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# Physics and biology collide

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So. I was reading my Biology textbook the other day and encountered something called "water potential." A simple summary of this term is water's potential energy , or it's capacity to perform work when free water moves from high water potential to low water potential. What? Physics in biology you say? Of course! :eagerness: Physics is everywhere.

Let's define water potential in depth. Water potential is given by the equation water potential (symbol = Greek letter psi) = potential due to solute concentration + potential due to pressure, or:

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The potential due to solute concentration, or solute potential, is directly proportional to the number of dissolved solute molecules. Solute binds water therefore reducing the number of free water molecules and decreasing it's capacity to do work. Because of this, solute potential is negative.

potential due to pressure, or pressure potential, is the physical pressure on a solution. Tension due to pressure is a negative pressure potential, whereas an applied pressure creates a positive water potential.

The biology part of water potential is that it is essential to a cell's well being (plant cells in particular). The water potential determines the direction of movement of water in/out of a cell. For plant cells, it determines the shape and stiffness of the cell. A plant cell is flaccid initially. It becomes turgid when it intakes free water in that the pressure from the water pushes on the cell wall, making the cell swell. The cell becomes plasmolyzed when free water leaves the cell, causing the cell to shrivel and the cell membrane to pull away from the cell wall. This state is dangerous for a plant (commonly known as wilting) and the plant may die. These conditions are created by unequal water potentials of the cell vs the cell's surrounding environment. If a cell has a lower water potential than the surrounding solution, it will intake free water and become turgid. If the cell has a higher water potential than it's surrounding environment, it will expel free water and become plasmolyzed.

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If you'd like to know more about water potential, didn't understand a thing I just said or would like background noise doing homework, the following link may be of use to you:

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This brings back wonderful memories about AP Bio

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