Chapter 6 Water Relations 6 1 Water Availability Chapter 6 Water Relations 1 Concentration gradients influence the movement of water between an organism and its environment The tendency of water to move down concentration gradients and the magnitude of those gradients from an organism to its environment determine whether an organism tends to lose or gain water from the environment Water Content of Air Water vapor is continuously added to air as water evaporates from the surfaces of oceans lakes and rivers On land evaporation accounts for much of the water lost by organisms As the amount of water vapor in the surrounding air increases the water concentration gradient from organisms to the air is reduced and the rate at which organisms lose water to the atmosphere decreases Evaporative air coolers work poorly in humid climates where the water content of air is high These mechanical systems work best in arid climates where there is a steep gradient of water concentration from the evaporative cooler to the air A steep water concentration gradient is conducive to a higher rate of evaporation Since air is rarely completely saturated with water vapor we can use its degree of saturation as a relative measure of water content Relative humidity is the water vapor density divided by saturation water vapor density The actual amount of water in air is measured directly as the mass of water vapor per unit volume of air Water vapor density is given as mg H2O L air or as g H2O m3 air The maximum quantity of water vapor that air at a particular temperature can contain is its saturation water vapor density Saturation water vapor density increase with temperature We usually think in terms of total atmospheric pressure the pressure exerted by all the gases in air Water vapor pressure expressing water vapor as an atmospheric pressure in Pascal Pa The pressure exerted by the water vapor in air that is saturated with water is called saturation water vapor pressure and increases with temperature Water vapor pressures can represent the relative saturation of air with water Vapor pressure deficit is the difference between the actual water vapor pressure and the saturation water vapor pressure at a particular temperature kPa kilopascals Water Movement in Aquatic Environments All aquatic environments contain dissolved substances These dissolved substances slightly dilute the water Water is more concentrated in freshwater environments than in the oceans The body fluid of all organisms contains water and solutes including inorganic ions and amino acids We can think of aquatic organisms and the environment that surrounds them as two aqueous solutions separated by a selectively permeable membrane Chapter 6 Water Relations 2 Diffusion occurs when the internal environment of the organism and the external environment differ in concentrations of water and salts these substances will tend to move down the concentration gradient Osmosis is the diffusion of water across a semipermeable membrane In the aquatic environment water moving down its concentration gradient produces osmotic pressure The strength of osmotic pressure pascals across a semipermeable membrane depends upon the difference in water concentrations across the membrane Larger difference generate higher osmotic pressure Isosmotic organisms with body fluids containing the same concentration of water and solutes at the external environment Salt and water diffuse at approximately equal rates into and out of an organism Hypoosmotic organisms with body fluids with higher concentration of water lower solute concentration than the external medium tend to lose water to the environment High internal concentration of water low internal concentration of salts Salt diffuses in at a higher rate water diffuses out at a higher rate Hyperosmotic organisms with body fluids with a lower concentration of water higher solute concentration than the external medium are subject to water flooding inward from the environment Low internal concentration of water and high internal concentration of salts Salt diffuses out at a higher rate and water diffuses in at a higher rate Water Movement between Soils and Plants of the air surrounding the organism On land water flows from the organism to the atmosphere at a rate influenced by the vapor pressure deficit Water moving from the soil through a plant and into the atmosphere flows down a gradient of water potential the capacity of water to do work higher to lower free energy The capacity of water to do work depends upon its free energy content Water flows from positions of Under the influence of gravity water flows downhill from a position of higher free energy at the top of the hill to a position of lower free energy the bottom of the hill Water flows down its concentration gradient from locations of higher water concentration hypo osmotic to locations of lower water concentration hyper osmotic The measureable osmotic pressure generated by water flowing down these concentration gradients show that water flowing response to osmotic gradients has the capacity to do work Water potential is measured in pascals represented by the symbol psi and the potential of pure water is set at 0 and highly negative water potential of dry air Within small spaces such as the interior of a plant cell or the pore spaces within soil other forces are significant Matric forces are a consequence of water s tendency to adhere to the walls of containers such as cell walls or the soil particles lining a soil pore Smaller pore sizes such as fine clay soils exert higher matric forces Larger pore sizes such as sand exert lower matric forces The higher potential of soil water compared to the water potential of roots induces water to flow from the soil Chapter 6 Water Relations 3 to the plant root together As water enters roots from the surrounding soil it joins a column of water that extends from the roots through the water conducting cells Hydrogen bonds between adjacent water molecules bind the water molecules in the water column As water molecules at the upper end of this column evaporate into the air at the surfaces of leaves they exert tension or negative pressure on the entire column This negative pressure helps power uptake of water by terrestrial plants 6 2 Water Regulation on Land Terrestrial plants and animals regulate their internal water by balancing water acquisitions against water loss When organisms moved into the terrestrial environment they faced