Ch 3 Movement of substances across cell membrane
Practical 3.1
Demonstration of osmosis using dialysis
tubing
Results (p. 3-2)
Set-up
Change in liquid level in the capillary tube
Experimental
Rises
Control
Lowers until it reaches the liquid level of water in the beaker
Questions (p. 3-3)
1
Its small bore gives a more obvious change in liquid level.
2
Sucrose solution on the outside of the tubing will affect the result. Rinsing the
tubing ensures no such sucrose solution is present.
3
a
There is a net water movement from distilled water to sucrose solution.
b
Osmosis.
c
Differential permeability.
4
After a certain period of time, the force produced by the weight of the liquid
column balances the force developed by the water potential gradient.
5
a
The liquid level will rise faster and higher.
b
The liquid level will drop and the tubing will eventually shrink.
Conclusion (p. 3-3)
When sucrose solution is separated from distilled water by a dialysis tubing, osmosis
takes place and there is a net movement of water molecules from distilled water to
sucrose solution.
Practical 3.2
Demonstration of osmosis using living
animal tissue
Results (p. 3-6)
Set-up
Change in liquid level in the thistle funnel
A
Rises
B
Lowers until it reaches that of water in the beaker
Questions (p. 3-6)
1
Set-up B is a control. It shows that any change in liquid level in set-up A is due to
the concentrated sucrose solution.
2
Distilled water has a higher water potential than concentrated sucrose solution, so
there is a net movement of water from the distilled water to concentrated sucrose
solution through the differentially permeable animal tissues by osmosis. The
volume of liquid inside the thistle funnel increases and the liquid level rises.
Conclusion (p. 3-7)
Living animal tissues are differentially permeable. When solutions with different
water potential are separated by living animal tissues, osmosis takes place.
Practical 3.4
Study of osmosis in living plant cells
Results (p. 3-12)
In concentrated sucrose
In less concentrated
In very dilute sucrose
solution
sucrose solution
solution
Questions (p. 3-13)
1
To prevent the evaporation of sucrose solution, which may change its water
potential and affect the results. This also provides a flat surface for observation
and keeps the objective lens of the microscope clean.
2
The cytoplasm swells up gradually until the cell membrane presses tightly against
the cell wall.
3
No. This is because the concentration of the content / water potential of each cell
varies.
Conclusion (p. 3-13)
When the surrounding fluid has a lower water potential than the plant cells, water
leaves the cells by osmosis. The cells finally become plasmolyzed and flaccid. When
the water potential of the surrounding fluid increases, water will enter the cells by
osmosis. The cytoplasm expands and the cells become turgid.
Practical 3.5
Study of osmosis in living plant tissue
Results (p. 3-15)
Initial weight (g)
Liquid
inside the
Strip
beaker
1
2
3
Distilled
Water
2.81
2.76
2.80
10%
sucrose
solution
2.76
2.87
2.84
2.83
Strip Strip
Final weight (g)
Average
Strip Strip Strip
Average
Percentage
change in
weight (%)
1
2
3
2.79
3.08
3.15
3.07
3.10
11.11%
2.83
2.82
2.79
2.80
2.81
2.80
-0.71%
2.85
2.84
2.57
2.62
2.55
2.58
-9.15%
20%
sucrose
solution
Questions (p. 3-16)
1
Osmosis cannot takes place across the potato peel because the peel is impermeable
to water. Any peel left on the potato strips will affect the result.
2
To prevent the evaporation of water which may change the concentration of the
liquids in the set-ups and affect the results.
3
To absorb the surplus water on the surface of the potato strips which may increase
the weight of the potato strips and affect the results.
4
To minimize the water loss from the potato strips by evaporation. Any water loss
will decrease the weight of the potato strips and affect the results.
5
The potato strips in distilled water become heavier. This is because the water
potential of distilled water is higher than that of the potato cells. Water enters the
potato strips by osmosis.
6
The weight of the potato strips in 10% sucrose solution changes very slightly. This
is because the water potential of 10% sucrose solution is nearly the same as that of
the potato cells. There is almost no net movement of water into or out of the potato
strips.
7
The potato strips in 20% sucrose solution become lighter. This is because the
water potential of 20% sucrose solution is lower than that of the potato cells.
Water leaves the potato strips by osmosis.
Conclusion (p. 3-17)
Living plant tissues are differentially permeable. When living plant tissues are placed
in solutions with different water potential, osmosis takes place.
Practical 3.6
Examination of phagocytosis in Amoeba
Results (p. 3-19)
Questions (p. 3-19)
1
Amoeba takes in food particles by phagocytosis: when Amoeba gets close to the
food particles, pseudopodium starts to form to surround the food particles. The
whole food particles are finally engulfed by the Amoeba.
2
Phagocytosis is important for:
1
the nutrition of some single-celled organisms, e.g. Amoeba engulfs food
particles by phagocytosis;
2
body defence against diseases, e.g. in humans and other mammals, certain
white blood cells engulf harmful microorganisms by phagocytosis.