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Excretion and Osmoregulation — Class 11 Maharashtra HSC Biology revision notes

A 7-page visual revision sheet for Excretion and Osmoregulation, written for Class 11 Maharashtra HSC Biology. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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Excretion and Osmoregulation in one look

what water can buy you: ammonia, urea or uric acid, and the kidney that concentrates the last drop

Excretion, Defined Against Digestion

  • metabolism (catabolic + anabolic) produces by-products that must leave the body: metabolic waste products
  • unlike digestive waste (unabsorbed food, never entered a cell), metabolic waste forms inside body cells
  • excretory products in humans: water, CO2, nitrogenous wastes (ammonia, urea, uric acid), creatinine
  • also excess mineral salts, and bile pigments: bilirubin via faeces, urochrome via urine
  • also excess vitamins, hormones and drugs, and volatile spice compounds via the lungs
  • excess amino acids cannot be stored, so they undergo deamination: the amino group is removed
  • deamination forms toxic ammonia, excreted as-is or converted to a less toxic form first

One Deamination Product, Three Exits

how much water can the animal afford to spend diluting ammonia?

  • plenty of water — Ammonotelism: excrete ammonia directly, ~300-500 ml water per gram; energy-cheap but water-expensive
  • some water, must conserve — Ureotelism: convert to urea first, ~50 ml water per gram; less toxic, moderately soluble
  • very little water — Uricotelism: convert to uric acid, ~5-10 ml water per gram or none; energy-expensive but almost water-free

Who Uses Which Route

  • Ammonotelic: aquatic invertebrates, bony fishes, aquatic/larval amphibians, protozoa (contractile vacuole only, no organ)
  • Ureotelic: mammals, cartilaginous fishes (sharks, rays), many aquatic reptiles, most adult amphibians. liver's ornithine/urea cycle, 3 ATP per urea
  • Uricotelic: birds, many reptiles, land snails, some insects, via the inosinic acid pathway in the liver

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