Fig. 1 shows a stained blood smear from a patient attending a health clinic. The large cell is a white blood cell and the many smaller disc-shaped cells are...
Question 1 Report
Fig. 1 shows a stained blood smear from a patient attending a health clinic. The large cell is a white blood cell and the many smaller disc-shaped cells are red blood cells. Table 1 gives counts from a separate measured sample of the patient's blood.
| cell type | number of cells per mm3 of blood |
|---|---|
| red blood cells | 5 100 000 |
| white blood cells | 7200 |
(a) Describe three visible features of a red blood cell in Fig. 1. [3]
(b) Explain how the structure of a red blood cell allows efficient transport of oxygen. [3]
(c) Calculate the number of white blood cells expected in 0.50 mm3 of this blood. [2]
(d) Explain why the body needs mitosis to maintain the number of blood cells. [2]
(e) Design two improvements to the student's method if they counted cells in only one microscope field. [3]
Answer Details
(a) Three visible features of red blood cells are any three of: they are disc-shaped; biconcave or thinner in the centre; have no visible nucleus; and many are similar in size. [3]
(b) The biconcave shape gives a large surface area for diffusion. The cell is thin, giving a short diffusion distance. Red blood cells contain haemoglobin, which binds and carries oxygen. [3]
(c)
\[7200\ \text{cells mm}^{-3}\times0.50\ \text{mm}^3=3600\ \text{cells}\]
The expected number is 3600 white blood cells. [2]
(d) Blood cells are continually lost and have limited lifespans. Mitosis produces genetically identical new cells to replace them and maintain blood-cell numbers. [2]
(e) Count cells in several different microscope fields rather than only one. Calculate a mean count. Use the same sample volume each time, ideally using a counting chamber with a known volume, so counts are comparable. [3]
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