A-LevelBiologyImmunity9700/22 · May/June 2019 · Q317 Marks

Fig. 3.1 is a photomicrograph of human blood cells from a healthy individual who lives at sea level. The cells labelled C, D and E are white blood cells. [Figure 3.1]

(a)Name cells C, D and E.[3]
(b)(i)In humans, an increase in the white blood cell count can be associated with leukaemias and with infectious diseases, such as measles. Chronic lymphocytic leukaemia (CLL) is a type of cancer that starts in the bone marrow. In the early stages, many people with CLL feel well. The disease is sometimes diagnosed by chance during a routine blood analysis, when a high white blood cell count is noticed. Many of these white blood cells are only partially mature. Suggest why CLL starts in the bone marrow and not in any other location in the body.[2]
(b)(ii)Explain why a high white blood cell count is a feature of measles and of CLL.[3]
(c)(i)Most of the oxygen that enters the mammalian circulatory system is transported by red blood cells. Describe and explain the passage of oxygen across the cell surface membrane of the red blood cell.[2]
(c)(ii)At a high altitude, the partial pressure of oxygen in the atmosphere is lower than at sea level. If a person travels from low altitude to high altitude and remains there for a few weeks, the red blood cell count increases. Explain why the body needs to respond to high altitude by increasing the number of red blood cells.[3]
(d)Polypeptide synthesis occurs before a red blood cell is released into the circulation. The HBB gene codes for the β-globin polypeptide of haemoglobin. There are two alleles of HBB, known as HbA and HbS. Describe the difference between the HbA allele and the Hbs allele and state how this difference affects: • the β-globin polypeptide • the haemoglobin molecule.[4]

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 (May/June 2019). The mark or mark type is shown in brackets.

[3] C =neutrophil; A polymorphonuclear leucocyte A basophil A eosinophil
D = lymphocyte ; B / T is neutral
E = monocyte ; A macrophage I leucocyte / white blood cell, throughout
[2] any two from:
lymphocytes / blood cells, produced in / originate from, bone marrow ;
ref. to stem cell(s);
ref. to cell division; e.g. bone marrow has cells that can carry out, mitosis / cell division A (stem cells are) multipotent
[3] any three from:
measles: immune response stimulated;
detail; e.g. either antigen recognition / presence / binding or lymphocyte / white blood cell, stimulated / triggered plus clonal, expansion / proliferation / cell division / mitosis or clonal selection / AW R incorrect pathogen name R bacteria
CLL: result of, uncontrolled mitosis;
A uncontrolled, division / cell cycle I rapid mitosis
detail; owing to, mutation / named mutation or cell cycle checkpoints not working or cells don't respond to signals to control division
[2] any two from:
(passive/simple) diffusion (of oxygen) across the phospholipid bilayer ; A movement from a higher to a lower concentration for diffusion A between phospholipids / between fatty acid chains / across hydrophobic core / across bilayer / AW, for phospholipid bilayer
small size;
non-polar/uncharged ;
[3] any three from:
less oxygen in inhaled air / AW ; A less oxygen taken up from alveoli
low(er) saturation of haemoglobin (with oxygen) ; A Hb for haemoglobin A haemoglobin carries less oxygen must be the context of oxygen R saturation, with air / carbon dioxide
haemoglobin has lower affinity for oxygen ;
(so) more haemoglobin required (so more red blood cells) / (more red blood cells) so more haemoglobin; AW
compensation / compensates ;
A so same, volume / quantity / amount, of oxygen reaches body cells (compared to lower altitude)
[4] any four from:
1 (alleles have) different nucleotide sequences; A different base sequences
2 base substitution / substitution of a base; A T replaced by A
3 (alleles have) different mRNA codons ; A altered mRNA codon R altered genetic code
4 (results in) one amino acid change or change from, glutamic acid / glutamate / glu, to, valine / val or changed primary structure; A changed sequence of amino acids R if implies whole sequence changes
5 AVP; e.g. HbA = CTC and Hbs = CAC (on template strand) A HbA = GAG and Hbs = GTG (on non-template strand) or HbA = CTT and Hbs = CAT (on template strand) A HbA = GAA and Hbs = GTA (on non-template strand) this is not also mp2
6 ref. to amino acid substitution at position 6 (this is not also mp4) A 6th, codon / triplet, changed
7 changed, (β-globin / Hb) tertiary structure; A polypeptide / (Hb molecule) quaternary structure R ẞ-globin / polypeptide
ref. to loss of globular structure / hydrophobic amino acid faces outside (instead of towards centre); A described e.g. (val) amino acid with hydrophobic R-group instead of (glu) amino acid with hydrophilic R-group accept hydrophilic/polar, amino acid accept hydrophobic/non polar, amino acid
8 sticky (haemoglobin) molecule formed / forms fibres with other haemoglobin molecules ;
9 oxygen taken up less easily / less oxygen carried by haemoglobin / AW ; A ẞ-globin takes up oxygen less easily / AW
10 AVP; (sickle cell) haemoglobin / ẞ-globin, less soluble / AW I insoluble