Water Movement in Plant Tissue (HL) (HL IB Biology)

• Water potential is represented by Ψ or Ψ_w
• The water potential of a solution is influenced by several factors, including solute potential and pressure potential
  • Solute potential, also known as osmotic potential, is represented by the symbol Ψ_s
    • Solute potential is the effect that solutes in a solution have on water potential:
      • Pure water with no dissolved solutes has a solute potential of zero
      • As solutes are added to a solution its solute potential decreases and becomes more negative
        • Provided that pressure potential (see below) remains constant, a decrease in solute potential will cause a decrease in water potential
    • The effect of solutes on water potential can be explained as follows:
      • Solute molecules bind to water molecules via hydrogen bonds as they dissolve
      • The potential energy available in the water is transferred to the hydrogen bonds
      • The reduction in potential energy means that the water potential is reduced
  • Pressure potential, also referred to as turgor potential or turgor pressure, is represented by Ψ_p
    • Pressure potential is the hydrostatic pressure to which water is subjected
    • Pressure potential inside plant cells is usually positive as the cytoplasm exerts pressure on the inside of the cell wall; this is turgor pressure and provides support for plant tissues

    • Negative pressure potential can occur in xylem vessels where water and dissolved minerals are transported under tension

• The total water potential of a solution is the sum of its solute potential and its pressure potential, as shown in the formula:

Ψ_w = Ψ_s + Ψ_p

Solute potential and pressure potential diagram

(Left) Water potential is affected by solute potential and pressure potential; here both of these are zero, so the total water potential is zero

(Centre) Solute potential inside plant cells is below zero due to the presence of solutes in the cytoplasm; this means that plant cells placed in pure water have a lower water potential than their surroundings and water moves in by osmosis

(Right) As plant cells fill with water the pressure potential increases due to the pressure of the cytoplasm on the inside of the plant cell wall; the cell will fill with water until the pressure potential cancels out the effects of the solute potential

• Changes occur when plant tissue is bathed in either a hypotonic or hypertonic solution
• When plant tissue is placed in a hypotonic solution:
  •  Plant cell cytoplasm contains dissolved substances which lower the solute potential of the plant cells
  • This contributes to a lower water potential inside the plant cell
  • Water moves from the surrounding solution into the plant cell down its water potential gradient
  • The inward movement of water leads to an increase in the volume of the cell cytoplasm and the pressure potential increases as the cytoplasm presses against the cell wall
  • Eventually the pressure potential reaches a point at which the water potential is equal inside and outside the cell, and the inward movement of water stops
  • Plant cells in this state are turgid and will provide structural support to the plant
• When plant tissue is placed in a hypertonic solution
  • The surrounding solution has a lower solute potential than that of the cell cytoplasm
  • This contributes to a lower water potential in the surrounding solution
  • Water moves out of the plant cell into the surrounding solution down its water potential gradient
  • The loss of water from the plant cell will result in a reduced volume of cytoplasm and a decreased pressure potential inside the cell
  • Plant cells will lose turgor pressure and the plant will begin to wilt