water potential. cells and their environment cells need to be able to move materials through...
Post on 18-Jan-2016
215 Views
Preview:
TRANSCRIPT
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)
top related