IB Biology HLCarbohydrates & LipidsPaper 1 & 2~9 min read
Fatty Acids
Butter is solid and olive oil is not. The reason is a single detail in the hydrocarbon tail — whether it contains a carbon–carbon double bond — and that detail decides how tightly the molecules can pack.
📚 What you need to know
Both triglycerides and phospholipids contain glycerol with fatty acids attached. Fatty acids have long hydrocarbon tails.
Saturated fatty acids have only single carbon–carbon bonds, so the tail is “saturated” with hydrogen.
They are straight molecules, so they pack tightly, giving a higher melting point and making them solid at room temperature.
Unsaturated fatty acids contain at least one carbon–carbon double bond, so they do not carry the maximum possible number of hydrogen atoms.
The double bond causes a kink, so unsaturated fatty acids cannot pack tightly and are often liquids at room temperature.
Monounsaturated means one C=C double bond; polyunsaturated means many.
You should be able to recognise from a diagram which type a fatty acid is — look for double bonds.
Two forms of the same thing
Fatty acids have long hydrocarbon tails. Hydrocarbons are molecules containing only hydrogen and carbon. Fatty acids occur in two forms: saturated and unsaturated, and unsaturated ones can be monounsaturated or polyunsaturated.
The red circle at the left of each chain is the carboxyl group, which is the end that bonds to glycerol. Everything to the right of it is the hydrocarbon tail.
Saturated fatty acids
The bonds between the carbon atoms in the hydrocarbon tail are all single bonds.
The fatty acid is said to be saturated with hydrogen: each carbon atom in the tail (except the final one) is bonded to two hydrogen atoms, which is the maximum possible.
They are straight molecules, so lipids containing them can pack tightly together.
Tight packing increases their melting point and causes them to be solid at room temperature.
For this reason, saturated fatty acids are often used as storage molecules in animals, for example the fats in meat and butter.
Unsaturated fatty acids
The bonds between the carbon atoms in the tail are not all single bonds.
The fatty acid is unsaturated because the tail does not contain the maximum number of hydrogen atoms possible: each carbon in a carbon–carbon double bond can only bond to one hydrogen atom instead of two.
These double bonds cause the tail to kink, or bend, so unsaturated fatty acids are not as straight as saturated ones.
They therefore cannot pack as tightly, so fats containing them are often liquids at room temperature.
Type
Carbon–carbon double bonds
Melting point
Typical state and use
Saturated
None
Higher
Solid; storage in animals, e.g. butter
Monounsaturated
One
Lower than saturated
Liquid oils; energy storage in some animals and plants
Polyunsaturated
Many
Low
Oils used for energy storage in plants
Straight chains can sit side by side like pencils in a box. Bent ones cannot, so there is more space between them and they come apart more easily.
Trace the logic in one line and you will never lose these marks: double bond → kink → poor packing → lower melting point → liquid at room temperature. Most questions here are asking you to walk along that chain in one direction or the other.
Reading a fatty acid diagram
You should be able to work out from a diagram whether a fatty acid is saturated, monounsaturated or polyunsaturated. There is one thing to look for: carbon–carbon double bonds.
🧠
Count, do not guess
No double bonds in the tail = saturated. One = monounsaturated. More than one = polyunsaturated. Ignore the double bond in the carboxyl group at the end — only carbon–carbon double bonds in the tail count.
Worked examples
WE 1
From structure to state
Explain why a fat containing mainly saturated fatty acids is solid at room temperature. (3 marks)
Point 1: the bonds
Saturated fatty acids contain only single carbon–carbon bonds.
Point 2: the shape
This makes the hydrocarbon tails straight, with no kinks.
Point 3: the consequence
Straight tails pack tightly together, which raises the melting point, so the fat is solid at room temperature.
Single bonds → straight → tight packing → high melting pointwrite the chain of reasoning; a bare statement that “saturated fats are solid” scores one mark at most
WE 2
Identifying from a diagram
A diagram shows a fatty acid tail containing three carbon–carbon double bonds. Identify the type of fatty acid and predict its state at room temperature. (2 marks)
Point 1: identify
More than one C=C double bond means it is polyunsaturated.
Point 2: predict
Three kinks prevent tight packing, so the melting point is low and it will be a liquid oil at room temperature.
Polyunsaturated, so a liquid oiljustify the prediction with packing; the identification alone is only half the answer
WE 3
Why “unsaturated”?
Explain the meaning of the term unsaturated when applied to a fatty acid. (2 marks)
Point 1: the hydrogen
The hydrocarbon tail does not contain the maximum number of hydrogen atoms possible.
Point 2: the reason
Each carbon in a carbon–carbon double bond can bond to only one hydrogen atom instead of two.
Not full of hydrogen, because of the double bondsthe word “saturated” is about hydrogen, not about fat in general — say so
💡 Exam tips
Define saturated and unsaturated by the bonds, not by whether the fat is solid.
Use the word kink — it is the term the mark schemes use.
Always explain the melting point through packing.
Count only carbon–carbon double bonds in the tail when classifying.
Have examples ready: butter and meat fats (saturated), plant and fish oils (unsaturated).
Remember that both triglycerides and phospholipids contain fatty acids.
⚠ Common mistakes
Counting the C=O double bond of the carboxyl group. Only the tail counts.
Saying unsaturated fats have “no hydrogen”. They simply do not have the maximum possible.
Saying saturated fats have stronger bonds. The difference is shape and packing, not bond strength.
Mixing up mono and poly. Mono is one double bond; poly is more than one.
Stopping at “it is solid”. The marks are in the reasoning, not the conclusion.
Up next: Phospholipids — replace one fatty acid with a phosphate group and you get the molecule that makes every cell membrane possible.
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