AP BiologyUnit 1: Chemistry of Life1.5
AP Biology · Unit 1 · Topic 1.5

Lipids

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Heads face water, tails hide. Straight tails pack solid; kinked tails stay fluid.

Lipids aren't built from one repeating unit. They're grouped by a property instead: mostly carbon and hydrogen, they're largely nonpolar and don't mix with water. That one property runs through fat storage, waxy waterproofing, and every cell membrane.

Key terms

  • Nonpolar — electrons shared almost evenly, as in C–H bonds. Nonpolar molecules can't hydrogen bond with water, so they barely dissolve.
  • Triglyceride — a fat: glycerol with three fatty acids attached by ester linkages. Stores over twice the energy per gram of carbohydrate.
  • Saturated — no C=C double bonds. The tail is straight and carries as many hydrogens as it can hold.
  • Unsaturated — one or more C=C double bonds, which usually put a kink in the tail.
  • Phospholipid — glycerol, two fatty acid tails, and a phosphate-containing head. It's amphipathic: polar head, nonpolar tails.
  • Steroid — four fused carbon rings. Cholesterol and hormones like testosterone and estrogen are steroids.

How it works

  1. To build a fat, three fatty acids each bond to glycerol by dehydration synthesis. Three ester linkages form, and three waters leave.
  2. The fatty acids attach to glycerol, not to each other. No repeating chain forms, so a fat is not a polymer.
  3. Tail shape sets packing. Straight saturated tails stack tightly, so butter is solid at room temperature. Kinked unsaturated tails can't, so olive oil is liquid.
  4. Drop phospholipids in water and they self-assemble. Polar heads face the water on both surfaces. Nonpolar tails hide in between. That's a bilayer.
  5. The bilayer is the basic structure of cell membranes. More unsaturated tails keep it fluid; more saturated tails stiffen it.
  6. In animal cells, cholesterol sits among the phospholipids as a buffer. When warm, it restrains their movement. When cold, it keeps tails from packing tightly.

Why it matters

The bilayer's nonpolar core decides what crosses a membrane. Small nonpolar molecules like O2 and CO2 slip through. Ions and large polar molecules like glucose need a protein. The same logic explains a waxy leaf: a nonpolar layer gives water nothing to hydrogen bond with, so less evaporates. On data questions, check the conditions first. If every sample was measured at one temperature, a difference in fluidity comes from the membranes' makeup.

Exam traps

TrapActually
A fat is a polymer of fatty acids.It's three fatty acids on one glycerol, a fixed size you can't extend. Lipids are the one major macromolecule class that isn't a polymer.
Saturated tails are the loose, fluid ones.Reversed. Saturated tails are straight and pack tightly. Kinked unsaturated tails pack loosely and keep a membrane fluid.
The tails face the water and the heads hide inside.Heads are polar, so they face water on both surfaces. Nonpolar tails hide in the middle of the bilayer.
Cholesterol always makes a membrane stiffer.It's a buffer. It lowers fluidity when warm and raises it when cold, keeping the membrane workable.
Worked example

Four fatty acids, all 18 carbons long, differ only in their number of C=C double bonds. With 0, 1, 2, and 3 double bonds, they melt at 69 °C, 13 °C, −5 °C, and −11 °C. Which are liquid at a room temperature of 22 °C, and what explains the trend?

  1. Anything that melts below 22 °C is liquid at 22 °C. That's the three fatty acids with at least one double bond.
  2. Length is the same in all four, so size can't explain the trend. The only variable is the number of double bonds.
  3. Each double bond here adds a kink. Kinked tails can't line up closely, so the attractions between neighboring molecules weaken.
  4. Weaker attractions between molecules → less heat needed to pull them apart → a lower melting point.

The three unsaturated fatty acids are liquid at 22 °C. Melting point falls with each added double bond because kinks keep the tails from packing tightly.

Melting separates whole molecules from each other; it breaks no covalent bonds. The near-miss is 'a double bond is stronger than a single bond, so more double bonds should raise the melting point.' That mixes up bonds inside a molecule with attractions between molecules.

Now try it on real exam-style questions

Ten questions on Lipids, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 1.5 Lipids

Which part of a phospholipid is hydrophilic?

  1. A The two fatty acid tails
  2. B The phosphate-containing head
  3. C Any double bonds within the tails
  4. D The entire molecule, end to end
Show the answer

B — The head carries a charged phosphate group, so it attracts water, while the two hydrocarbon tails are nonpolar and hydrophobic. Having both kinds of region is what makes a phospholipid amphipathic.

Question 2 · 1.5 Lipids

Cholesterol and testosterone belong to which class of molecules?

  1. A Phospholipids
  2. B Triglycerides
  3. C Steroids
  4. D Polypeptides
Show the answer

C — Cholesterol and testosterone are steroids, lipids built from four fused carbon rings. Cholesterol sits in animal cell membranes, but that location does not make it a phospholipid.

Question 3 · 1.5 Lipids

Which reaction splits a triglyceride into glycerol and fatty acids?

  1. A Dehydration synthesis, which removes water
  2. B Hydrolysis, which adds water across bonds
  3. C Hydrogenation, which adds hydrogen
  4. D Denaturation, which unfolds the molecule
Show the answer

B — Hydrolysis adds a water molecule across each ester bond, freeing glycerol and the fatty acids. Dehydration synthesis is the reverse reaction, the one that builds the triglyceride and releases water.