Water Potential

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

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

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 Ψp water 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. 2. 3. 4. Don’t freak out. Just plug the numbers in. Use the formulas given. Remember water will move from high concentration (highest, “most positive” Ψ to low concentration, “most negative Ψ)