Oil and water do not mix. That simple fact is the whole definition of a lipid: a molecule mostly of carbon and hydrogen that does not dissolve in water. It also explains how a cell can have a membrane at all.
Key idea
Lipids are hydrophobic ("water-fearing"). Fats and oils store energy compactly; phospholipids build membranes; steroids such as cholesterol (and some hormones) have four linked rings; waxes waterproof leaves and feathers.
- Fats (triglycerides): one glycerol joined to three fatty acids by dehydration synthesis. Saturated fatty acids have no C=C double bonds, so their straight tails pack tightly: solid at room temperature (butter). Unsaturated ones have one or more double bonds, which put kinks in the tail: liquid at room temperature (olive oil).
- Phospholipids: glycerol with two fatty acid tails and a phosphate head. The head is hydrophilic ("water-loving"), the tails hydrophobic.
- Steroids: four fused carbon rings. Cholesterol is part of animal cell membranes; cortisol is a steroid hormone.
Why it works
Fat stores about 9 kcal of energy per gram, more than twice the 4 kcal per gram of carbohydrate or protein. It packs so much because its long chains are mostly C–H bonds, and it is stored without water around it. That is why seeds, migrating birds and hibernating animals store fat.
Worked example: Energy in storage
A bird stores 12 g of fat before migrating. About how much energy is that?
- Fat stores about 9 kcal per gram.
- $12 \times 9 = 108$ kcal.
- The same energy as carbohydrate would take $108 \div 4 = 27$ g, more than twice the mass.
Answer: About $108$ kcal
Worked example: Saturated or unsaturated?
A fatty acid has two C=C double bonds. Is it saturated, and would a fat made of it more likely be solid or liquid at room temperature?
- Double bonds mean the chain is not "saturated" with hydrogen: it is unsaturated.
- Each double bond bends the tail, so the molecules cannot pack tightly.
- Loosely packed molecules stay liquid: an oil.
Answer: Unsaturated; liquid (an oil)
Common mistake
Thinking lipids are polymers made of repeating units, like the other three. A fat is built from parts (one glycerol and three fatty acids) by dehydration synthesis, but it is not a long chain of the same monomer, and steroids are not built from repeating units at all. What lipids share is a property, not a monomer: they do not mix with water.