WATER RELATIONSHIPS

Lecture 7.4

Why does water move through the plant?

• Photosynthesis

• Regulate temperature

• Move solutes/nutrients through plant

Water is a polar molecule

Oxygen is more electronegative than hydrogen.

It pulls shared electrons in the covalent bonds closer to itself.

Water is a polar molecule

Positive pole due to:

Fewer free electrons on one end of the molecule

Water is a polar molecule

Positive pole due to:

More free electrons on the other end of the H2O molecule.

Water properties:

Cohesion

Adhesion

Compressibility

Water properties:

Cohesion

• Molecule of water sticks to each other via (long) H-bonds

• important in water transport

Water properties:

Adhesion

Water sticks to other molecules via H bonds

Water properties:

Compressibility

Liquid water is incompressible because of H bonds.

How does water move in plants?

At a cellular level and molecular level by :

Diffusion.

How does water move in plants?

At a cellular level and molecular level by : Diffusion

Process by which molecules move from an area of higher concentration to an area of lower concentration, AKA: concentration gradient; until equilibrium is reached.

How does water move in plants?

At a cellular level and molecular level by : Diffusion

CONCENTRATION GRADIENT (down the gradient)

from an area of high concentration to an area of low concentration

How does water move in plants?

At a cellular level and molecular level by : Diffusion

How can we describe how fast they are moving?

Due to the FLUX = (quantity of substance transferred (mol)) (area x time)

How does water move in plants?

At a cellular level and molecular level by : Diffusion

How can we describe how fast they are moving? FLUX

How does water move in plants?

At a plant level by :

Bulk flow

How does water move in plants?

Water Potential

Quantifies the tendency of water to move from one area to another due to gravity, diffusion or mechanical pressure.

Water always moves from higher to lower potential.

Adding more solutes makes water potential more negative.

How does water move in plants?

Osmosis

Diffusion of water across a membrane. Passive movement across a selectively permeable membrane.

How does water move in plants?

Osmosis: Diffusion of water across a membrane. Passive movement across a selectively permeable membrane.

What are some characteristics of membranes?

• lipid bilayer

• selectively permeability

• Characteristic of the molecules (size, charge) will permit the flow through the lipid bilayer

• Transport proteins/ Aquaporins

How does water move in plants?

Complete:

Water can … through membranes by … but … make the process so much faster.

Water can diffuse through membranes by osmosis but aquaporins make the process so much faster.

How does water move in plants?

Osmosis: Diffusion of water across a membrane. Passive movement across a selectively permeable membrane.

Where the permeable membrane allow the passing of what types of molecules?

Small nonpolar molecules:

• Gases O2, N2, CO2

• fat soluble vitamins

• Steroids

Small uncharged polar molecules

• Water

• urea, ethanol, glycerol

How does water move in plants?

Osmosis: Diffusion of water across a membrane. Passive movement across a selectively permeable membrane.

Aquaporins

Specialized integral membrane proteins that form pores in cell membranes to rapidly facilitate the transport of water and small neutral solutes.

How does water move in plants?

Water moves from … water potential to … solute concentration.

Water moves from high water potential to low solute concentration.

Osmotic potential

Measure of the potential of water to move between regions of differing concentrations across a water-permeable membrane.

Water moves down/up osmotic gradients?

Water moves down osmotic gradients.

Water moves down osmotic gradients

So, what happens here?

Water moves across a semipermeable membrane into a compartment containing salt.

Outside there is a high concentration of water.

Water moves down osmotic gradients

So, what happens here?

The water flows inward until the force of its pressure inside balances the osmotic driving force.

Water moves down osmotic gradients

So, what happens here?

• The osmotic potential of pure water is 0.

• The addition of solutes lowers the water potential.

• Water moves towards a region with a lower water potential.

Turgor Pressure

Supports plant cells and tissues.

Turgor Pressure: Supports plant cells and tissues.

Process: How plants build up pressure

Step 1: Concentrate solutes

• The plant cell accumulates dissolved salts in its cytoplasm/vacuoles.

Step 2: Lower water potential

• More dissolved solutes make the cell’s water potential more negative

Step 3: Water enters

• Water moves into the cell by osmosis, from higher to lower water potential.

Step 4: Pressure builds up

• The cell expands, and water pushes the membrane against the cell wall, creating turgor pressure.

Turgor Pressure: Supports plant cells and tissues.

Process: How plants build up pressure

Turgor pressure allows plant to have:

• Structural support

• Cell expansion

• Stomata movement

Diffusion

• The movement of particles

• Different substances diffuse independently, each according to its own concentration gradient.

• Results in the net movement of molecules from higher to lower concentration.

• The rate of diffusion depends on:

-concentration gradient (d(c))

-diffusion coefficient (D).

Diffusion Flux

Amount of a substance that moves through a given area per unit of time due to diffusion.

Measured using Fick’s law

Diffusion Flux: Amount of a substance that moves through a given area per unit of time due to diffusion.

Fick’s law

J = -D (d[c]) / dx

J is the diffusion flux

D is the diffusion coefficient (depends on the molecule and medium)

d[c] is the concentration change

dx is distance

Bulk Flow

Molecules of water move all together from one place to another because of the differences in potential energy (water potential) → higher to lower

Pressure driven bulk flow mechanism responsible for long-distance transport of sap.

Quick reminder: Types of sap

Xylem sap:

• Water and minerals and its transport

• Mainly upward

• Driving force: transpiration pull and pressure differences

Phloem sap:

• Sugars and organic nutrients

• Phloem

• Food transport

• Pressure flow mechanism

Water rises to treetops by …, the process by which water evaporates from leaves through stomata.

Water rises to treetops by transpiration, the process by which water evaporates from leaves through stomata.

Water Potential Ψ_w

Measure of the potential energy in water.

• Difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient pressure)

• Quantifies tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure.

• measured in units of pressure: megapascals MPa

Water Potential Ψ_w

Which way does water move along a water-potential gradient?

From higher water potential to lower water potential.

Water Potential

By definition, the potential of pure water is Ψ_w is =

Ψ_w = zero

… Ψ_w of a solution is always going to be negative -MPa

Water Potential Ψ_w

What is the usual water potential of plant cells relative to pure water?

Usually negative because of dissolved solutes and other factors.

Water Potential Ψ_w

What is the usual water potential gradient in a transpiring plant?

Pure water/soil → root → leaf;

values generally become more negative.

Water Potential Ψ_w

Where is water potential usually most negative in a transpiring plant?

The leaf, because transpiration helps create a lower water potential.

Water Potential Ψ_w

Components of water potential in plant solutions:

solute concentration, pressure, gravity and other factors (matric effects)

Water Potential Ψ_w

Components of water potential in plant solutions: solute concentration, pressure, gravity and other factors (matric effects).

Ψ_system =

Ψ_system = Ψ_s + Ψ_p + Ψ_g + Ψ_m

where

s = solute, more solutes, more negative

p = pressure — such as turgor pressure

g = gravity

m = matrix ( the effect of water sticking to surfaces )

Water Potential Ψ_w

Water potential in a system refers:

Potential in the:

• soil water Ψ_soil

• root water Ψ_root

• leaf water Ψ_leaf

• water in the atmosphere Ψ_atmosphere

Water Potential Ψ_w

Water potential in a system refers to potential in the:

• soil water Ψ_soil

• root water Ψ_root

• leaf water Ψ_leaf

• water in the atmosphere Ψ_atmosphere

Water moves to equilibrate what?

equilibrate the system or compartment from higher Ψ to the one with lower Ψ until the difference is back to zero.

ΔΨ_w = 0

, where water moves in response to ΔΨ_w not individual components.

Solute Potential Ψ_s—Osmotic potential

Ψ_system = Ψ_s + Ψ_p + Ψ_g + Ψ_m

• Pure water has Ψ_s = 0 MPa

• Dissolved solutes lower potential, making Ψ_s negative

• Plant cell cytoplasm typically has Ψ_s aroundPressure -0.5 to -1.0 MPa

• Increase solute concentration makes Ψ_s more negative

• Difference in water potential contribute to osmotic water uptake from soil into root cells.

Pressure Potential Ψ_system = Ψ_s + Ψ_p + Ψ_g + Ψ_m

Effect of physical pressure on water potential.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

How does positive pressure affect total water potential?

It increases total water potential.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

What is the typical turgor pressure in living plant cells?

About 0.6 - 0.8 MPa some cells reach around 1.5 MPa.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

What does Ψ_p= 0 mean?

There is no pressure contribution to water potential.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

What happens when a plant cell loses turgor?

The cell becomes flaccid; loss of turgor can contribute to wilting.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

How does negative pressure (tension) affect water potential?

It decreases total water potential.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

How does transpiration help water move up the xylem?

It creates tension in xylem water, helping pull water upward from the roots.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

What is the difference between turgor pressure and xylem tension?

Turgor is positive pressure; xylem tension is negative pressure that helps pull water upward.

Pressure PotentialΨ_p
Effect of physical pressure on water potential.

What happens when Ψ_p = 0?

No pressure contribution to water potential.

Gravitational Potential Ψ_system = Ψ_s + Ψ_p + Ψ_g + Ψ_m

• Negative close to 0 in a plant with no height → not very important in small plants

• Taller trees become more important Ψ_g→ 100m, column of water is 1MPa

Matrix Potential Ψ_system = Ψ_s + Ψ_p + Ψ_g + Ψ_m

Adhesion of water surfaces.

→ Removes energy from the system because water molecules bind to the cellulose matrix of the plant’s cell walls.

The more humidity, Ψ_w in atmosphere =

The more humidity, Ψ_w in atmosphere = 0

Water moves through:

Diffusion and bulk flow

Transpiration

Loss of water vapor from plants from above ground tissue (mainly leaves)

-PASSIVE (no ATP)

-Driving energy = water potential between soil vs plant/atmosphere

-Uptake of CO2 for photosynthesis and the loose of water by transpiration are necessary for life.