Introduction to Cell Water Relations

CHAPTER: 4

Introduction to Cell Water Relations

Water:

•      Water being considered as universal solvent, occupies 75% of our planet in the form of oceans.

•      The evaporation of water from the surface of ocean, formation clouds and raining, is a natural cycle evolved during course of Evolution of this planet.

•      In the course of Chemical Evolution, the birth of life has chosen H2O as the medium of biochemical activities.  Thus water has become mother of life or “Solvent of Life”.

•      Cells of all organisms are made up 90% or more of water.  And all other components are either dissolved or suspended in water to form protoplasm, which is often referred to as physical basis of life.

•      Water is a remarkable compounded made up of Hydrogen and oxygen (2:1) and it has high specific heat, high heat of vaporization, high heat of fusion and expansion (colligative properties)

Importance of Water:

•      Water is the major component of living cells and constitutes more than 90% of protoplasm by volume and weight.

•      It acts as medium for all biochemical reaction that takes place in the cell, and also acts a medium of transportation from one region to another region. 

•      Water because of its bipolar nature acts as universal solvent for it dissolves more substances than any other solvent.  Electrolytes and non-electrolytes like sugars, and proteins dissolve very well.  Even some hydrophobic lipid molecules show some solubility in water.

•      Water acts as a good buffer against changes in the Hydrogen ion concentration (pH).  This is because of its ionization property.  Certain xerophytes use water as buffer system against high temperature.

•      Water is an important substrate in photosynthesis, for it provides reducing power in CO2 fixation; water is also used in breaking or making chemical bonds of polypeptides, poly-nucleotides, carbohydrates etc.

•      Water also exhibits viscosity and adhesive properties. 

•      Because of hydrogen bonds, water molecules are attracted towards each other, they are held to each other with considerable force.  This force of attraction is called cohesive force. 

•      Thus water possesses a high tensile strength.  If this water is confined in very narrow columns of dimensions of xylem vessels, its tensile and cohesive forces reach very high values (1000-1200 Gms).  And this force is very helpful in ascent of sap. 

•      Water is of great importance in osmoregulation, particularly in the maintenance of turgidity of cells, opening and closing of stomata and growth of the plant body.

Cell:

•      Cells are the basic structural units of organisms, and plant organization varies from single cells to aggregations of cells to complex multicellular structures.

•      With increasing complexity there are increasingly sophisticated systems for absorbing water, moving it large distances, and conserving it but fundamentally the cell remains the central unit that controls the plant response to water.

•      The driving forces for water movement are generated in the cells, and growth and metabolism occur in the aqueous medium provided by the cells.

•      The cell properties can change and result in acclimation to the water environment.

•      As a consequence, many features of complex multicellular plants can be understood only from a knowledge of the cell properties.

Cell Water Relation:

•      The term “Cell water relations” describes plant water status in a cell, individual organ (leaf, internode, flower) or whole plant level, furthering our understanding of basic plant growth and development, and plant response to the environment.

•       At the field level, water use and water use efficiency are the common means of evaluating a crop and its yield performance to seasonal water availability.

•      Water relations encompasses measurement techniques that describe general plant water status, the quantification of water in cell and tissue expansion, maintenance of turgor, and the overall stomatal gas exchange of plants according to moisture in the soil and aerial environment.

•      Water enters plants through the roots, and travels both through and around cells.

•      Water is moved up the plant mainly in the xylem vessels of the stem and in leaf veins, and water is transpired from the plant via stomata on the leaf surface.

•      From the 1960s, the discipline of water relations expanded mainly through the concept of water potential, allowing multidisciplinary research spanning soil, plant and aerial environments, and culminated in the practical management of crop water requirements.

•       In simple terms, water relations allow direct measurement of how much water is in the plant.

•      This information directly indicates how well the plant is performing and how the plant copes with stress – in contrast to indirect measurements of soil moisture content or rainfall deficit.

Ø  The cell water relation can be briefly discussed on the following headings:

1. Water Potential
2. Diffusion
3. Osmosis
4. Transpiration
5. Stomatal physiology
6. Ascent of Sap

Water Potential:

•      The free energy in water that is available to do work is described by water potential.

•      The water content in the soil, plants and atmosphere is usually described as water potential (Ψw).

•      Water in plants and soil moves in response to differences in water potential (Ψw).

•      Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as capillary action (which is caused by surface tension).

Ø  Water Potential (Ψw) = Ψs + Ψm + Ψt+ Ψg 

                               where, osmotic/solute potential (Ψs),

                                matrix potential (Ψm) ,

                                turgor potential (Ψt) or pressure potential &

                                gravitational potential (Ψg) significant in tall plant.

Ø  In this context, it is important to be familiar with the term called water potential (Ψw) which refers to the chemical free energy of water. The chemical free energy of pure water or solutes is always expressed in terms pressure units such as bars.

Ø  Purest form means there are no other molecules in it.  the potential of free pure water at atmospheric pressure and at a temperature of 25°C corresponds to 0 (zero) Mpa (mega pascal) or bar.

2. Diffusion:

•      Movement of molecules from an area of high concentration to an area of low concentration. movement from one side of a membrane to another .

•      Diffusion is the process by which substances move down a concentration gradient, from an area of high concentration to an area of low concentration. Diffusion happens in living systems, for example, it explains the movement of carbon dioxide in leaves.

Ø  Having own kinetic energy of water,  water molecules will be in constant motion randomly.

3. Osmosis:

•      If a solution & its pure solvent are separated by a semi permeable membrane the solvent molecule diffuse into solution. The diffusion of solvent molecule into the solution through a semi-permeable membrane is called osmosis  or osmotic diffusion.

•      Water, for that matter any solvent in its pure state has its own chemical potential by virtue of which it exhibits random movement.  This is referred to as chemical free energy or water potential. If such a solvent is separated from a solution (solvent + solute) by a semi permeable membrane, water molecules move from higher chemical or water potential to the lower water potential.  In this case pure water has higher chemical energy than the solution, for the solute present in water lowers the free chemical energy of pure solvent of the solution.

4. Transpiration:

•      Plant absorbs a large quantity of water from the soil by root hairs. Only a small part (1-2%) of this water is utilized by the plant in its life process. The remaining large part (98-99%) of water is lost in the form of vapor from the internal tissue. Loss of water in the form of vapor through the exposed aerial parts of plants is called Transpiration.

•      Transpiration helps in creating suction pressure, which facilitates the ascent of sap as well as absorption of water and to some extent absorption of minerals.  Transpiration keeps the cells in continuous flux.  It has cooling effect on the plant body and also helps in the development of good roof system and also helps in the development of good roof system and mechanical tissues.  Added to this, it helps in the development of good drainage in the soil.

•      Water absorbed by the root system is transported upwards and the same is always lost from the aerial surfaces of the plant body. In fact loss of water facilitates the absorption and translocation of water and minerals in the plant body.

•      If water is lost in the form of liquid, it is called Guttation; on the contrary if water is lost in the form of water vapors, it is considered as Transpiration.

5. Stomatal physiology:

•      When the water vapor escapes into the atmosphere through the stomata, it is called stomatal transpiration. The stomata constitute the chief pathways through which about 90% of the water vapor is lost by the aerial parts of the plant. The stomata are minute pores found on the epidermis. They allow water vapor to escape through the minute opening present between two guard cells.

•      Stomata plays a significant role in transpiration as a major part of water vapor is lost through the stomatal pore. Stomata exhibits periodic opening and closing during day. It depends upon heat, light, water content of the cell and humidity. Generally, stomata are open during the day and close at night. The opening and closing of the stomatal pore regulate the process of stomatal transpiration. The changes in turgor pressure of guard cells cause the opening and closing of stomatal pore.

6. Ascent of Sap:

•      The plants absorb a large quantity of water from the soil by root hairs.

•      From root hairs, it reaches to the top of the plant through xylem, where  approximately the same amount of water is transpired from the surface of the aerial parts of the plants to the atmosphere.

•      Water and minerals absorbed by the root hairs are called sap. The upward movement of sap from the root to the tip of plant is called ascent of sap.

•      Ascent of sap takes place against the gravitational force.