Activity of Water Plant Physiology UNI Spring 2006.

Activity of Water

Plant Physiology UNI

Spring 2006

Overview

• Physical activity of water– Effect of temperature– Effect of pressure

• Chemical activity of water– Includes physical effects (T, P)– Also other effects

• Combining to get water potential

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 Pressure

air

water

MEDIUM MOVEMENT

P

High Pressure

air

water

FAST MOVEMENT

P

To increase or decrease activity

• Increase or decrease pressure

• Increase or decrease temperature

Chemical activity of molecules

• Based on physical environment (T, P)

• Also 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

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

• The water in a 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, -