Chapter 7 · Macromolecules
Nucleic Acids and Identifying Molecules
Describe a nucleotide, then identify any macromolecule from its elements, monomer, diagram or test result.
Nucleic acids
Nucleic acids are CHONP molecules. Their monomers are nucleotides, and each nucleotide has 3 components.
Nucleotides are joined by covalent bonds. The usual examples are DNA, RNA and ATP. The main function is heredity storage and directions for making things like proteins. ATP is the main chemical energy your cells use.
The one table worth memorizing
| Macromolecule | Monomer | Elements | Clues |
|---|---|---|---|
Carbohydrate | Monosaccharide | CHO, ratio 1:2:1 | Rings, many –OH groups |
Lipid (fat) | Glycerol + fatty acids | CHO, little oxygen | Long C–H tails or 4 fused rings |
Protein | Amino acid | CHON | Amino and carboxyl groups, R group |
Nucleic acid | Nucleotide | CHONP | Phosphate + sugar + base |
Worked example
An unknown molecule
An unknown contains C, H, O, N and P. It has a repeating backbone with four kinds of units. What is it?
Elements: the N and P point toward a nucleic acid, but other molecules have those too.
Shape: a repeating backbone with four kinds of units fits nucleotides with four bases.
Two clues agree, so the answer is well supported.
Answer
Most likely a nucleic acid made of nucleotides. One clue suggests; two or more clues confirm.
Explore deeper: Elements are clues, not proof
Optional. This goes beyond what most courses ask for, and mastery here does not depend on it.
The element list is the fastest clue, and the easiest to over-trust. Phospholipids contain phosphorus and are lipids. ATP contains phosphorus and is a single nucleotide. Chitin, the material in insect shells, is a carbohydrate that contains nitrogen.
Key concept
How to be sure
Use elements to shorten the list, then confirm with the monomer, the shape or a test. A question that gives only the elements is asking for the most likely answer, so say most likely.
The macro tests
| Test | Detects | Positive | Negative |
|---|---|---|---|
Benedict's | Monosaccharides | Red, orange, yellow, green | Blue |
Iodine (Lugol's) | Starch | Black/blue | Orange/yellow |
Biuret | Proteins | Purple | Blue |
Sudan Red | Lipids | Red | None |
Worked example
Reading a results table
Benedict's: blue. Iodine: black. Biuret: blue. Sudan Red: no red. What is in the sample?
Benedict's blue is negative: no simple sugar.
Iodine black is positive: starch is present.
Biuret blue is negative: no protein. Sudan with no red: no lipid.
Answer
Starch only. Go row by row, and check each color against its own test.
Quick check
A juice stays blue with Biuret and turns orange with Benedict's. What does it contain?