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Biology Form 1
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Demonstrate Osmosis using Potato tuber and capillary tube

The diagram below shows a setup used to demonstrate a physiological process. Study it and answer the questions that follow.

(a) Name the physiological process being investigated in the setup above. 

  • Osmosis;

(b) (i) State the observation made after 60 minutes.        

  • Sugar solution rises in the capillary tube;

 (ii) Account for the observations made in the set up above. 

  • The sugar solution was hypertonic to the cells in contact hence water was drawn from the cells into the sugar solution by osmosis. In turn, the cell’s sap became hypertonic to adjacent inner cells hence drew in water from them by osmosis.
  • This successively continues from cell to cell until water is drawn from the water (hypotonic solution) in the Petri – dish. A continuous movement of water from the Petri – dish, through the cells and into the potato cavity is created. Resulting to increase in sugar solution in the capillary tube and a drop of water level in the Petri – dish.


Demonstrate Osmosis using Potato strips

In an experiment some slices were cut from a living potato tuber and were immersed in distilled water for one hour.  After that the slices were immersed in a concentrated salt solution for another one hour.  The slices were taken out of the liquid, weighed and immersed again in the liquid at regular intervals.  The results are shown in the graph below.  Point L shows the beginning of the experiment.

(a) Give the name given to the condition of the cells of the slices at point L (1 mark)

  • Hypertonic;

(b) (i) find the change in weight between L and M. (1 mark)

  • Change in weight = 20g – 18g = 2 g;

 (ii)  Which process brings about this change in weight?         (1 mark)

  • Osmosis;

(c) Explain the condition of the cells at point N.

  • Turgid cell with (hypotonic solution); the cell has maximally drawn in water by osmosis; turgor pressure equals wall pressure;

(d) The slices were removed from the concentrated salt solution at point P and washed in distilled water. Draw a curve on the graph, to show what would happen in the next one hour in the third section.    (1 mark)


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