Osmosis

Osmosis and TerminologyIon and Osmotic Balance Across Aquatic

Habitats and Animal Groups

2/19 and 2/25/08

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Osmosis Defined

Movement of some solvent across a selectively permeable membraneusually refers to the movement of water

cause most solutes can’t pass through the membrane

across a cell membranedown a concentration gradient (for solvent)

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Terminology Isosmotic

Equal osmolarityBut may still result in a change in cell volumeDue to differences in the electrochemical

gradientOr membrane permeability to particular solutes

IsotonicReference is cell responseSolution that does not cause shrinking or

swelling3

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Terminology (cont)

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Hypotonic solution

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Overview

Osmoregulation: solute and H20 balance Animals use different combinations of

tissues to control ion and water balance Representatives of most animal phyla live

in direct association with water Greater pressure for water/salt exchange than

terrestrial habitats Point: Animals cope with the ionic

concentration of the external environment using different mechanisms

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Obligatory Exchanges

Is there a gradient between the extracellular compartment and the external environment? Greater the gradient, greater tendency for

NET DIFFUSION 

Surface-to-volume ratio, higher for smaller animals Larger surface area = greater exchange Evaporative water loss, ion exchange, etc.

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Obligatory Exchanges (2)

Permeability of the integument or portions of the integument, esp. respiratory surfaces Covering external surfaces with

hydrophobic molecules, e. g. mucous, keratin, chitin

More aquaporin proteins increase water permeability

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Obligatory Exchanges (3)

Feeding Gain water and solutes from food In marine enviro. salt gain is a

problem Will have special means for excreting

excess salt Metabolic factors

End products of metabolism that cannot be used must be eliminated (nitrogenous waste) and this requires WATER! 8

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Ionic and Osmotic Regulation

Strategies Ionoconformer

exert little control over the solute profile within the extracellular space; exclusively marine

Ionoregulator control the ion profile of the extracellular space

Osmoconformer internal and external osmolarity are similar;

marine invertebrates Osmoregulator

osmolarity is constant regardless of the external environment

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Ionic and Osmotic RegulationVery similar to Figure 11.35 and Table 11.9 Willmer, 2/e

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Ionic and Osmotic Regulation (Cont.)

Ability to cope with changes in external osmolarity Stenohaline – tolerate a narrow

rangeGenerally conformers

Euryhaline – tolerate a wide rangeGenerally regulators

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Marine Invertebrates Marine inverts: internal osmotic concentration similar

to seawater Tend to be osmoconformers Exception is arthropods!

May regulate solute COMPOSITION to differ from their enviro, requires extensive regulation (= energy) Echinoderms – no significant regulation Jellyfish – regulate select ions

Lg size, active cells on outer surface Crustaceans – variable, but regulate ions

See Tables 11.3 and 11.4 in Willmer, 2/e

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Marine Invertebrates

Osmoregulation accomplished via: impermeable body surface thin surface membrane of the gills (rapid

exchange) Salt gained via:

INCOMPLETELY impermeable body surface thin surface membrane of the gills food and seawater (both containing some

solutes)

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Invasion of Other Habitats

Marine inverts, both conformers and regulators, can inhabit brackish water Oysters – tolerate dilution, even before

closing shell Various crabs (again) – fairly successful

regulators, although extremes may be too much

No FW Echinoderms or Cephalopods

Brackish Inverts + Fish Solid lines

arthropods Dashed lines

molluscs Black dotted lines

worms Teleosts

shaded area

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Figure 12.13

Freshwater Inverts + Fish Solid lines

arthropods Dashed green lines

molluscs Black dotted lines

worms Teleosts

shaded area

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Figure 13.9

Extreme Habitats - cryptobiosis Adaptation to

extremeenviro change

Drying in envirocan lead to increasedosmotic conc

Response may be extreme = cryptobiosis

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Figure 14.4

Extreme Habitats – regulation!

Regulation of internal conc over wide range of salinities!!!

Also see Figure 14.7 structures/mechanisms

used by Artemia at different life stages

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Figure 14.6

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Marine bony fishes

Marine bony fishes Few same or slightly above the conc. of

the external medium (hagfish), most about 1/3 the conc. of seawater

 General Osmotic Tendencies Osmotic efflux of water Influx of ions

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Marine bony fishes (2)

Active secretion of monovalent ions at the gills

Produce small amount of urine isosmotic to the blood but high in Mg++ and SO4=

Drink water Compare/contrast w/ freshwater bony

fish, see summary handout of vertebrates (slide 15) Osmotic issues are loss of ions/salts and water

gain!

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Bony fishes: marine vs freshH2O

Figure 11.36 Saltwater teleost

Figure 13.15 Freshwater teleost

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Freshwater Vertebrates

Figure 12.12

CompareTo Figure11.5

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Marine Vertebrates-elasmobranchs/chondrichthyes

Sharks and rays almost exclusively marine Solve problem of water efflux by being

slightly hyperosmotic (to the environment)

However, salt conc. about 1/3 that of SW High osmolarity from organic compounds

in a ratio of 2 urea: 1 TMAO Urea is an end-product of protein metabolism

and is known to destabilize many proteins (= ENZYMES)!!

TMAO has an inhibitory effect on the action of the urea

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Marine Vertebrates-elasmobranchs/chondrichthyes v(2)

Hyperosmotic internal environment solves problem of water efflux actually slight influx via gills! No need to drink SW (w/ additional salt load)

But salt conc. about 1/3 that of SW means that there is still an ion regulation issue

Solutions include- Excretion of salts in urine Excretion of Na+ and Cl- (hyperosmotic to

SW) via the rectal gland

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Marine Vertebrates-birds and reptiles Salt glands May eliminate excess salt load by using an

extrarenal salt gland Salt gland produces a highly concentrated

solution of salt Seawater = 470 mmol Na+ /L Seabird salt gland excretion = 600-1100 mmol

Na+ /L Also produce uric acid

Combines with ions Precipitates from solution (H2O) conservation

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Marine Vertebrates-mammals

Marine mammals have a HIGHLY EFFICIENT KIDNEY that can produce urine more conc. than SW

Some pinnipeds can live without drinking water on a diet of fish Remember that marine fish are NOT as

conc. as SW or MARINE INVERTS!!

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Marine Vertebrates-reptiles, bird, and mammals

Figure 11.38

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Moist Skinned Animals

Back to regulatory issues, these animals will have less control over water loss than others Worms, various phyla Gastropod molluscs, esp. slugs Amphibians (only vertebrates here)

Evaporation rates are 1-2 orders of magnitude higher than other animals (Table 8.8 and Fig 8.12 in Schmidt-Nielsen, 5th Ed)

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Moist Skinned Animals (2)

Solutions to minimize water loss include Live near water Humid habitats, soil or mud Active at night (lower evaporation rate) Active during or immediate following

precipitation

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Less Permeable Terrestrial Animals

These animals will have more control over water loss than moist skinned animals Arthropods

exoskeleton and cuticle Most higher vertebrates, except

amphibians epidermis, hair, scales, feathers

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Additional Information from Text

Willmer (2/e) Table 5.1, extracellular fluid

concentration of various animals Figure 5.2, responses to changing

environmental concentrations Table 5.2, tolerance to water loss Table 5.3, permeability across various

surfaces Figure 5.6, chloride cells of fish