Plant Physiology 2014

RETNO MASTUTI

TRANSPORT AND TRANSLOCATION OF WATER

AND SOLUTES

1 - 1Properties of Water

1 Water and Plant Cells

Most organisms are comprised of at least 70% or more water.

80-90% of a growing plant cell is water. This varies between types of plant cells

Carrot has 85-95% water Wood has 35-75% water Seeds have 5-15% water

Water is the limiting resource for crop productivity in most agricultural systems

Plant continuously absorb and lose water

Water is absolutely essential for all living organisms

Water

Water is polar and readily forms hydrogen bonds

Cohesion, adhesion → capillary action

The Properties of Water

1. a liquid at physiological temperatures (0 – 100C)

2. high heat of vaporization → cooling system 

3. high specific heat (heat capacity). → thermal buffer

4. high surface tension → capillary action, a meniscus forms. (negative presssure tension)

5. high tensile strength and incompressibility → water is good for hydraulic systems and produces positive pressures (hydrostatic pressures).  This pressure provides the driving force for cell growth and other plant movements

6. dissociates into protons and hydroxide ions. 

Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321

High heat of vaporization. Large quantities of energy (about 44 kJ mol-1) is required to change water from liquid phase to gas phase at constant temperature.

Specific heat is the specific amount of heat in calories (4.184 J g-1 C-1) needed to raise the temperature of 1 gram of water by 1 degree Celsius to beak the hydrogen bond

Surface tension and capillary action of water

Acids and Bases

1. Water ionizes to form a hydrogen ion (or proton/H+) and hydroxide ion (OH-)

2. An acid is a substance that increases the [H+], or as a proton donor.    eg. HCl → H+ + Cl-

3. A base is a substance that increases the [OH-]; or from the perspective of a proton, a base is a substance that decreases the proton concentration; it is a proton acceptor.    e.g. NaOH →  Na+ + OH- (accepts protons to make water)    e.g. NH3 (ammonia) + H+ →  NH4+ (ammonium ion)

4. pH increases, the [H+] decreases and the [OH-] increasespH decreases, the [H+] increases and the [OH-] decrease

Living systems are very sensitive to pH

Organisms must maintain pH within tolerable

ranges. This is a good example of homeostasis.

A buffer is a solution that resists fluctuations in

pH when additional OH- or H+ are added.

A buffer maintain a constant pH and usually

consist of a proton donor and a proton acceptor.

Functions of Water 1. a major component of cells

2. a solvent for the uptake and transport of materials - a universal solvent 

3. a good medium for biochemical reactions - chemically inert

4. a reactant in many biochemical reactions (i.e., photosynthesis) ; transparent to light → photosynthesis

5. cell elongation and growth

6. thermal buffer

7. provides structural support via turgor pressure (i.e., leaves)

8. the medium for the transfer of plant gametes (sperms swim to eggs in water, some aquatic plants shed pollen underwater)

9. offspring (propagule) dispersal (think "coconut")

10. plant movements are the result of water moving into and out of those parts (i.e., diurnal movements, stomatal opening, flower opening)

1 – 2 Water Potential

Water and Plant Cells

The factors affecting water moves

Its concentration (chemical potential): water will diffuse from a dilute solution to concentrated solution (osmosis).

Its pressure (mechanical potential): water will move from a high pressure system (hose-pipe) to a low pressure system (vacuum).

Its height (gravitational potential): water will flow downhill.

These factors sum together to give the overall free energy of a mass of water

Water Potential

Measure of the energy state of water.

This concept is important in plant physiology because it determines the

direction and movement of water.

A. Some definitions :

1. Free energy of water : energy available to do work (J = N m)

2. Chemical potential (µ) : free energy/unit quantity (per mole) (J mol-1)

3. Water potential (Ψw) - chemical potential of water, compared to pure water at the same temperature and pressure (atm pressure).

The water potential of pure water is zero

 

4. Water potential is defined as the chemical potential of water divided by the partial molar volume. 

J mol-1/ L mol-1 =  J / liter = energy / volume = (weight x distance)/area x distance = force/area = pressure  

 J mol-1/ L mol-1 = (N x m mol-1)/ (m3 mol-1) =  N  m-2 = MPa

5. Pressure is measured in MPa (megapascals). 1 MPa = 10 bars = 10 atm (1 atm = 760 mm Hg

= 14. 7 lbs sq in-1)

B. Equation for water potential

Water potential is the sum of the contributions of the various factors that influence water potential

Ψw = water potential Ψp = pressure potential Ψs = solute or osmotic potential Ψm = matrix potential, and Ψg = gravity potential

Ψw   = Ψp + Ψs + Ψm + Ψg

Pressure (Pressure Potential; Ψp) Due to the pressure build up in cells against to

the wall. It is usually positive, although may be negative

(tension) as in the xylem. Pressure can be measured with an osmometer.

a large positive pressure potential flaccid or plasmolysed cell

(right) have a pressure potential of zero

Solute (or osmotic) potential (Ψs) This is the contribution due to dissolved solutes. Solutes always decrease the free energy of

water, thus there contribution is always negative.

The solute potential of a solution can be calculated with the van’t Hoff equation:

Ψs = - miRT where,

m = molality (moles/1000 g); i = ionization constant (often 1.0); R = gas constant (0.0083 liter x MPa/mol) T = temperature (K).

Matric potential (Ψm)

This is the contribution to water potential due to the force of attraction of water for colloidal, charged surfaces.

It is negative because it reduces the ability of water to move.

In large volumes of water it is very small and usually ignored.

However, it can be very important in the soil, especially when referring to the root/soil interface.

Gravity (Ψg)Contributions due to gravity which is usually

ignored unless referring to the tops of tall trees.

Ψw   = Ψp + Ψs

Water and Plant Cells

1 – 3 Water Movement

Water Movement There are two major ways to move molecules:

A. Diffusion : spontaneous, random movement of molecules from an area of high free energy (higher concentration) to one of low free energy (lower concentration) → concentration gradient.

Does not require energy (exergonic). Net diffusion stops

when concentration on both sides equal (if crossing a membrane) or when there is a uniform distribution of particles. When equilibrium is reached : Molecules continue to move, but no net change in concentration.

B. Osmosis: diffusion through selective permeable membrane

C. Bulk (or Mass) Flow. : mass movement of molecules in response to a pressure gradient, from high to low pressure, following a pressure gradient.

Factors influencing the rate of diffusion

1. Concentration Gradient. directly proportional to the concentration gradient, indirectly proportional to resistance, inversely proportional to distance traveled

2. Molecular Speed : directly proportional to temperature and indirectly related to molecular weight

3. Temperature 4. Pressure 5. Solute effect on the chemical potential of the solvent. Solute

particles decrease the free energy of a solvent. The critical factor is the number of particles, not charge or particle size.

Fick’s Law Jv = (C1 - C2) r

Diffusion rate (Jv), concentration gradient (C1 – C2) and resistance (r)

Diffusion - Osmosis

- Osmosis - diffusion of water. - Osmosis affects the turgidity of cells, different solution can affect the cells internal water amounts

Turgor pressure occurs in plants cells as their central vacuoles fill with water.