Nitrogen Excretion

Question 1

How is ammonia excreted?

Answer 1

• NH3 out via gills
• diffuses out to mix with water
• NH3 + H20 = NH4 +OH
• Ammonia is in the form of glutarate when in the blood
• little energy needed to convert glutamate to ammonia but ammonia is very toxic

Example : Mainly fish and Invertebrates

Question 2

How is Urea Excreted?

Answer 2

In alkaline lakes there are not enough H= ions in the water to convert ammonia to ammonium so ornithine-urea cycle is used.

Example : Mammals, Amphibians, Fish in alkaline lakes

Question 3

Describe the Ornitin-Urea Cycle

Answer 3

Ornithine (PLUS HCO3- and NH3) –> Carbamyl phosphate –> Citrulline (PLUS Aspartate and 3ATP) –> Arginino Succinate (MINUS fumarate) –> Arginine (MINUS urea) –> Ornithine.

Question 4

How is Uric acid excreted?

Answer 4

Uric acid is excreted as a white powder by birds and reptiles ( a lot of energy is needed to produce uric acid from urea) However it is water insoluble and so reduces water loss. Low toxicity.