In the pulmonary circulation of a mammal, deoxygenated blood becomes oxygenated when red blood cells pass through alveolar capillaries and haemoglobin within the cells combines with oxygen. The blood returns to the heart to be pumped around the systemic circulation.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

1. [5] • consider all mps to award if response clearly addresses events in the left side / Ο mps 1,2,6,7,8 must be in the context of the left side of the heart / • otherwise allow mp 3, 4, and 5 to max 3 – these mark points are inset to the right / any five from: / 1 blood enters (heart) via / in, pulmonary vein(s); / 2 blood exits heart to aorta; / R arota / 3 blood flows from atrium to ventricle; / 4 atrial, systole / contraction, then, ventricular, systole / contraction ; / A atrium contracts then ventricle contracts / unclear / attempts / mixed / right and left / sides / 5 pressure in ventricle, increases / gets higher ; / I high pressure / 6 bicuspid / (left) atrioventricular valve closes ; A mitral valve / R if in context of events in the atrium / 7 semilunar / aortic, valve opens ; ecf from arota in mp 2 / R if in context of events in the atrium / 8 bicuspid / (left) atrioventricular, valve closes when (blood) pressure in ventricle higher than atrium / or / semilunar / aortic, valve opens when (blood) pressure in ventricle higher than aorta; A aortic valve
2. [2] if no marks gained allow one mark for four, polypeptides /polypeptide chains / two, (identical) alpha/α, chains / globins, and two, (identical) beta/β, chains / globins ; / A subunits / polypeptides, for chains / R beta-pleated sheets / globulin / plus one from; / each, chain / globin / polypeptide, incorporates / contains / AW, a haem (group); / A four haem groups / A molecule for polypeptide if mp1 gained / A ferrous ion / Fe2+ / iron (ion / atom), within porphyrin (ring) for haem group / A prosthetic group with, ferrous ion / Fe2+ / iron (ion / atom) / hydrogen bonds / hydrophobic interactions / ionic bonds, between, chains / polypeptides / globins; / context is interactions between chains R if disulfide bond also stated
3. [2] oxygen binds to, iron ion / ferrous ion / Fe2+ ; / A oxygen bonds / oxygen forms a bond, only if, iron ion / ferrous ion / Fe2+ stated / A oxygen binds to, iron / iron atom/haem (group) / R if more than one oxygen molecule binds to one iron ion / detail of saturated; e.g. / total of 4 oxygen molecules / 8 oxygen atoms (bind per haemoglobin molecule) / each, chain / globin / polypeptide, binds one oxygen molecule R bonds / ref. to cooperative binding / allostery A description / ref. to (fully saturated molecule is) in, high partial pressures of oxygen / high concentration of oxygen ; / ref. to T/tense, state to, R/ relaxed, state