Water Potential

Cells and Their Environment

• Cells need to be able to move materials through membranes and throughout the cytoplasm to maintain homeostasis

• Cellular membranes are selectively permeable which helps regulate the movement of materials

• The cellular environment is aqueous meaning there are solutes (such as various salts) dissolved in water

• Water can move through the cell membrane by osmosis or through special channels called aquaporins

• Larger molecules like certain ions and sugars rely on protein channels and transport proteins to help move across the cell membrane

Diffusion & Osmosis

• The simplest form of movement is diffusion (movement from high concentration to low concentration)

• Diffusion does not require energy input by the cells

• Osmosis is the diffusion of water through a membrane

• The terms hypertonic, hypotonic, and isotonic are used to describe solutions separated by selectively permeable membranes (meaning environments a cell can be found in)

• Water moves from areas of high potential (high free water concentration, low solute concentration) areas of low potential (low free water concentration, high solute concentration)

Hypertonic• A hypertonic solution has a

higher solute concentration and a lower water potential as compared to the other solution (the intracellular environment on the other side of the membrane)

• Therefore water will move into the hypertonic solution through the membrane by osmosis

• Meaning more water leaves the cell

Hypotonic• A hypotonic solution has a

lower solute concentration and a higher water potential than the solution on the other side of the membrane (the intracellular environment)

• Therefore water will leave the hypotonic solution, moving down the concentration gradient

• Meaning more water enters the cell

Isotonic

• Isotonic solutions have equal water potentials• Water moves back and forth between the cell

membrane at equal rates and the cell maintains it’s shape

• In non-walled, animal cells the cell has the potential to burst if too much water moves in the cell

• In walled plant and fungus cells water build up in the cell results in a pressure build up that can also affect the rate of osmosis

Water Potential

• Water potential (Ψ) is the potential energy of water in a solution or a cell as compared to pure water

• Calculating water potential allows scientists to make prediction of where water will flow

• Water will flow from higher potential lower potential (remember water diffuses from high concentration low concentration)

Ψ = Ψs + Ψpwater potential = solute potential + pressure potential

Solute Potential (Ψs)

• More solutes in a solution result in a lower Ψs

– Meaning water is more likely to move to that area• Solute potential is a negative number because

solutes lower the water potential of a system (remember a lower potential means water will want to go there!)

Pressure Potential (Ψp)• More pressure in a cell results in a

larger pressure potential • More pressure potential in a cell

means water will want to leave the cell to relieve some of the pressure

• Pressure potential tends to be positive a number (remember a higher potential means water will want to leave that area!)

Calculating (Ψs)

• Calculating solute potential Ψs = - iCRT• i = ionization constant – 2 for a salt like NaCl because in when dissolved in water

NaCl breaks apart into 2 ions (salt will increase i resulting in a lower potential meaning water will want to go there)

– 1 for sugars (sucrose, glucose, etc.) because when dissolved in water sugar remains sugar

• C = molar concentration (moles/liter)• R = pressure constant = 0.0831 (liters bars/mole°K)– A bar is a measure of pressure

• T = temperature = 273 + °C (kelvins)

What does this all mean!!!

1. Don’t freak out.2. Just plug the numbers in.3. Use the formulas given.4. Remember water will move from high

concentration (highest, “most positive” Ψ to low concentration, “most negative Ψ)