Water Relations
Chapter 6
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Outline
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Water Availability
Water Content of Air
Water Movement in Aquatic Environments
Water Movement Between Soils and Plants
Water Regulation on Land
Water Acquisition by Animals
Water Acquisition by Plants
Water Conservation by Plants and Animals
Water and Salt Balance in Aquatic
Environments
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Water Availability
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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.
Must consider an organism’s microclimate
in order to understand its water relations.
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Water Content of Air
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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.
Evaporative coolers work best in dry
climates.
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Water Content of Air
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Relative Humidity: (相對濕度)
相對濕度)
Water Vapor Density
Saturation Water Vapor Density
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(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
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Total Atmospheric Pressure
Pressure exerted by all gases in the air.
Water Vapor Pressure (蒸氣壓)
蒸氣壓)
Partial pressure due to water vapor.
Saturation Water Vapor Pressure (飽和水蒸氣壓)
飽和水蒸氣壓)
Pressure exerted by water vapor in air saturated
by water.
Vapor Pressure Deficit (蒸氣壓差)
蒸氣壓差)
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
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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
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If two environments differ in water or salt
concentrations, substances will tend to move
down their concentration gradients.
Diffusion (擴散)
擴散)
Osmosis: Diffusion through a
semipermeable membrane. (滲透壓)
滲透壓)
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Water Movement in Aquatic Environment
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Isomotic: Body fluids and external fluid are
at the same concentration. (等滲透壓)
等滲透壓)
Hypoosmotic: Body water is at a higher
concentration than the external environment.
(低滲透壓)
低滲透壓)
Hyperosmotic: Body water is at a lower
concentration than the external environment.
(高滲透壓)
高滲透壓)
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isomostic
hyperomostic
hypomostic
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Water Movement Between Soils and Plants
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Water moving between soil and plants flows
down a water potential gradient.
Water potential (Ψ) is the capacity to
perform work. (水勢能)
水勢能)
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 + Ψmatric + Ψpressure
Matric Forces: Water’s tendency to
adhere to container walls. (基勢力)
基勢力)
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
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Terrestrial organisms face two major
challenges:
Evaporative loss to environment.
Reduced access to replacement water.
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Water Regulation on Land - Animals
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Wia= Wd + Wf + Wa - We - Ws
Wia= Animal’s internal water
Wd = Drinking
Wf = Food
Wa = Absorbed by air
We = Evaporation
Ws = Secretion / Excretion (分泌/排泄)
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Water Regulation on Land - Animals
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Water Regulation on Land - Plants
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Wip= Wr + Wa - Wt - Ws
Wip= Plant’s internal water
Wr =Roots
Wa = Air
Wt = Transpiration
Ws = Secretions
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Water Regulation on Land - Plants
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Water Acquisition by Animals
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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.
(代謝水)
代謝水)
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Water Acquisition by Plants
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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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Digitaria
Eleusine
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Water Conservation by Plants and Animals
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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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表
皮
的
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Dissimilar Organisms with Similar
Approaches to Desert Life
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Camels
Can withstand water loss up to 20%.
Face into sun to reduce exposure.
Thick hair: Increased body temperature
lowers heat gradient. (可承受;禁得起)
Saguaro Cactus (北美巨型仙人掌;仙人掌)
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
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Scorpions
Slow down, conserve, and stay out of sun.
Low metabolic rates
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. (美國西部的矮松)
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Water and Salt Balance in Aquatic Environments
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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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Skates
Rays
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Slightly hyperosmotic
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hypoosmotic marine fish
hypoosmotic saltwater mosquitoes
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Water and Salt Balance in Aquatic Environments
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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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hyperosmotic freshwater fish
hyperosmotic freshwater
mosquitoes
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Review
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Water Availability
Water Content of Air
Water Movement in Aquatic Environments
Water Movement Between Soils and Plants
Water Regulation on Land
Water Acquisition by Animals
Water Acquisition by Plants
Water Conservation by Plants and Animals
Water and Salt Balance in Aquatic
Environments
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