## Water potential -

Word quiz by Anders Michael Nielsen, February 2020. Offline version w/o feedback here.
Video by Paul Anderson, also on his website Bozeman Science.

The Greek letter is used as a symbol of water potential. Remembering the ancient god Poseidon carrying a trident may help you remember the name of the letter (Ψ). Water always moves from a concentration of water to a concentration of water, and the movement is by the process of osmosis.
Most often, we talk about a concentration between one place and another. The concentration of a solute in a liquid is the molar concentration in mol/L, and the concentration of pure water is roughly . However, when deciding which direction water flows in biological systems, it is convenient to use water potential (Ψ) as Ψ as it also takes into account gravity, pressure, and surface tension. Water potential is the potential energy of water per unit compared to pure water, and water always flows in the direction of the place with the water potential. Pure water has a water potential of zero . However, if we add just one grain of salt (NaCl) to pure water, the NaCl splits into Na+ and Cl-, each attracting many H2O molecules due to their charge and the polar nature of H2O. This the water potential of that newly NaCl-polluted water. At the tops of trees, water evaporates from the leaves; the water potential in the leaves , and more water flows in the direction from the roots to the leaves at the top of the tree/scrub. It is the same with the stem; it has a more negative (Ψ) than the roots that have a more negative (Ψ) than the water surrounding the roots in order to absorb water from the soil. The pressure component of Ψ, Ψp in a leaf is from the cell walls exerting pressure on the cell as the cell tries to expand from water inside the cell. That Ψp is as the cell wall will put pressure on the cell forcing water the cell. Remember that water flows from an area of high Ψ to an area with a lower Ψ. Solute potential, the Ψs part of the equation can be calculated by the equation Ψs = where C is the concentration in mol/L, and i is the ionization constant. The temperature is measured in Kelvin, and R is a constant. In short, the ionization constant of glucose is 1 as it does not break up, and the ionization constant of NaCl is two as it breaks up in a Na+ and Cl- when added to the water. So, this equation tells us that the more ions you add to a solution, the more you Ψs. For the calculation of Kelvin, please remember that Kelvin equals . Overall, Ψ can be calculated as the sum of Ψs plus Ψp and the equation is Ψ = Ψs + Ψp. However, had there been more components in the calculation of Ψ such as a gravitation and humidity component, those would just have been added to the equation.