#18 Osmosis

Osmosis is the diffusion of water molecules from a region of their higher concentration to a region of their lower concentration, through a partially permeable membrane. 

Osmosis is a special form of diffusion and always involves the movement of H₂O across a membrane. Osmosis is:  

• the movement of H₂O
• across a selectively permeable membrane
• down a water potential gradient. 

In the picture below

- The concentration of sugar molecules is higher on the concentrated solution (L) and lower on the diluted one (R). 

- The concentration of water molecules is higher on the (R) and lower on the (L) (a lot of place is taken up by sugar molecules).

It is confusing to talk about the 'concentration of water', so we can say that a diluted solution (R) has a high water potential and a concentrated solution (L) has a low water potential. 

There is a water potential gradient between the 2 sides. The water molecules diffuse down this gradient, from a high water potential (R) to a low water potential (L).  

Cell membranes 

• partially permeable (let some substances pass through, but not others).
• separate 2 solutions: cytoplasm and solution around the cell. 
• If the solutions are of different concentrations, osmosis will occur. 

Effect of Osmosis on plant and animal cells

1. When placed in H₂O: 

Concentration of H₂O outside the cell is higher than inside it. Cells will take in H₂O by osmosis:

• plant cells become turgid (swollen) but do not burst (have tough cell wall which is fully permeable).

• animal cells will burst (no cell wall). 

2. When placed in concentrated sugar or salt solutions:

Concentration of H₂O inside the cell is higher than outside it. H₂O get out of the cells by osmosis:

• plant cells become flaccid (soft and limp), cytoplasm is no longer pressed against the cell wall. The plant loses it firmness and begin to wilt. 
• animal cells shrink, become crenated.

Common misconceptions

Sugar and salt do not move by osmosis. Cell membranes prevent them entering or leaving the cell. 

Try this

A potato was set up as shown in the figure below (left-hand side). The investigation was left for several hours. The results are shown on the right-hand side of the figure.

1. Describe what happened to

a. the water in the disk

b. the salt solution in the hollow in the potato.    [2 marks]

2.

a. Name the process that is responsible for the changes that have occurred.                                                                  [1 mark]

b. Explain why these changes have occurred.                 [3 mark]

c. Where does this process occur in a plant?                  [1 mark]

d. What is the importance to the plant of this process?   [1 mark]

Answers

1.   a. The volume of water in the dish decreased.

b. The volume of salt solution in the potato increased.

            2. a. Osmosis
                b. 3 points from:

- there was a higher concentration of water in the dish than in the potato
- so water moved into the potato.
- from a high concentration of water to a lower concentration of water
- by osmosis.

c. Root hairs, or in the roots.

d. Osmosis enables the plant to absorb water to maintain cell turgidity (or to replace water lost by transpiration).