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Lipids: Fats, Oils and More

Lipids are biomolecules that do not dissolve in water. The main ones are fats and oils (triglycerides): one glycerol joined to three fatty acids by ester bonds. Saturated fatty acids have straight tails and give solid fats; unsaturated ones have C=C double bonds, bent tails and give liquid oils. Phospholipids build cell membranes and steroids such as cholesterol make hormones.

🎬 Step-by-step story

  1. This is a fatty acid: a long tail of carbon beads with an acid group (–COOH) at one end. The tail hates water.
  2. A saturated tail has only single bonds, so it is straight. An unsaturated tail has a C=C double bond, which puts a bend (kink) in it.
  3. Glycerol has three –OH groups. Each one joins a fatty acid. We get one triglyceride (a fat) and three water molecules. The links are ester bonds.
  4. Straight tails stack tightly, so the fat is solid at room temperature, like butter or ghee. Bent tails cannot pack, so the fat stays liquid, like olive or mustard oil.
  5. Not all lipids are fats. Phospholipids have a water-loving head and two tails and form the cell membrane. Steroids, like cholesterol, are four joined rings.
  6. Your turn: choose the three tails of a fat. Predict: solid or liquid at 25 °C? Then check.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why do oil and water not mix?

Lipid tails are made of C and H only and have no charge, so water molecules prefer each other and push the oil away.

What does "unsaturated" actually mean?

The chain is not "full" of hydrogen. A C=C double bond means two carbons hold fewer H atoms, and the chain bends there.

Where does the water come from when a fat forms?

The –OH from glycerol and the H from the fatty acid's –COOH leave together as H₂O, three times.

Why is ghee solid but mustard oil liquid?

Ghee has mostly straight saturated tails that stack tightly. Mustard oil has bent unsaturated tails that cannot.

Is cholesterol a fat?

It is a lipid but not a fat. It is a steroid made of four rings, with no fatty acid tails.

What if a fat has one bent tail and two straight ones?

It is in between, often soft or semi-solid. Try it in free play.

What are lipids?

Lipids are a group of biomolecules that do not dissolve in water but dissolve in organic solvents such as ether or alcohol. They are made mostly of carbon, hydrogen and a little oxygen.

Jobs of lipids:

Fatty acids: saturated and unsaturated

A fatty acid is a long hydrocarbon chain with a carboxyl group (–COOH) at one end. General formula of a saturated one: CnH2n+1COOH.

How to tell them apart in the lab: shake with bromine water (orange). Oleic acid decolourises it, because bromine adds across the C=C bond. Palmitic and stearic acids leave it orange.

Shorthand: 18:1 means 18 carbons and 1 double bond.

Fats and oils: esters of glycerol

A fat (triglyceride) forms when glycerol (propane-1,2,3-triol) reacts with three fatty acids. Each –OH of glycerol and –COOH of an acid join and release one water molecule. The new link is an ester bond, so fats are esters.

glycerol + 3 fatty acids → triglyceride + 3 H2O

Classification:

Properties: lighter than water, insoluble in water, greasy. Unsaturated oils decolourise bromine water. Hydrogenation adds hydrogen to C=C bonds with a nickel catalyst and turns oil into solid fat (margarine, vanaspati). Hydrolysis splits a fat back into glycerol and fatty acids; with an alkali it gives soap (saponification).

Uses: food and energy, cooking, soap, candles and cosmetics, biodiesel.

Other lipids: phospholipids, steroids, waxes

Saponifiable lipids contain fatty acids joined by ester bonds, so an alkali can turn them into soap: fats/oils (triglycerides), phospholipids, waxes.

Non-saponifiable lipids have no fatty acids, so no soap forms:

Try it: solid or liquid?

Put a spoon of ghee or butter and a spoon of cooking oil in two cups in the fridge for an hour. Which one becomes hard first? Predict using the tail shapes, then compare with step 4 and free play in the 3D. (Ask an adult before using the kitchen.)

Key formulas and definitions

Worked examples

1. How many water molecules are released when one triglyceride forms?

Three: one for each ester bond between glycerol and a fatty acid.

2. A fat is made from glycerol + 3 stearic acids. Will it be solid or liquid at 25 °C?

Solid. Stearic acid is saturated, so the straight tails stack tightly.

3. How can you tell oleic acid from stearic acid?

Add bromine water. Oleic acid (C=C bond) turns it colourless; stearic acid does not.

4. Is cholesterol saponifiable? Why?

No. It is a steroid with no fatty acid or ester bond, so an alkali cannot make soap from it.

Common mistakes

Practice quiz

1. Lipids are:
2. A triglyceride is made from glycerol and:
3. Which is unsaturated?
4. The cell membrane is mainly made of:
5. Making soap from fat with an alkali is:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What are lipids in simple words?

Lipids are the fats, oils, waxes and steroids in living things. They do not mix with water.

What is the difference between fats and oils?

Both are triglycerides. Fats are solid at room temperature (more saturated); oils are liquid (more unsaturated).

Are all fats bad for health?

No. The body needs fats for energy, membranes and vitamins. Eating too much saturated and trans fat is linked to heart disease, so balance matters.

Where this is taught

PolandSzkoła podstawowa, klasa VIIIBiologically important substances
Spain2º BachilleratoBiomolecules
Ukraine10 класOxygen-containing organic compounds

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