Lipids

Lipids are a diverse group of biological molecules that share the common property of being hydrophobic (insoluble in water) and soluble in organic solvents such as ethanol and acetone. Unlike carbohydrates, proteins, and nucleic acids, lipids are not true polymers — they are not built from repeating monomer units joined by condensation reactions in the same way. However, triglycerides and phospholipids are assembled from smaller components (glycerol and fatty acids) via condensation reactions.

The main types of lipid at A-Level are triglycerides, phospholipids, and cholesterol.

By the end of this lesson you should be able to: distinguish saturated from unsaturated fatty acids and relate saturation to melting point; describe triglyceride formation from glycerol and three fatty acids by ester-bond condensation; explain how the amphipathic structure of phospholipids gives rise to the bilayer; account for cholesterol's role as a fluidity buffer; and interpret the emulsion test for lipids.

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Fatty Acids

A fatty acid consists of a carboxyl group (–COOH) attached to a long hydrocarbon chain (typically 14–22 carbon atoms). The hydrocarbon tail is non-polar and therefore hydrophobic.

Saturated Fatty Acids

• Every carbon–carbon bond in the hydrocarbon chain is a single bond (C–C).
• The chain is straight, allowing fatty acid molecules to pack closely together.
• Lipids rich in saturated fatty acids tend to be solid at room temperature (e.g., butter, lard, animal fats).
• Common examples: palmitic acid (C16), stearic acid (C18).

Unsaturated Fatty Acids

• One or more carbon–carbon double bonds (C=C) are present in the hydrocarbon chain.
• Monounsaturated: one C=C double bond (e.g., oleic acid).
• Polyunsaturated: two or more C=C double bonds (e.g., linoleic acid, linolenic acid).
• Each double bond introduces a kink (bend) in the chain, preventing close packing.
• Lipids rich in unsaturated fatty acids tend to be liquid at room temperature (e.g., olive oil, sunflower oil).

Exam Tip: If asked why unsaturated fats have lower melting points than saturated fats, explain that the kinks caused by C=C double bonds reduce the strength of van der Waals forces between adjacent fatty acid tails because the molecules cannot pack as closely together.

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Triglycerides

Key Definition: A triglyceride is a lipid formed from one molecule of glycerol and three fatty acid molecules, joined by three ester bonds through condensation reactions.

Formation

Glycerol is a three-carbon alcohol with three hydroxyl (–OH) groups. Each hydroxyl group reacts with the carboxyl group of a fatty acid in a condensation reaction, releasing water and forming an ester bond.

Glycerol + 3 fatty acids → triglyceride + 3H₂O

The reverse reaction (hydrolysis) breaks the ester bonds by adding water, regenerating glycerol and three fatty acids. This occurs during digestion (catalysed by lipase enzymes) and during lipolysis in adipose tissue.

Properties and Functions of Triglycerides

Property	Explanation	Biological Importance
High energy content	More C–H bonds per molecule than carbohydrates; higher ratio of hydrogen to oxygen	Yield approximately 2× more energy per gram than carbohydrates when oxidised during respiration
Insoluble	Non-polar hydrocarbon chains; no effect on water potential	Can be stored in cells without drawing in water by osmosis; adipose tissue is compact
Low density	Less dense than water	Helps aquatic mammals (whales, seals) maintain buoyancy
Thermal insulation	Subcutaneous adipose tissue	Reduces heat loss in endotherms; blubber layer in marine mammals
Protection	Fat deposits around organs	Kidney and heart are cushioned by adipose tissue
Waterproofing	Hydrophobic nature	Sebum on skin and fur; waxy cuticle on leaves (contains lipids)
Metabolic water	Oxidation of fats produces water	Desert animals (e.g., camel, kangaroo rat) obtain metabolic water from fat reserves

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Phospholipids

Key Definition: A phospholipid has a similar structure to a triglyceride, but one of the three fatty acid chains is replaced by a phosphate group (often with an additional polar molecule attached).

Structure

• Hydrophilic head: the glycerol, phosphate group, and any attached polar molecule (e.g., choline in phosphatidylcholine).
• Hydrophobic tails: the two fatty acid hydrocarbon chains.
• This dual nature makes phospholipids amphipathic (having both hydrophilic and hydrophobic regions).

Phospholipid Bilayer

When placed in an aqueous environment, phospholipids spontaneously arrange themselves into a bilayer:

• Hydrophilic heads face outward (toward the aqueous extracellular fluid and the aqueous cytoplasm).
• Hydrophobic tails face inward (away from water), forming the interior of the membrane.

This arrangement is the structural basis of all cell membranes (the fluid mosaic model). The hydrophobic core of the bilayer acts as a barrier to most polar and charged molecules, controlling what enters and leaves the cell.

Key properties of the phospholipid bilayer:

• Selectively permeable — small non-polar molecules (O₂, CO₂) can pass through freely; polar molecules and ions require transport proteins.
• Fluid — phospholipids can move laterally within their own layer, giving the membrane flexibility.
• Self-sealing — if the membrane is disrupted, phospholipids spontaneously rearrange to close the gap.

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Cholesterol

Cholesterol is a type of lipid with a distinctive structure: four fused hydrocarbon rings with a short hydrocarbon tail and a single hydroxyl (–OH) group.

Role in Cell Membranes

• Cholesterol molecules sit between the phospholipid tails in the membrane.
• At high temperatures: cholesterol reduces membrane fluidity by restricting phospholipid movement, preventing the membrane from becoming too fluid and permeable.
• At low temperatures: cholesterol prevents close packing of phospholipid tails, stopping the membrane from becoming too rigid and maintaining flexibility.
• Overall, cholesterol acts as a fluidity buffer, maintaining an optimal level of membrane fluidity across a range of temperatures.

Other Roles of Cholesterol

• Precursor for the synthesis of steroid hormones (e.g., testosterone, oestrogen, cortisol).
• Precursor for bile salts, which emulsify lipids during digestion.
• Component of vitamin D synthesis in the skin.

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The Emulsion Test for Lipids

The emulsion test is used to detect the presence of lipids in a sample:

1. Add the sample to a clean test tube.
2. Add an equal volume of ethanol and shake vigorously to dissolve any lipid present.
3. Pour the ethanol solution into a test tube containing an equal volume of cold water.
4. Positive result: a cloudy white emulsion forms. The lipid molecules come out of solution as tiny droplets suspended in the water, scattering light and producing the cloudy appearance.
5. Negative result: the solution remains clear.

Exam Tip: The emulsion test does not distinguish between triglycerides, phospholipids, or other lipids — it simply confirms the presence of lipid. Also note that no colour change occurs; it is the formation of a white emulsion (cloudiness) that indicates lipid.

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Comparing Triglycerides and Phospholipids

Feature	Triglyceride	Phospholipid
Glycerol	Yes (1 molecule)	Yes (1 molecule)
Fatty acids	3	2
Phosphate group	No	Yes
Ester bonds	3	2 (plus a phosphoester bond)
Solubility in water	Insoluble (fully hydrophobic)	Amphipathic (head soluble, tails insoluble)
Main function	Energy storage, insulation, protection	Cell membrane structure

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Worked Example: Ester Bonds and Energy Density

Triglyceride assembly and breakdown are common calculation contexts because the ester-bond count fixes the water arithmetic: three condensation reactions form one triglyceride, releasing three water molecules, and hydrolysis of that triglyceride by lipase consumes three water molecules to regenerate glycerol and three fatty acids.

Worked question: A triglyceride is formed from one glycerol molecule and three molecules of stearic acid ($\text{C}_{18}\text{H}_{36}\text{O}_2$C18​H36​O2​, $M_r = 284$Mr​=284). Glycerol has $M_r = 92$Mr​=92 and water has $M_r = 18$Mr​=18. (a) How many water molecules are released during formation? (b) Calculate the relative molecular mass of the triglyceride.

Solution:

• (a) Three ester bonds form, so three water molecules are released.
• (b) Combine the reactant masses and subtract the water lost: $M_r = 92 + (3 \times 284) - (3 \times 18) = 92 + 852 - 54 = 890.$Mr​=92+(3×284)−(3×18)=92+852−54=890.

Interpreting energy density. The reason a gram of triglyceride yields roughly twice the energy of a gram of carbohydrate is visible in the formulae. Compare glucose, $\text{C}_6\text{H}_{12}\text{O}_6$C6​H12​O6​, with a fatty-acid tail: glucose is already partly oxidised (one oxygen per carbon, giving many C–O bonds), whereas the hydrocarbon tail is highly reduced (long runs of C–H bonds and almost no oxygen). Oxidising a reduced C–H bond to $\text{CO}_2$CO2​ and $\text{H}_2\text{O}$H2​O releases more energy than oxidising an already-partly-oxidised C–O bond, so per unit mass the fat delivers more ATP — at the cost of consuming more oxygen. This is why the respiratory quotient (RQ = $\text{CO}_2$CO2​ produced ÷ $\text{O}_2$O2​ consumed) of a lipid-respiring tissue is about 0.7, compared with 1.0 for carbohydrate: fat oxidation demands proportionally more oxygen. Migrating and hibernating animals exploit exactly this high energy-per-gram, low-osmotic-cost storage.

Exam Tip: When a question asks you to explain (not just state) why lipids store more energy than carbohydrates, the mark-scheme discriminator is the phrase "more C–H bonds / more highly reduced" — followed by "release more energy on oxidation". Simply writing "fats have more energy" scores nothing.

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Summary

• Fatty acids can be saturated (no C=C, straight chains, solid fats) or unsaturated (one or more C=C, kinked chains, liquid oils).
• Triglycerides form by condensation of glycerol with three fatty acids via ester bonds; hydrolysis reverses this.
• Triglycerides store more energy per gram than carbohydrates and serve as thermal insulators and protective cushions.
• Phospholipids are amphipathic and form bilayers — the structural basis of cell membranes.
• Cholesterol regulates membrane fluidity across different temperatures and is a precursor for steroid hormones and bile salts.
• The emulsion test detects lipids by forming a cloudy white emulsion when an ethanol-lipid solution is added to water.

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A-Level Deep Dive

Spec mapping

This lesson is mapped to AQA 7402 Section 3.1.3 — Lipids (refer to the official AQA specification document for exact wording). It covers fatty acid saturation, triglyceride formation by ester-bond condensation, phospholipid amphipathic structure, the phospholipid bilayer as the membrane foundation, the role of cholesterol as a fluidity buffer, and the emulsion test for lipids. Examined directly on Paper 1 and synoptically on Paper 2 (membrane transport, photosynthesis, respiration) and Paper 3.

Why triglycerides are better energy stores than carbohydrates

A frequently examined AO2 question: explain why fats yield approximately twice as much energy per gram as carbohydrates. The reasoning chain is:

1. Fatty acids have many more C–H bonds per molecule than glucose (a long aliphatic chain).
2. C–H bonds, when oxidised, release more energy than C–O or C–C bonds.
3. Triglycerides also store less oxygen per molecule than carbohydrates, so when oxidised they consume more O₂ but release more energy per gram.
4. Triglycerides are hydrophobic — they can be stored in compact anhydrous droplets in adipocytes without dragging in water (osmotically inert), so the effective energy density per unit mass of tissue is even higher than the chemical energy per mass of pure triglyceride.

This explains the use of triglyceride stores for long-term energy reserves (hibernation, migration, fasting) rather than carbohydrate stores (which are osmotically expensive — every gram of glycogen retains ~3 g of water in cells).

Synoptic links

This lesson connects to:

1. AQA 7402 Section 3.2.3 — Cell-surface membranes (fluid-mosaic model): the entire architecture of every biological membrane depends on phospholipid bilayer self-assembly driven by the hydrophobic effect. Cholesterol's role as a fluidity buffer determines how membranes behave across a temperature range and is examined directly in 3.2.3 questions on membrane permeability.
2. AQA 7402 Section 3.5.2 — Respiration (β-oxidation): triglycerides are hydrolysed to glycerol and fatty acids; fatty acids enter the mitochondrion and are oxidised in 2-carbon increments (β-oxidation) to acetyl-CoA, which feeds the Krebs cycle. Lipid metabolism is therefore directly synoptic with respiration energetics.
3. AQA 7402 Section 3.6 — Stimuli and nerves: the myelin sheath wrapping axons is a multilayered phospholipid structure (sphingolipid- and cholesterol-rich) that insulates the axon and enables saltatory conduction. Demyelinating diseases (multiple sclerosis) directly disrupt action-potential propagation.
4. AQA 7402 Section 3.6.4 — Homeostasis (steroid hormones): cholesterol is the precursor for testosterone, oestrogen, progesterone, cortisol and aldosterone. Cell-surface glucocorticoid responses in homeostasis depend on this lesson's content.

Specimen question modelled on the AQA paper format

Question (6 marks): Triglycerides and phospholipids are both lipids found in living organisms. Compare and contrast their structures and explain how their structures relate to their biological functions.

Mark scheme decomposition: