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3.10.10 - Water balance and osmoregulation All animals must...

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Water balance and osmoregulation All animals must exchange materials and energy with their environments. External environments may be very different from the animal’s internal environment, and may change rapidly; usually the internal environment needs to stay fairly constant. Regulation of the animal’s internal composition within appropriate limits -- a process called homeostasis -- is largely done by monitoring and adjusting the composition of body fluids. Body fluids can be thought of as part of three fluid ‘compartments’: Extracellular fluid Intracellular fluid Blood Control points: Blood (vertebrates) Coelomic fluid (many invertebrates) Water balance and osmoregulation Besides energy, gas exchange, nutrition, and heat balance, what are the major regulatory problems? Nitrogen excretion Water balance Electrolyte (salt, or ion) balance (solute balance) All of these processes are interrelated, all depend on the external environment, and the way an animal handles one problem often influences the way it handles the other problems. } Osmoregulation Nitrogen metabolism and excretion All animals catabolize proteins in cellular respiration (more in carnivores, less in herbivores) ‘Burning’ amino acids to make ATP (or converting them into other compounds) releases N-containing amino groups via deamination : Glycolysis Proteins NH 3 Electron transport & Oxidative phosphorylation Krebs cycle Nitrogen metabolism and excretion Deamination releases ammonia (NH 3 ), the initial (primary) nitrogenous waste. This must be disposed of. • Some animals ( ammoniotelic ) excrete the ammonia directly • biochemically simple and energetically cheap, but…. • ammonia is highly toxic , so must be stored in low concentrations (i.e., lots of water needed for disposal ) • most common in aquatic species, which have plenty of water available (freshwater bony fish, many invertebrates)
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Nitrogen metabolism and excretion The large amount of water loss needed for ammonia excretion is a problem for animals with limited access to water. Solution: convert ammonia to less toxic nitrogen compounds; two commonly used: UREA (CN 2 OH 4 ) and URIC ACID (C 5 N 4 O 3 H 4 ) UREA ( ureotilic animals) • highly soluble; fairly low toxicity: high concentrations OK • excreting N as urea requires far less water than if ammonia were excreted • urea excretion is energetically expensive : it costs substantial amounts of ATP to convert ammonia into urea. • most important nitrogen excretion product in many land animals (mammals, adult amphibians); also used by some marine animals (marine mammals, sharks, sea turtles).
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