5.5 : Osmosis

Question 1

Define osmosis

Answer 1

The net movement of water from an area of high water potential to low water potential through a semi permeable membrane

Question 2

Define water potential

Answer 2

The pressure exerted by water molecules as they collide with a membrane or container.

Question 3

What is water potential measured in? Symbol?

Answer 3

Measured in units of pressure pascals ( Pa ) or kilopascals ( kPa )

Symbol : psi

Question 4

What is the difference between different water potentials?

Answer 4

Pure water water potential = 0

The more concentrated the more negative.

Therefore all solutions have negative water potentials.

Question 5

Define hydrostatic pressure?

Answer 5

The diffusion of water into a solution leads to an increase in volume of this solution. If the solution is in a closed system, such as a cell, this results in an increase in pressure ( called hydrostatic pressure ).

Question 6

Describe how animal cell react in different solutions

Answer 6

Placed in hypotonic solution ( high water potential ) : water moves into the cell by osmosis. No cell wall and thin cell surface membrane. It will break and cell will burst = cytolysis

Placed in hypertonic solution ( low water potential ) : cytoplasm loses water into solution via osmosis. Reduction in volume of cell -> crenation.

Question 7

Describe how plant cells react in different solutions

Answer 7

Placed in hypotonic solution (high water potential ) : water moves into cell via osmosis. Increases hydrostatic pressure pushes membrane against strong cellulose cell wall. This is called turgor pressure. As this increases it resists entry of more water and cell = turgid

Placed in hypertonic solution ( low water potential ) :
water leaves cell by osmosis. Reduction in volume of cytoplasm = pulls cell surface membrane away from cell wall = plasmoylsed

Question 8

How would you investigate the effects of solutions of different water potential on plant cells?

Answer 8

Pieces of potato or onion can be placed into sugar or salt solutions with different concentrations, and therefore different water potentials.

Water will move in or out of cell depending on the solution.

It will therefore increase or decreases in mass and size, which can be measured.

Question 9

How would you investigate the effects of solutions of different water potential on animal cells?

Answer 9

Eggs can be used to demonstrate osmosis in animal cells.

Eggs without their shells are placed in different concentrations of sugar syrup.

Over time, osmosis takes place.

( If the egg is hard boiled for easier handling , the membrane will be damaged ).