Not every macromolecule has nitrogen, phosphorus or sulfur
- The idea
- Predict
- See it
- Why
- Check
The idea
“All the macromolecules are made of the same elements, so a missing element should affect them all.”
You learned the elements of life as one list, and every macromolecule is built around carbon. So it seems fair that each one uses the whole list.
The short version
The mistake: All the macromolecules are made of the same elements, so a missing element should affect them all.
What's actually true: Starch and fat have only C, H and O. Proteins add nitrogen and usually sulfur. Nucleic acids add nitrogen and phosphorus.
- Starch, glycogen, cellulose and fats contain only carbon, hydrogen and oxygen. Phospholipids are different. Their head carries a phosphate group.
- Every amino acid has an amino group, so every protein contains nitrogen. The amino acids cysteine and methionine also bring sulfur.
- Nucleic acids have nitrogen in their bases and phosphorus in the phosphate groups that link nucleotides into a backbone.
- So a missing or labeled element points to particular molecules. A cell given no nitrogen can still make starch and fat, but not much new protein or DNA.
On the exam: Before you answer, ask which molecules actually contain that element. Starch and fat have only C, H and O.