1. Lecture #11 – Animal Osmoregulation and Excretion

2. Key Concepts Water and metabolic waste
The osmotic challenges of different environments
The sodium/potassium pump and ion channels
Nitrogenous waste
Osmoregulation and excretion in invertebrates
Osmoregulation and excretion in vertebrates

3. Water and Metabolic Waste
Osmoregulation ≠ Excretion
All organismal systems exist within a water based environment
The cell solution is water based
Interstitial fluid is water based
Blood and hemolymph are water based
All metabolic processes produce waste
Metabolic processes that produce nitrogen typically produce very toxic ammonia

4. Critical Thinking
The cellular metabolism of _____________ will produce nitrogenous waste.

5. Critical Thinking
The cellular metabolism of proteins, nucleic acids, and ATP will produce nitrogenous waste.

6. Water and Metabolic Waste
All animals have some mechanism to regulate water balance
All animals have some mechanism to regulate solute concentration
All animals have some mechanism to excrete nitrogenous waste products
Osmoregulation and excretion systems vary by habitat and phylogeny (evolutionary history)

7. Animals live in different environments
Marine….Freshwater….Terrestrial
All animals must balance water uptake vs. water loss and regulate solute concentration within cells and tissues

8. The osmotic challenges of different environments – water balance
Water regulation strategies vary by environment
Body fluids range from 2-3 orders of magnitude more concentrated than freshwater
Body fluids are about one order of magnitude less concentrated than seawater for osmoregulators
Body fluids are isotonic to seawater for osmoconformers
Terrestrial animals face the challenge of extreme dehydration

9. The osmotic challenges of different environments – solute balance
All animals regulate solute content, regardless of their water regulation strategy
Osmoregulation always requires metabolic energy expenditure

10. The osmotic challenges of different environments – solute balance
In most environments, ~5% of basal metabolic rate is used for osmoregulation
More in extreme environments
Less for osmoconformers
Strategies involve active transport of solutes and adaptations that adjust tissue solute concentrations

11. Water Balance in a Marine Environment
Marine animals that regulate water balance are hypotonic relative to salt water (less salty)
Where does water go???

12. Critical Thinking
Marine animals that regulate water balance are hypotonic relative to salt water – where does water go???

13. Critical Thinking Ψ = P - s
Marine animals that regulate water balance are hypotonic relative to salt water – where does water go???
Remember water potential!
Ψ = P - s

14. Critical Thinking
Marine animals that regulate water balance are hypotonic relative to salt water – where does water go???
Water will always move from high ψ to low ψ
Pressure is not important in this instance (no cell wall)
Solute concentration is much higher in the saltwater environment than in the cytoplasm
Water is constantly moving out of the animal by osmosis

15. Water Balance in a Marine Environment
Marine animals that regulate water balance are hypotonic relative to salt water
They dehydrate and must drink lots of water
Marine bony fish excrete very little urine
Most marine invertebrates are osmoconformers that are isotonic to seawater
Water balance is in dynamic equilibrium with surrounding seawater

16. Solute Balance in a Marine Environment
Marine osmoregulators
Gain solutes because of diffusion gradient
Excess sodium and chloride transported back to seawater using metabolic energy, a set of linked transport proteins, and a leaky epithelium
Kidneys filter out excess calcium, magnesium and sulfates
Marine osmoconformers
Actively regulate solute concentrations to maintain homeostasis

17. Specialized chloride cells in the gills actively accumulate chloride, resulting in removal of both Cl- and Na+
Figure showing how chloride cells in fish gills regulate salts
From Gorka, by 13 November 2008

18. Solute Balance in a Marine Environment
Marine osmoregulators
Gain solutes because of diffusion gradient
Excess sodium and chloride transported back to seawater using metabolic energy, a set of linked transport proteins, and a leaky epithelium
Kidneys filter out excess calcium, magnesium and sulfates
Marine osmoconformers
Actively regulate solute concentrations to maintain homeostasis

19. Water Balance in a Freshwater Environment
All freshwater animals are regulators and hypertonic relative to their environment (more salty)
Where does water go???

20. Critical Thinking
All freshwater animals are regulators and hypertonic relative to freshwater – where does water go???

21. Critical Thinking
All freshwater animals are regulators and hypertonic relative to freshwater – where does water go???
Solute concentration is much lower in the freshwater environment than in the cytoplasm
Water is constantly moving by osmosis into the animal

22. Water Balance in a Freshwater Environment
All freshwater animals are regulators
They are constantly taking in water and must excrete large volumes of urine
Most maintain lower cytoplasm solute concentrations than marine regulators – helps reduce the solute gradient and thus limits water uptake
Some animals can switch environments and strategies (salmon)

23. Some animals have the ability to go dormant by extreme dehydration
The waterbear song by MalWebb

24. Solute Balance in a Freshwater Environment
Large volume of urine depletes solutes
Urine is dilute, but there are still losses
Active transport at gills replenishes some solutes
Additional solutes acquired in food

25. Marine osmoregulators dehydrate and drink to maintain water balance; regulate solutes by active transport
Freshwater animals gain water, pee alot to maintain water balance; regulate solutes by active transport
Figure showing a comparison between osmoregulation strategies of marine and freshwater fish

26. Water Balance in a Terrestrial Environment
Dehydration is a serious threat
Most animals die if they lose more than 10-12% of their body water
Animals that live on land have adaptations to reduce water loss

27. Critical Thinking
Animals that live on land have adaptations to reduce water loss – such as???

28. Critical Thinking
Animals that live on land have adaptations to reduce water loss – such as???
Waxy cuticle on arthropod exoskeletons
Mollusk and reptile shells and scales
Layers of dead skin cells
Fur that develops an insulating boundary layer
Eating wet food
Retaining metabolic water
Small openings from respiratory surfaces to outside environment

29. Solute Balance in a Terrestrial Environment
Solutes are regulated primarily by the excretory system
More later

30. The sodium/potassium pump and ion channels in transport epithelia
ATP powered Na+/Cl- pumps regulate solute concentration in most animals
First modeled in sharks, later found in other animals
Position of membrane proteins and the direction of transport determines regulatory function
Varies between different groups of animals
Figure showing the Na/K pump and membrane ion channels. This figure is used in the next 9 slides.

31. The Pump
Metabolic energy is used to transport K+ into the cell and Na+ out
This produces an electrochemical gradient

32. Critical Thinking
What kind of electrochemical gradient???

33. Critical Thinking What kind of electrochemical gradient???
Two K+ in vs. 3 Na+ out…..

34. Critical Thinking What kind of electrochemical gradient???
Two K+ in vs. 3 Na+ out…..
Cell interior becomes more negative in charge and lower in Na+ concentration

35. The Na+/Cl-/K+ Cotransporter
A cotransporter protein uses this gradient to move sodium, chloride and potassium into the cell

36. The Na+/Cl-/K+ Cotransporter
Sodium is cycled back out
Potassium and chloride accumulate inside the cell

37. Selective Ion Channels
Ion channels allow passive diffusion of chloride and potassium out of the cell
Placement of these channels determines direction of transport – varies by animal

38. Additional Ion Channels
In some cases sodium also diffuses between the epithelial cells
Shark rectal glands
Marine bony fish gills

39. Additional Ion Channels
In other animals, chloride pumps, additional cotransporters and aquaporins are important
Membrane structure reflects function

40. Nitrogenous Waste
Metabolism of proteins and nucleic acids releases nitrogen in the form of ammonia
Ammonia is toxic because it raises pH
Different groups of animals have evolved different strategies for dealing with ammonia, based on environment
Figure showing different forms of nitrogenous waste in different groups of animals

41. Critical Thinking
Why does ammonia raise pH???
Remember chemistry……

42. Critical Thinking Why does ammonia raise pH???
Remember chemistry..…ammonia is NH3…..a base…..protons are abundant…..

43. Critical Thinking Why does ammonia raise pH???
Remember chemistry..…ammonia is NH3…..a base…..protons are abundant…..
Ammonia readily acquires a proton to become ammonium – NH4+
This reduces proton concentration = raises pH
Higher pH disrupts enzyme function
Base – a substance that accepts protons

44. Nitrogenous Waste
Metabolism of proteins and nucleic acids releases nitrogen in the form of ammonia
Ammonia is toxic because it raises pH
Different groups of animals have evolved different strategies for dealing with ammonia, based on environment

45. Nitrogenous Waste
Most aquatic animals excrete ammonia or ammonium directly across the skin or gills
Plenty of water available to dilute the toxic effects
Freshwater fish also lose ammonia in their very dilute urine

46. Nitrogenous Waste
Most terrestrial animals cannot tolerate the water loss inherent in ammonia excretion
They use metabolic energy to convert ammonia to urea
Urea is 100,000 times less toxic than ammonia and can be safely excreted in urine
Urea is produced in the liver, carried in blood to the kidneys

47. Nitrogenous Waste
Insects, birds, many reptiles and some other land animals use even more metabolic energy to convert ammonia to uric acid
Uric acid is excreted as a paste with little water loss
Energy expensive