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The following points highlight the top ten properties of water molecules. The properties are: 1. Temperature and Physical State 2. Absorption and Dissipation of Heat 3. Melting and Vaporizing Water 4. Water as a Solvent 5. Cohesion and Adhesion 6. Nature of Cellular Water 7. Factors Affecting the Chemical Potential of Water 8. Water in the Soil 9. Entry of Water into Cells 10. Entry of Water into Roots.
Properties of Water Molecules:
- Temperature and Physical State
- Absorption and Dissipation of Heat
- Melting and Vaporizing Water
- Water as a Solvent
- Cohesion and Adhesion
- Nature of Cellular Water
- Factors Affecting the Chemical Potential of Water
- Water in the Soil
- Entry of Water into Cells
- Entry of Water into Roots
Water Molecules: Property # 1. Temperature and Physical State:
Water remains in a liquid state over the range of temperatures. The melting and boiling points of water are higher than expected when compared with other molecules of similar size e.g. NH3, CH4. Table 7-1 details some physical properties of water and these are compared with other molecules of similar molecular size. The values are shown in units of jouls g-1.
Ammonia, methane and ethane require-less energy to change their state. Methanol and ethanol with additional oxygen raises their boiling points, nearing that of H2O.
Water Molecules: Property # 2. Absorption and Dissipation of Heat:
The thermal capacity of a substance or the amount of energy that can be absorbed for a given temperature rise is termed specific heat. Table compares specific heat of water with other substances. Liquid water has highly organised structure and hence has high thermal conductivity i.e. it conducts heat away from its point of application.
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Water has high specific heat and thermal conductivity and hence can absorb and dissipate large amounts of heat energy without enhancing temperatures. This characteristic enables the plant to maintain stable temperature.
At the cell level, biochemical reactions are leading to overheating, are permitted to proceed. Good amount of heat can be exchanged between cells and their environment without causing extreme variations of temperature at the cell level.
Water Molecules: Property # 3. Melting and Vaporizing Water:
Changes in the state of any substance e.g., solid to liquid or liquid to gas require energy. However, such changes should not alter temperature. The energy required to convert solid into liquid state is called heat of fusion.
The data given in Table 7-1 show value of heat of fusion for water is highest, second only to ammonia. The high value is attributed to high state of energy needed to brear strong intermolecular forces due to hydrogen bonding.
Another important attribute is the density of ice which at 0°C is less than that of liquid water. Compared with ice, molecules of water in liquid from are packed more tightly. As a result ice floats on the surface of lakes. Hydrogen bonding increases energy needed to evaporate water. The energy required to convert one mole of water to one mole of water vapour is the heat of vaporization. (44 kJ mol-1 at 25°C).
Evaporation from moist surface cools the surface since energetic molecules leave the surface, leaving behind the lower-energy molecules. Consequently, plants lose enormous amount of heat following evaporation from the surface of leaf cells. In land plants which experience intense sunlight, evaporation regulates temperature of leaves.
Water Molecules: Property # 4. Water as a Solvent:
The highly polar attribute of the water molecule makes it as an excellent solvent. It can neutralize electrical attractions between charged solute molecules by surrounding the molecules with a hydration shell.
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The polarity of molecules is estimated by dielectric constant and water has highest dielectric constant. Table 7-2 compares dielectric constant of some common solvents at 25°C.
Water Molecules: Property # 5. Cohesion and Adhesion:
Water molecules exhibit strong mutual attraction between themselves due to hydrogen bonding. This is known as cohesion. Due to cohesion, water has high surface tension. Consequently water molecules at the surface are being pulled into the bulk water. Water drops tend to be spherical due to high surface tension.
Cohesion also contributes to the tensile strength of water, and water columns are capable of withstanding high tension viz. 30 megapascal (MPa). Cohesion of water molecules attracts it towards solid surfaces and is known as adhesion.
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The latter process causes capillary of water in vessels (Fig. 7-3). The continuity of water columns in assets of plants is due to cohesion, adhesion and tensile strength. These aspects shall be discussed subsequently.
Water Molecules: Property # 6. Nature of Cellular Water:
The nature of water in biological systems is attracting lot of attention and several techniques are currently being employed. In general, most of the water appears to be ‘free’ and can be identified in vacuoles. This vacuolar water can be compared with a dilute salt solution. It is subjected to hydrostatic pressures of many bars.
Some amount of water is also held tightly by many plant cell constituents. This is referred to as adsorbed water and most of it is present near the cell membranes. Water molecules are also regarded as integral components of cell membranes and surfaces of cellulose microfibrils and polysaccharide colloids appear to be coated with a layer of water molecules.
Water Molecules: Property # 7. Factors Affecting the Chemical Potential of Water:
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Water is spontaneously transferred from a region of higher chemical potential of water to a region of lower chemical potential of water provided no barrier separates the two regions. Several factors affect the energy content of water.
Some of these factors are temperature, presence of solutes and imbibants in the system; existence of tension in the system. Elsewhere we have discussed that conventionally it is preferred to use the term water potential in place of chemical potential of water. The water potential values are recorded in bars.
Water Molecules: Property # 8. Water in the Soil:
Water in the soil is retained in many ways and is open to various types of stresses. Hydrostatic pressure or absorptive forces hold the water in the soil. Water which moves downward under the force of gravity is called gravitational water. This water is not of much use to a plant.
However, bulk quantity of the water in the soil remains as capillary water after that gravitational water is drained away. This is the main source of water available to the plants. It is held as thin films on the surface of the soil particles and also in small capillaries between the soil particles.
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This water can freely move in all directions, especially towards the direction of greatest capillary tension. Hygroscopic water is held hygroscopically as thin film surrounding colloidal particles of soil or organic matter. This water is also not available to the plants.
The residual water which lacks vapour pressure is called combined water or water of chemical constitution. This water is held through chemical forces and can be driven off by heating only. Several factors like evaporation, gravity or root absorption tend to decrease the level of water in the soil.
Water in the soil diffuses through water potential. Its components are the same as those of a cell. Furthermore, the matric and the osmotic potential interact strongly in the soil. The tendency of the soil to absorb water is called water tension.
There is considerable variation in the water potential of the soils. A field capacity (FC) of the soil is when the soil is wetted and then allowed to drain till capillarity movement stops. Clay soils, in general, hold more of water at field capacity than sandy soils. Clay soils dry out slowly but its water potential is low. Water moves very fast in sandy soils whereas it moves slowly in the clay soils.
Plants fail to absorb enough water and replace the one lost through transpiration. When there is fall in the water potential sufficiently, then the leaves begin to recover and are said to be in a state of incipient wilt. If water in the soil continues to be low then the leaves wilt to a point of no recovery even though it is enclosed by water vapours.
At this point the water content of the soil is referred to as the permanent wilting percentage. (PWP). It may be added that field capacity is purely a physical value whereas PWP is basically a physiological value.
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In brief, the readily available soil water represents the amount of water retained in a soil between FC and PWP. The range of soil water between FC and the PWP constitutes an important attribute of soil and also deer-mines the agricultural value of the soils.
Total soil moisture stress (TSMS) is called upon to indicate the mean potential of water in the soil resulting from all the factors which affect it. These include gravitation, matric, hydrostatic, and osmotic forces.
Water Molecules: Property # 9. Entry of Water into Cells:
It is generally accepted that water enters the cells osmotically i.e., it moves down a potential gradient. However, from time to time ‘active’ water uptake concept has also been advanced. In some instances expenditure of respiratory energy has been shown to be involved in water uptake.
From the soil, water diffuses directly into the free space of roots. Free space is that part of the root or tissue to which the solution in question (e.g. the solution bathing the tissue) has direct and unhindered access.
It is extremely difficult to measure this space but can be expressed as follow:
It is difficult to obtain this value with accuracy but in general the apparent free space of roots is in the range of 6-10% of the total tissue.
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It may be mentioned that the water-filled intercellular spaces and cell walls in root tissue, except vacuoles, are nearly 7-10% of the tissue volume.
In summary, the apparent free space value of roots is basically the cell walls and intercellular spaces.
It may be added that the apparent free space does not include vacuoles which are separated from the surrounding fluid through the cytoplasm and the cellular system of plasmalemma and tonoplast.
Water Molecules: Property # 10. Entry of Water into Roots:
Terrestrial plants obtain their major water supply from the soil. Most part of water is absorbed by the younger parts of the root system and especially the region bearing root hairs. The root hairs arise as cylindrical outgrowths from the outer walls of the cells of piliferous layer in the maturation region of a young root. They penetrate between the particles of the soil.
A root hair consists of a cell w all lined internally by a thin film of cytoplasm surrounding a central vacuole (Fig. 7-4).
The nucleus of the cell passes in the cytoplasm of the root hair. A gummy softening of the wall near the distal end of the root hair leads to a most intimate connection with the soil particles facilitating absorption of water present as films around them.
They increase the absorptive surface of the root. The root hair arise in an acropetal order and function for a short while. Each root hair cell forms an osmotic system. The cell sap has an osmotic pressure of about 2-5 atmospheres or more. The cytoplasmic membrane acts as a semipermeable membrane.
The soil water has small quantities of salts such as, nitrates, carbonates, phosphates and sulphates of sodium, potassium and calcium, etc. dissolved in it and their total concentration is usually less than 0.2 per cent.
The water enters the vacuole of the root hair by osmosis. The root hair, while absorbing water has to face a certain resistance from the soil due to the physical forces of the soil capillarity, adsorption and also the existing osmotic gradient or the soil water system.
However, the absorptive forces of root hair are adequate to overcome these resistances. Plants chiefly utilize the capillary water. They cannot absorb the water adsorbed on the colloidal particles of soil, because the adsorbed water is held tenaciously and the absorptive forces of the root hairs are not sufficient to overcome these forces of adsorption.
