A-LevelBiologyBiological molecules9700/23 · Oct/Nov 2019 · Q112 Marks

Fig. 1.1 is a diagram of a molecule of haemoglobin.

(a)(i)Name the structure labelled A on Fig. 1.1.[1]
(a)(ii)State the function of structure A.[1]
(b)Haemoglobin is described as a globular protein. Explain why this protein is described as globular.[2]
(c)The gene HBB codes for the ẞ-globin polypeptide. State why a polypeptide, such as ẞ-globin, is described as a polymer.[2]
(d)A single base change in the DNA of the gene HBB results in a change to the amino acid sequence of ẞ-globin. In the sequence, a single glutamic acid is replaced by valine. Outline the effects of this change in the amino acid sequence of ẞ-globin on the structure and function of a haemoglobin molecule.[3]
(e)Haemoglobin interacts with carbon dioxide and carbon monoxide. Outline the role of haemoglobin in the transport of carbon dioxide.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

1(a)(i) haem; A heme / prosthetic group I iron / iron ion / Fe / Fe2+ I porphyrin ring
1(a)(ii) one from combines with / binds / carries / transports / AW, oxygen (in lungs); R forms bonds releases / AW, oxygen, in tissues / at low oxygen concentrations ; allows haemoglobin to transport oxygen ;
1(b) two from 1 spherical / ball-like / rounded / AW ; R circular/round ignore 3D shape 2 (water) soluble / forms H bonds with water ; 3 hydrophilic R-groups on outside of molecule / hydrophobic R-groups on inside ; R ref. to 'tails' 4 dynamic / metabolic / physiological / AW, function ;
1(c) made of amino acids ; one from joined by peptide bonds ; R 'peptide bond between two amino acids' R 'dipeptide bond(s)' macromolecule / long-chain (molecule) / large molecule ; repeated / many, (sub-)units / monomers;
1(d) three from 1 (R-group of) glutamic acid / glu, is polar / hydrophilic and (R-group of) valine / val, is non-polar / hydrophobic ; 2 change in tertiary structure ; A 'change in globular shape / less globular in shape' 3 change in quaternary structure of haemoglobin ; R of ẞ-globin 4 ref. to haemoglobin forms fibres (with other Hb) ; A ‘sticky molecules' 5 haemoglobin is less (water) soluble ; I insoluble 6 haemoglobin is less efficient at, binding / transporting, oxygen ; A less oxyhaemoglobin A haemoglobin / ẞ-globin, has lower affinity for oxygen A reduced oxygen carrying capacity I haemoglobin, does not / cannot, bind oxygen 7 AVP ;
1(e) three from 1 haemoglobin combines with carbon dioxide ; 2 carbon dioxide reacts with (terminal), amine group(s)/–NH2/−NH ; 3 to form carbaminohaemoglobin ; R carboxyhaemoglobin / carbonylhaemoglobin 4 each polypeptide can carry a molecule of carbon dioxide / haemoglobin can carry four molecules of carbon dioxide ; 5 carbon dioxide remains, bound / AW, to Hb until in, region of low pCO2 or high pO2/ pulmonary circulation / lungs / alveoli; 6 ref. to (carbonic acid dissociates to form) hydrogen ions, which bind to / AW, haemoglobin ; A H+ forms haemoglobinic acid or HHb I hydrogen unqualified 7 AVP; e.g. hydrogencarbonate ions to plasma