20 April 2015

#82 Question 5

The diagram below shows a small part of a human lung as it appears through a microscope.

(a) Name the type of blood vessel in which the red blood cell is present.
(1 mark)
(b) Describe and explain two ways in which the structure of the alveoli, shown in the diagram, enables gas exchange to take place rapidly.
(4 marks)
(c) Explain why large organisms such as mammals need specialised gas exchange surfaces, whereas small organisms such as a single-celled Amoebo do not.
(2 marks)
(Total 7 marks)

Candidate A

(a) capillary ü
* This is correct. 1/1

(b) They have a large surface area ü
     They are thin, so oxygen can diffuse across quicklyü

* The statement about a large surface area is correct, but the answer also needs to say why this enables gas exchange to take place rapidly (because the question asks you to 'explain'). The second answer is not sufficiently clear - what is thin? It is not the whole alveoli that are thin, but their walls. The second part of this answer does give a clear explanation of why this helps gas exchange to
take place quickly. 2/4

(c) Large organisms have small surface areas compared to their volume,ü so they need extra surface üto be able to get enough oxygen.

* There is a correct and clear statement about surface area to volume ratio, and the answer also lust gets a second mark. However, this isn't really very clear - see candidate B for a better explanation. 2/2

Candidate B

(a) capillary ü
* Correct. 1/1

(b) large surface area ü - so more oxygen and carbon dioxide molecules can diffuse across at the same time  ü
      good supply of oxygen - to maintain a diffusion gradient between the alveoli and the blood

* The first way is correct and well explained. However, the second, although true, does not answer the question which is about the structure of the alveoli. So just 2/04.

(c) They have small surface area to volume ratios, übut an Amoeba has a large surface area to volume ratio. The oxygen that diffuses in across the surface has to supply the whole volume ü of the animal, so in a large animal that is not enough and they have specialised gas exchange surfaces to increase the surface areaü and let more oxygen diffuse in.

* This is a good answer. All the important points are there and it is clearly expressed. 2/2

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