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Water Relationships. Outline. 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

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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



  • 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

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    Water and Creatures in Terrestrial Environments narrow limits

    • Terrestrial organisms face (2) major challenges:

      • Evaporative loss to environment.

      • Reduced access to replacement water.


    Water and Plants narrow limits

    • 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 narrow limits

    • 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
    Dermal Tissue narrow limits

    • 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 narrow limits

    Guard cells flaccid


    Plant adaptations: narrow limits

    • Root system development

    • Low growth habit (reduced wind exposure)

    • Hirsute leaves

    • Leaf coloration

    Water acquisition by plants
    Water Acquisition by Plants narrow limits

    • 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
    Water Regulation on Land - Animals narrow limits

    • Wia= Wd + Wf + Wa - We - Ws

    • Wia= Animal’s internal water

    • Wd = Drinking

    • Wf = Food

    • Wa = Absorbed by air

    • We = Evaporation

    • Ws = Secretion / Excretion


    Physiological Regulation of Tissue-Water Balance in Animals narrow limits

    • Kidneys evolved to produce hypertonic uring in terrestrial organisms

    • Other mechanisms exist as well – eg. Certain sea birds

    Osmoregulation in animals
    Osmoregulation in Animals narrow limits

    • 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 ofK+, water is drawn osmotically, reabsorbtion in the hindgut

    • Vertebrates: Kidneys

      • Hydrostatic filtration (blood under pressure)

      • Selective reabsorption

      • Mammals, birds produce hypertonic urine



    • Water Summary salt through specialized glands

    • 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