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Water Potential Calculator (Biology Guide)

Water potential defines the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects.

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v2.0 Logic

Reference Water Potentials (MPa)

CategoryValue/Price
Pure Water (Open Container)0 MPa
Typical Plant Cell-0.5 to -1.5 MPa
Dry Soil<-1.5 MPa

Water always moves from an area of higher (less negative) water potential to lower (more negative) water potential.

Technical Overview

In biology and plant physiology, water potential is represented by the Greek letter Psi (Ψ). It is typically measured in megapascals (MPa). Pure water in an open container at standard sea-level pressure has a water potential of exactly zero. Any addition of solutes lowers the concentration of free water molecules, generating a negative solute potential (Ψs). In plant cells, the rigid cell wall exerts a physical pressure against the expanding protoplast, creating a positive pressure potential (Ψp) known as turgor pressure. This tool accurately derives these properties for AP Biology and college-level botanical sciences.

Professional Applications

  • AP Biology preparation
  • Plant physiology labs
  • Osmosis predictions

What is Ψs (Solute Potential)?

Solute potential mathematically determines how solutes lower the water's free energy. It is calculated using Ψs = -iCRT. The negative sign is crucial: as solute concentration increases, water potential becomes more negative.

Role of Ionization (i)

Sucrose doesn't ionize in water, so its constant is 1. Salt (NaCl) splits into Na+ and Cl- ions, making its constant 2. This is why salt has double the osmotic draw of sugar per mole.

Direction of Water Flow

Water physically moves toward the more negative number. If a plant cell falls to -0.8 MPa and the surrounding soil dries out to -1.2 MPa, the plant will literally lose water to the dirt and wilt.

Scientific Formula

Ψ = Ψs + Ψp | Ψs = -iCRT

Frequently Asked Questions

What is water potential?

The quantifiable measure of free energy of water in a system.

Why is water potential negative?

Pure water is 0. Adding any solute decreases the free energy of the water molecules, driving the number into the negative range.

What happens at 0 MPa?

This is pure distilled water at atmospheric pressure. Water cannot have a positive potential without physical pressure.

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