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Activity of Water =Water Potential
Plant Physiology UNI
Spring 2009
Overview
• Physical part– Effect of temperature– Effect of pressure
• Chemical part– Effects of solutes– Effects of hydrophilic solids
• Combining to get water potential– Potential to do “work” (push, move)
Physical: activity of water
• Molecules have energy
• Molecules move around
air
water
Temperature effect
• Absolute zero
• Cold
• Medium
• Hot
Absolute Zero
air
water
NO MOVEMENT
Cold Temperature
air
water
SLOW MOVEMENT
Medium Temperature
air
water
MEDIUM MOVEMENT
Hot Temperature
air
water
FAST MOVEMENT
Effect of Pressure
• Low pressure
• Medium pressure
• High pressure
PPP
Low Pressure
air
water
SLOW MOVEMENT
P
Medium Temperature
air
water
MEDIUM MOVEMENT
P
High Pressure
air
water
FAST MOVEMENT
P
To increase or decrease activity
• Increase or decrease pressure
• Temperature also counts, but minor effect over short distances
Chemical activity of molecules
• Based on chemical environment– Determines what a molecule interacts with– Determines the effect of the interactions
Physical and chemical effects
• Molecules have physical activity
• Also chemical activity– Interact with other chemicals – Each molecule behaves in its own way
• Water acts like water• Ca acts like Ca
– What affects the chemical activity?– How much?
Without solutes
air
water
WATER MOLECULE NOT TIED DOWN BY SOLUTES
STANDARD WATER ACTIVITY
++ +
+
__
_
Add solutes
air
water
SLOWER MOVEMENT
WATER MOLECULE PARTLY COVERED
WATER ACTIVITY IS LOWER
Change Water Activity
• Increase– ↑ temperature– ↑ pressure
• Decrease– ↓ temperature– ↓ pressure– add solutes
• More solutes = more effect• But we are still focusing on water
activity, not solute activity
Pure Water, Standard T, P
air
water
WATER ACTIVITY OF PURE WATER = STANDARD
STANDARD = 0
THIS WATER ACTIVITY = 0
COMPARE ALL WATER TO THIS STANDARD
Water Potential
• Water activity of your solution compared to water activity of standard solution
• Standard solution = 0
• Your solution can be – Same activity (WP = 0)– Lower activity (WP < 0)– Higher activity (WP > 0)
• Doesn’t occur in biology
Need to get some numbers
• Water potential scale
• Like temperature scale
Absolute & Relative Scales: Temperature
Relative Scales: Water Potential
Water potential scale
• Water activity in cell (beaker, bucket, trunk)– Not the other chemicals
• Distributed characteristic (all the same)– Like temperature
• Compared to standard (0)• Can be positive, zero or negative• Expresses effect of P, T, solutes
– On water, compared to standard
Summary
• Physical activity of water molecules changes with– (Temperature,) pressure
• Chemical activity of water molecules changes with solutes
• Water potential tells us the total activity– Compared to pure water, standard TP
• WP can be +, 0, -
Properties of gases & liquids
gases liquids
density (L/H)
Properties of gases & liquids
gases liquids
density (L/H) lower higher
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F)
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
interaction between molecules(near 0/lots)
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
interaction between molecules(near 0/lots)
zero for ideal gas
yes, holds molecules together
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
interaction btwn molecules (0/lots)
zero for ideal gas
yes, holds molecules together
Change in V with temp (little/lots)
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
interaction btwn molecules (0/lots)
zero for ideal gas
yes, holds molecules together
Change in V with temp (little/lots)
lots little
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
interaction btwn molecules (0/lots)
zero for ideal gas
yes, holds molecules together
Change in V with temp (little/lots)
lots less
Change in V with P(little/lots)
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
interaction btwn molecules (0/lots)
zero for ideal gas
yes, holds molecules together
Change in V with temp (little/lots)
lots less
Change in V with P(little/lots)
lots almost none
Properties of gases & liquids
gases liquids
density (L/H) lower higher
diffusion (S/F) faster slower
interaction btwn molecules (0/lots)
zero for ideal gas
yes, holds molecules together
Change in V with temp (little/lots)
lots little
Change in V with P(little/lots)
lots almost none
Water: gas, liquid & solid
• Life mostly water: properties critical
• Boil or evaporate: big change in volume
• Small volume change when we make ice
• Polar molecules– Not stuck together in gas– Stuck together loosely in liquid– Stuck together tightly in solid
Acting out a gas
• Student volunteers
• Gas molecules don’t hold onto each other
• Can compress if contained (can push) – big volume change
• Can't pull (no connection between molecules)
• Gases: can push but not pull
• Gases: under positive pressure only
Acting out a liquid (water)
• Same volunteers• Can push or pull: molecules bound to each other
– Especially true for water– More like solid than like gas
• Positive or negative P (“pressure” or “tension”)– Compression: P is + – Tension: P is - (like solid)
• Must contain liquid to push (like gas)
Gas in liquid—acting out in class
• Same volunteers
• Make the middle molecule a gas– Can push – Can't pull
• Gas bubble is called embolism – may occur naturally
• Embolisms stop transport based on pulling
Archaic use of “tension”
• Hypertension, oxygen tension in blood, high tension electrical wires
• “Tension” was used to mean + pressure – Still use this sense in medicine
• Gas pressure can be lower than atmospheric, but that is still positive – Low pressure is positive– Vacuum is zero pressure – No negative pressures in gases
Demonstrations
Syringe sealed at bottom
Liquid
Air
Pull up
What is holding up this part of the water drop?
What will happen to the liquid when we pull up?What happens to air & liquid pressures?