osmosis

Question 1

what is osmosis?

Answer 1

the passive movement of water from an area of high water potential (hypertonic) to an area of low water water potential (hypotonic) across a selectively-permeable membrane.

Question 2

what is meant by the phrase ‘water potential’?

Answer 2

the pressure created by water particles, how freely water particles can move.

Question 3

what is the unit of water potential

Answer 3

represented by Ψ (psi)

measured in units of pressure, usually kilopascals (kPa)

Question 4

what are the standard conditions of pressure and temperature?

Answer 4

temperature = 25°C
pressure = 100 kPa

Question 5

under standard conditions of pressure and temperature, pure water has a water potential of…

Answer 5

ZERO - water particles can move freely

Question 6

what is the highest rate of water potential?

Answer 6

zero

Question 7

what effect will the addition of a solute to pure water have on WP?

Answer 7

it will lower its water potential (become more negative) - the water particles are attracted to the solute and so are less free to move around.

Question 8

water potential of a solution must always be…

Answer 8

negative

Question 9

the more concentrated a solution is…

Answer 9

the lower its WP

Question 10

describe a way of finding the WP of cells/tissues?

Answer 10

1. place them in a series of solutions of different WP’s.
2. no net gain or loss of WP of cell/tissue - WP of external (solution) and internal (cell/tissue) environment are the same.

Question 11

solute and water particles move in a…

Answer 11

random motion due to kinetic energy.

Question 12

what do red blood cells contain?

Answer 12

a variety of solutes in their watery cytoplasm.

Question 13

what will happen if a red blood cell is placed in pure water?

Answer 13

it wall absorb water via osmosis because the RBC has a lower water potential than pure water. Although cell surface membranes are thin and flexible, they cannot stretch to any extent, therefore the CSB will break, the red blood cell will swell and burst (haemolysis). Contents including haemoglobin are released.

Question 14

what is done to prevent animal cells from bursting?

Answer 14

animals cells live in liquid conditions with the same WP as the cells.

Question 15

what will happen if the red blood cell has a higher WP than the solution?

Answer 15

water will leave the cell via osmosis and the cell will shrink and become shrivelled. Haemoglobin will become more concentrated giving cell darker appearance.

Question 16

what will happen if a plant cell is placed in pure water?

Answer 16

water will enter the cell via osmosis and the cell will swell and become turgid. It will not burst due to the rigid protection of the cell wall.

Question 17

what will happen if the plant cell has a higher WP than the solution?

Answer 17

water will leave the cell via osmosis and the cell will shrink (plasmolysed).

Question 18

what is meant by the phrase ‘selectively permeable’?

Answer 18

Membrane only allows certain molecules through; it is only permeable to water and other small molecules.

Question 19

during an osmosis practical, how and why do you dry the potato chips?

Answer 19

with a paper towel - to remove surface water - this would add to the mass - change water potential

Question 20

you must control the temperature during your osmosis practical - HOW?

Answer 20

using a water bath and check the temperature at regular intervals using a thermometer

Question 21

give 2 similarities in the movement of substances by diffusion and osmosis

Answer 21

1. down a conc. gradient (high to low)

2. passive transport - does not require ATP