Lipids

AS · 10 min

Lipids are a varied group of molecules defined by one shared property: they are insoluble in water but dissolve in organic solvents such as ethanol. The two lipids on the syllabus are triglycerides (fats and oils), which store energy and insulate, and phospholipids, which form every cell membrane. As with carbohydrates, the exam focus is on relating molecular structure to function, and on the ester bond formed by condensation.

Why lipids do not dissolve in water

Water molecules are polar: they have a slightly negative oxygen and slightly positive hydrogens, and they attract other polar or charged molecules. Lipids are made mostly of long hydrocarbon chains (chains of carbon and hydrogen), in which the electrons are shared evenly. These chains are non-polar, so they cannot form hydrogen bonds with water. Water molecules attract each other more strongly than they attract the hydrocarbon chains, so the lipid is pushed out of solution and clumps together. A molecule that behaves like this is hydrophobic ("water-hating").

Definition

Hydrophilic molecules or groups are polar or charged and interact with (dissolve in) water. Hydrophobic molecules or groups are non-polar and do not interact with water.

Building blocks: glycerol and fatty acids

Glycerol is a small molecule with three carbon atoms, each carrying a hydroxyl (–OH) group: $\text{CH}_2\text{OH–CHOH–CH}_2\text{OH}$ .

A fatty acid has a carboxyl group ( $-\text{COOH}$ ) at one end, attached to a long hydrocarbon chain (often 14 to 22 carbons, most commonly 16 or 18). The general formula can be written $\text{R–COOH}$ , where R is the hydrocarbon chain.

Saturated and unsaturated fatty acids

A saturated fatty acid has no carbon–carbon double bonds in its hydrocarbon chain. Every carbon carries as many hydrogen atoms as possible: it is "saturated" with hydrogen. The chain is straight.
An unsaturated fatty acid has one or more C=C double bonds. A monounsaturated fatty acid has one; a polyunsaturated fatty acid has two or more. Each double bond (in its usual natural form) puts a kink in the chain.

Key result

	Saturated	Unsaturated
C=C double bonds	none	one (mono) or more (poly)
Shape of chain	straight	kinked at each double bond
Packing of molecules	close together	cannot pack closely
Melting point	higher	lower
State at room temperature (as triglycerides)	usually solid (fats)	usually liquid (oils)
Typical source	animals	plants and fish

The kinks stop unsaturated chains from lying neatly side by side, so the attractions between molecules are weaker and less energy is needed to separate them. That is why olive oil is liquid and butter is solid.

Triglycerides

A triglyceride is made from one glycerol and three fatty acids. Each fatty acid reacts with one of glycerol's –OH groups in a condensation reaction: the –OH of glycerol and the –OH of the fatty acid's carboxyl group combine, a molecule of water is removed, and an ester bond forms.

Key result

\text{glycerol} + 3\ \text{fatty acids} \xrightarrow{\text{condensation}} \text{triglyceride} + 3\,\text{H}_2\text{O}

An ester bond is the $-\text{COO}-$ link between glycerol and a fatty acid. Three ester bonds form in each triglyceride, releasing three molecules of water. Hydrolysis of the ester bonds (by adding water, catalysed in digestion by lipase) reverses the reaction.

The three fatty acids in one triglyceride may be the same or different, saturated or unsaturated. Glycerol's –OH groups have been used up in the ester bonds, and the remaining molecule is mostly hydrocarbon. Therefore:

Definition

Triglycerides are non-polar, hydrophobic molecules. They are insoluble in water.

A triglyceride is a macromolecule but not a polymer: it is not made of a chain of repeating, similar subunits.

Drawing the formation of an ester bond

\text{R–COOH} + \text{HO–CH}_2\text{–(glycerol)} \longrightarrow \text{R–COO–CH}_2\text{–(glycerol)} + \text{H}_2\text{O}

When drawing, show the –OH from the fatty acid's carboxyl group and the –H from glycerol's hydroxyl group leaving as water, and label the $-\text{C(=O)–O}-$ link "ester bond".

Key result

Function	Structural reason
Energy store (in adipose tissue of animals; in seeds such as sunflower and castor oil seeds)	many C–H bonds in the long hydrocarbon chains; oxidation in respiration releases about twice as much energy per gram as carbohydrate (about 37–39 kJ g⁻¹ against about 16–17 kJ g⁻¹)
Store does not affect water potential	triglycerides are insoluble (hydrophobic), so they have no osmotic effect and are stored compactly as droplets without water
Source of metabolic water	oxidation in respiration produces a lot of water (important for desert animals such as camels and kangaroo rats, and for embryos in eggs)
Thermal insulation	fat (adipose tissue under the skin, blubber in whales and seals) conducts heat poorly
Buoyancy	fat is less dense than water, helping aquatic mammals float
Protection	fat around organs such as kidneys cushions them against physical damage
Electrical insulation	lipids in the myelin sheath around some nerve axons insulate them, speeding up nerve impulses
Waterproofing	hydrophobic oils and waxes (related lipids) on fur, feathers and leaf cuticles repel water

The high energy content per gram makes lipids ideal where mass matters: in animals that move about, in migrating birds, and in small seeds that must be light enough to be dispersed. Plants store most of their energy as starch, which is less dense in energy but can be mobilised more quickly.

Mass of a triglyceride (routine)

Palmitic acid ( $M_r = 256$ ) is a saturated fatty acid. Glycerol has $M_r = 92$ and water $M_r = 18$ . Calculate the $M_r$ of a triglyceride containing three palmitic acid molecules.

Solution

Three ester bonds form, each releasing one water molecule:

M_r = 92 + 3 \times 256 - 3 \times 18 = 92 + 768 - 54 = 806

Energy storage calculation

A person stores 10 kg of triglyceride. Fat releases about 39 kJ g⁻¹ when respired and glycogen about 17 kJ g⁻¹. Calculate the mass of glycogen that would store the same energy, and explain why animals store most of their long-term energy reserve as fat.

Solution

Energy in the fat $= 10\,000\ \text{g} \times 39\ \text{kJ g}^{-1} = 390\,000\ \text{kJ}$ .

Mass of glycogen $= 390\,000 \div 17 = 22\,900\ \text{g} \approx 23\ \text{kg}$ (2.3 times the mass).

In reality the difference is even bigger, because glycogen is stored with water bound to it, while fat is stored with almost none. Fat stores far more energy per unit mass, so an animal carrying its reserves as fat is much lighter, which matters for movement. The fat is also insoluble, so it has no osmotic effect.

Phospholipids

A phospholipid is like a triglyceride in which one of the three fatty acids has been replaced by a phosphate group (often linked to another small polar group, such as choline).

Key result

Structure of a phospholipid:

glycerol, joined by ester bonds to
two fatty acids: the hydrophobic (non-polar) fatty acid tails, and
a phosphate group: the hydrophilic (polar) phosphate head, which carries a negative charge.

The molecule is therefore partly hydrophilic and partly hydrophobic (amphipathic).

In water, phospholipids arrange themselves so that the hydrophilic heads face the water and the hydrophobic tails are hidden from it. They form a single layer on the surface of water, or spherical micelles, or, most importantly, a phospholipid bilayer: two layers with the tails pointing inwards towards each other and the heads facing the watery cytoplasm on one side and tissue fluid on the other. The bilayer is the basis of every cell membrane (see The fluid mosaic model).

Feature	Triglyceride	Phospholipid
Glycerol	1	1
Fatty acids	3	2
Phosphate group	none	1 (often with another polar group)
Polarity	entirely non-polar, hydrophobic	hydrophilic head and hydrophobic tails
Behaviour in water	forms droplets	forms bilayers or micelles
Main function	energy storage, insulation	membrane structure

The hydrophobic core of the bilayer is what makes membranes a barrier to water-soluble substances, such as ions and glucose: these are repelled by the non-polar tails and need protein channels or carriers to cross. Small non-polar molecules, such as oxygen and carbon dioxide, and lipid-soluble molecules, such as steroid hormones, dissolve in the core and diffuse straight through.

Relate structure to function (4 marks)

Explain how the structure of phospholipids allows them to form cell membranes.

Solution

A phospholipid has a hydrophilic (polar) phosphate head and two hydrophobic (non-polar) fatty acid tails.
In water, the heads interact with water while the tails are repelled by water.
So the molecules form a bilayer, with heads facing the aqueous cytoplasm and tissue fluid and tails facing inwards.
The hydrophobic core is a barrier to polar molecules and ions, making the membrane partially permeable.
The tails can move relative to each other, making the membrane fluid (and unsaturated tails increase fluidity).

Exam-hard: interpreting data on melting point

The table shows the melting points of three fatty acids, each with 18 carbon atoms.

Fatty acid	C=C double bonds	Melting point / °C
stearic	0	70
oleic	1	13
linoleic	2	−5

(a) Describe the relationship shown. (b) Explain the relationship. (c) Suggest why the membranes of fish living in cold seas contain a high proportion of unsaturated fatty acids.

Solution

(a) As the number of C=C double bonds increases, the melting point decreases (from 70 °C to −5 °C). Note the biggest decrease is from 0 to 1 double bond.

(b) Saturated chains are straight and pack closely, with many attractions between neighbouring chains, so more energy is needed to separate them. Each C=C double bond produces a kink, preventing close packing, so attractions are weaker and the melting point is lower.

(c) At low temperatures saturated fatty acids would make membranes rigid (solid-like). Unsaturated fatty acids with kinked tails keep the phospholipid bilayer fluid at low temperature, so membrane proteins can move and the membrane can function (for example, for transport and vesicle fusion).

Watch out

Triglycerides are joined by ester bonds, not glycosidic or peptide bonds.
"Saturated means it has single bonds" is incomplete: the definition is no C=C double bonds in the hydrocarbon chain.
Fat releases more energy per gram than carbohydrate; it does not "contain more energy" in absolute terms. Always state "per gram" or "per unit mass".
Phospholipid heads are hydrophilic, tails hydrophobic. Reversing these is a frequent slip.

Exam tip

Syllabus phrasing to use: "triglycerides are non-polar hydrophobic molecules"; phospholipids have "hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails".
To describe triglyceride structure, give: one glycerol, three fatty acids, joined by three ester bonds formed in condensation, three water molecules removed; fatty acids may be saturated (no C=C) or unsaturated (one or more C=C).
For functions, link each to a property: energy store because of many C–H bonds (more energy per gram than carbohydrate) and insolubility (no osmotic effect).
The emulsion test is the test for lipids (see Testing for biological molecules).

Summary

Lipids are insoluble in water and soluble in organic solvents because of their non-polar hydrocarbon chains.
Fatty acids: carboxyl group plus hydrocarbon chain; saturated (no C=C, straight) or unsaturated (one or more C=C, kinked).
Triglyceride: glycerol + 3 fatty acids, joined by 3 ester bonds formed by condensation, releasing 3 water molecules.
Triglycerides are non-polar and hydrophobic.
Functions: energy store (about twice the energy per gram of carbohydrate, insoluble), metabolic water, thermal insulation, buoyancy, protection, electrical insulation.
Phospholipid: glycerol + 2 fatty acids + phosphate; hydrophilic phosphate head and hydrophobic fatty acid tails; forms bilayers, the basis of membranes.

Practice questions

Question

Name the bond that joins a fatty acid to glycerol, and the reaction that forms it.
Distinguish between a saturated and an unsaturated fatty acid.
How many water molecules are released when one triglyceride is formed?
State two differences between the structure of a triglyceride and a phospholipid.
Explain why triglycerides are insoluble in water.
Give three functions of triglycerides in animals and link each to a property of triglycerides.
A triglyceride contains three oleic acid molecules ( $M_r = 282$ ). Calculate its $M_r$ ( $M_r$ glycerol $= 92$ , water $= 18$ ).
Explain why the phospholipids in a membrane form a bilayer. (3 marks)
Seeds such as sunflower seeds store energy mainly as oil rather than starch. Suggest two advantages to the plant. (2 marks)
Camels store fat in their hump. Explain how this helps them survive in the desert, considering both energy and water. (4 marks)

Answers

Ester bond; condensation (water removed).
Saturated: no C=C double bonds in the hydrocarbon chain (straight chain). Unsaturated: one or more C=C double bonds (kinks in chain).
Three.
Triglyceride has three fatty acids, phospholipid two; phospholipid has a phosphate group (hydrophilic head), triglyceride has none; triglyceride entirely non-polar, phospholipid has a polar head and non-polar tails.
They consist mainly of non-polar hydrocarbon chains; the polar –OH groups of glycerol are used in ester bonds; non-polar molecules cannot form hydrogen bonds with water, so they are hydrophobic.
Any three, each with a reason: energy store, many C–H bonds release about twice the energy per gram of carbohydrate; insulation, fat conducts heat poorly; buoyancy, less dense than water; protection of organs, cushioning; source of metabolic water when oxidised; electrical insulation in myelin.
$92 + 3 \times 282 - 3 \times 18 = 92 + 846 - 54 = 884$ .
Phospholipids have hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails; in an aqueous environment (both inside and outside the cell) heads face the water; tails face inwards, away from water, forming two layers.
More energy stored per unit mass, so seeds are lighter (easier to disperse by wind or animals); oil is insoluble, so no osmotic effect; compact storage; releases metabolic water on germination.
Fat releases a large amount of energy per gram when respired, so it is a concentrated energy reserve for when food is scarce; oxidation of fat in respiration produces a large amount of metabolic water, helping when water is scarce; storing fat in one place (the hump) rather than all over the body reduces insulation elsewhere, helping heat loss across the rest of the body.

Why lipids do not dissolve in water

Building blocks: glycerol and fatty acids

Saturated and unsaturated fatty acids

Triglycerides

Drawing the formation of an ester bond

Functions of triglycerides, related to structure

Phospholipids

Practice questions

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