60 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.3 Cell Membranes And Transport)

Question 1

Most cell membranes are permeable to —— and to ——-

Answer 1

Most cell membranes are permeable to water and to certain solutes.

Question 2

Osmosis is a special case of ——————–

Answer 2

Osmosis is a special case of diffusion which involves the movement of water molecules only.

Question 3

Water potential is a measure of ———–
It is the —————

Answer 3

Water potential is a measure of the free energy of water molecules.

It is the tendency for water to move.

Question 4

Water potential is measured in ————

Answer 4

It is measured in kilopascals (kPa).

Question 5

What is the water potential of pure water?

Answer 5

There is no tendency for water molecules to move into pure water, so pure water has a water potential of zero.

Question 6

What happens to water potential when you add a solute to pure water?

Answer 6

The addition of a solute to pure water tends to bring water molecules in.

As the force pulls inwards, it has a negative sign and so the addition of a solute to pure water lowers the water potential and gives it a negative value.

The higher the concentration, the more strongly water molecules are pulled in, and the lower, i.e. the more negative, the water potential.

Question 7

Explain why the water potential becomes neagtive when you add solute to water in terms of energy.

Answer 7

1. Where there is a high concentration of water molecules, i.e. a dilute solution, water molecules have a high potential energy because they are free to move.
2. In a solution, water molecules are weakly bound to the solute so fewer are free to move. The system has a lower potential energy.
3. External water molecules, with higher potential energy, will move down an energy gradient to the lower potential energy.
4. This is the pulling force they experience, which is the osmotic pull inwards, i.e. the water potential.
5. A more concentrated solution has even fewer free water molecules. Consequently, the pull on water molecules is greater so the water potential is more negative, i.e. lower.

Question 8

What does this diagram show?

Answer 8

The diagram shows the movement of water between cells with different water potentials.

Water moves from an area of higher water potential, e.g. -100 kPa, to an area of lower water potential, e.g. -200 kPa or -800 kPa.

Question 9

Describe solute potential?

Answer 9

When the water potential is related only to the concentration of the solution it could be called the solute potential.

Question 10

What does solute potential measure?

Answer 10

Solute potential measures how easily water molecules move out of a solution.

The more solute present, the more tightly water molecules are held, the lower the tendency of water to move out.

So a higher concentration solution has a lower, more negative solute potential.

Question 11

Osmosis (Key-Term)

Answer 11

The net passive diffusion of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential

Question 12

Water potential (Key-Term)

Answer 12

The tendency for water to move into a system; water moves from a solution with higher water potential (less negative) to one with a lower water potential (more negative).

Water potential is decreased by the addition of solute.

Pure water has a water potential of zero.

Question 13

Solute potential (Key-Term)

Answer 13

A measure of the osmotic strength of a solution.

It is the reduction in water potential due to the presence of solute molecules.

Question 14

What is the water potential for pure water and for solutions?

Answer 14

For pure water, water potential = 0.

For solutions, the water potential is negative.

Question 15

What is the symbol for water potential?

Answer 15

Question 16

How does water molecules travel across cell membranes?

Answer 16

Water molecules travel across cell membranes through specialised channels called aquaporins.

A cell may have thousands of aquaporins and transfer billions of water molecules each second.