A-LevelBiologyTransport in mammals9700/23 · Oct/Nov 2021 · Q311 Marks

Fig. 3.1 is a transmission electron micrograph showing red blood cells in a capillary of a healthy adult. [Figure 3.1]

(a)(i)Explain how the cells in the capillary shown in Fig. 3.1 can be identified as red blood cells.[3]
(a)(ii)Capillaries are surrounded by tissue fluid. Outline the ways in which the composition of tissue fluid differs from blood.[2]
(b)(i)Scientists compared features of the physiology of three groups of people who live at different altitudes. These people were selected from populations that have lived at these altitudes for many thousands of years. The scientists took blood samples from people in each location and measured: • the concentration of haemoglobin in the blood • the percentage saturation of haemoglobin with oxygen in blood leaving the lungs • the oxygen concentration in the blood leaving the lungs. The results are shown in Table 3.1. [Table 3.1] Describe the changes in mean percentage saturation of haemoglobin with oxygen and the mean haemoglobin concentration in blood as altitude increases.[3]
(b)(ii)With reference to Table 3.1, suggest how the people living at high altitude can have an oxygen concentration in blood leaving the lungs similar to that of the people living at sea level.[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 2021). The mark or mark type is shown in brackets.

[3] any three from:
1 no, nucleus / organelles ; ora e.g. 'if they were white blood cells ...'
2 cytoplasm is, homogeneous / AW ;
3 about same, size / width / diameter, as lumen of capillary ;
4 cells about 7 µm in diameter / AW ;
5 flexible/variety of shapes / irregular shapes ;
6 AVP; e.g. rouleau
[2] assume answers are about tissue fluid unless told otherwise any two from:
1 no red blood cells ;
2 no platelets;
3 fewer (named) protein(s) / no large proteins / no plasma proteins; A no named plasma proteins (albumen / fibrinogen) A fewer plasma proteins only if stated that they are, small / leave blood
4 less, glucose / amino acids / fatty acids ;
5 fewer, white blood cells / leucocytes / neutrophils / monocytes ; A more macrophages
6 less/lower concentration of, oxygen or more / higher concentration of, carbon dioxide; I urea except in liver and muscle tissue R 'waste'
[3] ignore any explanations
(mean) percentage saturation of haemoglobin with oxygen (in blood leaving the lungs) decreases ;
(mean) haemoglobin concentration in blood increases ;
any suitable comparative data quote with altitude in m and mean Hb concentration in g 100 cm⁻³ or percentage saturation;
[3] any three from:
1 more red blood cells ;
2 larger red blood cells ;
3 more haemoglobin per red blood cell ;
4 more alveoli/larger surface area for gas exchange ; A bigger lungs / larger chest volume / broader chest / larger capacity of lungs (total or vital)
5 higher, cardiac output/ stroke volume / AW; A higher blood pressure in pulmonary artery
6 AVP;
7 AVP; e.g. higher tidal volume / deeper breaths higher ventilation rate (minute volume) I faster breathing ref. to erythropoietin / EPO, stimulating production of (more) red blood cells more red blood cells through alveolar capillaries per unit time