Water potential

Solvent

A substance capable of dissolving another substance to form a solution

Water as universal solvent

Water can dissolve a wide range of polar and charged particles as it is a polar molecule.

Solvation

The interaction of a solvent with the molecules and ions that dissolve in it

Solvation due to ion-dipole interraction

• Ionic compounds dissolve in water because there are strong ion-dipole attractions between the ions and the water (dipole) molecules.

• The closer the opposite charges can get to each other, the more stable the system, which is why solvation stabilizes the ions.

Solvation due to hydrogen bonding

Many organic compounds have polar groups. This enables them to dissolve in water due to the formation of hydrogen bonds between water molecules and polar groups.

Example of solvation due to ion-dipole interraction

When sodium chloride dissolves in water, the Na⁺ ions are attracted to the slightly negative oxygen atoms and the Cl⁻ ions are attracted to the slightly positive hydrogen atoms. The ions become surrounded by a hydration shell of water molecules.

Example of solvation due to hydrogen bonds

A glucose molecule has five OH groups. During solvation, water molecules form hydrogen bonds with all these groups. A hydration shell sorrounds the glucose molecules.

Hypotonic solution

Lower solute concentration than the inside of the cell

Isotonic solution

Same solute concentration as the inside of the cell

Hypertonic solution

Higher solute concentration than the inside of the cell

Osmosis in cells with cell wall in hypotonic solution

1. There is higher solute concentration in the cell compared to outside

2. Water moves into the cell by osmosis

3. The cell appears swollen

4. Plasma membrane presses against cell wall

5. Cell is turgid

6. Cell wall prevents cell from bursting

Osmosis in cells with cell wall in isotonic solution

1. There is equal solute concentration in the cell compared to outside

2. No net movement of water into or out of the cell by osmosis

3. Plasma membrane presses lightly against cell wall

4. Cell may become flaccid

Osmosis in cells with cell wall in hypertonic solution

1. There is lower solute concentration in the cell compared to outside

2. Water moves out of the cell by osmosis

3. The volume of the cytoplasm shrinks

4. Plasma membrane tears away from the cell wall

5. Cell is plasmolysed

6. The hypertonic solution fills the space between the cell wall and membrane

Osmosis in cells without cell wall in hypotonic solution

1. The solute cocentration is higher in the cell compared to the outside

2. Water moves into the cell by osmosis

3. Cell swells and loses biconcave shape

4. Cell bursts due to the build up of internal pressure and leave empty ‘ghost cells‘

Osmosis in cells without cell wall in isotonic solution

• Solute cocentration is equal in and out of the cell

• No net movement of water into or out of the cell by osmosis

• The cell mantains its shape

Osmosis in cells without cell wall in hypertonic solution

• The solute concentration is lower in the cell compared to the outside

• Water moves out of the cell by osmosis

• The cell shrinks and shrivels

• Some cells become crenated (used for red blood cells)

Importance of regulating concentration of body fluids in animals

Animal cells don’t have a cell wall, so the concentration of body fluids has to be regulated to prevent cells gaining or losing too much water and being damaged.

Osmosis in fresh water

Fresh water has a lower solute concentration than the cytoplasm of cells, so there is a continuous inflow of water by osmosis.

Adaptations of unicellular organisms in fresh water

Unicellular organisms that lack a cell wall are in danger of bursting. Some have contractile vacuoles, which fill with excess water contained in cytoplasm and then contract to squeeze it out of the cell.