D.2.3 HL Flashcards
Explain water potential as the unit for potential energy of water per unit volume.
The net movement of water by osmosis is determined by differences in water potential between two solutions connected by a partially permeable membrane. Water potential can be defined as the pressure exerted by water molecules as they collide with a membrane. It is denoted as psi and measured in kilopascal. It is impossible to measure the absolute quantity of the potential energy of water so relative values are used. Pure water has the highest water potential and has a value of 0 kPa at standard temperature. Solutions have negative water potential, lower than pure water.
Why does water always move with the gradient and how does pressure potential affect this?
Water molecules always move from a region of high water potential to a region of low water potential. This happens because water always aims to achieve balance. however, pressure potential is the pressure exerted on water. If it’s high, it resists water movement but if its low water enters easily to allow balance.
Explain solute and pressure potential.
Solute potential refers to how a solution is affected by the amount of solute it contains. The greater the amount of solute, the lower the water potential. This is because water molecules bind to the solute molecules reducing the number of water molecules which are free to diffuse. The solute potential ranges from 0 downwards.
Pressure potential refers to how a solution is affected by the pressure applied to it. The greater the pressure, the higher the water potential. This potential tends to be a positive value inside the cell but it can be negative in the xylem cells due to sap. In plant cells the pressure potential is a result of the cell wall exerting pressure on the cytoplasm. The formula for water potential = solute potential + pressure potential
Explain water movement and potential in plant tissues.
In hypotonic solutions, the solute potential of the tissue is more negative than the solute potential of the solution. This causes water to move into the tissue and consequently push the plasma membrane against the cell wall, increasing turgor pressure. This causes the pressure potential to be positive which can offset the negative solute potential resulting in an equilibrium.
In hypertonic solutions, the solute potential of the solution is more negative than the solute potential of the tissue. This causes water to move out from the tissue to try to equalize the solute potentials on both sides.
