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Water Balance of Plants
People are watery creatures. More precisely, we are salt-watery creatures, though not as salty as the oceans.Water, the fluid medium of the human body, makes up 55% to 75% of the total body weight. Electrolytes are the positive and negative ions present in body fluids. In previous chapters we have called these salts or, in some cases, trace elements. Many of these ions are minerals
that are already familiar to you. They each have specific functions in body fluids, and some of them are also involved in the maintenance of the normal pH of the body fluids. In this chapter we will first discuss fluid–electrolyte balance, then review and summarize the mechanisms involved in acid–base balance

WATER PLAYS A CRUCIAL ROLE in the life of the plant. For every gram of organic matter made by the plant, approximately 500 g of water is absorbed by the roots, transported through the plant body and lost to the atmosphere. Even slight imbalances in this flow of water can cause
water deficits and severe malfunctioning of many cellular processes.Thus, every plant must delicately balance its uptake and loss of water.This balancing is a serious challenge for land plants. To carry on photosynthesis,they need to draw carbon dioxide from the atmosphere, but doing so exposes them to water loss and the threat of dehydration.Amajor difference between plant and animal cells that affects virtually all aspects of their relation with water is the existence in plants of the cell wall. Cell walls allow plant cells to build up large internal hydrostatic pressures, called turgor pressure, which are a result of their normal water balance. Turgor pressure is essential for many physiological processes,
including cell enlargement, gas exchange in the leaves, transport in the phloem, and various transport processes across membranes. Turgor pressure also contributes to the rigidity and mechanical stability of nonlignified plant tissues. In this chapter we will consider how water moves into and out of plant cells, emphasizing the molecular properties of water and the physical forces that influence water movement at the cell level. But first we will describe the major functions of water in plant life.


note;Many of plant activities are determined by the properties of water and of substances dissolved in water;Hydrogen bonding  is the one among the properties of water

Definition:  A hydrogen bond is a type of attractive (dipole- dipole) interation between an electronegative atom and a hydrogen atom bonded to another electronegative atom.
This bond always involves a  hydrogen atom.
Hydrogen bonds can occur between moldecules or within parts of a single molecule.A hydrogen bond tends to be stronger than van der Waals forces, but weaker than covalent bonds or ionic bond.
           WATER IN PLANT LIFE
Water makes up most of the mass of plant cells, as we can readily appreciate if we look at microscopic sections of mature plant cells: Each cell contains a large water-filled vacuole. In such cells the cytoplasm makes up only 5 to 10% of the cell volume; the remainder is vacuole. Water typically constitutes 80 to 95% of the mass of growing plant tissues. Common
vegetables such as carrots and lettuce may contain 85 to 95% water.Wood, which is composed mostly of dead cells, has a lower water content;sapwood, which functions in transport in the xylem,75% water; and heartwood has a slightly lower water content.Seeds, with a water content of 5 to 15%, are among the driest of plant tissues, yet before germinating they must absorb a considerable amount of water.Water is the most abundant and arguably the best solvent known. As a solvent, it makes up the medium for the movement of molecules within and between cells and greatly influences the structure of proteins, nucleic acids,polysaccharides, and other cell constituents. Water forms
the environment in which most of the biochemical reactions of the cell occur, and it directly participates in many essential chemical reactions.Plants continuously absorb and lose water. Most of the water lost by the plant evaporates from the leaf as the CO2 needed for photosynthesis is absorbed from the atmosphere.On a warm, dry, sunny day a leaf will exchange up to 100% of its water in a single hour. During the plant’s lifetime,water equivalent to 100 times the fresh weight of t he
plant may be lost through the leaf surfaces. Such water loss is called transpiration.
Transpiration is an important means of dissipating the heat input from sunlight. Heat dissipates because the water molecules that escape into the atmosphere have higherthan-average energy, which breaks the bonds holding them in the liquid. When these molecules escape, they leave
behind a mass of molecules with lower-than-average energy and thus a cooler body of water. For a typical leaf, nearly half of the net heat input from sunlight is dissipated by transpiration. In addition, the stream of water taken up by the roots is an important means of bringing dissolved
soil minerals to the root surface for absorption.Of all the resources that plants need to grow and function,water is the most abundant and at the same time the most limiting for agricultural productivity. Thefact that water is limiting is the reason for the practice of crop irrigation. Water availability likewise limits the productivity of natural ecosystems. Thus an understanding of the uptake and loss of water by plants is very important.We will begin our study of water by considering how its structure gives rise to some of its unique physical properties.We will then examine the physical basis for water movement, the concept of water potential, and the application
of this concept to cell–water relations.



THE STRUCTURE AND PROPERTIES OF WATER
Water has special properties that enable it to act as a solvent and to be readily transported through the body of the plant. These properties derive primarily from the polar structure of the water molecule. In this section we will examine how the formation of hydrogen bonds contributes
to the properties of water that are necessary for life.The Polarity of Water Molecules Gives Rise to
Hydrogen Bonds The water molecule consists of an oxygen atom covalently
bonded to two hydrogen atoms. The two O—H bonds form an angle of 105° ). Because the oxygen
atom is more electronegative than hydrogen, it tends to attract the electrons of the covalent bond. This attraction results in a partial negative charge at the oxygen end of the molecule and a partial positive charge at each hydrogen.The Polarity of Water Makes It an Excellent Solvent
Water is an excellent solvent: It dissolves greater amounts of a wider variety of substances than do other related solvents.This versatility as a solvent is due in part to the small size of the water molecule and in part to its polar nature.The latter makes water a particularly good solvent for ionic
substances and for molecules such as sugars and proteins that contain polar —OH or —NH2 groups.
Hydrogen bonding between water molecules and ions,and between water and polar solutes, in solution effectively decreases the electrostatic interaction between the charged
substances and thereby increases their solubility. Furthermore,the polar ends of water molecules can orient themselves next to charged or partially charged groups in macromolecules, forming shells of hydration. Hydrogen bonding between macromolecules and water reduces the
interaction between the macromolecules and helps draw them into solution.The Thermal Properties of Water Result from

Hydrogen Bonding
The extensive hydrogen bonding between water molecules results in unusual thermal properties, such as high specific heat and high latent heat of vaporization. Specific heat is the heat energy required to raise the temperature of a substance by a specific amount.When the temperature of water is raised, the molecules vibrate faster and with greater amplitude. To allow for this motion, energy must be added to the system to break the hydrogen bonds between water molecules. Thus, compared
with other liquids, water requires a relatively large energy input to raise its temperature. This large energy input requirement is important for plants because it helps buffer temperature fluctuations.
Latent heat of vaporization is the energy needed to separate molecules from the liquid phase
and move them into the gas phase at constant temperature—a process that occurs during transpiration. For water at 25°C, the heat of vaporization is 44 kJ mol–1—the highest value known for any liquid. Most of this energy is used to break hydrogen bonds between
water molecules. The high latent heat of vaporization of water enables plants to
cool themselves by evaporating water from leaf surfaces, which are prone to heat up because of
the radiant input from the sun.Transpiration is an important component of temperature regulation
in plants

WATER COMPARTMENTS
Most of the water of the body, about two-thirds of the total water volume, is found within individual cells and is called intracellular fluid (ICF). The remaining third is called extracellular fluid (ECF) and includes blood plasma, lymph, tissue fluid, and the specialized fluids such as cerebrospinal fluid, synovial fluid, aqueous humor, and serous fluid.Water constantly moves from one fluid site in the
body to another by the processes of filtration and osmosis. These fluid sites are called water compartments. The chambers of the heart and all of the blood vessels form one compartment, and the
water within is called plasma. By the process of filtration in capillaries, some plasma is forced out into tissue spaces (another compartment) and is then called tissue fluid. When tissue fluid enters cells by the process of osmosis, it has moved to still another compartment
and is called intracellular fluid. The tissue fluid that enters lymph capillaries is in yet another
compartment and is called lymph.The other process (besides filtration) by which
water moves from one compartment to another is osmosis, which, you may recall, is the diffusion of
water through a semi-permeable membrane. Water will move through cell membranes from the area of its greater concentration to the area of its lesser concentration.Another way of expressing this is to say that water will diffuse to an area with a greater concentration of dissolved material. The concentration of electrolytes present in the various water compartments determines just how osmosis will take place





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