Session 1: Plants and Water PDF

BYU4300 - Unit I Session 1: Plants and Water Session 1: Plants and Water Contents Introduction, p1 1.1 Functions of water, p2 1.2 Unique properties of water, P3...

BYU4300 - Unit I Session 1: Plants and Water Session 1: Plants and Water Contents Introduction, p1 1.1 Functions of water, p2 1.2 Unique properties of water, P3 1.3 Water content, p9 1.4 The structure of liquid water - A model, p10 1.5 Principles of water movement, p11 Summary, p17 Objectives, p18 Introduction Water is essential for the existence of life and it is the most abundant constituent of living things. In this session, you are going to learn about the importance of water to plant life and movement of water within a plant. The living tissues of plants usually contain more than 70% of water by weight and water may therefore, be called the “fluid of life”. Life in all cells and hence of all living organisms depends on water. Maintenance of a satisfactory water content is essential for the plants to remain alive. Living organisms on this planet first evolved in water and for hundreds of millions of years all plants lived totally immersed in water. One of the major hindrances for the colonization of land appears to be the difficulty in controlling the water content of plant tissues exposed to the desiccating influence of air. As you are aware, the plants capable of such control did eventually evolve and the distribution of vegetation on the earth’s surface today is controlled more on the availability of water than on any other environmental factor. 1 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water The most successful land plants of today can sustain a continued water loss from aerial parts by maintaining an efficient uptake of water from the soil. Thus, the tissue water-balance of a land plant is a very dynamic one and there is a continuous balancing of gain and loss of water. Our main interest in this session is to understand the cell-water relations and the various concepts necessary to the understanding of water relations at the whole plant level. Section on Functions of water focuses on the main functions performed by water in plants. Water has many unique properties due to its structure. Section Unique properties of water describes the structure of the water molecule and its unique properties and their importance to plant life. Water content differentiates between bound water and water that is free to move or unbound water. The Structure of liquid water – a model, describes a theoretical model for the structure of water proposed by Benson and Siebert. Physical properties involved in entry of water into cells and movement between cells, are explained in Principles of water movement. 1.1 Functions of water Water performs numerous functions in plants. Let us first review briefly the functions and properties of water which would help us in the study of cell water relations. These could be grouped under four general headings: Constituents of protoplasm A solvent A reactant Maintenance of turgidity Constituent of Protoplasm Water comprises 70-90% of the fresh weight of most herbaceous plants and over 50% of the fresh weight of woody plants. Reduction of water below a critical level is harmful to the plant. This is because the most vital molecules such as proteins are hydrated in their natural state and the presence of water is essential for maintenance of their structure and activity. The relatively high permeability of cell walls and cell membranes to water results in a continuous liquid phase extending throughout the 2 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water plant. The transport and communication of various kinds take place through this pathway. A Solvent Water functions as the solvent in which gases, minerals and other solutes enter plant cells. It is also the medium in which chemical reactions take place in the cell protoplasm. A Reactant Water is a reactant in many of the chemical reactions of the cell. Example: Water is a reactant in the hydrolysis of starch to glucose; (C6H10O5)n + nH2O nC6H12O6 Starch Glucose It is a reactant in the synthesis of malic acid from fumaric acid in the Krebs’cycle; HOOC-CH=CH-COOH+H2O HOOC-CH2-CH2-COOH Fumaric acid Malic acid and in the reduction of carbon dioxide to carbohydrate in photosynthesis. Light H2O + CO2 (CH2O)n + O2 Chloroplast Maintenance of Turgidity Uptake of water and consequent stretching of cell wall make cells, tissues or organs firm or turgid. Maintenance of turgidity is extremely important in cell enlargement and growth and to maintain the form in non-woody tissues. Turgidity is also important in stomatal movement. 1.2 Unique properties of water Water possesses several unique properties. These unique properties of water arise from its molecular structure (Fig.1). The structure of a molecule of water is not linear but V-shaped. Water molecule is made up of two hydrogen atoms covalently bonded to an oxygen atom. The angle between the two covalent bonds is 104.5 o. Oxygen attracts electrons within the covalent bond more strongly than does hydrogen. Thus, an uneven distribution of charges occurs within each O-H bond of the molecule. Due to this uneven distribution of charges, oxygen bears a partial negative charge (-) and hydrogen a partial positive charge (+). 3 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water This uneven distribution of charges within a bond is known as a dipole and the bond is said to be polar. Figure 1.1 - The structure of a water molecule with asymmetric distribution of charges. The polarity of a molecule depends both on the polarity of its covalent bonds and the geometry of the molecule. The angled arrangement of the polar O-H bonds of water creates a permanent dipole for the molecule as a whole. The water molecules get attracted to each other due to the polarity of the water molecule. A relatively weak bond is produced due to the attraction between slightly positive hydrogen atoms of one water molecule with slightly negative oxygen atom of another. This bond is called a hydrogen bond. Many of the properties of water are due to the presence of these hydrogen bonds. The energy values of different types of bonds are given in table 1.1. You can see that H. bonds are stronger than Van der Waals forces. Table 1.1: Energies of some bonds in kJ mol-1 attractive forces 4 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water The presence of hydrogen bonds between water molecules results in water possessing several unique properties. Let us learn about these unique properties of water and their significance to life. Water is a liquid at room temperature The higher the molecular weight of an element or a compound, the greater is it’s likelihood to be a solid or a liquid at room temperature. Lower the molecular weight of an element or a compound more likely for it to be a liquid or a gas. For example, the following hydrocarbons with relatively low molecular weights (MW) are gases. Methane Ethane Propane Butane MW 16 30 44 58 The following hydrocarbons with high molecular weights are liquids. Hexane Heptane Octane MW 86 110 114 CO2 and H2S with molecular weights of 44 and 34 respectively are also gases. However, H2O with a molecular weight as low as 18 is a liquid at room temperature. This is because the hydrogen bonds provide strong attractive forces among water molecules, and do not allow them to separate and escape as vapour. Thus, water remains as a liquid at room temperature. Hydrocarbon molecules are attracted by relatively weak Van der Waals forces (Table 1) and little energy (temperature lower than room temperature) is adequate to separate them and drive them into gaseous phase. We have already discussed the importance of water to be in the liquid phase in section functions of water on page 2. Water has a high specific heat The specific heat of water is the amount of energy required to raise the temperature of 1g of water by 1oC. The specific heat of water is equal to 4.184 Joules (J) or 1 Calorie. It is higher than that of any other substance except liquid ammonia, but ammonia is a liquid at - 33oC. 5 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water The high specific heat of water is due to the way in which molecules are arranged, an arrangement which allows the hydrogen and oxygen atoms to vibrate freely, almost as if they were free ions. Thus, they can absorb large quantities of energy without any appreciable increase in temperature. High specific heat of water makes it possible for plants to absorb a large amount of solar radiation without an injurious rise in temperature (remember that plants contain about 90% of water). In other words, plants have a relatively high temperature stability. Water has a high latent heat of vaporisation The latent heat of vaporisation is the amount of energy required to change 1g of liquid water to vapour without changing the temperature. The latent heat of vaporisation of water is 2452 Jg-1 (580 cal) at 100oC. This unusually high latent heat of vaporisation is again due to the presence of hydrogen bonds. Because of the presence of hydrogen bonds, a large amount of energy is required to separate water molecules from one another. The significance of this property of water to plant life is in that the leaves are cooled as they lose water by transpiration. Condensation, the reverse of vaporisation has a warming effect because during condensation of water a considerable amount of energy is released. Water has a high latent heat of fusion The amount of energy required to melt 1g of ice at OoC is called the latent heat of fusion of water. The latent heat of fusion of water is high and is equal to 335Jg-1 (80 cal). The high latent heat of fusion of water is again due to the presence of hydrogen bonds. This property of water is significant to plants in that a relatively large amount of heat must be lost from water before it freezes. Contents of cells and their environment are therefore less likely to freeze. Water expands on freezing During the conversion from solid to liquid state, molecules of water move slightly further apart. This should result in an increase in volume. But, water is extremely unusual because its total volume decreases during melting or expands on freezing. This is because the molecules are packed more efficiently in the liquid water than in the solid state. Efficient packing always reduces volume. Each molecule in the liquid is surrounded by 5 or more other molecules whereas in ice, it is surrounded only by 4 others. Because of this disorder ice has a volume about 9% 6 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water greater than liquid. Volume increase reduces density, as volume and density are inversely proportional to each other. As density of ice is lower than liquid water, ice floats on water. This floating ice layer insulates the water below and prevents it from being cooled down to the freezing point. This helps in the survival of aquatic life in cold climates, when ice is formed in lakes and rivers, thus, life can continue in a liquid medium at lower depths. Transparency of water Water is fairly transparent to visible radiation, which falls in the range of 380 -750 nm (1nm= 10-9m) wavelengths of the electromagnetic spectrum. It allows light to penetrate and makes it possible for aquatic plants to photosynthesise at considerable depths in water bodies. This property of water is also important in the penetration of visible light into deep tissues of leaves. While water is transparent to the wave lengths essential for plant life, it absorbs the infra-red wave lengths. This makes water a fairly good heat insulator. Adhesion and Cohesion of water molecules The attraction of like molecules for each other is called cohesion and attraction between unlike molecules is called adhesion. Because of the polar nature of water, it develops high cohesive forces due to hydrogen bonding between water molecules and high adhesive forces due to hydrogen bonding with other molecules. Both these forces are important in the transport of water. You will learn about its importance when you study ascent of sap in session 6. High surface tension of water Water molecules at an air-water interface are more strongly attracted to neighbouring water molecules due to cohesive forces than to the gas phase on the other side of the surface. Because of this unequal attraction, the air- water interface assumes a shape that minimizes its surface area. In effect, the water molecules exert a force at the surface of the air-water interface. This force influences the shape of the surface and also creates a pressure in the rest of the liquid. The condition that exists at the interface is known as surface tension. The surface tension of water is higher than that of any other liquid except mercury. Surface tension of water is very important in generating the forces needed to pull water to the top of the trees. You will learn about this later when you study ascent of sap. Under normal pressure, high surface tension prevents the passage of air bubbles through the minute pores and pits in the cell walls. The formation of droplets of water on leaf surfaces after rain or irrigation is also due to 7 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water high surface tension of water. Entry of water into intercellular spaces of leaves through open stomata is prevented due to the formation of water droplets. Capillarity Cohesion, adhesion and surface tension give rise to a phenomenon known as capillarity. Water tends to move up a glass capillary tube due to capillarity. The upward movement of water is due to the attraction of water at the periphery to the polar surface of the glass tube (adhesion) and to the surface tension of water which tends to minimize the surface area. Adhesion and surface tension of water molecules at and just below the surface cause water to move up the tube until the force of adhesion is balanced by the weight of the water column. Viscosity The viscosity of a liquid could be understood as its resistance to flow. Water should have a high viscosity because of hydrogen bonding between water molecules. But water has a comparatively low viscosity and it flows readily in plants. The reason for this low viscosity is that in the liquid state each hydrogen bond of a water molecule is shared on the average by two other water molecules. This makes individual bonds somewhat weakened and fairly easily broken. Hydrogen bonds between water molecules break and reform about 500 billion times every second. Dielectric constant Water has one of the highest known dielectric constants which is a measure of the capacity to neutralise the attraction between electrical charges. This property makes water a powerful solvent especially for electrolytes and polar molecules. The positive side of the water molecule is attracted to negative ions and negative side to the positive ions. Thus, water molecules form a ‘cage’ around ions or polar molecules so that the ions are unable to unite with each other and crystallise (Fig. 1.2). Figure 1.2 : “Cages” produced among positive and negative ions, which keep them apart, not allowing to unite and crystallise. 8 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water Activity 1 1. Which of these properties of water is not due to the hydrogen bonding between water molecules? a) Stabilises temperature inside and c) It is a universal solvent outside cell d) Ice floats on water. b) Molecules are cohesive 2. Complete the following table. Properties of water Importance to plant life Liquid at room temperature High specific heat High latent heat of evaporation Expand in freezing High di-electric constant Adhesion and cohesion of water molecules Answers 1) d 2) Refer section 1..2 1.3 Water content We learnt at the beginning of this session that water is the most abundant constituent of plants. A part of this water is bound to cell surfaces and some held in the micro-capillaries of the cell walls. The water that is held in micro-capillaries is referred to as bound water. However, most of the water in the plant is free to move and is called free or unbound water. About 40 to 55% of the water in a cell is found in the wall depending on age, thickness and composition of the cell wall. In meristematic tissues where the vacuole is small and cell walls are thin much of the water is found in the cytoplasm. However, in mature cells, the cytoplasm usually forms only a thin layer and may contain as little as 5% to 10% of the water in the cell. In a mature cell 50% to 80% or more of the water is found in the vacuole. The water content is usually determined by drying the plant material to a constant weight at a temperature of 70 - 85oC. Care should be taken to avoid charring while drying as charring is an indication of loss of dry 9 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water matter. However, a very small amount of water associated with organic substances is not removed by this procedure. Water content is usually expressed as a percentage of dry matter. (Please read the article: Saura- Mas, S., & Lloret, F. (2007). Leaf and Shoot Water Content and Leaf Dry Matter Content of Mediterranean Woody Species with Different Post-fire Regenerative Strategies. Annals of Botany, 99(3), 545–554. http://doi.org/10.1093/aob/mcl284) Fresh weight-oven dry weight Water content = X 100 Oven dry weight 1.4 The structure of liquid water - A model A theoretical model for the structure of a liquid was proposed by Benson and Siebert in 1992. This model is called Octemer- Tetromer model and helps to explain some of the unusual properties of water. According to them the individual molecules of the liquid water do not simply move randomly but are organised into tiny cubes composed of eight molecular units and rings composed of four units. The octametric cubes and tetrametric rings are joined into chains and networks as shown in Figure 1.3. 10 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water Figure 1.3: Structure of liquid water as proposed by Benson and Siebert. 1.5 Principles of water movement So far, we studied the functions and properties of water as an introduction to the study of water relations of plant cells. In a study on water relations of plant cells we are concerned with the entry of water into plant cells andits movement from cell to cell. It has been recognised that several physical processes known as: diffusion, osmosis, bulk flow and imbibition are involved in both entry and movement of water into and between cells. We shall now turn our attention to examine these basic phenomena. 11 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water Diffusion Water molecules possess kinetic energy; in a solution, they are not static but in continuous motion and they collide with one another and exchange their kinetic energy. The net movement of molecules from one point to another due to their random kinetic activities is called diffusion. As a result of this movement of molecules, they tend to be uniformly distributed throughout the available space. The molecules or ions move from regions of higher concentration to regions of lower concentration i.e., along a concentration gradient. We can observe diffusion easily by carefully placing a crystal of a dye in a beaker of still water. You will be able to see the colour spreading throughout the liquid slowly from the crystal of dye. Here the spread of colour is from the region of higher concentration to a region of lower concentration. In the case of gases diffusion could also occur along a pressure gradient. Diffusion is an important process for living organisms. Photosynthesis depends to a larger extent on the diffusion of CO 2. Water loss from plants (transpiration) is largely a diffusion process. Uptake of minerals by roots from the soil solution depends on diffusion. In 1855, a German physiologist A. E. Fick discovered that the rate of solute transport by diffusion is directly proportional to the concentration gradient. This relationship can be represented by the following equation. Js = - Ds. dc/dx The rate of solute transport or the flux (Js) is the amount of substance s crossing a unit area per unit time. J s may have the units mol m-2s-1. Diffusion coefficient (Ds) measures how easily a substance s moves through a particular medium. The concentration gradient (dc) is the difference in concentration of substance s between two points separated by the distance (dx). The negative sign in the equation indicates that the flux moves down a concentration gradient. Ds = k T/m u Where, k = a constant 12 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water T = absolute temperature m = molecular weight of the diffusing substance u = viscosity of the medium through which the diffusion occurs According to equation ➁ Diffusion coefficient Ds is a function of temperature. As the temperature rises the rate of diffusion increases. Temperature directly influences molecular motion and therefore, kinetic energy. Many gases diffuse 1.03 times faster when temperature is raised by 10oC. Further, the diffusion coefficient varies inversely with the molecular weight of the diffusing substance i.e., when molecular weight increases the rate of diffusion decreases. Water (MW=18gmol-1) vapour will diffuse faster than CO2 (MW= 44gmol-1). The relative rate of diffusion for water in comparison with CO2 is 1.56. Osmosis The diffusion of solvent molecules (usually water) through a differentially permeable membrane from a place where solvent concentration is high to a place where the concentration is low could be called osmosis. This special case of diffusion is often the process by which water moves from the soil into the plant and from one living plant cell to another when membranes are crossed. A simple set up to demonstrate osmosis and osmotic pressure is shown in Figure 1.4. Container A is separated from B by a differentially permeable membrane. Container A contains pure water and B an aqueous solution. Water molecules (the solvent) move from A to B across the membrane by osmosis because solvent concentration in A is greater than that in B. If a pressure (P in the figure) is applied to the solution in container B the entry of water from A to B could be prevented. The additional pressure that must be applied to the aqueous solution to prevent the entry of water is called the osmotic pressure. 13 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water Differentially permeable membrane Figure 1.4: A simple set up to demonstrate osmosis and osmotic pressure. The movement of water or solvent molecules is affected by the number of solute particles (molecules or ions) present in a given volume of water. One mole of a non-electrolyte gives 6.02 x 1023 particles whichis also known as the Avogadro number. Electrolytes such as NaCl give double the Avogadro number because NaCl dissociates in water to Na+ and Cl-. According to the Van’t Hoff relationship we can express osmotic pressure as follows. P = nRT/V where, n = number of solute molecules in solution of volume V R = gas constant (0.0821 L atm K-1 mol-1) From this relationship, a molar solution of a nonelectrolyte should have an osmotic pressure of 22.4 atm or 2.27 Mpa (Mpa = Mega pascal). We refer to the properties of a solution which depend on the number of particles, as colligative properties. Therefore, osmosis is a colligative property. Other colligative properties are boiling point, freezing point and vapour pressure. Bulk flow or Mass flow A second process of movement of water is known as bulk flow or mass flow. When the flow occurs in response to differences in pressure, and the moving fluid (in plants it is an aqueous solution) carries all what is dissolved in it and suspended in it, the movement is called bulk or mass flow. Sometimes the differences in pressure, which drives the flow, 14 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water are established by hydrostatic pressures or by mechanical forces, both of which are parameters indicative of free energy. The rate of flow, dv/dt (v = volume and t = time) depends on the hydrostatic pressure difference ( P) and on the resistance R, offered by the path of flow. dv/dt = P/R Among many common examples of bulk flow are convection currents, water moving through a pipe and the flow of a river. Hydration or Imbibition The surface of certain substances such as proteins, cell wall polysaccharides and particles in soil possess electrical charges. These surfaces attract polar water molecules and show great affinity for water molecules. Substances, which have an affinity towards water, are called hydrophilic substances. The process by which water is adsorbed by hydrophilic substances is called hydration or imbibition. The tenacity with which the water molecules are adsorbed depends on the nature of the adsorbing surface and the distance between the surface and the adsorbed water molecule. Those molecules located directly on the adsorbing surface will be held extremely tight and those at some distance from the surface will be held much less tight. Imbibition may be considered as a special type of diffusion since the net movement of water is along a concentration gradient. As a result of imbibition, the substance imbibes water and swells. However, after imbibition has taken place the total volume of the system is always less than the original volumes of water imbibed and the hydrophilic substance. Imbibition is primarily responsible for the first phase of water uptake by a seed prior to germination. Further, imbibition and gravity are the main causes of water flow through soils. Evaporation When a pan of water or any surface saturated with water is exposed to the atmosphere, water will diffuse from the pan of water or the surface saturated with water into the atmosphere in the form of vapour. This process is called evaporation. Here again diffusion is from an area of higher water concentration to an area of lower water concentration. Since evaporation occurs in the form of vapour, a vapour pressure gradient (e) between the evaporation surface and the atmosphere is considered as the driving force. If we assume that the evaporating 15 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water surface is pure water, the potential for evaporation could be expressed as e = eo -e Where, eo = vapour pressure of the evaporating surface (free water) e = vapour pressure of the water in the air. 16 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water We must remember that vapour pressure is an indicator of free energy. This saturated vapour pressure (e o) is often expressed as a function of temperature in a graphical form (could be obtained from any physics text book). If relative humidity (RH - the actual amount of water in the air expressed as a percentage of the saturated value) is known e could also be calculated by the following equation. e = eo * RH%/100 In most instances, the evaporation surface is not pure water e.g., evaporation of water from plants. Thus, if molar fraction of the solvent is known (you will learn about mole fraction of a solvent in the next Session;) using the Raoult’s law, vapour pressure of the solution could be calculated. e=x  eo Where, X = mole fraction of solvent Passive and active absorption of water Diffusion, bulk flow, osmosis, imbibition and evaporation occur in response to a concentration or a pressure gradient. There is no expenditure of metabolic energy in these movements. When a movement or a process takes place independent of metabolic energy, it is said to be a passive movement or a passive process. Any movement where metabolic energy is used could be called an active movement or an active process. Many claims have been made over the years that there is active transport of water in plants. However, it is thought that the contribution made by active transport can be considered to be negligible. 17 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water Activity 2 1. A small amount of red dye is placed at the bottom of a beaker of water as shown below. Having diagrams only show the process of diffusion of the dye in water. 2. What are the factors that affect the rate of diffusion? Check your answers by referring section 3.5 Summary Water is essential for existence of life and it is the most abundant constituent of living things. Water is a polar molecule and hydrogen bonding occur between water molecules. These two features account for most of the unique properties of water. These properties allow cellular activities to occur and life to exist on earth. Water molecules possess kinetic energy and they move from a region of high concentration to one of lower concentrations, the process is termed diffusion. Osmosis is the movement of solvent molecules through a semi permeable membrane from a region of higher concentration of solvent to a region of lower solvent concentration. Bulk or mass flow is when the flow occurs in response to a difference in pressure and the moving fluid carries with it all what is dissolved and suspended in it. The movement of water molecules from a water body or a surface saturated with water into the atmosphere is termed evaporation. 18 Copyright © 2010, The Open University of Sri Lanka (OUSL)  BYU4300 - Unit I Session 1: Plants and Water Although many claims have been made, the contribution of active transport to the movement of water in plants is considered to be negligible. Objectives Now you should be able to Explain why the water molecule is polar and how the water molecule confers unique properties to water To list the unique properties of water and discuss their importance to plant life. Describe the different physical processes by which water moves into and out of plant cells. Review Questions 1. Most of the properties of water are due to the existence of hydrogen bonds between water molecules. Justify this statement. 2. Explain how the properties of water make it uniquely suited to the role it plays in plant life. 19 Copyright © 2010, The Open University of Sri Lanka (OUSL)