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Water potentialWater potential is a concept that helps to describe the tendency of water to move from one area to another, particularly into or out of cells.

oWater molecules move randomly. oWhen water is enclosed by a membrane some of the moving water molecules will hit the membrane, exerting pressure on it. oThis pressure is known as water potential.

It is measured in units of pressure, can be measured in kPa, MPa, bar.

Pure water has a water potential of zero.

A solution will have a lower concentration of water molecules so it will have a negative water potential.

Water PotentialWater Potential•We look at water movement in

terms of water potential. (ψ psi)•Two factors:

– Solute concentration and pressure•Pure water ψ =0•The addition of solute lowers the water potential. (negative number)

Water potential determines the rate and direction of osmosis.

Pressure potential is important in plant cells because they are surrounded by a cell wall which, is strong and rigid. When water enters a plant cell, its volume increases and the living part of the cell presses on the cell wall. The cell wall gives very little and so pressure starts to build up inside the cell. This has the tendency to stop more water entering the cell and also stops the cell from bursting. When a plant cell is fully inflated with water, it is called turgidturgid. Pressure potential is called turgor pressure in plants)

ψp

ψp

ψp

ψp

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.

Water potential (ψ) =

pressure potential (ψp )

+ solute potential (ψs)(osmotic)

This is an open container, so the ψp = 0

This makes the ψ = ψsThe ψs =-0.23, so ψ is -0.23MPa, and water moves into the solution.

Water moves from a place of high water potential to a place of low water potential.

Can a solution with a molarity of 0.2 be in equilibrium with a solution with a molarity of 0.4? YES!PressureTwo solutions will be at equilibrium when the water potential is the same in both solutions. This does not mean that their solute concentrations must be the same, because in plant cells the pressure exerted by the rigid cell wall is a significant factor in determining the net movement of water.

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 concentration

R = Pressure constant = 0.0831 liter bar/mole K

T = Temperature in degrees Kelvin (273 + °C) of solution Example Problem: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??????

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So, we can now define osmosis as the movement of water molecules from a region of higher water potential to a region of lower water potential through a semi-permeable membrane.

 

MolarityAnother way of expressing

concentration, the way that we will use most in this course, 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 x1 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

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