Lipids: fats, oils, phospholipids and steroids – Chemical Basis of Life (Senior 4 Biology) | Ganzaa

Chemical Basis of Life

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Senior 4 · Biology

Chemical Basis of Life

5 lessons

Lesson 4 of 5

Lipids: fats, oils, phospholipids and steroids

25 min

Solid ghee, liquid groundnut oil

On a cool morning, ghee (clarified cow butter) in a jar is a soft solid, while groundnut oil in a bottle beside it pours freely. Both are almost pure triglycerides, and both store about twice as much energy per gram as sugar. Why should one be solid and the other liquid? The answer lies in a single kind of bond in their fatty acid chains, and the same bond decides how "healthy" a fat is considered to be.

Definition

Lipid: a member of a varied group of organic compounds that are insoluble in water but dissolve in organic solvents such as ethanol. They include fats and oils, phospholipids, waxes and steroids.

Fatty acid: a hydrocarbon chain with a carboxyl group, $\mathrm{-COOH}$ , at one end. Saturated: no carbon–carbon double bonds, so the chain holds the maximum number of hydrogen atoms. Unsaturated: one (monounsaturated) or more (polyunsaturated) C=C double bonds.

Triglyceride: one glycerol molecule joined to three fatty acids by ester bonds, formed by condensation.

Phospholipid: glycerol joined to two fatty acids and a phosphate-containing group.

Steroid: a lipid built on four carbon rings joined together, for example cholesterol.

Triglycerides

Glycerol, $\mathrm{C_3H_8O_3}$ , has three OH groups. Each reacts with the COOH group of a fatty acid in a condensation reaction, so a triglyceride forms with three ester bonds and three molecules of water are released (Figure 1). Hydrolysis by lipase enzymes, in digestion, reverses this.

Figure 1. A triglyceride forms from glycerol and three fatty acids by condensation; each ester bond releases one molecule of water. The kinked middle chain is unsaturated.

The properties of a triglyceride depend on its fatty acids:

Saturated chains are straight, so the molecules pack closely and attract each other strongly: the fat has a higher melting point and is solid at room temperature (most animal fats such as ghee and lard, and also palm and coconut oil, are rich in them).
Unsaturated chains are kinked at each double bond, so the molecules cannot pack closely: oils such as groundnut, sunflower and soybean oil are liquid at room temperature.

A saturated fatty acid with $n$ carbon atoms has the formula $\mathrm{C}_n\mathrm{H}_{2n}\mathrm{O}_2$ . Each C=C double bond removes two hydrogen atoms.

Worked example 1: how many double bonds?

Linoleic acid, found in sunflower oil, has the formula $\mathrm{C_{18}H_{32}O_2}$ . Is it saturated, monounsaturated or polyunsaturated?

A saturated 18-carbon fatty acid would be $\mathrm{C_{18}H_{36}O_2}$ (since $2 n = 36$ ).
Linoleic acid has $36 - 32 = 4$ fewer hydrogen atoms.
Each double bond removes 2 H, so it has $\frac{4}{2} = 2$ C=C bonds: it is polyunsaturated.

Phospholipids and steroids

In a phospholipid, one fatty acid of a triglyceride is replaced by a phosphate-containing group. The molecule now has a charged, hydrophilic head and two non-polar, hydrophobic tails (Figure 2). In water, phospholipids arrange themselves with their heads facing the water and their tails hidden inside: this is how the bilayer of every cell membrane forms, as you saw in Unit 4.

Steroids look nothing like fats, but they are lipids because they are insoluble in water. Cholesterol sits between the phospholipids of animal cell membranes and controls their fluidity; the body also uses it to make steroid hormones (testosterone, oestrogen, cortisol), vitamin D and bile salts.

Waxes are esters of a fatty acid with a long-chain alcohol. They are very water-repellent: the waxy cuticle of a leaf cuts water loss, and insects are coated with a thin layer of wax.

Figure 2. A phospholipid (simplified) and a cholesterol molecule. Only the phosphate head of the phospholipid and the OH group of cholesterol are attracted to water.

What lipids do

Function	Explanation	Example
Energy store	Oxidation releases about $38\,\mathrm{kJ\,g^{-1}}$ , about twice as much as carbohydrate or protein (about $17\,\mathrm{kJ\,g^{-1}}$ ), because fatty acids have many C–H bonds and little oxygen; insoluble, so stored without osmotic effects	Adipose tissue under the skin; oil in groundnut and sunflower seeds
Source of metabolic water	Oxidising fat produces more water per gram than oxidising carbohydrate	Embryos in bird and reptile eggs; animals of dry regions
Thermal insulation	Fat conducts heat slowly	Fat layer (blubber) of whales and seals
Electrical insulation	The lipid-rich myelin sheath insulates nerve axons	Fast nerve impulses in mammals
Protection and buoyancy	Fat cushions organs; it is less dense than water	Fat around the kidneys; buoyancy of aquatic mammals
Waterproofing	Lipids are hydrophobic	Leaf cuticle; oil from a bird's preen gland spread on its feathers
Membranes	Phospholipid bilayer; cholesterol controls fluidity	Every cell membrane
Hormones and vitamins	Made from cholesterol, or are lipids themselves	Testosterone, oestrogen; vitamins A, D, E and K are fat-soluble

Worked example 2: why store fat and not glycogen?

An adult has about 15 kg of stored fat (an illustrative figure). Using $38\,\mathrm{kJ\,g^{-1}}$ for fat and $17\,\mathrm{kJ\,g^{-1}}$ for glycogen, (a) how much energy does the fat store hold, and (b) what mass of glycogen would store the same energy?

(a) $15\,000\,\mathrm{g} \times 38\,\mathrm{kJ\,g^{-1}} = 570\,000\,\mathrm{kJ}$ .
(b) $\frac{570\,000}{17} = 33\,529\,\mathrm{g} \approx 34\,\mathrm{kg}$ .
The same energy as glycogen would weigh more than twice as much, and glycogen is stored with water, making it heavier still. For an animal that must move, fat is the lighter store.

Worked example 3: metabolic water

The fat tripalmitin is oxidised completely: $\mathrm{C_{51}H_{98}O_6} + 72.5\,\mathrm{O_2} \longrightarrow 51\,\mathrm{CO_2} + 49\,\mathrm{H_2O}$ Compare the mass of water produced per gram with that from glucose: $\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \longrightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O}$ . ( $A_{r}$ : C = 12, H = 1, O = 16.)

$M_{r}$ of tripalmitin $= 51 \times 12 + 98 \times 1 + 6 \times 16 = 612 + 98 + 96 = 806$ . Water formed $= 49 \times 18 = 882\,\mathrm{g}$ per 806 g, that is $\frac{882}{806} = 1.09\,\mathrm{g}$ of water per gram of fat.
Glucose: $\frac{6 \times 18}{180} = \frac{108}{180} = 0.60\,\mathrm{g}$ of water per gram.
Fat yields almost twice as much water per gram, because it contains so much hydrogen. A developing chick, sealed in its egg, relies on this water from the fat of the yolk.

Testing for lipids: the emulsion test

Crush or mix the sample with about $2\,\mathrm{cm^3}$ of ethanol and shake well, so that any lipid dissolves.
Pour the ethanol (leave any solid behind) into a test tube containing about $2\,\mathrm{cm^3}$ of water.
A cloudy white emulsion shows a lipid: the lipid comes out of solution as tiny droplets that scatter light. If the mixture stays clear, no lipid is present.

Ethanol is highly flammable: keep it well away from any flame.

Fats and health

The World Health Organization's "Healthy diet" fact sheet (January 2026) advises adults to take no more than 30 % of their daily energy from fat, no more than 10 % from saturated fat (fatty meat, butter, ghee, palm and coconut oil, cream, cheese) and no more than 1 % from trans fat, and to replace them with unsaturated fats from plant oils, fish, avocado and nuts. Diets high in saturated and trans fats raise blood cholesterol, which contributes to fatty deposits in arteries (atherosclerosis), heart attacks and strokes.

Khan Academy: "Lipid overview". Watch for how the fatty acid chains of a triglyceride are drawn, the difference between saturated and unsaturated chains, and the shapes of phospholipids and steroids.

Warning

Common mistakes

Calling lipids polymers. A triglyceride is made of four molecules, not a long chain of repeating monomers.
Writing "glycosidic bond" for a lipid. Glycerol and fatty acids are joined by ester bonds.
Saying that all plant fats are unsaturated. Palm and coconut oil are rich in saturated fat.
Saying the emulsion test works in water alone. The lipid must first be dissolved in ethanol; the emulsion forms only when that solution is added to water.

Tip

Memory hook. "Straight chains stack; kinks keep apart." Saturated chains are straight and stack into solid fats; the kinks of unsaturated chains keep molecules apart, giving liquid oils. For lipid functions, think "SWIMP": Store energy, Water (metabolic), Insulate, Membranes, Protect.

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Practice: lipids

Name the type of reaction and the bond formed when a fatty acid joins glycerol. How many water molecules form when one triglyceride is made?
Stearic acid is $\mathrm{C_{18}H_{36}O_2}$ and oleic acid is $\mathrm{C_{18}H_{34}O_2}$ . Classify each, and explain which would give a fat with the higher melting point.
Describe how you would test a crushed groundnut for lipid, and give the positive result.
Explain how the structure of a phospholipid lets it form a bilayer in water.
A seed stores 2.0 g of oil. Using $38\,\mathrm{kJ\,g^{-1}}$ , how much energy is available to the seedling? What mass of starch ( $17\,\mathrm{kJ\,g^{-1}}$ ) would store the same energy?
Suggest why many small seeds that are dispersed by wind store oil rather than starch.
A cooking-oil advert says "cholesterol is bad for you, so our oil has none". Evaluate this claim, using what you know about the roles of cholesterol.

Show answersHide answers

Condensation; an ester bond. Three water molecules.
Stearic acid is saturated ( $2 n = 36$ ); oleic acid is monounsaturated (2 H fewer, one C=C). A fat rich in stearic acid has the higher melting point, because its straight chains pack closely.
Shake the crushed groundnut with about $2\,\mathrm{cm^3}$ of ethanol, then pour the ethanol into about $2\,\mathrm{cm^3}$ of water: a cloudy white emulsion shows lipid.
The phosphate head is charged and hydrophilic, the fatty acid tails hydrophobic. In water the molecules form two layers, with heads facing the water on each side and tails facing each other inside, away from water.
$2.0 \times 38 = 76\,\mathrm{kJ}$ . Starch: $\frac{76}{17} = 4.5\,\mathrm{g}$ (2 s.f.).
Oil stores about twice as much energy per gram as starch, so the seed can be lighter for the same food store and be carried further by wind.
Partly misleading. Plant oils never contain cholesterol, so the claim makes the oil seem special when it is not. Cholesterol is also needed: the body makes it for membranes, steroid hormones, vitamin D and bile salts. High blood cholesterol is a health risk, and that is raised mainly by saturated and trans fats in the diet; so the better question is how much saturated fat the oil contains.

Quick check

Why is groundnut oil liquid at room temperature while ghee is solid?

Key points

A triglyceride is glycerol + 3 fatty acids joined by ester bonds (condensation, 3 $\mathrm{H_2O}$ released).
Saturated fatty acids ( $\mathrm{C}_n\mathrm{H}_{2n}\mathrm{O}_2$ ) are straight and give solid fats; unsaturated ones have kinking C=C bonds and give oils.
Phospholipids have a hydrophilic head and hydrophobic tails and form membranes; steroids such as cholesterol have four fused rings.
Lipids store energy (about $38\,\mathrm{kJ\,g^{-1}}$ ), give metabolic water, insulate, protect, waterproof and make hormones. The emulsion test detects them.

Sources and credits

1.3.4: Lipids, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the structure of triglycerides, phospholipids, steroids and waxes. Adapted by Ganzaa.
Dietary advice: World Health Organization, "Healthy diet" fact sheet (26 January 2026). Facts cited, not copied.
Figures 1 and 2, the table, worked examples (the 15 kg fat store is illustrative), practice and quick check: written by Ganzaa.
Photos: Rainer Z ..., CC BY-SA 3.0; Bambi Cia, CC BY-SA 4.0; Berkshire Community College Bioscience Image Library, CC0; all via Wikimedia Commons.
Video: "Lipid overview", Khan Academy, embedded from YouTube.

1Solid ghee, liquid groundnut oil

2Triglycerides

3Phospholipids and steroids

4What lipids do

5Testing for lipids: the emulsion test

6Fats and health