Biology › Section A · The study of life and its chemistry › Topic A.5
Carbon and organic molecules
Life chose carbon for a reason that lives in the periodic table: four valence electrons and four vacancies, so a carbon atom can bond in four directions at once. That single fact builds chains, rings, and branches — and with a few substitutions, everything from fuel to DNA.
In this topic
- The four-valent backbone
- Chains, branches, and rings
- Hydrocarbons: skeletons in their purest form
- Substituted hydrocarbons: chemistry bolted on
- Structural isomers: same atoms, different wiring
- Geometric isomers: cis and trans around a double bond
- Enantiomers: molecules with handedness
- Functional groups: the parts that do the work
- Worked example: reading a molecule's groups
- Worked example: classifying an isomer pair
- Common mistakes to avoid
- Key takeaways
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