Water and Plant Cells Syed Abdullah Gilani

Water plays a crucial role in the life of plants For every gram of organic matter absorbed by plants, approx. 500 g of water is absorbed by the roots, transported through the plant body and lost to the atmosphere. Every plant must delicately balance its uptake and loss of water. Turgor Pressure: Plant cell walls allow plant cells to build up large internal hydrostatic pressures due to their normal water balance, called turgor pressure.

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 non-lignified plant tissues.

Water in Plant Life Water makes up most of the mass of plant cells. Each cell contains a large water-filled vacuole. The cytoplasm makes upto 5 – 10 % of the cell volume; the remainder is vacuole. Water typically constitutes 80 – 95 % of the mass of growing plant tissues. Common vegetables such as carrots and lettuce may contain 85 – 95 % water.

Woods – composed of dead cells – has lower water content. Seeds are among the driest plant tissues with 5 – 15 %. Water is the most abundant and arguably the best known solvent. As a solvent, it makes up the medium for the movement of molecules within and between cells. water influences structures of proteins, nucleic acids, polysaccharides, and other cell constituents.

Plants continuously absorb and lose water. Transpiration: Most of the water lost by the plant evaporates from the leaf as CO2 needed for photosynthesis is absorbed from the atmosphere, known as 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.

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 The Polarity of Water Molecules Gives Rise to Hydrogen Bonds Makes It an Excellent Solvent Hydrogen Bonding results in Thermal Properties of Water The Cohesive and Adhesive Properties of Water Water Has a High Tensile Strength

Properties of Water Water is an excellent polar solvent. Water is reactive (not only dissolves but reacts with solutes, e.g., hydrolysis, dehydration etc.). Water has high specific heat Water has high latent heat of vaporization Water demonstrates adhesion and cohesion. Water has high tensile strength. Water is not compressible (produces turgor pressure upon compression). Water is commonly available. Water can be limiting to growth (e.g., in drought or warm dry sunny day)S.

The Polarity of Water Molecules Gives Rise to Hydrogen Bonds 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.

These partial charges are equal, so the water molecule carries no net charge. This separation of partial charges, together with the shape of the water molecule, makes water a polar molecule, The opposite partial charges between neighboring water molecules tend to attract each other. The weak electrostatic attraction between water molecules, known as a hydrogen bond, is responsible for many of the unusual physical properties of water.

In aqueous solutions, hydrogen bonding between water molecules leads to local, ordered clusters of water that, because of the continuous thermal agitation of the water molecules, continually form, break up, and re-form

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. Polarity 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.

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. 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. 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.

The Cohesive and Adhesive Properties of Water Are Due to Hydrogen Bonding water molecules at an air-water interface are more strongly attracted to neighboring water molecules than to the gas phase in contact with the water surface. To increase air-water interface, hydrogen bonds must be broken, which requires an input of energy. The energy required to increase the surface area of a gas-liquid interface is known as Surface Tension. Cohesion: The extensive hydrogen bonding in water also gives rise to the property known as cohesion, the mutual attraction between molecules. Adhesion: Adhesion, is the attraction of water to a solid phase such as a cell wall or glass surface. Cohesion, adhesion, and surface tension give rise to a phenomenon known as capillarity, the movement of water along a capillary tube.

Water Has a High Tensile Strength Cohesion gives water a high tensile strength, defined as the maximum force per unit area that a continuous column of water can withstand before breaking.

Water Transport Processes When water moves from the soil through the plant to the atmosphere, it travels through a widely variable medium such as Cell wall Cytoplasm Membrane Air spaces The mechanisms of water transport also vary with the type of medium.

Water can cross plant membranes by: Diffusion of individual water molecules through the membrane bilayer, Bulk flow of water molecules through a water-selective pore formed by integral membrane proteins such as aquaporins. Aquaporins are integral membrane proteins that form water-selective channels across the membrane. Because water diffuses faster through such channels than through a lipid bilayer, aquaporins facilitate water movement into plant cells.

Although, the presence of aquaporins may alter the rate of water movement across the membrane, they do not change the direction of transport or the driving force for water movement.

Diffusion Diffusion is the movement of molecules (particles) by random thermal agitation. Water molecules in a solution are not static, they are in continuous motion, colliding with one another, and exchanging kinetic energy. This random motion of molecules is called diffusion. Diffusion causes the movement of molecules from regions of high concentration to low concentration – that is down a concentration gradient.

Thermal motion of molecules leads to diffusion – the gradual mixing of molecules and eventual dissipation of concentration differences. Diffusion is rapid over short distances but extremely slow over long distances.

Bulk Flow Bulk flow is the concerted movement of groups of molecules en masse, most often in response to a pressure gradient. Pressure-driven bulk flow of water is the predominant mechanism responsible for long-distance transport of water in the xylem. In contrast to diffusion, pressure-driven bulk flow is independent of solute concentration gradients

Osmosis Osmosis is the passive movement of water across a membrane. Osmosis is the movement of solvent molecules (water and other small uncharged substances) through a semi-permeable membrane. Movement of molecules occur through concentration gradient, i.e., from lower concentration to higher concentration. Membranes of plant cells are selectively permeable, that is, they allow the movement of water and other small uncharged substances across them more readily than the movement of larger solutes and charged substances.

Chemical Potential of Water All living things require continuous input of free energy to maintain and repair their organized structures, as well as to grow and reproduce. The chemical potential of water is a quantitative expression of the free energy associated with water. Free energy = potential to perform work.

The chemical potential is a relative quantity: expressed as difference between the potential of a substance in a given state and the potential of the same substance in a standard state. Unit = energy per mole of substance (J mol-1) Water potential: chemical potential of water/1 molal volume of water (18 x 10-6 m3 mol-1). Water potential is a measure of the free energy of water per unit volume (J m-3).

The main driving force behind the movement of water is the difference between free energies of water molecules on two sides of the semi-permeable membrane. For non-electrolytes, free energy is known as chemical potential (psi, Ψ). For water, free energy is called as water potential (Ψw).

The major factors influencing the water potential in plants are: Concentration (solutes) Pressure Gravity Water potential is symbolized by the letter Ψw (Greek letter psi) Ψw = Ψs + Ψp + Ψg The reference state used to define water potential is pure water at ambient pressure and temperature  

Solutes Ψs: solute potential or osmotic potential. It is the effect of solutes on water potential. Solutes reduce the free energy of water by diluting the water. Mixing solutes and water increases the disorder of the system, thereby lowering free energy, known as Entropy effect. The osmotic potential is independent of the specific nature of the solute.

Pressure Ψp: hydrostatic pressure of the solution (pressure potential). Positive pressures raise the water potential. Negative pressures reduce it. The positive pressure within cells is referred as Turgor pressure. Ψp can also be negative, as in the case of xylem and walls between cells, where a tension, or negative hydrostatic pressure, can develop. Water in the reference state is at ambient pressure, so Ψp = 0 MPa, in the standard state.

Thus, Ψw” for pure water is 0 MPa, at 1 Atmosphere and a particular temperature. Water potential is usually expressed in pressure units such as bars or atmospheres (1 bar = 0.987 atmospheres). Water potential is also expressed in unit of pressure called pascals (Pa); 1 Pa = 1 Newton/square meter (Nm-2) 1 MPa (one megapascal) = 10 bars = 9.87 atmospheres.

Gravity Gravity causes water to move downward unless the force of gravity is opposed by an equal and opposite force. Ψg depends on the height (h) of the water above the reference-state water, the density (pw) and the acceleration due to gravity (g) Ψg = pwgh At the cell level, Ψg is insignificant in comparison to Ψs and Ψp. Therefore, the equation for Ψw is simplified to Ψw = Ψs + Ψp.

Water Potential in the Plant Cell growth, photosynthesis, and crop productivity are all influenced by water potential and its components. Like the body temperature of humans, it is a good indicator of plant health. Water enters the cell along a water potential gradient. Water potential is lowered by the addition of solutes and because water potential value is zero for pure water, all other water potential values will be negative.

In osmotic or other systems, the movement of water will take place from a region of higher water potential (less negative) to a region of lower water potential (more negative). Water potential values of plant cells under different osmotic conditions are as follows:

In plasmolysed or flaccid cell: Ψw (lowest) = Ψs (as Ψp = nil) In partially turgid cell: Ψw (higher) = Ψs + Ψp In fully turgid cell: Ψw (highest) = zero (as Ψp numerically equals Ψs but both having opposite signs).

Example: Two cells, A and B, in contact with each other. Cell A has a pressure potential (turgor pressure) of 4 bars and contain cell sap with an osmotic potential of -12 bars. Cell B has a pressure potential of 2 bars and contains sap with osmotic potential of -5 bars. Then: Ψw of cell A = Ψs + Ψp = - 12 + (+4) = - 8 bars Ψw of cell B = Ψs + Ψp = - 5 + (+2) = - 3 bars

Hence, water will move from cell B to cell A (i. e Hence, water will move from cell B to cell A (i.e. towards lower or more negative water potential). In an open osmotic system, (e.g. plasmolysed plant cells), the water potential and the osmotic potential values are numerically similar and also have the same sign (i.e. negative). In a closed osmotic system, (e.g. turgid plant cells), a pressure is imposed on water which increases the water potential, this pressure is called as turgor pressure.

A flaccid cell (in air) is dropped in the 0.1 M sucrose solution. Because the starting water potential of the cell is less than the water potential of the solution, the cell takes up water. After equilibration, the water potential of the cell rises to equal the water potential of the solution, and the result is a cell with a positive turgor pressure.

Increasing the concentration of sucrose in the solution makes the cell lose water. The increased sucrose concentration lowers the solution water potential, draws water out from the cell, and thereby reduces the cell’s turgor pressure. In this case, the protoplast is able to pull away from the cell wall (i.e, the cell plasmolyzes) because sucrose molecules are able to pass through the relatively large pores of the cell walls.

Plasmolysis: contraction or shrinkage of the protoplasm from the cell wall. In a plasmolyzed cell, the space in between the cell wall and plasma membrane is filled with outer hypertonic solution.   When plant tissue is placed in hypotonic solution, endosmosis occurs, the water moves into the cell, the cell becomes turgid, and turgor pressure develops. When plant tissue is placed in hypertonic solution, the water will move out cell, the cells become flacid, plasmolysis occurs. Hypotonic solution is the one with lower concentration of solutes outside the cell. Hypertonic solution is with higher concentration of solutes outside the cell.