Plant phsiology
Plant phsiology
Plant – water relations- is the movement of water and other substances across membranes, throughout the plant, and between the plant and its environment
This subject involves the cellular processes which influence water relations.
Growth also depends upon water uptake, and much of plant water relations depends upon cell interacting with their environment.
Hydrogen bond in water molecules
E.g Carbon dioxide (MW = 44) and n-butane (MW = 58 grams per mole are all gases at room temperature.
Water (MW= 18 grams per mole ) is liquid at room temperature
Reason for that:
Hydrogen bonds provides a disproportionately high attractive force among water molecules, inhibiting their separation and escape as vapor
The hydrocarbons on the other hand, have only the relatively weak van der Waals forces among their molecules in the liquid state. Little energy (low temperature) is needed to drive them into the gaseous phase
Other liquids with low molecular weights are also polar molecules with hydrogen bonding between them are Methanol CH3OH, MW = 32 , Methanoic acid CHOOH, MW = 46
Water is an incompressible Fluid
The normal form of a plant is maintained by the pressure of water in its protoplasts pressing against its cell walls.
This is also observed in growth of plants due to absorption of water into their cells
Closing and opening of stomata which is controlled by the movement of water into and out their guard cells
Various substances are transported in moving fluids in Plants
Pascal’s law states that increase in pressure on the surface of a confined fluid is transmitted undiminished throughout the confining vessel or system. This is the basic principle of hydraulics.
Specific heat
The amount of energy required to raise the temperature of a unit mass of a substance 1 oC is called its specific heat.
Specific Heat Capacity (C or S ) - The quantity of heat required to raise the temperature of a substance by one degree Celsius is called the specific heat capacity of the substance. The quantity of heat is frequently measured in units of Joules(J). Another property, the specific heat, is the heat capacity of the substance per gram of the substance. The specific heat of water is 4.18 J/g° C.
Specific heat
This slightly variation results into absorbing large quantities of energy without much temperature increase.
The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The relationship between heat and temperature change is usually expressed in the form shown below where c is the specific heat. The relationship does not apply if a phase change is encountered, because the heat added or removed during a phase change does not change the temperature.
Q = CmΔT
Where Q = heat added
C = Specific heat
m = mass
ΔT = change in temperature
The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C which is higher than any other common substance. As a result, water plays a very important role in temperature regulation. The specific heat per gram for water is much higher than that for a metal, as described in the water-metal example. For most purposes, it is more meaningful to compare the molar specific heats of substances.
Latent Heats of Vaporization and Fusion
Some 2,452 joules (586 cal) are required to convert 1g of water at 20 oC to 1g of water vapor at 20oC. This is high latent heat of vaporization .
Melting of 1 g of ice at 0 oC, 335 j (80 cal) must be supply which is also high latent heat of fusion caused by hydrogen bonds.
When a kettle is put on a boil the temperature of the water steadily rises until it reaches 100 °C. At this temperature it starts to boil, that is to say bubbles of vapour form at the bottom and rise to the surface, where they burst and escape as
Once the water has begun to boil, the temperature remains constant at 100 °C. But at the same time, heat is being steadily absorbed by the water from the gas flame or heating element.
This heat, which is going into the water but not increasing its temperature, is the energy needed to convert the water from the liquid state to the vapour state.
Definition of the specific latent heat of vaporization
The specific latent heat of vaporization of a substance is the quantity of heat required to change unit mass of the substance from the liquid to the vapour state without change of temperature. ( Symbol = L ).
The SI unit of specific latent heat of vaporization is the Joule per kilogram ( J / kg ). However, in order to avoid having to write every large numbers the alternative units kj / kg or MJ / kg may be used instead.
1 kj = 1000 j
1 Mj = 100000 j
So we may express the specific latent heat of vaporization of water as 2260 kj / kg or 2.26 Mj / kg. ( The old thermal unit was calorie per gram ( cal / g ) ).s
Latent heat of fusion
Just as latent heat is taken in when water changes to vapour at the same temperature, so the same thing occurs when ice melts to form water.
But in this case the latent heat is not so great. It requires only 336000 j to convert 1 kg of ice at 0 °C to water at the same temperature.
Likewise, when water at 0 °C freezes into ice, the same quantity of heat is given out for every 1 kg of ice formed. This is called the specific latent heat of ice.
As already mentioned, the phenomenon of latent heat is not confined to water alone. Other substances also absorb latent heat when they melt; conversely, they give out latent heat on solidifying. This heat is called latent heat of fusion.
The specific latent heat of fusion
The specific latent heat of fusion of a substance is the quantity of heat required to convert unit mass of the substance from the solid to the liquid state without change in temperature. ( Symbol = L. ).
The SI units, j / kg, or alternatively kj / kg or Mj / kg, are used for fusion as for vaporization.
Adhesive and Cohesive Forces of Water
Water is polar molecule and hence attracted to many other substances, that is wets them.
This attraction between unlike molecules is called adhesion.
While the attraction of like molecules for each other is called cohesion.
Cohesion confers upon water an unusually high tensile strength, which is the ability to resist stretching (tension) without breaking. E.g pulling of the water by Xylem element of a stem to the top of tree without breaking
Cohesive and Adhesive Forces
When a liquid comes into contact with a surface ( Such as the walls of a graduated cylinder or a table top), both cohesive and adhesive forces will act on it.
These forces govern the shape which the liquid takes on.
Due to the effects of adhesive forces, liquid on a surface can spread out to form a thin, relatively uniform film over the surface, a process known as wetting.
In the presences of strong cohesive forces, the liquid can divided into a number of small, roughly spherical beads which stand on the surface, maintaining minimal contact with the surface
Cohesive forces are the intermolecular forces such as those from hydrogen bonding and Van der Waals forces which cause a tendency in liquids to resist separation
Adhesive forces are the attractive forces between unlike molecules.
They are caused by forces acting between two substances, such as mechanical forces (sticking together) and electrostatic forces (attraction due to opposing charges).
In the case of a liquid wetting agent, adhesion causes the liquid to cling to the surface on which it rests. When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the glasses surfaces. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules.
When liquid is placed on a smooth surface, the relative strengths of the cohesive and adhesive forces acting on that liquid determine the shape it will take (and whether or not it will wet the surface)
If the adhesive forces between a liquid and a surface are stronger, they will pull the liquid down, causing it to wet the surface.
However, if they cohesive forces among the liquid itself are stronger, they will resist such adhesion and cause the liquid to retain a spherical shape and bead the surface
Cohesion among water molecules also accounts for surface tension;
The molecules at the surface of a liquid are continuously being pulled into the liquid by the cohesive hydrogen-bonding ) forces, where those in the vapor phase are too few and too distant to exert any force on the molecules at the surface. As a result, a drop of water acts as if it were covered by a tight elastic skin; surface tension is what makes a falling drop spherical . The surface tension of water is higher than that of most other liquids.
Surface tension plays many roles in the physiology of plants. For example, under normal pressures, surface tension prevents the passage of air bubbles through the minute pores and pits in cell walls. The water surface of bubbles cannot deform enough to pass through the small openings.
Water as a Solvent
Water will dissolve more substances than any other common liquid.
This is partially because it has one of the highest known dielectric constants, which is a measure of the capacity to neutralize the attraction between electrical charges.
Water is an especially powerful solvent for electrolytes and polar molecules such as sugars.
The positive side of the water molecule is attracted to the negative ion or molecular surface of a polar molecules, and likewise the negative side to the positive ion or surface.
Water molecules thus form a “cage” around ions or polar molecules, so the ions are often unable to unit with each other and crystallize into a precipitate.
The importance of water as solvent in living organisms is evident in various processes such as Osmosis and protoplasmic activities in the cells.
Osmosis process depend on dissolved materials in the cell’s water.
All molecules of protoplasm perform their specific chemical activities to the water surroundings in which they exist.
Water molecules actively enter into the chemistry that is the basis of life. Water and carbon dioxide are the raw materials for photosynthesis.
Movements of various materials in plants either by diffusion and bulk flow depend on dissolved materials.
Diffusion, Osmosis, and Movement Across a Membrane
Diffusion
Spontaneous movement of particles from an area of high concentration to an area of low concentration
Does not require energy (exergonic)
Occurs via random kinetic movement
Net diffusion stops when concentration on both sides equal (if crossing a membrane) or when there is a uniform distribution of particles
Equilibrium is reached
Molecules continue to move, but no net change in concentration (hence the phase "net diffusion" above
Diffusion of one compound is independent to diffusion of other compounds
Factors Affecting Diffusion Across a Plasma Membrane
Diffusion directly through lipid bilayer
The greater the lipid solubility of the diffusing particle, the more permeable the membrane will be
All else being equal, smaller particles will diffuse more rapidly than larger particles
O2, H2O, CO2 rapidly diffuse across lipid bilayer
Diffusion of Hydrophilic Molecules Across a Plasma Membrane
Plasma membrane is semipermeable
Water, while polar, is small enough to freely move across the plasma membrane
Larger hydrophilic uncharged molecules, such as sugars, do not freely diffuse
Charged molecules cannot diffuse through lipid bilayer
Ion channels and specific transporters are required for charged molecules and larger, uncharged molecules
Osmosis, the Passive Transport of Water
Osmosis = the diffusion of water across a semi-permeable membrane
Plasma membrane permeable to water but not to solute
Solute = dissolved particle
Solvent = liquid medium in which particles may be dissolved
Water moves from solution with lower concentration of dissolved particles to solution with higher concentration of dissolved particles
Water moves from dilute solution to concentrated solution
Osmotic potential is the total of all dissolved particles
Solution Types Relative to Cell
Hypertonic Solution: Solute concentration of solution higher than cell
More dissolved particles outside of cell than inside of cell
Hyper = more (think hyperactive); Tonic = dissolved particles
Water moves out of cell into solution
Cell shrinks
Hypotonic Solution: Solute concentration of solution lower than cell
Less dissolved particles outside of cell than inside of cell
Hypo = less, under (think hypodermic, hypothermia); Tonic = dissolved particles
Water moves into cell from solution
Cell expands (and may burst)
Isotonic Solution: Solute concentration of solution equal to that of cell
No net water movement
Osmosis Produces a Physical Force
Movement of water into a cell can put pressure on plasma membrane
Animal cells will expand and may burst
Some cells, such as Paramecium have organelles called contractile vacuoles which are basically little pumps which pump excess water out of cell
You can alter the rate of contractile vacuole pumping by placing it in increasingly hypotonic solutions
Organisms with a cell wall, such as plants, do not burst
Cell membrane pushes against cell wall
The rigid cell wall resists due to its own structural integrity
These opposing forces create turgidity, which keeps plants upright
If you don't water a plant, it wilts (this is called plasmolysis). Water it, the leaves will come back up do to the reestablishment of turgidity.
What part of the plant is responsible for drawing water into the plant cell?
Facilitated Diffusion
Allows diffusion of large, membrane insoluble compounds such as sugars and amino acids
Does not require energy (passive)
Highly Selective
Substance binds to membrane-spanning transport protein
Binding alters protein conformation, exposing the other surface
Fully reversible - molecules may enter the cell and leave the cell through the transport protein.
Particles move from areas of high concentration to areas of low concentration.
Movement rate of particles will saturate
Maximum rate limited by number of transporters
Once all transporters are operating at 100%, an increase in concentration will not increase rate
Diffusion Versus Bulk Flow
Fluid are substances , such as liquids or gases, that flow or conform to the shape of their container.
When the flow occurs in response to differences in pressure and involves groups of atoms or molecules moving together, it is called bulk flow.
The differences in pressure is sometimes due to differences in pressure which is established by gravity (the weight of fluid); these are hydrostatic pressures.
Also the pressure is due to mechanical forces which applied to all or part of the systems.
In Plants fluids flow through the vascular tissues by bulk flow in response to pressure differences that are created by diffussion
Water and the substances dissolved in it move into and out of cells not by bulky flow
Each molecule move at a time and their net movement from one point to another is due to random kinetic activities or thermal motions of molecules or ions which is called diffusion
Diffusion often occurs in response to differences in concentration of substances between one point and another
Concentration differences are common in living cells in particular and in organisms in general
In order to understand the process of diffusion within the plant cells we need to understand the kinetic theory
Transport in Plants
Transport in plants occurs on three levels
The uptake and release of water and solutes by individual cells
Absorption of water and mineral from the soil by root cells
Short-distance transport of substances from celsl to cell
Loading of sucrose from photosynthetic cells into the sieve tube cells of the phloem
Long –distance transport of sap within the xylem and phloem
This is a whole plant phenomena-transport of photosynthate from leaf to root
Survival of the plant depends on balancing water uptake and water loss
In an animal cell, water flows from hypotonic to hypertonic solutions, but in a plant cell,
there is the added presence of the pressure created by the cell wall.
The combination of solute concentration differences and physical pressure are incorporated into water potential.
Energy
Energy - the ability to do work, that is, to move matter against opposing forces such as gravity and friction
kinetic energy - the energy of motion.
potential energy - stored energy, the capacity to do work
Thermodynamics - the study of energy transformation The First Law of Thermodynamics - Energy can be transferred and transformed, but it can neither be created nor destroyed
The total energy of the universe is constant
The Second Law of Thermodynamics - Every energy transfer or transformation increases the entropy of the universe
There is a trend toward randomness
Energy must be spent to retain order - this spending of energy usually releases heat, which increases the entropy elsewhere
Free Energy - the portion of a system's energy the can perform work It is called "free" energy because this is the energy which can perform work, not because there is no energy cost to the system
Exergonic Reaction - a process with a net release of free energy Sometimes called spontaneous, but that doesn't mean that it will occur rapidly
Burning paper is exergonic, but paper just doesn't ignite when it is exposed to air - it requires an initial input of energy to start the reaction
Kinetic theory
The kinetic theory states that the elementary particles (atoms, ions, and molecules) are in constant moving at temperatures above absolute zero.
The average energy of a particle in a homogenous substance rises as temperature increases but is constant for different substances at a given temperature.
References:
Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego, CA
Peter H. Raven, Ray F. Event, Susan E. Eichhorn (2005) Biology of Plants, 7th. Ed. W.H. Freeman and Co., New York.
Textbooks:
References:
Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego, CA
—Dickson, W.C. 2000. Integrative plant anatomy, Academic
Press
—Fahn, A. 2003. Plant Anatomy, Pergamon Press
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