Chapter 7 · Macromolecules
Lipids
Describe what makes a lipid, compare saturated and unsaturated fats by structure, and recognize steroids and phospholipids.
What makes a lipid a lipid
Lipids are hydrophobic (nonpolar), which means they avoid water. That shared behavior is what groups them, even though their shapes differ a lot. Fats and steroids contain only C, H and O. Phospholipids also contain phosphorus.
| Job | Which lipids |
|---|---|
Boundaries of cells | Phospholipids |
Chemical signals | Steroids such as estrogen and testosterone |
Long-term energy | Fats |
Fats
A fat has a 3-carbon glycerol backbone attached to 3 fatty acids. Fatty acids are long hydrocarbon chains.
Saturated or unsaturated: follow the structure
| Saturated | Unsaturated |
|---|---|
All single bonds between carbons | At least one C=C double bond |
Holds the maximum hydrogen | Less than the maximum hydrogen |
Straight tails pack tightly | Kinked tails cannot pack |
Usually solid at room temperature | Usually liquid at room temperature |
Typical of animal fats: butter, lard | Typical of plant and fish oils: olive oil |
Worked example
Reading the structure
A fatty acid chain has one C=C double bond partway along. Predict its shape and state.
One double bond means unsaturated.
A double bond puts a kink in the tail, so the tail is bent.
Bent tails cannot stack tightly, so the fat is most likely liquid at room temperature.
Answer
Kinked and most likely liquid. If a question gives you the state instead, run the steps backward.
Steroids and cholesterol
A steroid has a carbon skeleton of 4 fused rings. Steroids differ in the kinds and locations of the functional groups attached. They count as lipids because they are hydrophobic, not because they look like fats.
Cholesterol is a steroid carried in blood by lipoproteins. HDL is the good form: it carries excess cholesterol away from arteries. LDL is the bad form: it sticks to artery walls and adds to plaque.
Explore deeper: Phospholipids
Optional. This goes beyond what most courses ask for, and mastery here does not depend on it.
Many courses show the double layer first and come back to it later. Here is the part worth knowing now. A phospholipid is a glycerol with 2 fatty acid tails and a phosphate head. The head attracts water and the tails avoid it, so one molecule has two personalities.
In water, phospholipids line up in a double layer with heads facing the water on both sides and tails tucked inside. That bilayer is the boundary of every cell.
Quick check
Which part of a phospholipid faces the watery inside of a cell?