Phosphorus doesn't mean DNA. Nitrogen doesn't mean protein.
- The idea
- Predict
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- Why
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The idea
“If a molecule has phosphorus, it's a nucleic acid. If it has nitrogen, it's a protein.”
DNA has phosphorus and every amino acid has nitrogen. Those elements just show up in other molecules too.
The short version
The mistake: If a molecule has phosphorus, it's a nucleic acid. If it has nitrogen, it's a protein.
What's actually true: Nitrogen and phosphorus each turn up in more than one kind of molecule, so you identify a macromolecule by its mix of elements.
- Nitrogen is in every amino acid and in the bases of DNA and RNA. It's also in chitin and some phospholipids.
- Phosphorus is in nucleic acid backbones, every phospholipid, and ATP. Some proteins get phosphate added after they're made.
- Lots of nitrogen with little phosphorus fits a protein or chitin. Lots of both fits a nucleic acid.
- Phosphorus with no more nitrogen, plus lots of carbon and hydrogen, fits a phospholipid.
On the exam: One element only narrows the list. Read nitrogen and phosphorus together before you name the molecule.