2.1.6Inquiry Process
Fatty Acids — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 41 of 160
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Section 1 of 6Structure of a Fatty Acid
Fatty acids are the building blocks of most lipids.
Carboxyl end
- A fatty acid has a carboxyl group at one end, which is polar and able to join other molecules.
Hydrocarbon tail
- A long unbranched chain of carbon and hydrogen atoms, usually 12 to 20 carbons, that is non-polar and hydrophobic.
Saturated
- Contains only single carbon-carbon bonds, so the chain is straight and holds the maximum number of hydrogen atoms.
Unsaturated
- Contains one or more C=C double bonds; one double bond is monounsaturated and several are polyunsaturated.
| Type | C=C bonds | Shape | State at room temperature |
|---|---|---|---|
| Saturated | None | Straight | Usually solid |
| Monounsaturated | One | Bent (cis) | Usually liquid |
| Polyunsaturated | Two or more | Several bends | Liquid |
Section 2 of 6Cis and Trans Isomers
The geometry around a double bond affects the physical properties of the lipid.
Cis and Trans Isomers
Cis arrangement
- Hydrogens on the same side of the double bond make a kink, which stops chains packing closely.
Trans arrangement
- Hydrogens on opposite sides keep the chain nearly straight, so it behaves similarly to a saturated fat.
Melting point
- Kinked chains pack loosely and have lower melting points than straight chains of the same length.
Health
- High intake of trans fats is associated with raised risk of cardiovascular disease in epidemiological studies.
Section 3 of 6Making a Triglyceride
1
A glycerol molecule has three hydroxyl groups, each able to react with a fatty acid.
2
The hydroxyl group of glycerol and the carboxyl group of a fatty acid undergo condensation.
3
One water molecule is released and an ester bond forms.
4
The process is repeated twice more, giving three ester bonds.
5
The result is a triglyceride and three molecules of water: glycerol + 3 fatty acids → triglyceride + 3H₂O.
Section 4 of 6Functions of Triglycerides
Energy store
- Lipids release about twice as much energy per gram as carbohydrate, so they are a compact long-term store.
Insulation
- A fat layer beneath the skin conducts heat poorly, reducing heat loss in animals such as seals.
Buoyancy
- Lipids are less dense than water, which helps aquatic animals float.
Protection
- Fat deposits around organs such as the kidneys cushion them against impact.
Section 5 of 6Worked Example
Question Compare the energy released by 10 g of fat and 10 g of carbohydrate, using values of 37 kJ per g for fat and 17 kJ per g for carbohydrate.
- Fat10 g × 37 kJ g⁻¹ = 370 kJ.
- Carbohydrate10 g × 17 kJ g⁻¹ = 170 kJ.
- Ratio370 ÷ 170 = 2.18, roughly double.
Answer Fat releases 370 kJ and carbohydrate 170 kJ, about 2.2 times as much.
Section 6 of 6Exam Checklist & Tips
2.1.6 – Key Points for the Exam
- Saturated fatty acid A fatty acid with no carbon-carbon double bonds.
- Unsaturated fatty acid A fatty acid with one or more carbon-carbon double bonds.
- Triglyceride A lipid made of one glycerol and three fatty acids joined by ester bonds.
- Ester bond A bond formed between a carboxyl group and a hydroxyl group in condensation.
- Hydrophobic Not attracted to water; non-polar.
Common Mistakes
- Saying double bonds are found in saturated fats; they are in unsaturated ones.
- Counting the hydrocarbon tail as polar; only the carboxyl end is polar.
- Forgetting that three water molecules are released when a triglyceride forms.
- Claiming that correlation between trans fats and heart disease alone proves causation.
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