Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Lipids are mostly carbon and hydrogen. C–H bonds are nonpolar, so most lipids are hydrophobic. They are grouped by this property, not by a shared monomer.
- Saturated fatty acid: only C–C single bonds, straight tail. Unsaturated: at least one C=C double bond, which kinks the tail. More double bonds = more unsaturated.
- Fat (triglyceride) = glycerol + 3 fatty acids, joined by dehydration synthesis (3 water released). Hydrolysis reverses it.
- Phospholipid = glycerol + 2 fatty acids + a phosphate group. Hydrophilic head, hydrophobic tails (amphipathic). In water they form a bilayer, heads out, tails in.
- Steroids have four fused carbon rings. Steroid hormones control growth and development, energy metabolism and homeostasis. Cholesterol gives animal cell membranes structural stability.
- You need the general shapes only. The exact molecular structure of specific lipids is not tested.
Key relationships
| Structure | Consequence | Biological outcome |
|---|---|---|
| straight saturated tails | pack tightly, many attractions between tails | solid at room temperature (butter, lard) |
| kinked unsaturated tails | pack loosely, fewer attractions | liquid at room temperature (most plant and fish oils); fluid membranes in the cold |
| many C–H bonds, little oxygen | about 37 kJ g⁻¹, against about 17 kJ g⁻¹ for carbohydrate | light, compact energy store |
| fat layer conducts heat poorly | slows heat loss | blubber insulates whales and seals |
| hydrophilic head + hydrophobic tails | heads face water, tails hide inside | bilayer of plasma and cell membranes |
| four fused rings, hydrophobic | small molecule that suits a signal or a membrane | steroid hormones as signals; cholesterol stabilises animal membranes |
| Equation | Use |
|---|---|
| mass = energy ÷ energy per gram | compare fat and carbohydrate stores |
Assumptions behind the numbers
- 37 and 17 kJ g⁻¹ are average values for complete respiration; individual fats and sugars differ a little.
- Comparisons of stored mass ignore any water stored with the fuel.
- “Solid” or “liquid” depends on the temperature you choose; compare lipids at the same temperature.
Mistakes to avoid
- Calling lipids polymers.
- Saying unsaturated tails have weaker covalent bonds; it is the packing that changes.
- Drawing phospholipid heads pointing into the bilayer.
- Saying cholesterol is in all cell membranes; this course links it to animal cells.
- Forgetting that steroids are lipids.
- Predicting an effect without the mechanism: always link structure → packing or polarity → function.
Quick self-check
- Why does adding double bonds lower the melting point of a fat? (Each double bond kinks the tail, so tails pack loosely, with fewer attractions between them to overcome.)
- Mass of fat that stores 740 kJ? And of carbohydrate? (740 ÷ 37 = 20 g of fat; 740 ÷ 17 = 43.5 g, about 44 g, of carbohydrate.)
- Why do phospholipids, but not fats, form bilayers? (Phospholipids have a hydrophilic head and hydrophobic tails; fats are hydrophobic all over, so they clump into droplets instead.)
- Predict what happens to an animal cell’s membrane if its cholesterol is removed. (It loses structural stability: it is more easily damaged and less able to keep its shape as conditions change.)
Next: practice questions.