2.1.6Additional lessons
Fatty Acids — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Structure of Fatty Acids
General structure
- General structure A fatty acid has a long hydrocarbon chain, usually 14 to 20 carbons, ending in a carboxyl group, -COOH.
Saturated
- Saturated All carbon-carbon bonds in the chain are single, so the chain is straight and molecules pack tightly.
Monounsaturated
- Monounsaturated One carbon-carbon double bond is present, which bends the chain and prevents tight packing.
Polyunsaturated
- Polyunsaturated Two or more carbon-carbon double bonds are present, producing multiple kinks in the chain.
Cis and trans
- Cis and trans In cis fatty acids hydrogens lie on the same side of the double bond, giving a bend;
- trans forms are straighter.
| Type | Double bonds | Shape | Usual state at room temperature |
|---|---|---|---|
| Saturated | None | Straight | Solid fat, e.g. butter |
| Monounsaturated | One | Bent | Liquid oil, e.g. olive oil |
| Polyunsaturated | Two or more | More bent | Liquid oil, e.g. sunflower oil |
Part 2 of 5Triglycerides
Triglycerides are the main form of long-term energy storage in animals and in many seeds.
1
One molecule of glycerol, a three-carbon alcohol with three hydroxyl groups, provides the backbone.
2
Each hydroxyl group reacts with the carboxyl group of a fatty acid in a condensation reaction.
3
An ester bond forms at each reaction and three water molecules are released in total.
4
The resulting triglyceride is non-polar, so it is insoluble in water and stored in fat tissue.
5
Hydrolysis by lipase adds water and releases glycerol and fatty acids when energy is needed.
Part 3 of 5Functions and Health
Functions and Health
Energy storage
- Energy storage Fats contain more energy per gram than carbohydrates, so they are an efficient and compact store.
Insulation
- Insulation A layer of fat under the skin reduces heat loss and can also cushion organs.
Melting point
- Melting point Saturated fats pack tightly and melt at higher temperatures, while unsaturated fats stay liquid.
Health evidence
- Health evidence Trans fats raise the risk of coronary heart disease, and claims about other fats should be judged against data.
Part 4 of 5Worked Example
Question
- Glycerol (C₃H₈O₃) reacts with three molecules of palmitic acid (C₁₆H₃₂O₂) to form a triglyceride.
- Find the formula of the triglyceride.
- Add the atoms of the reactants
- carbon 3 + 3 × 16 = 51;
- hydrogen 8 + 3 × 32 = 104;
- oxygen 3 + 3 × 2 = 9.
- carbon 3 + 3 × 16 = 51;
- Three water molecules (H₆O₃) are released by condensation.
- Subtracthydrogen 104 - 6 = 98 and oxygen 9 - 3 = 6, with carbon unchanged at 51.
Answer The triglyceride is C₅₁H₉₈O₆, with three molecules of water released.
Part 5 of 5Exam 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 molecule of glycerol joined to three fatty acids by ester bonds.
- Ester bond The bond formed between a hydroxyl group and a carboxyl group in a condensation reaction.
- Cis isomer A form of an unsaturated fatty acid in which hydrogens are on the same side of the double bond.
Common Mistakes
- Saying saturated means "full of water"; it means fully saturated with hydrogen atoms.
- Describing unsaturated fats as straight; the double bond puts a bend in the chain.
- Stating that two water molecules are released when a triglyceride forms; three are released.
- Calling fats polymers; they are assembled from different parts and do not have repeating monomers.
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