Osmosis

1. Osmosis Osmosis and Terminology Ion and Osmotic Balance Across Aquatic Habitats and Animal Groups 2/19 and 2/25/08

2. Osmosis Defined • Movement of some solvent across a selectively permeable membrane • usually refers to the movement of water • cause most solutes can’t pass through the membrane • across a cell membrane • down a concentration gradient (for solvent) 2

3. Terminology • Isosmotic • Equal osmolarity • But may still result in a change in cell volume • Due to differences in the electrochemical gradient • Or membrane permeability to particular solutes • Isotonic • Reference is cell response • Solution that does not cause shrinking or swelling 3

4. Terminology (cont) Hypotonic solution 4

5. 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 5

6. 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. 6

7. 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 7

8. 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

9. 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 9

10. Ionic and Osmotic Regulation Very similar to Figure 11.35 and Table 11.9 Willmer, 2/e 10

11. Ionic and Osmotic Regulation (Cont.) • Ability to cope with changes in external osmolarity • Stenohaline – tolerate a narrow range • Generally conformers • Euryhaline – tolerate a wide range • Generally regulators 11

12. 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

13. 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)

14. 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

15. Brackish Inverts + Fish Figure 12.13 • Solid linesarthropods • Dashed linesmolluscs • Black dotted linesworms • Teleostsshaded area

16. Freshwater Inverts + Fish Figure 13.9 • Solid linesarthropods • Dashed green linesmolluscs • Black dotted linesworms • Teleostsshaded area

17. Extreme Habitats - cryptobiosis Figure 14.4 • Adaptation to extremeenviro change • Drying in envirocan lead to increasedosmotic conc • Response may be extreme = • cryptobiosis

18. Extreme Habitats – regulation! Figure 14.6 • Regulation of internal conc over wide range of salinities!!! • Also see Figure 14.7 • structures/mechanisms used by Artemia at different life stages

19. 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

20. 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!

21. Bony fishes: marine vs freshH2O Figure 13.15 Freshwater teleost Figure 11.36 Saltwater teleost

22. Freshwater Vertebrates Figure 12.12 Compare To Figure 11.5

23. 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

24. 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

25. 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

26. 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!!

27. Marine Vertebrates-reptiles, bird, and mammals Figure 11.38

28. 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)

29. 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

30. 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

31. 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