogen bonds, ionic interactions or hydrophobic interactions. ✔

It can change the enzyme or active-site conformation. ✔

Such inhibition reduces enzyme activity OR reduces effective enzyme–substrate complex formation. ✔

(c) Maximum 4:

Haemoglobin loads oxygen in lungs OR unloads it in respiring tissues. ✔

One molecule can carry four oxygen molecules OR binding is cooperative. ✔

At low oxygen partial pressure it favours unloading OR at high oxygen partial pressure oxygen binds readily. ✔

This allows release to oxygen-consuming tissues. ✔

The delivered oxygen supports aerobic respiration. ✔

Some tissue carbon dioxide binds haemoglobin. ✔

Raised CO₂ or reduced pH shifts the dissociation curve right OR promotes oxygen release. ✔

The Bohr effect lowers oxygen affinity OR facilitates oxygen unloading. ✔

Construction:1 mark for clear, concise organization of the complete response, without substantial repetition or irrelevant material.

Award distinct points up to the individual part caps 4,7,4; maximum15 content marks plus1 independent construction mark, total16.