Water Relations

Chapter 5

Water Availability

•        Water content in organisms ranges from 50-90%

•        An organism will loose or gain water from their its environment as a result of:

v    The tendency of water to move down concentration gradients

v    The magnitude of those gradients

•        Must consider an organism’s microclimate in order to understand its water relations.

Terrestrial Environments

•       Evaporation accounts for much of water lost.

•       As water vapor in the air increases:

v  The water concentration gradient from organisms to air is reduced

v  Evaporative loss is decreased.

•       Evaporative coolers work best in dry climates.

Water Content of Air

•       Relative Humidity:

Water Vapor Density

Saturation Water Vapor Density 

•       Water vapor density = water vapor per unit volume of air.

•       Saturation water vapor density = quantity of water vapor air can potentially hold.

v  Changes with temperature.

Water Content of Air

•       Water Vapor Pressure (WVP) = Partial pressure due to water vapor.

•       Saturation Water Vapor Pressure (SWVP) = Pressure exerted by water vapor in air saturated by water.

•       Vapor Pressure Deficit

v  Difference between WVP and SWVP at a particular temperature.

Evaporative Water Loss

Aquatic Environments

•       Water moves down concentration gradient.

v  Water is more concentrated in freshwater environments than in the oceans.

•       Aquatic organisms can be viewed as an aqueous solution bounded by a selectively permeable membrane floating in an another aqueous solution.

Water Movement in Aquatic Environment

•       Isosmotic: Body fluids and external fluid are at the same concentration.

•       Hyperosmotic: Body fluids are at a lower concentration than the external environment.

•       Hypoosmotic: Body fluids are at a higher concentration than the external environment.

Water Movement Between Soils and Plants

•        Water moving between soil and plants flows down a water potential gradient.

•        Water potential (ψ) is the capacity to perform work.

v    Dependent on free energy content.

v    Pure Water ψ = 0.

§    ψ in nature generally negative.

§    ψ _solute measures the reduction in Ø due to dissolved substances.

Water Movement Between Soils and Plants

_v     ψ _plant = ψ _solute + ψ _matric + ψ _pressure

Water Regulation on Land

•       Terrestrial organisms face (2) major challenges:

v   Evaporative loss to environment.

v   Reduced access to replacement water.

Water Regulation on Land - Animals

_•       W_ia= W_d + W_f + W_a - W_e - W_s

Water Acquisition by Animals

•        Most terrestrial animals satisfy their water needs via eating (W_f) and drinking (W_d).

v     Can also be gained via metabolism through oxidation of glucose:

•        C₆H₁₂O₆ + 6O₂ à 6CO₂ + 6H₂O

Ø   Metabolic water refers to the water released during cellular respiration.

Water Regulation on Land - Plants

•       W_ip= W_r + W_a - W_t - W_s

Water Acquisition by Plants

•       Extent of plant root development often reflects differences in water availability.

v   Deeper roots often help plants in dry environments extract water from deep within the soil profile.

Water Conservation by Plants and Animals

•       Many terrestrial organisms equipped with waterproof outer covering.

•       Concentrated urine / feces.

•       Condensing water vapor in breath.

•       Behavioral modifications to avoid stress times.

•       Drop leaves in response to drought.

•       Thick leaves

•       Few stomata

•       Periodic dormancy

Osmoregulation by Marine Organisms

Osmoregulation by Freshwater Organisms