The small intestine - structure and function

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					   The small intestine - structure and function
                    The small intestine - structure and function
    When you are using a microscope to examine the structure of a cell or an organ, it is
    easy to forget that it has a particular function. The ways in which cells, tissues and
    organs are adapted for their functions is a theme that recurs frequently in A-level
    biology. In this unit you will be looking at the structure of the small intestine.
    You should have been provided with three worksheets. This one contains a number of
    tasks and questions which should help you to look in detail at some aspects of the
    structure and function of the organ. Worksheet Ws2 contains some drawings made from
    slides seen under a light microscope. Worksheet Ws3 is a tracing of an electron
    micrograph of an epithelial cell, showing the position of its nucleus and the mitochondria
    present in its cytoplasm.
   As a starting point, think about the following question and keep the answer firmly in mind
   while you are working through this unit.
 1 What are the functions of the small intestine?
    Examine Figure 4.3 on worksheet Ws2. Look at the muscle layers labelled B and C.
2 (a) Which layer, -B or C, is circular muscle? How can you tell?
      (b) What will happen to the lumen of the small intestine
      when: (i) the circular muscle contracts;
        (ii) the longitudinal muscle contracts?
    (c) Suggest how these muscle layers
    help to move substances through the
   Measure the total thickness of muscle
   layers B and C, and the thickness of
   the small intestine wall, from the
   outside to the bottom of the villi. Use
   these     figures    to    calculate    the
   percentage of the total thickness of the
   small intestine wall that is muscle tissue.

3 Similar measurements made from
   sections taken through the wall of the
   stomach showed that approximately
   75% of its total thickness was muscle
   tissue. Suggest a reason for the
   difference between this figure and the one you calculated for the small intestine.
   An investigation was carried out into factors that influence the rate of uptake of glucose by
   pieces of rat intestine. This was measured for a range of different concentrations of
   glucose. In one set of experiments, the glucose was stirred; in the other, it was left unstirred.
   The graph in Figure 4.1 shows the results.

4 (a) Look at the curve that shows the rate of uptake of glucose when the contents of the
      intestine were unstirred.
       (i) Describe the relationship between the rate of uptake and the concentration of
           glucose in the intestine.
       (ii) What is the evidence from this graph that a mechanism other than simple diffusion
           is involved in uptake of glucose from the small intestine?
    The small intestine - structure and function

   (b) The villi in the small intestine are constantly moving. This is a result of contraction of the muscle in
      layer A and of smooth muscle cells in the villi. What do the results of this investigation suggest about
      the importance of villi being able to move?
   Now look at worksheet Ws3. This is a tracing of a single epithelial cell from one of the villi. The
   magnification of the drawing is x 20 000.

5. (a) Calculate the actual length of one microvillus in micrometres. Show your working.
   (b) By how many times does the possession of microvilli increase the surface of the cell along the
      transect shown on the drawing? Explain how you arrived at your answer.

  Escherichia coli (E. coli) is a species of
  bacterium commonly found in the intestines
  of humans and other animal In healthy
  individuals these bacteria use some of the
  food molecules in the intestine but also
  synthesise vitamins which are of use to their
  hosts. Some strains of E. coli, however, are
  harmful. Figure 4.2 is a drawing of an
  electron micrograph of part of an epithelial
  cell from the intestine of a rabbit which ha
  been infected with one of these strains of E.

6 (a) Describe how the bacteria have damaged this tissue.
    (b) Suggest how this damage might lead to diarrhoea in the infected animal.
   Inside the cell on Worksheet Ws3, the position of the nucleus is shown as well as all the
   mitochondria that were visible in the original photograph. The number of mitochondria in different
   cells can be compared by estimating their volume density. Place the transparent grid over the
   drawing of the cell. The grid lines intersect to give a number of points. Calculate the volume density
   from the ratio:
                               Number of points touching mitochondria
                               Total number of points touching the cell

7 Suggest a reason for the relatively high volume density of mitochondria in this cell.
   Finally, use all the information given in these worksheets to produce an answer to question 8.
   In producing your answer try to keep the following points in mind.
     What does the question really want you to do? It is asking you to explain so you are going to have to
      do a little bit more than just describe the various features. You need to explain how they are
      involved in the absorption of digested food.
     How much do you need to write? This is where the mark allocation comes in. Eight marks means
      that you should try to give eight valid points that you have learnt as part of your A-level course.
      When you have finished, check your work and make sure you have.
8 Explain how the structure of the small intestine is adapted to its function in absorbing digested food.
                                                                                                      (8 marks)

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