The Fundamental Unit of Life

Activity 5.1

• Let us take a small piece from an onion bulb. With the help of a pair of forceps, we can peel off the skin (called the epidermis) from the concave side (inner layer) of the onion. This layer can be put immediately in a watch glass containing water. This will prevent the peel from getting folded or getting dry.

Question: What do we do with this peel?

Answer: We will observe the epidermis of onion peel under the microscope.

• Let us take a glass slide, put a drop of water on it, and transfer a small piece of the peel from the watch glass to the slide. Make sure that the peel is perfectly flat on the slide. A thin camel hair paintbrush might be necessary to help transfer the peel. Now we put a drop of safranin solution on this piece followed by a cover slip. Take care to avoid air bubbles while putting the coverslip with the help of a mounting needle. Ask your teacher for help. We have prepared a temporary mount of onion peel. We can observe this slide under low power followed by high powers of a compound microscope.

Question to be answer

Question: (1) What do we observe as we look through the lens?

Answer

Observation through the lens:

(a) Many similar continuous rectangular structures with a dot in the middle have been seen. These structures look similar to hone a honeycomb.

Conclusion: These are the cells of the onion peel with the nucleus in the middle.

Question: (2) Can we draw the structures that we are able to see through the microscope, on an observation sheet?

Answer

Yes, the structures seen through the microscope can be drawn through the microscope.

Question: (3) Does it look like Fig. 5.2?

Answer

Yes, the structures seen through the microscope look like figure 5.2.

Activity 5.2

• We can try preparing temporary mounts of leaf peels, tips of roots of onions, or even peels of onions of different sizes.

• After performing the above activity, let us see what the answers to the following questions would be:

Question: (a) Do all cells look alike in terms of shape and size?

Answer

Yes, all cells look alike in terms of shape and size.

Question: (b) Do all cells look alike in structure?

Answer

Yes, all cells look alike in structure.

Question: (c) Could we find differences among cells from different parts of a plant body?

Answer

Yes, there are some differences in shapes and sizes. But there is no difference among the cells from different parts of a plant body in their basic structures. All cells are rectangular in shape with a nucleus near the middle.

Question: (d) What similarities could we find?

Answer

All cells have similar basic structures with nuclei in the middle.

Activity 5.3

Osmosis with an egg

(a) Remove the shell of an egg by dissolving it in dilute hydrochloric acid. The shell is mostly calcium carbonate. A thin outer skin now encloses the egg. Put the egg in pure water and observe after 5 minutes.

Question: What do we observe?

Answer

The egg swells because water passes into it through the process of osmosis.

Conclusion

The egg swells because it was kept in a hypotonic solution. Since the concentration is higher in the egg cell thus water molecule passes through osmosis in the egg cell. Resulting in egg swells. This process is known as plasmolysis.

Question: (b) Place a similar de-shelled egg in a concentrated salt solution and observe for 5 minutes. The egg shrinks. Why?

Water passes out of the egg solution into the salt solution because the salt solution is more concentrated.

Answer

The egg shrinks because water passes out from the egg cell to the concentrated salt solution. This happens because of osmosis.

Here the concentrated salt solution is called a hypertonic solution. If the concentration of a solution is higher than the other solution, such a solution is called a hypertonic solution. The passing out of the water from the egg cell to the concentrated salt solution is called deplasmolysis or cytolysis.

Thus, because of cytolysis egg cell shrinks.

Activity 5.4

Put dried raisins or apricots in plain water and leave them for some time. Then place them into a concentrated solution of sugar or salt. You will observe the following:

(a) Each gains water and swells when placed in water.

(b) However, when placed in the concentrated solution it loses water and consequently shrinks.

Question: (1) Why do dry raisins swell when placed in water?

Answer

Dry raisins get swollen because they absorbed water. This happens because the molecules of water pass to the dry raisins' cells through osmosis by the process of plasmolysis.

Question: (2) Why do dry raisins shrink when placed in concentrated salt solution?

Answer

Dry raisins get swollen because they absorbed water. This happens because the molecules of water pass to the concentrated solution from the raisins' cells through osmosis by the process of deplasmolysis, and dry raisins shrink.