A-LevelBiologyInherited change9700/42 · Feb/Mar 2022 · Q314 Marks

The red blood cells of people with sickle cell anaemia have reduced oxygen-carrying capacity.

(a)Explain the relationship between the gene mutation that causes sickle cell anaemia and the reduced oxygen-carrying capacity of red blood cells.[5]
(b)(i)People with sickle cell anaemia may need a blood transfusion. One risk associated with a blood transfusion is a condition known as transfusion-associated circulatory overload (TACO). TACO is caused by a large increase in blood volume over a short period of time. This increase in blood volume can be harmful. (i) Predict the effect of an increase in blood volume on ADH secretion and state one consequence for kidney function of this change in ADH secretion.[1]
(b)(ii)One feature of TACO is an increase in fluid entering the alveoli, which makes it difficult to breathe. This increase in fluid can be caused by an increase in blood pressure in the pulmonary capillaries that surround the alveoli. Suggest how an increase in blood pressure in the pulmonary capillaries can cause fluid to enter the alveoli.[1]
(c)People with kidney disease may be at a higher risk of TACO following a blood transfusion. A study carried out in 2019 investigated whether there is a link between kidney disease and TACO. This study included data from: • 200 people who developed TACO after a blood transfusion • 405 people who did not develop TACO after a blood transfusion. The people in the study were put into one of four categories: • people with acute (short-term) kidney injury (AKI) • people with chronic (long-term) kidney disease (CKD) who do not require dialysis • people with severe chronic kidney disease who do require dialysis (CKD + D) • people who do not have kidney disease. In dialysis, a machine is used to carry out the function of the kidneys. The results of this study are shown in Fig. 3.1. [Figure 3.1] With reference to Fig. 3.1, describe the effect of having kidney disease on the risk of developing TACO. Suggest reasons for this effect.[4]
(d)Patients who are at high risk of developing TACO following a blood transfusion can be given a type of drug called a loop diuretic. In normal urine production, 99% of sodium ions in the glomerular filtrate are reabsorbed and 1% are excreted. In urine production of people who take loop diuretics, 80% of sodium ions in the glomerular filtrate are reabsorbed and 20% are excreted. Explain how loop diuretics affect water reabsorption in the kidneys and suggest why this reduces the risk of developing TACO following a blood transfusion.[3]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2022). The mark or mark type is shown in brackets.

[5] any five from: max four from mp1–mp6: 1 (sickle cell anaemia is) homozygous recessive ; 2 ref. to beta / β, globin / polypeptide or HBB (allele / gene) ; 3 base/nucleotide, substitution or mis-sense mutation or change in, triplet / codon or GAG to GTG / A to T; 4 causes, change in amino acid (sequence) / glutamic acid to valine ; 5 valine is non-polar; 6 causes change in, 3D shape / tertiary structure / quaternary structure / globular structure (of haemoglobin) ; 7 haemoglobin has lower affinity for oxygen ; 8 (in red blood cells) haemoglobin, less soluble / sticks together / forms fibres ; 9 (red blood) cells become sickle shaped ;
[1] decrease (in ADH) (accept no ADH secreted) and reduce water reabsorption (in the collecting duct) or decrease (in ADH) and increased volume of/ more dilute, urine (formed);
[1] idea that fluid / water, is forced (from blood) into alveoli; R diffusion / osmosis
[4] any four from: describe: 1 having kidney disease increases the risk of TACO; ora 2 AKI has highest risk (of TACO) ; 3 data quote to support mp1 (comparison); reasons: 4 kidney disease reduces ability to remove excess water ; 5 more water retained increases blood volume ; 6 AVP; e.g. CKD + D less risk as dialysis helps to restore water volume
[3] any two from: 1 fewer sodium (ions) reabsorbed / more sodium (ions) in filtrate ; 2 decreases water potential of filtrate or increases water potential of the medulla ; 3 less water reabsorbed, by osmosis / down water potential gradient ; 4 increases volume of urine ; plus: 5 so blood volume (from transfusion) is reduced ;