Question

Define and explain water potential.

Answer 1

Hint: Water potential is the energy of water per unit volume respective to pure water in reference conditions. Plants have the ability to bring water to the leaves in order to allow for photosynthesis. It works on the laws of thermodynamics.

Complete answer:
At constant temperature and pressure, the difference between free energy of a water in a system compared to pure water is called water potential. Slatyer and Taylor gave the concept of water potential for the first time. It is founded on two key principles of freedom of energy and chemical potential. The thermodynamics laws govern the potential of water.

The measurement of how freely water molecules can move in a particular environment or system can also be represented by water potential. It is measured in kilopascals (kPa) and represented by the Greek letter, Psi. The water potential is never positive, but it has a zero maximum value (same value as pure water at the atmospheric pressure). The more solutes present in the solution, the more negatively this water potential becomes, as impure water or water that has solutes tends to reduce the freedom of movement of water molecules by attracting water towards itself.

Osmosis, gravity, mechanical pressure or matrix effects like surface tension are responsible for causing water potential. It is important to consider the flow of water between plants, animals and soil. Also, water potential is highly used in electric power generation.

The water potential of plant solutions is determined by solute concentration, heat, gravity, and factors called matrix effects such as:
> Matric potential: Binding of water to a matrix either eliminates or absorbs potential energy from the device. The matrix potential is similar to the solute potential since it involves linking the energy in the aqueous system by creating hydrogen bonds between the water and some other portion. However, in solute potential, the other components are hydrophilic, soluble solute molecules, while in matrix potential, the other components are hydrophilic, insoluble plant cell wall molecules.

> Osmotic or solute potential: Solutes decrease the potential of water (resulting in a negative value) by absorbing some of the potential energy present in the water. Solute molecules can dissolve in water because water molecules can bind to them through hydrogen bonds; a hydrophobic molecule like oil that cannot bind to water cannot be dissolved.

> Gravitation potential: In a plant without height, the gravitational potential is always negative to 0. It often takes away or absorbs the system's potential energy. The gravitational force drags water down to the ground and decreases the overall energy content of the plant water.
> Pressure potential: Pressure potential, also known as turgor potential, may be both positive and negative. Since pressure is an energy term, the higher the pressure, the greater the energy potential of the system and vice versa.

Note: Water potential can be defined as the relative propensity of water to pass from one location to the other and is commonly depicted by the Greek letter, Psi. It is the indicator of potential energy of water that aids in the movement of water through plants.