IB Biology SLTopic 2 — Carbohydrates & LipidsPaper 1 & 2Structure and function~9 min read
Fatty Acids
Butter is solid. Olive oil is liquid. Both are made of glycerol and fatty acids, and both are sitting in your kitchen at the same temperature. The difference is a handful of double bonds putting bends in the tails. This page is about how one bend changes whether a fat pours or sits in a block.
📘 What you need to know
A fatty acid is a long hydrocarbon tail with a carboxyl group (—COOH) at one end and a methyl group (—CH3) at the other.
Saturated fatty acids have only single carbon–carbon bonds. Every carbon carries as many hydrogens as it can.
Unsaturated fatty acids have at least one C=C double bond, so they carry fewer hydrogens than they could.
One double bond makes it monounsaturated; more than one makes it polyunsaturated.
Double bonds put a kink in the tail, so the molecules cannot pack closely together.
Poor packing means a lower melting point, so unsaturated lipids are usually liquid oils at room temperature.
Saturated tails are straight, pack tightly, have a higher melting point, and are usually solid fats found in animals.
The shape of a fatty acid
Every fatty acid follows the same plan. At one end is the carboxyl group, which is the reactive part — that is what bonds to glycerol. Stretching away from it is a chain of carbons, each carrying hydrogens, ending in a methyl group. The tail is pure hydrocarbon, which is why it is non-polar and hates water.
The general layout
CH3 — long hydrocarbon tail — COOH
Where “saturated” comes from. A carbon in the middle of a chain has four bonds: two to its neighbours and two spare for hydrogen. If all the C—C bonds are single, every carbon is holding the maximum hydrogen it can — it is saturated with hydrogen. A carbon in a double bond has used two bonds on its neighbour, so it can only take one hydrogen.
Chemists leave the hydrogens off drawings like these. Each corner is a carbon, and you are expected to know it is holding hydrogens as well.
Why the kink changes the melting point
This is the bit worth understanding rather than memorising. Melting point is really a question of how easily molecules can be pulled apart from each other.
Straight saturated tails lie against each other like pencils in a box. Being close together means strong attractions between neighbouring molecules, so you need more heat energy to separate them — a high melting point, and a solid at room temperature.
Now bend one tail. It no longer fits neatly against its neighbours, so the molecules sit further apart, the attractions between them are weaker, and less heat is needed to pull them apart. Lower melting point, liquid at room temperature.
Nothing about the bonds inside the molecule changes when a fat melts. What changes is whether the molecules can be pulled away from each other, and that depends entirely on packing.
The pattern in real numbers
All three of these fatty acids have 18 carbon atoms, so the only thing changing is the number of double bonds. Watch the melting point fall.
Fatty acid
Carbon atoms
C=C double bonds
Melting point
State at 20°C
Stearic acid
18
0 (saturated)
about 69°C
Solid
Oleic acid
18
1 (monounsaturated)
about 13°C
Liquid
Linoleic acid
18
2 (polyunsaturated)
about −5°C
Liquid
If a data question gives you a table like this, do not just describe it — explain it. “As the number of double bonds increases, the melting point decreases, because more kinks mean the molecules pack less closely.”
SATURATED — MOSTLY ANIMAL
All C—C bonds are single
Straight tails, pack tightly
Higher melting point, solid at room temperature
Found in meat fat and butter, used as animal storage
UNSATURATED — MOSTLY PLANT
One or more C=C double bonds
Kinked tails, cannot pack tightly
Lower melting point, liquid oil at room temperature
Found in olive, sunflower and other seed oils
🧠 A way to remember it
Saturated = straight = solid. Three s-words in a row. Everything else is the opposite: unsaturated tails are bent, so they stay liquid.
Worked examples
WORKED EXAMPLE
A fatty acid has 18 carbon atoms and 3 carbon–carbon double bonds. Classify it and predict its state at room temperature. [2]
Step 1: count the double bonds3 double bonds → more than oneStep 2: apply the rule
More than one C=C means polyunsaturated, and 3 kinks stop the molecules packing.
Polyunsaturated, and liquid (an oil) at room temperatureThe number of carbons is a distractor here. Only the C=C count decides the class.
WORKED EXAMPLE
Stearic acid melts at about 69°C and oleic acid at about 13°C. Both have 18 carbons. Explain the difference. [3]
Point 1
Oleic acid has one C=C double bond; stearic acid has none.
Point 2
The double bond kinks the tail, so oleic acid molecules cannot pack closely together.
Point 3
Attractions between the molecules are therefore weaker, so less heat energy is needed to melt it.
Kink → poor packing → weaker attraction → lower melting pointThree links in a chain. Miss the middle one and you lose a mark.
WORKED EXAMPLE
A carbon in the middle of a saturated hydrocarbon tail is bonded to two neighbouring carbons. State how many hydrogen atoms it carries, and explain your answer. [2]
Step 1: total bonds
Carbon always forms 4 covalent bonds.
Step 2: subtract the ones used4 − 2 = 2 bonds left for hydrogen.
2 hydrogen atoms — the maximum possible, which is what “saturated” meansIn a C=C double bond that carbon uses 3 bonds on carbons, so only 1 hydrogen fits.
💡 Exam tip
To classify from a diagram, hunt for double lines between carbons. Zero, one, or more than one — that is the whole decision.
Never stop at “unsaturated fats have a lower melting point”. Add why: kinks, poor packing, weaker attractions.
Say carbon–carbon double bond in full. “Double bond” on its own can mean the C=O in the carboxyl group.
Watch the wording: fats are solid, oils are liquid. Both are triglycerides.
In data questions, describe the trend first, then explain it. Two separate marks.
Remember the carboxyl group is the end that bonds to glycerol. The tail does not react.
⚠ Common mix-up
Thinking “saturated” means full of fat. It means saturated with hydrogen — as many hydrogens as the carbons can hold.
Saying double bonds make the molecule weaker. The C=C bond is strong. It is the attraction between molecules that is weakened, by poor packing.
Counting the C=O in the carboxyl group as making it unsaturated. Only carbon-to-carbon double bonds count.
Mixing up monounsaturated and polyunsaturated. Mono = one. Poly = many.
Assuming all plant lipids are unsaturated. It is a general pattern, not a rule — coconut oil is largely saturated.
Writing that unsaturated fatty acids have fewer carbons. They can have exactly the same number, as the table above shows.
Up next: Phospholipids — swap one fatty acid for a phosphate group and you get the molecule that every cell membrane on Earth is built from.
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