3.3 Osmosis

Question 0

Osmosis is a special case of…

Answer 0

Diffusion

Question 1

Define osmosis.

Answer 1

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane.

Question 2

What is a partially permeable membrane?

Answer 2

A membrane which is permeable to some molecules but not others.

Question 3

Cell-surface membranes and other plasma membranes such as those around organelles are partially permeable in the way that…

Answer 3

They are permeable to water molecules and a few other small molecules but not to larger molecules.

Question 4

What is a solute?

Answer 4

Any substance which is dissolved in a solvent e.g water. The solvent and solute together form a solution.

Question 5

What is water potential represented by?

Answer 5

The Greek letter psi (looks like a fork) measured in pressure. It is the pressure created by water molecules.

Question 6

Under standard conditions of temperature (25 degrees) and pressure (100kPa) what is pure water said to have?

Answer 6

A water potential of zero

Question 7

How would you lower pure waters water potential?

Answer 7

Add a solute

Question 8

The water potential of a solution (water+solute) must always be less than…

Answer 8

Zero

Question 9

The more solute added to a solution…

Answer 9

The lower it’s water potential

Question 10

Water will move by osmosis from a region of higher water potential to one of…

Answer 10

Lower water potential

Question 11

One way of finding the water potential of cells or tissues is…

Answer 11

To place them in a series of solutions of different water potentials. Where there is no net gain or loss of water from the cells or tissues , the water potential inside the cells or tissues must be the same as that of the external solution.

Question 12

The highest value of water potential is…

Answer 12

Zero (pure water)

Question 13

Animal cells contain a variety of solutes dissolved in their watery cytoplasm. If a red blood cell is placed in pure water…

Answer 13

It will absorb water by osmosis because it has a lower water potential. The cell-surface membrane (very thin- 7nm) cannot stretch and it will break, releasing its contents.

Question 14

What is it called when red blood cells burst and release their contents?

Answer 14

Haemolysis

Question 15

What is done to prevent animal cells from bursting?

Answer 15

They are bathed in a liquid which has the same water potential as the cells.

Question 16

Blood cells have the same water potential as…

Answer 16

Blood plasma

Question 17

If water potential of external solution and cell solution are equal what happens to the cell?

Answer 17

There is no change - water neither enters nor leaves the cell.

Question 18

If water potential of external solution is lower than the water potential of the cell solution what happens to the cell?

Answer 18

Water leaves the cell, it shrinks and the haemoglobin is more concentrated, giving the cell a darker appearance.

Question 19

In plant cells, what is the central vacuole?

Answer 19

The central vacuole contains a solution of salts, sugars and organic acids in water.

Question 20

In plant cells, what is the protoplast?

Answer 20

It consists of the cell-surface membrane, nucleus, cytoplasm and inner vacuole membrane

Question 21

What is the cellulose cell wall?

Answer 21

A tough, inelastic covering that is permeable to even large molecules.

Question 22

Plant cells cannot control the composition of the…

Answer 22

Fluid around them unlike animal cells - they are permanently bathed in almost pure water which is constantly absorbed by the plants roots.

Question 23

What does water entering a plant cell by osmosis cause the protoplast to do?

Answer 23

The protoplast swells and presses on the cell wall. The cell wall is only capable of very limited expansion - a pressure builds up which on it which resists the entry of further water - the cell is said to be TURGID.

Question 24

What happens to a plant cell which is placed in a solution with an equal water potential and then a lower water potential than it’s own?

Answer 24

In the equal, water neither enters nor leaves. There is no change.

Then the protoplast no longer presses on the cell wall. The protoplast begins to pull away front the cell wall. The cell is said to be at incipient plasmolysis.

Water leaves by osmosis. The volume of the cell decreases. Further loss and the cell shrinks further and the protoplast begins to pull away completely from the cell wall. The cell is said to be plasmolysed.