XII BIOLOGY
PRACTICAL-1
Make a temporary potato osmoscope to study osmosis.
Que.
1-………………………….4 Marks
Make
a temporary potato osmoscope to study osmosis. Sketch & label the diagram showing
the change in the water level.
OR
Make
a temporary slide to study plasmolysis. Sketch & label the diagram.
OR
Make
a temporary mount of leaf epidermis to study the structure & distribution
of stomata. Sketch & label the structure of stomatal apparatus.
OR
Find
out the adulteration of the given milk sample, for water and starch.
OR
Perform
the experiment to isolate DNA from given sample. Show your result to the
examiner.
Q.
1 Make a temporary potato osmoscope to study osmosis. Sketch & label the
diagram showing the change in the water level.
Preparation of
potato osmoscope……… 2Marks
Observation /
Conclusion…………………….1 Mark
Diagram of
Osmoscope………………………..1 Mark
Total-4 Marks
Aim:-To study the process of osmosis
using potato osmoscope.
Requirements:-
A fresh potato,
scalpel, glass beaker, pins, conc. Sugar solution, water, etc.
Procedure:
(1)
Take a fresh peeled potato tuber and cut it from one side to make a flat base.
(2)
Scoop the tuber with scalpel so as to make a cavity leaving a thin base.
(3)
Add concentrated sugar solution in the cavity and mark the initial level using
a pin.
(4)
Place the potato in a petridish or a glass bowl
containg coloured water.
(5)
This set up is called potato osmoscope or potato osmometer.
(6)
Leave the set up undisturbed for a few hours.
(7)
Observe the level of sugar solution.
Observation: -
After a few hours the level of sugar solution in the
potato asmoscope rises and thesolution becomes coloured.
Conclusions:
(1)
The rise in the sugar solution is due to osmosis.
(2)
The membranes of the living cells of potato tuber function as semipermeable
membrane.
(3)
Water from outside (hypotonic) enters the cavity filled with concentrated sugar
solution (hypertonic) thus demonstrating osmosis.
Procedure
Slice the potato tuber into two equal halves
with the help of a scalpel or a blade.
The
outer skin is to be peeled off. Since the tuber shape is irregular, slice the
halves into squares
From the mid-region of the tuber, scoop from
the soft parenchyma, so as to form a tiny cavity of a square or a circular
shape.
At the base, the cavity prepared should have a
minimum thickness.
Fill up half the cavity with the freshly
prepared 20% sugar solution.
Into the cavity, fix a pin in a way that the
mark is in the same line with the layer of the sucrose solution.
Set up the osmometer in a Petri dish/beaker
that is filled with water in a way such that 75% of the potato osmometer is
immersed in water
The set up should remain uninterrupted for
close to 1 hour.
Notice the sugar solution in the osmometer
towards the end of the experiment
Carry out the experiment with the help of
water in the cavity and the sucrose solution in the petri dish/beaker.
Observation-
After a period of
time, within the osmoscope, the sugar solution rises and is seen coloured.
Conclusion-
An increase in the level of sucrose solution
is observed in the osmometer. It is because of the entrance of water due to
endosmosis from the beaker.
Also, a water potential gradient is built
between the sucrose solution in the external water and the osmometer.
Though both the liquids are divided by living
cells of the potato tuber, they allow the entrance of water into the sugar
solution.
This demonstrates the entrance of water into the sugar solution through the tissues of potato serving as a selectively permeable membrane.
What is Osmosis?
Osmosis is the phenomena in which solvent
molecules pass through a semi-permeable membrane from an area of higher
concentration to an area of lower concentration.
The process continues until the quantity of
fluid is balanced or equalized in both regions, the region of higher
concentration and the region of lower concentration of the semipermeable
membrane.
In
other words, osmosis is
the diffusion or movement of water from a region of higher water potential to a
region of lower water potential.
In osmosis, what are solvent and solute?
The fluid that
permeates through the semipermeable membrane is called the solvent, whereas the
solute is the dissolved particles in the fluid.
What is the solution?
The mixture of solute
and solvent form the solution.
List
the different types of solutions.
The
following are the types of solutions:
1.Hypertonic solution – It is a solution with a high solute level. If living cells
are placed in a hypertonic solution, because of lower concentration water moves
out of the cell causing it to shrink and becomes plasmolyzed.
2.Hypotonic solution – It is a solution with low concentration levels of solute.
If living cells are placed in this solution, water passes into the cells
because of higher water concentration in comparison to the cell causing the
cells to swell and turn turgid.
3.Isotonic solution – A solution is said to be isotonic if both solutions have
an equal concentration of solute. If living cells are placed in an isotonic
solution, no change is shown as there is the equal concentration on both the
regions hence the cell retains its original shape.
VIVA VOCE-
(1)
Potato is used to prepare an osmoscope. Justify.
Ans:
Peeled
potato represents a living system to demonstrate the process of osmosis. The
cell membranes of potato act as the semipermeable membranes and allow the
passage of water from hypotonic solution to hyperonic solution.
(2)
Define osmotic pressure
Ans:
Osmotic
pressure is equivalent to the pressure which must be exerted upon the solution
to prevent the flow of solvent across a semi-permeable membrane.
(3)
Define permeability.
Ans:
Permeability
is defined as the property of membranes to allow passive diffusion of
molecules.
(4)
Do you find any role of osmosis in water purification?
Ans:
In
water purification, reverse osmosis is used. It is the reverse movement of
water through a semipermeable membrane from a more concentrated solution to a
more dilute solution by applying external presure on the more concentrated
solution. It is used for removing salts from saline water.
(5)
Osmosis plays important role in stomatal movement. Explain.
Ans:
Stomatal
movement depends on the turgidity of guard cells. Guard cells become turgid due
to endosmosis and the stoma opens. Guard cells become flaccid due to exosmosis
and the stoma closes.
(6)
What is the use of osmosis to plant cell.
Ans:
The
cells maintain their turgidity by this process, roots absorb water from the
soil by this process.
(7)
Why did you use sugar solution?
Ans:
Any
strong solution such as of salt also can be used. It is used to develop osmotic
difference/suction pressure.
(8)
Why the set up of potato is called osmometer / osmoscope?
Ans:
It
is so called because the difference in the initial and find levels of solution
in the potato is cavity can be measured/seen which is due to osmosis.
MULTIPLE
CHOICE QUESTIONS
(1)
In living cells, an osmosis specifically refers to the __________.
(a)
diffusion of solvent
(b)
diffusion of solute
(c)
diffusion of water
(d)
diffusion of solute and solvent both
(2)
The direction and rate of osmosis is controlled by ____________.
(a)
nature of property of membrane
(b)
nature of solvent
(c)
pressure gradient and concentration gradient
(d)
cell structure and composition of cytoplasm
(3)
The structure that contributes to the solute potential of a cell is
____________ .
(a) vacuolar sap
(b) cell sap
(c) water
(d) solute particles
(4)
Osmotic potential refers to _________ .
(a)
movement of water molecules from hypotonic solution to hypertonic solution
(b)
movement of water molecules from hypertonic solution to hypotonic solution
(c)
pressure potential
(d)
movement of water molecules from hypotonic to isotonic solution
(5)
Which of the following is not semipermeable?
(b) Egg membrane
(c)
Bladder membrane
(d)
Cell wall
(6)
Which acid is used to extract egg membrane?
(a)Acetic
acid
(b) Dilute HCl
(c)
Concentrated HCl
(d)
Dilute H2SO4
(7)
With reference to tonocity the solution outside the osmoscope should be _______
.
(a)Hypertonic
(b) Hypotonic
(c) Isotonic
(d) Dilute
Ans:
(1) - (c), (2) - (c), (3) - (d),
(4) - (a),
(5) - (d), (6) - (a), (7) - (b).