Page 63 - Understanding NCERT Science 09
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The movement of water across the plasma Activity ______________ 5.3
membrane is also affected by the
amount of substance dissolved in water. Thus, Osmosis with an egg
osmosis is the net diffusion of water across a (a) Remove the shell of an egg by dissolving
it in dilute hydrochloric acid. The shell
selectively permeable membrane toward a
is mostly calcium carbonate. A thin
higher solute concentration.
outer skin now encloses the egg. Put
What will happen if we put an animal cell the egg in pure water and observe after
or a plant cell into a solution of sugar or salt 5 minutes. What do we observe?
in water? The egg swells because water passes
One of the following three things could into it by osmosis.
happen: (b) Place a similar de-shelled egg in a
concentrated salt solution and observe
1. If the medium surrounding the cell has
for 5 minutes. The egg shrinks. Why?
a higher water concentration than the Water passes out of the egg solution
cell, meaning that the outside solution into the salt solution because the salt
is very dilute, the cell will gain water by solution is more concentrated.
osmosis. Such a solution is known as We can also try a similar activity with dried
a hypotonic solution. raisins or apricots.
Water molecules are free to pass Activity ______________ 5.4
across the cell membrane in both
directions, but more water will come • Put dried raisins or apricots in plain
into the cell than will leave. The net water and leave them for some time.
Then place them into a concentrated
(overall) result is that water enters the
solution of sugar or salt. You will
cell. The cell is likely to swell up. observe the following:
2. If the medium has exactly the same (a) Each gains water and swells
when placed in water.
water concentration as the cell, there
(b) However, when placed in the
will be no net movement of water across
concentrated solution it loses
the cell membrane. Such a solution is water, and consequently shrinks.
known as an isotonic solution. Unicellular freshwater organisms and
Water crosses the cell membrane in most plant cells tend to gain water through
both directions, but the amount going osmosis. Absorption of water by plant roots
in is the same as the amount going out, is also an example of osmosis.
Thus, diffusion is important in exhange
so there is no overall movement of
of gases and water in the life of a cell. In
water. The cell will stay the same size.
additions to this, the cell also obtains
3. If the medium has a lower nutrition from its environment. Different
concentration of water than the cell, molecules move in and out of the cell
meaning that it is a very concentrated through a type of transport requiring use
solution, the cell will lose water by of energy.
osmosis. Such a solution is known as The plasma membrane is flexible and is
made up of organic molecules called lipids
a hypertonic solution.
and proteins. However, we can observe the
Again, water crosses the cell
structure of the plasma membrane only
membrane in both directions, but this
through an electron microscope.
time more water leaves the cell than
The flexibility of the cell membrane also
enters it. Therefore the cell will shrink. enables the cell to engulf in food and other
Thus, osmosis is a special case of diffusion material from its external environment. Such
through a selectively permeable membrane. processes are known as endocytosis. Amoeba
Now let us try out the following activity: acquires its food through such processes.
52 SCIENCE
Rationalised 2023-24
