Tonicity (College Board AP Biology)

• More scientifically, tonicity of a solution refers to the concentration of solutes in that solution
• An important consideration is concentration differences on either side of a membrane
• This governs the movement of water by osmosis
• Tonicity can be referred to as being 
  • hypertonic
  • hypotonic
  • isotonic

Osmosis and Osmolarity

• Osmosis was introduced in 2.6 Membrane Transport as a method of passive transport
• Osmosis can also be described as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane
  • Water potential describes the tendency of water to move out of a solution

• The term osmolarity can be used to describe the solute concentration of a solution
• A solution with high osmolarity has a high solute concentration and a solution with low osmolarity has a low solute concentration.
  
  • Water will move from a solution of low osmolarity to a solution of high osmolarity across a selectively permeable membrane

How Osmosis Works Diagram

How osmosis works. The water moves from the region of higher water potential (dilute solution) to the region of lower water potential (concentrated solution).

Hypertonic

• 'Hyper-' means more
• A hypertonic solution has a higher solute concentration than the solution on the other side of the membrane
  • Therefore it has a higher osmolarity....
  • ...and a lower water potential
• An example is an increase in salinity of a lake, which makes the lake water hypertonic
  • This may cause the cells of organisms in the lake to lose water by osmosis, shrivel and die

Hypotonic

• 'Hypo-' means less
• A hypotonic solution has a lower solute concentration than the solution on the other side of the membrane
  • Therefore it has a lower osmolarity
  • ...and a higher water potential
• An example is the danger of injecting pure water into the blood
  • The blood plasma becomes hypotonic, causing red blood cells to gain water by osmosis, to lyze (burst) and die
  • Intravenous injections and drips are always carefully controlled to be isotonic

Isotonic

• 'Iso-' means the same
• An isotonic solution has the same solute concentration than the solution on the other side of the membrane
  • Therefore it has an equal water potential and osmolarity
• An example is seawater, which is isotonic to the cell contents of many sea animals eg. marine invertebrates

The Effect of Tonicity on Animal Cells Diagram

The effect of hypertonic, hypotonic and isotonic surroundings on animal cells

The Effect of Tonicity on Plant Cells Diagram

The effect of hypertonic, hypotonic and isotonic surroundings on plant cells

Contributors to Water Potential

• There are two distinct drivers of the water potential in cells
  • Water potential is given the symbol Ψ
• These are:
  • Solute potential Ψ_s
  • Pressure potential Ψ_p
• The relationship is given by this equation

Ψ = Ψ_p + Ψ_s

• Solute potential (Ψ_s) is caused by dissolved substances (solutes)
• Each solute molecule is bound by water molecules, effectively reducing the number of water molecules that can move around in the solution
• The more solute molecules are present, the lower the water potential
• For this reason, the water potential of a solution is always expressed as a negative number
  • The water potential of pure water is zero

• Pressure potential (Ψ_p) is the physical pressure of a solution in an enclosed area such as the inside of a cell
• This value can be positive or negative
  • Positive Ψ_p occurs in most plant cells due to osmotic uptake of water
    • This is sometimes called turgor pressure
  • Negative Ψ_p occurs in the hollow xylem vessels during transpiration as water and minerals are drawn up the plant's stem during transpiration
• Water always moves from areas of high water potential to areas of low water potential