Molecular modelling of urea formation

Use molecule models to show the chemical reaction that makes urea from ammonia. Show the other molecules that other animals make to remove nitrogenous waste from their bodies.
Science content
Biology: Sensing, Organ Systems (4, 5, 6)
Lessons activity is in
  • molecular models (see resource): one ammonia molecule per student (one nitrogen three hydrogens three bonds) and one carbon dioxide per student pair (one carbon two oxygens four bonds)

When we digest food, we absorb energy from it.
When proteins are broken down, we extract what we need from them (energy and building blocks for growth), but a toxic by-product containing nitrogen is made: ammonia (show molecule and point out nitrogen atom).
Our body has evolved to remove this toxin before it poisons us.
In our liver, it is converted to urea, which is 100,000 times less toxic.

Students use molecule models to show how urea is made from ammonia.
Each student receives an ammonia molecule, and each pair receives a carbon dioxide molecule.
Show them a urea molecule, and project the image.
Ask students to build a urea molecule with their partner from their ammonia molecules and the carbon dioxide molecule.
Ask them to use the rest of their atoms and bonds to make other molecule.
Help them figure out that the other molecule made with urea is H2O, or water.

Urea circulates in our blood until it is taken out of the blood by the kidneys.

Mammals and amphibians excrete urea. (not monotremes, egg-laying mammals).
Other animals deal with nitrogenous waste in other ways.
Fish and aquatic animals simply excrete ammonia through their gills or skin. (Some frogs even switch from making ammonia to urea as they leave tadpole stage to adult frog.)
Birds, insects and many reptiles excrete uric acid. It can be excreted as a solid, or a paste - we know what bird poo texture is. Monotremes excrete uric acid. These animals started in eggs, where solid uric acid can be stored without harming the baby, and left behind when they hatch.

Grades taught
Gr 6