Unit 3 - Osmosis
Plants use osmosis to prevent wilting.

Unit 3 - Osmosis

Osmosis refers specifically to the movement of water molecules through partially permeable membranes, and in this GCSE Biology quiz we take a close look at how the process works.

Partially permeable membranes allow water molecules to pass through them in both directions, and this is how osmosis occurs.. If there are solutions of different concentrations on opposite sides of the membrane, it is important that you realise that water molecules are passing through in both directions. It is a special case of diffusion as it involves only water molecules and a partially permeable membrane. To understand it properly involves your knowledge of the particle theory of matter (kinetic theory - atoms and molecules are constantly in motion).

Because there are more water molecules in a less concentrated solution than a more concentrated solution, more of them will be crossing through the membrane at any given time. The result of this is that the more concentrated solution will gain water molecules from the less concentrated one. You have probably seen this happening in lessons at school where a concentrated sugar solution in placed into a tube made from a partially permeable membrane or by placing a piece of potato into a glucose solution. The level of the concentrated sugar solution rises as it gains water molecules whereas the piece of potato shrinks since it loses water to the glucose solution surrounding it.

Eventually the concentrations will become equal. At this point, there will be the same number of water molecules passing through the partially permeable membrane in both directions. Equilibrium has been reached and so in practical terms, osmosis stops as there is no further change in concentration, but you must remember that the water molecules are still passing through the cell membranes.

In living organisms, the partially permeable membrane is the...
cell membrane
This contains the cytoplasm and when there is a concentration gradient between the outside of the cell and the cytoplasm, osmosis occurs, restoring the equilibrium
Plants use osmosis to prevent...
Plant cells become flaccid when they are short of water. Osmosis makes them turgid once more
Water molecules move...
down the concentration gradient
along the concentration gradient
against the concentration gradient
Along the gradient is always marked as incorrect in exams
Water molecules move from...
dilute to a more dilute solution
dilute to a more concentrated solution
concentrated to a more dilute solution
concentrated to a less concentrated solution
It is a special case of diffusion as it involves a movement of only water particles from a higher concentration to a lower concentration across a partially permeable membrane
Osmosis takes place in which of the following?
The atmosphere
Water is taken up by root hair cells from the soil as there is a concentration gradient from soil to the root hair cell
Potato cylinders are put into a strong sugar solution. After an hour they are...
the same
sweet potato
Water leaves the potato cells by osmosis, so the potato cylinders shrink in size due to water loss
Osmosis is the movement of which molecules?
Osmosis is only about water molecules
Osmosis takes place in this important structure.
Water is reabsorbed by osmosis in the nephrons of the kidneys, keeping the blood plasma at the right concentration to avoid the lysis (bursting) of the red blood cells in particular
Why is osmosis not the same thing as diffusion?
A partially permeable membrane is needed for osmosis
A partially permeable membrane is needed for diffusion
Osmosis involves oxygen only
Diffusion involves water only
True diffusion occurs when two substances are in direct contact with one another, here they are physically separated by the partially permeable membrane
Dilute means...
a little solute and a lot of water
a lot of solute and little water
the same
Dilute means weak and watery. A small amount of solute, e.g. salt, dissolved in a lot of water
You can find more about this topic by visiting BBC Bitesize - Osmosis and plant transport (CCEA)

Author:  Donna Maria Davidson

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