Osmosis (SL IB Biology)

• A solution typically consists of a solute dissolved in a solvent
• Water is a very good solvent because it is dipolar
  • The hydrogen side of the molecule is slightly positive while the oxygen side is slightly negative
• This enables water molecules to form hydrogen bonds with other polar solute molecules and ions
• Hydrogen bonding between water molecules is also considered at the start of the course, the notes can be found here
• The interaction between a solvent, such as water, and a solute is known as solvation

Hydrogen bond and electron arrangement in water diagram

Water molecules are dipolar because electrons are distributed unevenly between the hydrogen and oxygen atoms

• Polar solvents, such as water, can orientate themselves towards polar solutes and ions to form hydrogen bonds or ion-dipole forces
  • This creates hydration shells around each solute particle

Dipolar nature of water diagram

The dipole nature of water molecules allow them to form hydration shells around polar solutes and ions

• All cells are surrounded by a cell membrane which is partially permeable
• Water can move in and out of cells by osmosis
• Osmosis is the diffusion of water molecules from a less concentrated (dilute) solution to a more concentrated solution across a partially permeable membrane
  • In doing this, water is moving down its concentration gradient
• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Partially permeable membrane diagram

Osmosis and the partially permeable membrane.

• Osmosis can also be described as the net movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, through a partially permeable membrane

Movement of water diagram

The water moves from the region of lower solute concentration (dilute solution) to the region of higher solute concentration (concentrated solution)

• If a cell is placed in a solution with a lower solute concentration (i.e. more dilute) than the cytoplasm of the cell, then there will be a net movement of water into the cell by osmosis
  • Solutions like this is referred to as being hypotonic
• If however, the solution outside the cell has a higher solute concentration (i.e. more concentrated) than the cytoplasm of the cell, then there will be a net movement of water out of the cell
  • These solutions are said to be hypertonic
• If the solute concentration is the same on both sides of the cell membrane, there will be no net movement of water into or out of the cell by osmosis
  • An solution with a similar concentration as the cytoplasm of a cell is referred to as an isotonic solution

Tonicity of solutions diagram

The net movement of water is determined by the relative solute concentration of the solution outside the cell