01Where the atoms come from
Living things don't create atoms — they take them in from the environment and rearrange them. Plants pull CO₂ from the air and nitrogen/phosphorus from soil; animals get theirs by eating. Those atoms are rebuilt into new molecules for growth, repair and reproduction.
About 96% of your body mass is just four elements: carbon, hydrogen, oxygen and nitrogen (CHON).
02The element → molecule matching AP expects
Carbon, hydrogen, oxygen — in all four macromolecules (carbohydrates, lipids, proteins, nucleic acids).
Nitrogen — in proteins (every amino acid has an amino group, –NH₂) and nucleic acids (the nitrogenous bases).
Phosphorus — in nucleic acids (the phosphate backbone), phospholipids, and ATP.
Sulfur — in proteins, specifically the amino acids cysteine and methionine. Cysteine sulfurs form disulfide bridges that help hold protein shape.
03Why carbon is the backbone
Carbon has four valence electrons, so it forms four covalent bonds. That lets it build long chains, branches and rings — the skeletons of every organic molecule. Small attached groups (functional groups like –OH, –COOH, –NH₂, –PO₄) give each molecule its chemical personality.
04Notebook box
Quick ID trick: see S → protein. See P → nucleic acid or phospholipid. See N but no P or S → likely protein or a base. Only C, H, O → carbohydrate or fat.
Worked example: Researchers feed bacteria radioactive ³⁵S. Which molecule becomes labeled? → Protein, because only proteins (via cysteine/methionine) contain sulfur. (This is exactly the logic of the Hershey–Chase experiment.)