IMAT Biology · Biomolecules and Enzymes

Fatty Acids, Triglycerides, Phospholipids and Steroids

Lipids are non-polar molecules: triglycerides store energy, phospholipids build membranes, and steroids such as cholesterol have a ring structure with no fatty acids at all.

All Biomolecules and Enzymes notes

Why this matters

Lipid questions so far come from the older papers: the formula of a triglyceride (2021), the water involved in breaking up a phospholipid (2013), and which molecules carry carboxyl and hydroxyl groups (2019). They test the chemistry of the ester bond more than lists of facts.

Concept 1 of 3: Fatty acids: saturated and unsaturated

A fatty acid is a long chain of carbon and hydrogen with an acid group at one end. If every carbon in the chain holds as many hydrogens as it can, the chain is saturated and straight, and the molecules pack tightly into a solid fat. A carbon to carbon double bond puts a kink in the chain, the molecules pack loosely, and the fat is a liquid oil.

Definition

A fatty acid is a hydrocarbon chain (usually 14 to 22 carbons) with a carboxyl group $\mathrm{-COOH}$ at one end. Saturated fatty acids have the formula $\mathrm{C}_n\mathrm{H}_{2n}\mathrm{O_2}$ .
Saturated: no C=C double bonds; straight chains; animal fats, solid at room temperature.
Unsaturated: one (monounsaturated) or more (polyunsaturated) C=C double bonds; each double bond means two fewer H atoms; kinked chains; plant oils, liquid at room temperature.
Most natural unsaturated fats are cis. Trans fats (mainly from industrial hydrogenation) have straighter chains and are linked to heart disease.
Glycerol, the other building block of fats, is a three-carbon alcohol with three $\mathrm{-OH}$ groups and no carboxyl group.

Feature	Saturated fatty acid	Unsaturated fatty acid
C=C double bonds	None	One or more
Shape of chain	Straight	Kinked at each cis double bond
State of the fat at room temperature	Usually solid (butter, lard)	Usually liquid (olive oil)
Hydrogen atoms for the same chain length	The maximum possible	Two fewer for each double bond

Practice this conceptself-check · 3 quick reps

Unsaturated means fewer hydrogens, not more

Saturated means saturated with hydrogen: the most H atoms the chain can hold. Each C=C double bond removes two hydrogen atoms. So an unsaturated fatty acid always has fewer hydrogens than the saturated one of the same length.

Concept 2 of 3: Triglycerides: ester bonds and their formula

Each of glycerol's three OH groups reacts with the carboxyl group of one fatty acid. Each reaction is a condensation: it makes an ester bond and releases a water molecule. So a triglyceride is glycerol plus three fatty acids minus three waters, and digesting it with lipase puts three waters back.

Definition

A triglyceride is one glycerol joined to three fatty acids by three ester bonds.
An ester bond forms by condensation between an $\mathrm{-OH}$ group and a $\mathrm{-COOH}$ group: $\mathrm{-COOH} + \mathrm{HO-} \rightarrow \mathrm{-COO-} + \mathrm{H_2O}$ .
Forming one triglyceride releases 3 water molecules; full hydrolysis uses 3.
Triglycerides are the body's main long-term energy store (about twice the energy per gram of carbohydrate or protein, because they carry so many C-H bonds and little oxygen). They also insulate and protect organs. They are insoluble, so they do not affect osmosis.

Formula of a triglyceride

\mathrm{C_3H_8O_3} + 3\,(\text{fatty acid}) - 3\,\mathrm{H_2O}

$\mathrm{C_3H_8O_3}$ glycerol
$3\,\mathrm{H_2O}$ one water released per ester bond

Worked example

Lauric acid is a saturated fatty acid,

\mathrm{C_{12}H_{24}O_2}

. What is the formula of the triglyceride made from glycerol

(\mathrm{C_3H_8O_3})

and three lauric acid molecules?

Practice this conceptself-check · 4 quick reps

Triglycerides are joined by ester bonds, not glycosidic or peptide bonds

Each name belongs to one class: glycosidic bonds in carbohydrates, peptide bonds in proteins, ester bonds in lipids, phosphodiester bonds in nucleic acids. Mixing them up is a favourite distractor.

Concept 3 of 3: Phospholipids and steroids

Swap one fatty acid of a triglyceride for a charged phosphate group and you get a molecule with a water-loving head and two water-fearing tails. In water these molecules turn their tails away from the water and form a double layer: the basis of every cell membrane. Steroids look nothing like this: they are four rings of carbon fused together.

Definition

A phospholipid is glycerol joined to two fatty acids and one phosphate group (often carrying a small extra group such as choline). It is amphipathic: a hydrophilic head and two hydrophobic tails.
In water, phospholipids form a bilayer, tails inwards. This is the basis of the fluid mosaic membrane.
Breaking the simplest phospholipid into glycerol, two fatty acids and phosphate is hydrolysis of its three ester bonds (two to the fatty acids, one to the phosphate): it uses three water molecules and releases none.
Steroids are lipids built from four fused carbon rings. They contain no fatty acids and no glycerol, and are not made by condensation of repeating units.
Cholesterol sits among the phospholipids of animal membranes and regulates fluidity; it is also the starting material for steroid hormones (testosterone, oestrogen, cortisol), bile salts and vitamin D.
Waxes (a long fatty acid joined to a long alcohol) waterproof leaves and insect cuticles.

Lipid	Built from	Key property	Main role
Triglyceride	Glycerol + 3 fatty acids	Non-polar, insoluble	Energy store, insulation
Phospholipid	Glycerol + 2 fatty acids + phosphate	Amphipathic (polar head, non-polar tails)	Membrane bilayer
Cholesterol (steroid)	Four fused carbon rings	Mostly non-polar, one OH group	Membrane fluidity; makes steroid hormones
Wax	Fatty acid + long-chain alcohol	Very water-repellent	Waterproofing of leaves and cuticles

Practice this conceptself-check · 3 quick reps

Cholesterol contains no peptide bonds and no fatty acids

Cholesterol is a steroid lipid made of fused rings. It is not a protein, so it has no peptide bonds, and it is not a fat, so it has no fatty acids or ester bonds in its ring frame. It is typical of animal membranes; plant membranes use related sterols instead.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Triglycerides: ester bonds and their formula
Formula of a triglyceride
$\mathrm{C_3H_8O_3} + 3\,(\text{fatty acid}) - 3\,\mathrm{H_2O}$

Reference tables (2)

Fatty acids: saturated and unsaturated4 rows

Feature	Saturated fatty acid	Unsaturated fatty acid
C=C double bonds	None	One or more
Shape of chain	Straight	Kinked at each cis double bond
State of the fat at room temperature	Usually solid (butter, lard)	Usually liquid (olive oil)
Hydrogen atoms for the same chain length	The maximum possible	Two fewer for each double bond

Phospholipids and steroids4 rows

Lipid	Built from	Key property	Main role
Triglyceride	Glycerol + 3 fatty acids	Non-polar, insoluble	Energy store, insulation
Phospholipid	Glycerol + 2 fatty acids + phosphate	Amphipathic (polar head, non-polar tails)	Membrane bilayer
Cholesterol (steroid)	Four fused carbon rings	Mostly non-polar, one OH group	Membrane fluidity; makes steroid hormones
Wax	Fatty acid + long-chain alcohol	Very water-repellent	Waterproofing of leaves and cuticles

Watch out for (3)

Unsaturated means fewer hydrogens, not more
→ Fatty acids: saturated and unsaturated
Triglycerides are joined by ester bonds, not glycosidic or peptide bonds
→ Triglycerides: ester bonds and their formula
Cholesterol contains no peptide bonds and no fatty acids
→ Phospholipids and steroids

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