Biology Exercise 3/Ex_TransportAcrossmembrane/ymt/AOGHSS
Name: _________________ 4D (
)
Biology Exercise—Transport across membrane
1.
Define the term water potential of a solution.
Water potential is defined as the tendency for water molecules to move from one region to
to another
2.
Define the term osmosis in terms of water potential.
Osmosis is defined as the movement of water molecules from the region of higher water
potential to the region of lower water potential through a selectively permeable membrane.
3.
The diagram below shows an apparatus which is often used to demonstrate osmosis. This
setup is called osmometer. The dialysis membrane is permeable to water molecules but not to
sugar molecules.
(a)
Name the term used to describe the dialysis membrane with respect to its permeability
to water and sugar molecules.
The membrane is selectively permeable.
(b)
Describe the change in the initial level of sugar solution in the thistle funnel.
The level of sugar solution in the thistle funnel rises.
(c)
The following description explain how the change in (b) occurs. Cross out the wrong
words used.
Sugar solution in the dialysis tubing has a [higher/lower] water potential than the
distilled water outside. Water moves [into/out of] the dialysis tubing by
[osmosis/diffusion]. This causes the solution level in the thistle funnel to [rise/fall].
1
Biology Exercise 3/Ex_TransportAcrossmembrane/ymt/AOGHSS
4.
(a)
Name: _________________ 4D (
)
If the sugar molecules can pass through the dialysis tubing, will the change in solution
level in the thistle funnel be observed. Why?
The sugar solution level in the thistle funnel remains unchanged. Sugar molecules
diffuses out while water molecules diffuse inwards until the water potentials of the
solution on both sides of the dialysis tubing are equal.
(b)
If the positions of the sugar solution and distilled water are reversed [對調], describe
the change in water level in the thistle funnel. Explain your answers
Water level in the thistle funnel falls. Water has a higher water potential than the sugar
solution outside. Water moves out of the dialysis tubing by osmosis.
(c)
If the sugar solution in the dialysis tubing is replaced by an insoluble substance (e.g.
starch), will any change in the liquid level occur? Why?
There will be no change in liquid level because insoluble substances do not affect
the water potential of the medium in which they occupy.
5.
(a)
If the dialysis tubing is replaced by a living membrane (e.g. fish’s swim bladder), will
the same result as 3(a) be observed? Why?
Yes. The result will be the same because a living membrane is a selectively permeable
membrane.
(b)
What will be the change in the sugar solution level in the thistle funnel if the living
membrane is heated at 80℃ before use? Why?
The sugar solution level will be unchanged because the selective permeability of the
living is destroyed on heating. No osmosis occurs.
6.
Complete the following table which distinguishes between diffusion, osmosis and active
transport.
Diffusion
Osmosis
Active transport
Direction of
movement of
particles
External input of
energy
Particles diffuse from
the region of higher
concentration to the
region of lower
concentration
not needed
Nature of membrane membrane not
involved, if needed. necessary
water molecules move
from the region of
higher water potential
to the region of lower
water potential
not needed
particles move against
the concentration
gradient
needed
selectively permeable living membrane is
membrane needed
needed.
2