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BIOL 3240 Plant and Animal Ecology

Water Relations

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Water Availability

• The tendency of water to move down concentration gradients, and the magnitude of those gradients, determine whether an organism tends to lose or gain water from its environment.– Microclimate important factor!

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Water Content of Air• Evaporation accounts for much of water lost

by terrestrial organisms.– As water vapor in the air increases, the water

concentration gradient from organisms to air is reduced, thus evaporative loss is decreased.

– Would you buy a “swamp cooler” to help you keep cool in the cypress swamps of southern U.S.?

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Water Content of Air• Relative Humidity:

Water Vapor DensitySaturation Water Vapor Density (x 100)

• Water vapor density is measured as the water vapor per unit volume of air.

• Saturation water vapor density is measured as the quantity of water vapor air can potentially hold.– Changes with temperature.

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Water Content of Air• Total Atmospheric Pressure– Pressure exerted by all gases in the air.

• Water Vapor Pressure– Partial pressure due to water vapor.

• Saturation Water Vapor Pressure– Pressure exerted by water vapor in air saturated

by water.• Vapor Pressure Deficit– Difference between WVP and SWVP at a

particular temperature.

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Evaporative Water Loss

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Water Movement in Aquatic Environments

• Water moves down concentration gradient.– 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.

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Water Movement in Aquatic Environments

• If two environments differ in water or salt concentrations, substances will tend to move down their concentration gradients.

– Diffusion• Osmosis: Diffusion through a semipermeable

membrane.

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Water Movement in Aquatic Environment

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

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

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

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Water and Salt Balance in Aquatic Environments

• Freshwater Fish and Invertebrates– Hyperosmotic organisms that excrete excess

internal water via large amounts of dilute urine.• Replace salts by absorbing sodium and chloride at base

of gill filaments and by ingesting food.

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Osmoregulation by Freshwater Organisms

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Water and Salt Balance in Aquatic Environments

• Marine Fish and Invertebrates– Isomotic organisms do not have to expend energy

overcoming osmotic gradient.• Sharks, skates, rays - Elevate blood solute concentrations

hyperosmotic to seawater.– Slowly gain water osmotically.

• Marine bony fish are strongly hypoosmotic, thus need to drink seawater for salt influx.

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Osmoregulation by Marine Organisms

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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.– Dependent on free energy content.– Pure Water ψ = 0.• Ψ in nature generally negative.• Ψsolute measures the reduction in Ψ due to dissolved

substances.

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Water Movement Between Soils and Plants

– Ψplant = Ψsolute + Ψmatrix + Ψpressure

– Matrix Forces: Water’s tendency to adhere to container walls.

– Ψpressure is the reduction in water potential due to negative pressure created by water evaporating from leaves.

– As long as Ψplant < Ψsoil, water flows from the soil to the plant.

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Water Regulation on Land

• Terrestrial organisms face (2) major challenges:– Evaporative loss to environment.– Reduced access to replacement water.

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Water Regulation on Land - Animals

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Water Regulation on Land - Animals

• Wia= Wd + Wf + Wa - We - Ws

• Wia= • Wd = • Wf = • Wa = • We = • Ws =

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Water Acquisition by Animals

• Most terrestrial animals satisfy their water needs via eating and drinking.– Can also be gained via metabolism through

oxidation of glucose:• C6H12O6 + 6O2 6CO2 + 6H2O

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

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Water Regulation on Land - Plants

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Water Regulation on Land - Plants

• Wip= Wr + Wa - Wt - Ws

• Wip=

• Wr =

• Wa =

• Wt =

• Ws =

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Water Acquisition by Plants

• Extent of plant root development often reflects differences in water availability.– Deeper roots often help plants in dry

environments extract water from deep within the soil profile.• Park found supportive evidence via studies conducted

on common Japanese grasses, Digitaria adscendens and Eleusine indica.

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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

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Dissimilar Organisms with Similar Approaches to Desert Life

• Camels– Can withstand water loss up to 20%.• Face into sun to reduce exposure.• Thick hair: Increased body temperature lowers heat

gradient.

• Saguaro Cactus– Trunk / arms act as water storage organs.– Dense network of shallow roots.– Reduces evaporative loss.

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Two Arthropods with Opposite Approaches to Desert Life

• Scorpions– Slow down, conserve, and stay out of sun. – Long-lived – Low metabolic rates

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• Cicadas (Diceroprocta apache) – Active on hottest days. – Perch on branch tips (cooler microclimates). – Reduce abdomen temp by feeding on xylem fluid of

pinyon pine trees.

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