Water Movement in Xylem through TACT Mechanism and opening and closing of stomata

 Water Movement in Xylem through TACT Mechanism. 

Four important forces combine to transport water solutions from the roots, through the xylem elements in the stem, and into the leaves. These TACT forces are: transpiration, adhesion, cohesion and tension.

a. Transpiration

Involves the pulling of water up through the xylem of a plant utilizing the energy of evaporation and the tensile strength of water.

b. Adhesion 

Is the attractive force between water molecules and other substances. Because both water and cellulose are polar molecules there is a strong attraction for water within the hollow capillaries of the xylem.

c. Cohesion

 is the attractive force between molecules of the same substance. Water has an unusually high cohesive force due to the hydrogen bonding. It is estimated that water's cohesive force within xylem give it a tensile strength equivalent to that of a steel wire of similar diameter. A combination of adhesion, cohesion, and surface tension allow

water to climb the walls of small diameter tubes like xylem. This is called capillary action. The U shaped surface formed by water as it climbs the walls of the tube is called a meniscus

 Tension 

It can be thought of as a stress placed on an object by a pulling force. This pulling force is created by the surface tension which develops in the leaf's air spaces.

Tension is a negative pressure - a force that pulls water from locations where the water potential is greater. The bulk flow of water to the top of a plant is driven by solar energy since evaporation from leaves is responsible for transpiration pull.

Mechanism of Opening and Closing of Stomata

Mechanism of opening and closing of stomata can be studied by the following most acceptable theories.

1. Stareh Sugar Theory

According to this hypothesis photosynthesis occurs in light by absorbing carbondioxide which lowers the H+ ion of cell sap and pH of guard cell is increased. High pH favours the activity of enzyme phosphorylase which converts starch into glucose and phosphate. It dissolves in the medium and increases the concentration of cell sap. This causes an increase in the osmotic pressure of guard cells and its diffusion pressure deficit (DPD) also increases which results in the

movement of water into the guard cells from surrounding cells. Guard cells become turgid and swell. Thus the stomata open. During dark, the level of carbondioxide in substomatal cavity is increased which results in the decrease in the pH of guard cells. At low pH glucose is converted back to starch in the presence of enzyme

phosphorylase. Synthesis of starch leads to the dilution of cell sap by consuming its dissolved glucose molecule. Thus osmotic pressure of cell sap is decreased and its DPD (diffusion pressure deficit) is decreased. The

turgid cells lose water to surrounding cells and becomes flaccid and stomata closes.

2. Theory of k+ ion transport

In the presence of light starch is converted into phosphorylated hexoses and then to phosphoenol pyruvic acid, which combines with carbondioxide to produce malic acid. Malic acid dissociate into malate anion and H+ ion in the guard cell. H+ ions are transported to epidermal cells and K+ions are taken into the guard cells in exchange of H+ ions. Increased concentraton of K ions and malate ions in the vacuole of guard

cells causes sufficient osmotic pressure to absorb water from surrounding cells. It results in the opening of stomata.

In the dark carbondioxide concentration is increased in the substomatal cavity which prevents proton gradient across the protoplasmic membrane in guard cells. As a result active transport of K+ ions into guard

cells ceases. As soon as the pH of guard cells decreases the abscissic acid inhibits K+ion uptake by changing the diffusion and permeability of guard cells. Malate ion in the guard cell cytoplasm combine with H+ ion to

produce malic acid. These changes cause reversal of concentration movement. So the K+ ion is transported out of guard cells into the surrounding epidermal cells. The osmotic pressure of guard cell is decreased

which results in the movement of water from guard cells to surrounding cells

and guard cells becomes flaccid and stomata closes. 

Translocation of organic solutes

Green leaves are the photosynthetic machinery of the plant. These green leaves are regarded as "source of assimilates " because these are the sites of production of sugar during the process of photosynthesis. This sugar is converted into sucrose which is transported out of the leaf to the stem and a

then upwards to the buds ,fruits or seeds and downwards to the roots or the

underground stems. The buds, seeds, fruits, roots and the underground stems are together called "sinks of assimilates”. They utilize or store sugar. This transport of is

organic solutes from the source of assimilates to the sinks of assimilates is

called translocation of organic solutes.

a. Pressure flow mechanism: ( Mechanism of translocation of organic to solutes)

Food, primarily sucrose is transported by the vascular tissue called phloem from a source to a sink. Unlike transpiration's one-way flow of water sap, food in phloem sap can be transported in any direction needed so long as there is a source of sugar and a sink able to use, store or remove the sugar. The source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots may be mobilized to become a source of food in the early spring when the buds of trees, the sink, need energy for growth and

development of the photosynthetic apparatus. Phloem sap is mainly water and sucrose, but other sugars, hormones

and amino acids are also transported. The movement of such substances in the plant is called translocation.

b. The Pressure Flow or Mass Flow Hypothesis

The accepted mechanism needed for the translocation of sugars from source to sink is called the pressure flow hypothesis. As glucose is made at the source it is converted to sucrose (a dissacharide). The sugar is then moved into companion cells and into the living phloem sieve tubes by active transport. This process of loading at the source produces a hypertonic conditinosis. he philoem. Water in the adjacent xylem

moves into the phloem by osmosis. As osmotic pressure builds the phloem

At the sink osmotic pressure must be reduced. Again active transport

is necessary to move the sucrose out of the pholem sap and into the cells

which will use the sugar -- converting it into energy, starch, or cellulose. As sugars are removed osmotic pressure decreases and water moves out of the phloem.

 Homeostasis in Plants

Plants are present in diverse environmental conditions. In order to survive plants have to adopt various measures. Such measures are part of the homeostasis mechanism. Homeostasis is the ability of living organisms to maintain or nearly maintain constant internal conditions. It provides the

organism with a certain degree of independence from variations in the

external environmental conditions. Homeostasis does not mean to keep a

fixed internal environment as changes with in a specific range are necessary for normal body functions. It refers to the fact that the composition of the tissue fluid in the body is kept within narrow limits. Most of plant

mechanisms are related with the presence or absence of water. Osmoregulation or osmotic regulation is the homeostasis of water i.e. the control of gain or loss of water and dissolved salts. Plants are confronted

with different situations in terms of their water availability.

Isotonic, Hypotonic, and Hypertonic Solutions

Water moves readily across cell membranes through special protein- lined channels, and if the total concentration of all dissolved solutes is not equal on both sides, there will be net movement of water molecules into or

out of the cell. Whether there is net movement of water into or out of the cell

and which direction it moves depends on whether the cell's environment is

isotonic, hypotonic, or hypertonic.

When two environments are isotonic, the total molar concentration of dissolved solutes is the same in both of them. When cells are in isotonic solution, movement of water out of the cell is exactly balanced by movement of water into the cell.. In a hypotonic solution the total molar concentration of all dissolved solute particles is less than that of another solution or less

than that of a cell. If concentrations of dissolved solutes are less outside the

cell than inside, the concentration of water outside is correspondingly greater. When a cell is exposed to such hypotonic conditions, there is net water movement into the cell. Cells without walls will swell and may burst

(lyse) ifexcess water is not removed from the cell.

Cell with walls often benefit from the torgor pressure that developes in hypotonic environments.