AP Biology · Unit 1 · Topic 1.2
Every biological molecule is built from a short list of elements. Which ones are present tells you what kind of molecule you are looking at.
Learning objective 1.2.A — Describe the composition of macromolecules required by living organisms.
Pick a molecule. Before you reveal anything, press the elements you predict are in it. Then press Reveal composition to check yourself and count the atoms.
Scan every molecule in this tool and you will find C, H and O in all of them. That is the CED's first claim: these three are the most prevalent elements used to build carbohydrates, proteins, lipids and nucleic acids.
Because they are in everything, they cannot tell the four classes apart. A molecule made of only C, H and O could be a sugar, a glycerol backbone, or a fatty acid tail. You need more evidence.
Scan cysteine and methionine. Both are amino acids, and both carry an S in the R group. No carbohydrate, lipid or nucleic acid in this set contains sulfur.
So if an experiment shows sulfur in a molecule, protein is the class to name. Two cysteines can also join their sulfurs into a disulfide bond, which is part of how a protein holds its folded shape.
Scan the two nucleotides and the phospholipid. All three carry P. In nucleotides the phosphate links one nucleotide to the next, forming the sugar–phosphate backbone. In a phospholipid the phosphate is the water-loving head that faces out of a membrane.
Phosphorus therefore narrows the answer to two possibilities, not one. Compare palmitic acid, a plain fatty acid with no phosphorus at all, to see the difference a phosphate head makes.
Scan alanine and the nucleotides. N sits in the amino group of every amino acid and in the nitrogenous bases of every nucleotide. Carbohydrates and simple fats have none.
This is the reasoning an exam question wants: nitrogen present means the molecule is built from amino acids or from nucleotides.
Each card gives you a piece of evidence about which elements are present. Name the macromolecule class it points to — the same move an exam question asks for.
Answer these in your own words before you open the answers.
1. C, H and O are present in all four macromolecule classes, so they cannot discriminate between them. Glucose, glycerol, ribose and palmitic acid all scan as C, H, O only, yet they belong to two different classes. You would need to know whether N, P or S is present, or to look at the structure itself.
2. Proteins and nucleic acids. Nitrogen sits in the amino group of every amino acid and in the nitrogenous bases of every nucleotide, so without a nitrogen source neither polymer can be built. Carbohydrate and simple lipid synthesis can continue, because those need only C, H and O.
3. DNA. Nucleic acids contain phosphorus in the sugar–phosphate backbone but no sulfur; proteins contain sulfur in cysteine and methionine but no phosphorus. Phosphorus entering the cell and sulfur staying outside means the injected molecule was the nucleic acid, not the protein coat.
4. Cysteine is C3H7NO2S and alanine is C3H7NO2 — cysteine carries one sulfur atom in its R group and alanine carries none. Two cysteine R groups can form a disulfide bond that links distant parts of the chain and helps hold the folded shape in place.