Plasma Membrane
Activity-1:
Get in-go out
Let us look at the substances in the
table. Some are used by the cells while
some are discarded by the cells.
Preparing Solutions:
To prepare sugar solution we need sugar
and water. In a sugar solution sugar is the
solute and water is the solvent. Sugar
dissolves in water forming sugar solution.
Preparation of saturated solution :
Take 100 ml of water in a beaker.
Add sugar/salt. Stir till it is dissolved.
Repeat it till little amount of it is left at
the bottom of the beaker which will not
dissolve. This is the saturated solution of
sugar/salt (in cold water).
Which one is more concentrated
solution?:
Take three beakers with one hundred ml.
of water in each. Add half teaspoon of sugar
to the first beaker, one teaspoon to the
second and one and a half teaspoon to the
third. Compare the three solutions and
answer the following. The solution of
which beaker will be most sugary? What is
the reason? Can we convert the solution of
beaker I into solution of beaker III?
How? How can we make the solution of the
third beaker indicated to that of the
first? How much water should we add to the
solution in the third beaker to make it
similar to solution of the first beaker?
Solutions with different amount of solute
dissolved in them are solutions of different
concentrations. The amount of sugar present
in the 100 ml of water is the concentration
of the sugar.
Which beaker has the most
concentrated solution?
Lab Activity:
Aim : Observation of material in different
solutions
Material: 1. Two beakers 2. Tap water
3. Sugar 4. Dry grapes or kishmish
Procedure: Take 100 ml of water in a
beaker. Keep dry raisin (kishmish) in it.
Kishmish kept in tap water
Leave it for one hour. Observe what has
happened. Take it out and compare it with
the dry raisin. Is there any change in the
size of kishmish. (You may try the same
activity with slightly dried carrots and other
such vegetables) Did you ever observe that
your mother dipped wilted vegetables in
water. Can you identify the reason.
Then take 100 ml of saturated solution
of sugar in a beaker, which was already
prepared.
Swollen kishmish kept in tap water
Keep swollen kishmish of the previous
activity or fresh grape or carrot in it.
Leave it overnight. And observe in the
next morning what happens. Do you find
any change in the size of kishmish.
In the lab activity - 2a, water is
moving from …...............…… to
………….............
In the lab activity - 2b, water is moving
from …....……..… to ………......….
Let us think how water goes in and
comes out of Kishmish. Does the layer
permits water to pass through?. How does
it works? The cells in the outer layer of
kishmish permit water to pass through.
Learn more about the process. Let us
do the following activity.
Lab Activity:
Osmosis
Materials needed: One raw potato,
beaker / bowl, two pins, Water, Sharp knife.
Procedure: Take the raw potato and boiled
potato. Peel off the skins. Cut cube shaped
cups from them as shown in the figure (you
could make some other shapes as well).
Prepare a few amount of saturated sugar
solution.
Pour the sugar solution in the potato
cup to a level as shown in the figure.
Mark the level of sugar solution by
piercing the pin.
Keep the potato cup in the bowl/beaker
scoop to make a cup
Pour water in the beaker / bowl to half
of the height of the potato cup. Ensure that
the potato cup does not float or submerge
in water.
Leave the arrangement for half an hour
and note your observations
Repeat the above experiment by taking
sugar solution in the beaker and water in
the potato cup. Note your observation and
compare with that of your previous one.
By comparing potato and kishmish
activities, do you find anything in common
among them? What is it?
From your observations you can
conclude that water always moved towards
the sugar solution. What might be the
reason? Such process, in which water moves
towards sugar solution is called Osmosis.
(In Greek Osmos means pushing).
We see that the movement of water
through membrane in osmosis i.e. from
less solute concentrated to more solute
concentration. To understand how materials may be
moving in and out of cells through the cell
membrane, let us do the following activity.
Activity-2:
Filtration
To perform this activity we need the
following materials:
Two beakers, Funnel, Filter paper,
Retort stand, Sugar, Iodine and Wheat / rice
flour, 500ml plastic bottle, cotton cloth.
Procedure :
1. Arrange the filtering apparatus as
shown in the figure (4a) or the
alternative method as in figure (4b).
2. Prepare wheat or rice powder
solution in a beaker by adding one
tea spoon of powder in 100ml of
water.
3. Add a drop of tincture iodine to the
solution.
4. Now pour the solution into the
funnel.
Then observe,
What remains on the filter paper or
the cloth?
What did the filter paper/ cloth
allow to pass through?
Which substance is not allowed by
the filter paper to pass through?
Why are certain substances not
allowed to pass through the filter
paper? Guess.
Cells also act in the same way while
allowing the substances to pass through the
plasma membrane.
With the understanding of above
activities let us try to understand the nature
of the Plasma membrane.
a) It allows water to pass through it.
b) It allows certain materials dissolved in
water to pass through.
c) It will not allow certain materials to
pass through it.
Allowing materials to pass through is
called permeability.
In the previous activities we have
observed the movement of water and
materials in plants.
To know more details about plasma
membrane or cell membrane, let us go
through the following paragraphs.
The outermost, extremely delicate,
elastic and membranous covering of the
cell that separates its contents from the
external environment is called plasma
membrane. Protoplasm of a cell consists
of two parts, cytoplasm and nucleus.
Plasma membrane is selectively
permeable because it allows entry of
certain substances, exit of some
substances while preventing passage to
remaining substances. Plasma membrane is
a living flexible membrane.
Functions of plasma membrane
Shape: It provides a definite shape to semi
fluid contents of the cell.
Mechanical Barrier: It functions as a
mechanical barrier that protects the internal
contents of the cell.
Selective Permeability: The membrane
determines what substances are to be
allowed entry or exit from the cell.
Endocytosis: The flexibility of the
membrane enables the cell to engulf food
and other substances (foreign particles)
from its external environment by
endocytosis. Amoeba acquires its food by
this process.
Recognition: It has substances over its
surface which function as recognition
centres and points of attachment. They help
in tissue formation, distinction of foreign
substances and defence against microbes.
Flow of Information :Helps for flow of
information amongst different cells of the
same organism.
Osmosis: It occurs due to presence of tiny
water channels in the plasma membrane.
Cell Continuity: At places plasma
membranes of adjacent cells become
continuous to form plasmodesmata and cell
junctions.
Specialization: Plasma membrane gets
modified to perform different functions,
e.g. absorption in microvilli.
Diffusion
There are other ways in which materials
move in a medium. Let us study one such
process by the following activity.
If a bottle of scent is opened in one
corner of a room, what will happen?
How do we feel? Its smell spreads in
the entire room. Let us think of the
following.
How does the smell spread in the
entire room?
Is the smell spreading uniformly in
the entire room?
Can you reach the scent bottle if
you are blind folded?
When blind folded how you are
able to locate the scent bottle?
Can you give any other such
examples?
There are other processes in which
substances move in a medium. Let us study
another such process by the following
activity.
Activity-4:
Diffusion with coffee powder
Experiment with coffee powder. Let us
Take half bowl of water. Prepare a small
ball of coffee powder. Slowly put it in
water. Observe what happens. Write a note
on what you observed.
Do this activity as many ways as you
could like.
Putting the pinch of coffee powder
first and adding water slowly.
Pouring hot water on the pinch of
coffee powder etc.
Note your observations for each situation
and write down what you infer.
Let us see whether other substances
also behave in the same manner.
Activity-5
Keep a small Crystal of KMnO4
(Potassium permanganate) in the
centre of the Petridish with the help
of a Forceps.
Carefully fill the Petridish with
water. (It is better to use filler or
dropper)Observe the movement of Pink
Colour in the Petri dish everyminute.
Also observe the spreading of colour
from center to periphery.
Repeat the experiment with other
substances (E.g.: copper sulphate) and
compare the result.
Do you find any relation between
Time and Movement?
For better understanding, the
movements of molecules in water, observe
the following activity.
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