A student used a simple model to investigate how the folded lining of the small intestine increases the surface area available for absorption. Three tubes o...
Question 1 Report
A student used a simple model to investigate how the folded lining of the small intestine increases the surface area available for absorption. Three tubes of the same length were lined in different ways. One tube was left smooth, one was lined to represent villi, and one was lined to represent villi that also carried microvilli on their surface. The total surface area of each tube lining was then measured. Fig. 7.1 gives the surface areas that were obtained, and Fig. 7.2 shows the three tube linings that were tested.
| Surface of tube | Surface area (cm2) |
|---|---|
| Smooth tube (no villi) | 200 |
| Tube lined with villi | 2000 |
| Tube lined with villi and microvilli | 40000 |
(a) State how many times greater the surface area of the villus-lined tube is than that of the smooth tube. [1]
(b) Calculate how many times greater the surface area is when microvilli are also present, compared with the smooth tube. Show your working. [2]
(c) Name the tiny projections that add the extra surface area. [1]
(d) Explain why a large surface area is an advantage for absorption. [3]
(e) Suggest one reason why results from a model tube may differ from those of a real small intestine. [2]
Answer Details
(a) \( \dfrac{2000}{200} = \mathbf{10}\ \text{times} \) greater. [1]
(b) Compare the villi-plus-microvilli area with the smooth tube: \( \dfrac{40\,000}{200} \) [1] \( = \mathbf{200}\ \text{times} \) greater. [1] This shows how folding (villi) and then tiny projections (microvilli) multiply the surface area enormously.
(c) The tiny projections that add the extra surface area are the microvilli. [1]
(d) A large surface area is an advantage because (max [3]): more digested food can be absorbed [1]; absorption happens faster / more can be absorbed at the same time [1]; and more food molecules are in contact with the absorbing surface at once [1].
(e) Results from the model differ because a real small intestine has features the simple tube lacks (one point developed for 2 marks): a real intestine has a moving blood supply / mucus / digestive enzymes / a good capillary network [1] which keeps a concentration gradient and carries absorbed food away, whereas the simple model tube has none of these [1].
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