Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- To build new molecules, organisms must take in atoms and molecules from their environment.
- Carbon, hydrogen and oxygen are the most common elements in all four classes. Carbon forms four bonds, so it makes chains and rings.
- Sulfur is in proteins, in the side chains of cysteine and methionine.
- Phosphorus is in phosphate groups: in the head of every phospholipid and in every nucleotide of DNA and RNA.
- Nitrogen is in the bases of nucleic acids and in the amino group of every amino acid.
- Plants take in CO₂ from the air and nitrate, phosphate and sulfate ions from soil water. Animals get all of these from food.
Key relationships
| Class | Elements | Where the “extra” element sits |
|---|---|---|
| Carbohydrates | C, H, O | none |
| Fats and steroids | C, H, O | none |
| Phospholipids | C, H, O, P (often N) | P in the phosphate head |
| Proteins | C, H, O, N, S | N in amino groups; S in cysteine and methionine side chains |
| Nucleic acids | C, H, O, N, P | N in the bases; P in the sugar–phosphate backbone |
| Tool | How to use it |
|---|---|
| % by mass | (atomic mass × number of atoms) ÷ molar mass × 100 |
| Tracers | ³⁵S labels proteins; ³²P labels nucleic acids and phospholipids |
Assumptions behind the rules
- “Carbohydrates are C, H, O” covers almost all of them; a few modified sugars (as in chitin) carry nitrogen.
- Tracer experiments assume a radioactive isotope is used by cells exactly like the ordinary atom.
- Subtract the background count before comparing tracer readings.
Mistakes to avoid
- Saying proteins contain no sulfur, or that DNA does.
- Saying phosphorus is only in DNA (forgetting phospholipids, RNA and ATP).
- Saying all lipids contain phosphorus.
- Saying plants build their mass mainly from soil minerals.
- Using the number of atoms instead of their mass in a percentage calculation.
Quick self-check
- Which element would you use to label proteins but not DNA? (Sulfur, as ³⁵S.)
- Percentage of carbon in glucose, C₆H₁₂O₆? (6 × 12.01 = 72.06; 72.06 ÷ 180.18 × 100 = 40.0%)
- Why does a plant short of phosphate grow slowly? (It cannot make enough nucleotides for new DNA and RNA, or phospholipids for new membranes.)
Next: practice questions.