Page 64 - Understanding NCERT Science 09
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Activity ______________ 5.5 What do we infer from this activity? It
appears that only living cells, and not dead
• Find out about electron microscopes cells, are able to absorb water by osmosis.
from resources in the school library or Cell walls permit the cells of plants, fungi
through the internet. Discuss it with
your teacher. and bacteria to withstand very dilute
(hypotonic) external media without bursting.
In such media the cells tend to take up water
uestions
Q 1. How do substances like CO and by osmosis. The cell swells, building up
pressure against the cell wall. The wall exerts
2
water move in and out of the cell?
an equal pressure against the swollen cell.
Discuss.
Because of their walls, such cells can
2. Why is the plasma membrane
called a selectively permeable
surrounding medium than animal cells.
membrane? withstand much greater changes in the
5.2.3 NUCLEUS
5.2.2 CELL WALL
Remember the temporary mount of onion peel
Plant cells, in addition to the plasma we prepared? We had put iodine solution on
membrane, have another rigid outer covering the peel. Why? What would we see if we tried
called the cell wall. The cell wall lies outside observing the peel without putting the iodine
the plasma membrane. The plant cell wall is solution? Try it and see what the difference
mainly composed of cellulose. Cellulose is a
complex substance and provides structural is. Further, when we put iodine solution on
strength to plants. the peel, did each cell get evenly coloured?
When a living plant cell loses water According to their chemical composition
through osmosis there is shrinkage or different regions of cells get coloured
contraction of the contents of the cell away differentially. Some regions appear darker
from the cell wall. This phenomenon is known than other regions. Apart from iodine solution
as plasmolysis. We can observe this we could also use safranin solution or
phenomenon by performing the following methylene blue solution to stain the cells.
activity: We have observed cells from an onion; let
Activity ______________ 5.6 us now observe cells from our own body.
• Mount the peel of a Rhoeo leaf in water Activity ______________ 5.7
on a slide and examine cells under
the high power of a microscope. Note • Let us take a glass slide with a drop of
the small green granules, called water on it. Using an ice-cream spoon
chloroplasts. They contain a green gently scrape the inside surface of the
substance called chlorophyll. Put a cheek. Does any material get stuck on
strong solution of sugar or salt on the the spoon? With the help of a needle
mounted leaf on the slide. Wait for a we can transfer this material and
minute and observe under a
spread it evenly on the glass slide kept
microscope. What do we see? ready for this. To colour the material
• Now place some Rhoeo leaves in boiling we can put a drop of methylene blue
water for a few minutes. This kills the
cells. Then mount one leaf on a slide solution on it. Now the material is ready
and observe it under a microscope. Put for observation under microscope. Do
a strong solution of sugar or salt on not forget to put a cover-slip on it!
the mounted leaf on the slide. Wait for • What do we observe? What is the
a minute and observe it again. What shape of the cells we see? Draw it on
do we find? Did plasmolysis occur now? the observation sheet.
THE FUNDAMENTAL UNIT OF LIFE 53
Rationalised 2023-24
