Water and plant cells (chapter 3)

I. Background on water in plants

II. The properties of water

III. Understanding the direction of water movement: Water potential

I. Water

• Plant cells are mostly water; 80 - 95% of the mass of growing cells, (less in wood and seeds)

• Living cells must maintain a positive water pressure, or “turgor” to growand function properly.

• Plants lose large quantities of water in transpiration, the evaporation from the interior of leaves through the stomata.

Water passes easily through biological membranes, particularly through aquaporins - low resistance pores.

II. The properties of water Polar molecule that forms hydrogen bonds. Polar molecule that forms hydrogen bonds.

1) good solvent1) good solvent2) cohesive properties - attraction to like molecules2) cohesive properties - attraction to like molecules3) adhesive properties - attraction to unlike molecules3) adhesive properties - attraction to unlike molecules

Cohesion of water molecules gives water high tensile strength - it can withstand high tension

(negative pressure) without shearing apart.

Water in the xylem is under negative pressure (more on this in Chapter 4)

Properties of water, continued•Cohesion is the attraction of like molecules (H2O here) that gives water its tensile strength.

•Adhesion is the attraction of unlike molecules. Water adheres to cell walls, soil particles, glass tubes, etc. Adhesion explains capillarity & surface tension.

Water’s thermal properties

•High specific heat = 4.18 kJ kg-1 0C-1 Why don’t saguaros overheat?

•High latent heat of vaporization44 kJ mol-1 or 2.44 kJ g-1

Leaves are like swamp coolers!

What’s a sling psychrometer?

III. What factors determine the direction of water movement (through the soil, between cells, from roots to leaves, from leaves into air)?

How can we describe these factors in a consistent way?

We’ll use the concept of water potential.

“Potential” indicates the energetic state.

What factors determine the direction of water movement?

1. Gravity

2. Pressure

3. Concentration

0.0 0.2 0.4 0.6 0.8 1.0

Height, meters

0

10

20

30

40

50

60

70

80

90

100

but it flows upwardin trees.

How does this work?

How do we relate the energetic statusof water to height?

PressureWater moves from regions of higher to lower pressure

garden hosestrawthrough xylem of plants

Water moves from higher to lower pressure

Water pressures in plant cells can be positive (turgor), or negative, (tension).

Living cells ≥ 0 MPa to ≈ +3 MPa)Dead xylem cells ≤ 0 MPa, to as low as -12 MPa.

3) ConcentrationWater moves by diffusion from regions of higher to lower water concentration.

Solutes added to pure water dilute the water concentration.

Osmosis is the diffusion of water across a selectivelypermeable membrane from a region of higher to lower water concentration.

How does reverse osmosis purify water?

Solutes reduce the concentration of water. Think of the effect of solutes in terms of water concentration.

How can we bring together the influences of gravity, pressure, and solutes in understanding thestatus of water?

Is there a consistent set of units?

The concept of water potential, , brings together the influences of gravity, pressure, and concentration (solutes) in describing the energy state of water and the direction of water movement.

The water potential equation:

WWSSPPgg

WW = total water potential = total water potentialSS = solute potential = solute potential

P P = pressure potential = pressure potentialgg = gravitational potential = gravitational potential

All units will be pressure, pascals, Pa.MPa is megapascal, 106 Pa

We’ve been talking about the “energy state” of water, but now water potential in terms of pressure.

What’s the relationship?

Recall from before:

pressure x volume = energyPa x m3 = joules

pressure = energy/volume

The reference condition for water potential thinking:

Pure water (S= 0), at ground level (g = 0) and atmospheric pressure (P = 0) has a total water potential, W, of 0 MPa.

WWSSPPgg

How do we express S, P, & g in units of pressure?

S, the solute pressure or solute potential.

S = -RTCS

Where R is the gas constant, T is Kelvin temp.,and CS is the solute concentration.

R = 0.008314 MPa liters oK-1 mol-1

Cs = mol liter-1

Bottom line: adding solutes to water decreases the solute potential.

S = -RTCS

What is the solute (osmotic) potential of sea water?assume 25 oC or 298 oK

CS = 1.15 mole liter-1 of Na+ + Cl- + other ions

S = (-0.008314MPa liter oK-1 mol-1)(298oK)(1.15 mol liter-1)

S = -2.84 MPa

WWSSPPgg

The pressure potential P is just what we wouldmeasure with a pressure gauge.

Dimensional analysis= density x g x height= kg m-3 x m s-2 x m

= N m-2

= pressure, Pa

Example: what is gravitational potential of water at 100 m in a tree?

g = 1000 kg m-3 x 9.8 m s-2 x 100m

= 9.8 x 105 Pa or 0.98 MPa

So, to hold water at that height, there must be a counteracting negative pressure of at least -0.98 MPa in the xylem

What do various values of W mean for plant function?