AP Biology Water Potential Notes Essay


Water potential is the potential energy of water per unit area compared to pure water. Which allows you to figure out where water’s going to flow due to osmosis, gravity, pressure and even surface tension. It also allows you to figure out if water will flow in the first place and if it will flow into the cell or not.

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Water Potential (symbol psi) = Solute Potential (symbol psi s) + Pressure Potential (symbol psi p) Solute Potential= -i (ionization constant) C (concentration [ ]) R (pressure constant) T (temperature)

“water potential” (?) is a measure of the free energy of water; pure water (which has a high amount of free energy) is arbitrarily assigned a water potential of zero; units pressure: MPa; can be positive or negative factors that determine plant water potential:

1) amount of solutes- increasing concentrations will lower
the free energy (water potential); termed osmotic potential (?s) 2) turgor pressure (?p) in plant cell- positive pressure inside plant cells; increases free energy; loss of turgor = wilting 3) surfaces of macromolecules (e.g. cellulose) exerts an attractiveforce on water- matric potential (?m); lowers the free energy; usually a minor component, and often ignored Total water potential of a plant:

?plant =??s + ?p + ?m
Soil water potential
same components as plants, but with no turgor, and a gravitational componentadded for saturated soils (?g) ?plant =??s + ?g + ?m
for most terrestrial ecosystems, the matric potential (?m) is the most important component of soil water potential; in saline soils ?s is the most significant component. Atmospheric water potential

determined by the relative humidity of the air; from plant perspective, it is extremely low below 90% RH Solute potential is also known as osmotic

When solute molecules are dissolved into water, the concentration of water molecules is reduced, and therefore so is the water potential. Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules. The more solute molecules are present, the lower (and more negative) is ?s. Solute potential is always negative.