Excretion and osmoregulation in different environments

• What do freshwater fish excrete?

  Ammonia

  it is toxic but soluble in water

  ammonium ions diffuse out of gills, where it is diluted in water to be less toxic
• Plants are producers: they make proteins when they need them
• Plants use active transport and facilitated diffusion to take up nitrate and ammonium ions from the soil
• What is transamination in plants?

  • plants combine ammonium ions with a-keto glutarate, which makes the amino acid glutamate
  • glutamate is then converted into any other amino acid by transamination of other a-keto acids
• Why do plants not excrete any nitrogenous waste?

  they synthesise the amino acids that they need, so they do not excrete any nitrogen containing molecules
• What do mammals excrete?

  urea
• Why do mammals excrete urea?

  • they need to save as much water as possible
  • urea is less toxic than that ammonia
  • it requires some energy to produce
  • the body can tolerate it for short periods of time
• What do birds/insects/reptiles excrete?

  uric acid
• Why do birds/insects/reptiles excrete uric acid?

  • has a low toxicity and is insoluble in water
  • requires lots of energy to produce
  • uric acid can be stored in cells without harmful effects
  • it allows organisms to conserve water, as little is needed for its excretion
  • so organisms can survive in environments with little water
• What do marine fish excrete?

  urea