Water Relationships
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
Water Availability
• Water moves from greater to lesser
concentration
• Relative concentrations of water
– In air: relative humidity
–In water: osmolarity or salinity
• Balance of water gain and loss impacts
survival of organisms in a particular
environment.
Evaporation – Loss of Water from Organism
to Atmosphere
• Important for terrestrial organisms
–Provides cooling
–Represents major loss of water.
• Greatest in dry climates – water vapor in
air less – where ‘humidity’ is lower
–Concentration gradient greater
• Cooling from evaporation greatest in dry
climates.
Dry Environment:
• Great water pressure
deficit:
– High saturated vapor
pressure (air could
hold a lot of water)
– Low actual vapor
pressure
• Greater movement of
water from organism
to environment
Evaporative Water Loss
Dew
• Condensation of water on surfaces cooled by
radiation of heat to atmosphere
• Impacted by absolute humidity – amount of water
vapor in atmosphere
• Fog: Forms at dew point on nucleation sites, small
particles
– Important source of moisture in summer along
California coast – some other dry environments
Water Movement in Aquatic Environments
• Water moves down concentration gradient by
diffusion.
– 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
– Osmosis – special case of diffusion water movement across a membrane.
Water concentration in solutions
• Osmolarity: a measure of concentration of
dissolved substances in water
– Salinity: concentration of dissolved salts - salt
water solution contains relatively less water
than fresh water
• That means ?
– Water moves from area of less dissolved salts to
more dissolved salts
Concentration of solutes and cells
Hyperosmotic or Hypertonic – more dissolved substances
outside cell – like a pickle
water leaves the cell  crenation
Hypoosmotic or Hypotonic – less dissolved substances outside
water enters the cell  cell swells and bursts
Isoosmotic or Isotonic – same concentration inside and out,
the cell is at dynamic equilibrium
Organisms must maintain cellular/tissue osmolarity
within narrow limits
– Osmoregulation
– Adapted to particular osmotic
environment
• Generally water moves in and out of cells freely
• Salt movement is restricted due to charged nature
of salt ions
Water and Creatures in Terrestrial
Environments
• Terrestrial organisms face (2) major
challenges:
– Evaporative loss to environment.
– Reduced access to replacement
water.
Water and Plants
• Earliest plant lived in moist habitats
– No means to transport water within
structure
– Required moisture film for
reproductive processes
• Later plants developed vascular system
– Movement by transpiration
• Water moves from soil
to top of plant in
unbroken stream
• Transpiration provides
the pull
• Rate of transpiration is
regulated by action of
guard cells
• > 90% of water taken in
by roots is lost to the
atmosphere
• Adaptations include
reduced leaf area,
modifications to leaf
surface
Dermal Tissue
• The epidermis of a plant is often covered with a thick waxy layer
called the cuticle
• Guard cells
– Paired cells with
openings between them
(stomata)
– Allow gas exchange
Guard cells
turgid
Guard cells
flaccid
Water Regulation on Land - Plants
Plant adaptations:
• Root system
development
• Low growth habit
(reduced wind
exposure)
• Hirsute leaves
• Leaf coloration
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.
Water Regulation on Land Animals
• Wia= Wd + Wf + Wa - We - Ws
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Wia= Animal’s internal water
Wd = Drinking
Wf = Food
Wa = Absorbed by air
We = Evaporation
Ws = Secretion / Excretion
Water Regulation on Land Animals
Physiological Regulation of TissueWater Balance in Animals
• Kidneys evolved to produce hypertonic
uring in terrestrial organisms
• Other mechanisms exist as well – eg.
Certain sea birds
Osmoregulation in Animals
• Osmoregulation is the regulation of the body’s osmotic
(water and salt) composition
• Adaptation to dry terrestrial habitats: produce hypertonic
urine
• Insects: Malpighian tubules Active transport of K+, water
is drawn osmotically, reabsorbtion in the hindgut
• Vertebrates: Kidneys
– Hydrostatic filtration (blood under pressure)
– Selective reabsorption
– Mammals, birds produce hypertonic urine
– Marine birds (Procellarids) minimize water loss by
excreting salt through specialized glands
– Additional water is absorbed in the cloaca
• Water Summary
• Number of unique properties due to chemical nature
• Concentration of water drops as percent salt in solution
rises
• Movement of water from greater to lesser
concentrations occurs by specialized diffusion:
osmosis
• Development of various strategies to deal with water
has lead to patterns of distribution of species of plants
and animals