Chapter 7 · Macromolecules
Carbon and the Big Four
Tell organic from inorganic, explain why carbon is the skeleton of life, and recognize the four functional groups.
Organic or inorganic?
Organic molecules always contain carbon. Carbohydrates, lipids, proteins and nucleic acids are all organic. These four are the macromolecules, also called biomolecules.
The trap is that this rule only runs one way. Inorganic molecules can still have carbon. Carbon dioxide (CO₂), baking soda (NaHCO₃) and diamond all contain carbon and are all inorganic. Water, ammonia, table salt and silver are inorganic too.
| Organic | Inorganic |
|---|---|
DNA | Water |
Table sugar | Table salt |
Methane | Carbon dioxide (has carbon!) |
Ethanol | Diamond (pure carbon!) |
Worked example
Sorting a mystery list
A sample contains ammonia, glucose and baking soda. Which are organic?
Ammonia is NH₃. No carbon, so it cannot be organic.
Glucose is C₆H₁₂O₆. It is a sugar, which is a carbohydrate, so it is organic.
Baking soda is NaHCO₃. It has carbon, but biology courses usually list it as inorganic, with carbon dioxide.
Answer
Only glucose is organic. Carbon is required for organic molecules, but carbon alone is not enough.
Why carbon?
Carbon has 4 valence electrons, so it can form up to 4 bonds. Those bonds are stable and can connect to many other elements, which lets carbon build long chains and rings. That is why carbon is called the skeleton of organic molecules.
Key concept
Why four matters
An atom that makes one bond can only sit at the end of a chain. An atom that makes four can branch in several directions at once, which is what a skeleton needs.
Functional groups
A functional group is a small cluster of atoms attached to the carbon skeleton. It makes a molecule react in a predictable way. You need to know four.
Hydroxyl is the one to connect to water. It is hydrophilic, so molecules that have it attract water. Carboxyl is really a carbonyl and a hydroxyl sharing one carbon, which is why its name sounds like both.
Quick check
A molecule is covered in hydroxyl groups. What do you predict about it?
The Big Four
| Macromolecule | Examples | Main job |
|---|---|---|
Carbohydrates | Sugars, starch | Energy |
Lipids | Fats, steroids, cholesterol, phospholipids | Boundaries, signals, long-term energy |
Proteins | Hair, muscle, enzymes | The cell's tool kit |
Nucleic acids | DNA, RNA | Heredity and directions |
| Term | Meaning |
|---|---|
Macromolecule | A large carbon-based molecule of life; also called a biomolecule. |
Functional group | A small group of atoms that makes a molecule react in a predictable way. |
Hydrophilic | Water-loving. Attracts water. |
Hydrophobic | Water-avoiding. Does not mix with water. |