Water Water Water Potential

What are you water potential?

• Measures the tendency of water to leave one place in favor of another– Measured in bars

• High water potential = water molecules can easily move about

• Low water potential = water molecules cannot easily move about

Water Potential• Pure water water potential of zero

• Determines where water is going to flow due to– Osmosis– Gravity– Pressure– Surface tension

Lets go to our packet ‘o funA B

So What Does This Mean?• The solvent is pulling the water

toward it

• High solute concentration = low water potential

• Low solute concentration = high water potential

Review• Moves from hypotonic to hypertonic

Review• Hypotonic

– Solution= less solute; more water– Cell= more solute; less water

• Hypertonic – Solution= more solute; less water– Cell= less solute; more water

Lets go back to our packet ‘o fun

Y Z

Temperature• High temperature = more water

potential– Why?

• Low temperature = less water potential– Why?

Water molecules can move easily

Water molecules cannot move easily

What Will Happen?• When you put a pipette in water?

• What do you have to do to get the water out?

Water will go naturally up, against gravity, without pressure (-)

Put pressure on it (+)

Water • Concentration of solute is related to

osmotic pressure…

• Pressure related to pressure potential…

• Pressure raises water potential

The 3 Factors• What affects water potential?

– Temperature– Solutes– Pressure

Pressure Potential • A force that resists water flow due to a

container (cell) being full.

• Important in plant cells because they are surrounded by a cell wall

• Water enters plant cell, its volume increases and the living part of the cell presses on the cell wall

ψp

Pressure Potential• The cell wall gives very little so pressure

builds up…oh no!

• This has the tendency to stop more water entering the cell and stops cell from bursting

• When plant cell is inflated with water, it is turgid– Pressure potential is called turgor pressure

ψp

Solute (osmotic) potential (ψs )= –iCRTi = The number of particles the molecule will make in

water; for NaCl this would be 2; for sucrose or glucose, this number is 1

C = Molar concentrationR = Pressure constant = 0.0831 liter bar/mole K T = Temperature in degrees Kelvin (273 + °C) of solution Example Problem:The molar concentration of a sugar solution in an open beaker has been determined to be 0.3M. Calculate the solute potential at 27°C degrees. Round your answer to the nearest hundredth. What is the water potential? Answer: -7.48

Solute potential = -iCRT= -(1) (0.3 mole/1) (0.0831 liter bar/mole K)

(300 K)= -7.48 bar

Water potential = -7.48 + 0, so water potential = -7.48

Yikes, what's that??????

Water potential (ψ ) = pressure potential (ψp ) + solute (osmotic)

potential (ψs)

Pressure potential (ψp):

In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake Solute potential

(ψs): The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also. *As solute is added, the water potential of a solution drops, and water will tend to move into the solution.

MolarityAnother way of expressing

concentration, is called molarity. Molarity is the number of moles of solute dissolved in one liter of solution. The units, therefore are moles per liter, specifically it's moles of solute per liter of solution.

Be very careful to distinguish between moles and molarity.

So when you're given a problem or some information that says the concentration of the solution is 0.1 M that means that it has 0.1 mole for every liter of solution; it does not mean that it is 0.1 moles

molarity =

moles of solute liter of solution

"Moles" measures the amount or quantity of material you have; "molarity" measures the concentration of that material.

What is the molarity of a solution made by dissolving 2.5 g of NaCl in enough water to make 125 ml of solution?

molarity =moles of

solute liter of

solution

2.5 g NaCl x 1 mole NaCl58.5 g NaCl = 0.0427 mole

molarity = 0.0427 mole NaCl       0.125 L

= 0.34 M NaCl

1 mole of any substance has a mass = to the atom’s atomic mass.

Molarity is number of moles per Liters of solution

1- An AP Biology student dissolves 98.4 g of FeSO4 in enough water to make 2.000 L of solution. What is the molarity of the solution?

0.324 M

EXAMPLE PROBLEMS

1 mole .

151.91gx 98.4g = 0.647 moles ÷ 2.00L= 0.324 mol/L

151.91g

• Calculate the solute potential of a .1M NaCl solution at 22 degrees Celsius.

• If the concentration of NaCl inside the plant cell is .15M, which way will the water flow if the cell is placed in the .1M solution?

EXAMPLE PROBLEMS