Lipids

In a nutshell

Lipids are a group of biological molecules that are insoluble in water. The two you must know are triglycerides (the cell's main energy store) and phospholipids (the building block of every cell membrane).

This subtopic is about how each one is built from glycerol and fatty acids, how saturated and unsaturated fatty acids differ, and how each molecule's structure gives it the properties it needs for its job.

Assumed knowledge: Monomers and polymers, Carbohydrates.

Core content

Triglycerides: one glycerol and three fatty acids

A triglyceride is formed by the condensation of one molecule of glycerol and three molecules of fatty acid.

Each fatty acid joins to the glycerol by a condensation reaction that forms an ester bond and releases one molecule of water. Three fatty acids join, so three ester bonds form and three molecules of water are released.

Adding water back reverses the process: hydrolysis of a triglyceride breaks the ester bonds and releases the glycerol and three fatty acids.

A triglyceride is not a polymer. Its glycerol and fatty acids are not many repeating identical monomers, so do not call them monomers or the triglyceride a polymer.

Fatty acids: saturated and unsaturated

A fatty acid is a hydrocarbon chain (written R) ending in a carboxyl group, so its general formula is RCOOH. The chain, the R-group, may be saturated or unsaturated.

• A saturated fatty acid has no carbon-to-carbon double bonds. Every carbon is joined to the maximum number of hydrogen atoms. The chains are straight and pack closely, so these lipids tend to be solid at room temperature (animal fats).
• An unsaturated fatty acid has at least one carbon-to-carbon (C=C) double bond in its hydrocarbon chain. Each double bond puts a kink in the chain, so the chains cannot pack closely and these lipids tend to be liquid at room temperature (plant oils).

Still don't get it? · why a C=C double bond changes the whole molecule

Imagine a box of drinking straws. If every straw is dead straight, you can lay them side by side with no gaps and press the bundle down flat and hard, so the bundle behaves like a solid block. Now bend a kink into some of the straws. They no longer lie flat against each other, gaps appear, and the bundle stays loose and floppy.

A fatty acid chain is a straw. A saturated chain is straight, so the chains pack tightly and the fat is solid. A single C=C double bond cannot rotate, so it locks a bend into the chain. Kinked chains cannot pack closely, they stay loose, and the lipid is a liquid oil.

In the exam: the mark is not "it is bendy". The mark is that an unsaturated fatty acid contains at least one carbon-to-carbon double bond (C=C) in its hydrocarbon chain. Say "double bond" alone and you often lose the mark; you must say the double bond is between carbon atoms.

Phospholipids: swap one fatty acid for a phosphate group

A phospholipid has the same layout as a triglyceride except that one of the three fatty acids is replaced by a phosphate-containing group. So a phospholipid is one glycerol joined to two fatty acids and one phosphate group.

This gives the molecule two ends with opposite behaviour towards water:

• The phosphate head is hydrophilic (it is polar, so it interacts with, or is attracted to, water).
• The two fatty acid tails are hydrophobic (they are non-polar, so they repel, or are pushed away from, water).

A molecule with both a hydrophilic and a hydrophobic region like this is described as amphipathic.

Because of this split personality, phospholipids arrange themselves in water so the heads face the water and the tails hide away from it. In a cell, with water on both sides, they form a bilayer: two rows of phospholipids, tails pointing inwards to each other and heads facing the water outside and inside the cell. This bilayer is the basic structure of every cell membrane.

Still don't get it? · why phospholipids form a bilayer by themselves

Think of a swimming pool full of tiny people, each wearing a waterproof coat (the head) but with bare legs that hate getting wet (the tails). Nobody organises them. Each one just turns so its coat faces the water and tucks its legs away from it. With water above and below, the quickest way for everyone to satisfy that at once is to line up in two rows, coats out on both surfaces and all the bare legs tucked together in the middle, dry.

A phospholipid is one of those people. The hydrophilic phosphate head "wants" to be in water; the hydrophobic fatty acid tails "want" to avoid it. In water on both sides, that forces the molecules into two layers, heads facing the water, tails pointing inwards away from it.

In the exam: the creditable idea is that a phospholipid has a hydrophilic (phosphate) head and hydrophobic (fatty acid) tails, and that this is why it forms a bilayer in water, giving the structure of the cell membrane. The two regions are the point; do not just say "phospholipids make membranes".

Properties related to structure

The whole reason AQA asks about both molecules is to see whether you can link structure to property. Do not just describe the molecule; say what the structure lets it do.

Reading the table as structure to property:

• A triglyceride has a high proportion of energy-rich carbon-hydrogen bonds in its fatty acid tails, so when respired it releases a large amount of energy per gram, more than the same mass of carbohydrate. This makes it an excellent energy store.
• A triglyceride is hydrophobic and insoluble in water, so it can be stored in cells without affecting the cell's water potential or causing water to move in by osmosis.
• A phospholipid is amphipathic, so in water its molecules form a bilayer with hydrophilic heads facing the water and hydrophobic tails facing inwards. That bilayer is exactly what a cell membrane needs to separate the watery inside of a cell from the watery outside.

The bar chart makes the storage point concrete: a gram of lipid releases roughly twice as much energy as a gram of carbohydrate, so a lipid store packs more energy into less mass. (Values are approximate and vary a little between sources.)

Required practical: the emulsion test for lipids

The emulsion test shows whether a lipid is present in a sample. The order of steps matters and the result must be described precisely.

1. Take the sample and mix (shake) it with ethanol so any lipid dissolves.
2. Then pour the solution into an equal volume of water.
3. A white (milky) emulsion forms if a lipid is present. If no lipid is present, the mixture stays clear.

Key points that examiners insist on:

• Add ethanol first, then water. Adding water before ethanol does not work.
• Do not heat the mixture.
• The positive result is a white / milky emulsion, not a "precipitate", and "cloudy" on its own without the colour is not enough.
• This is a qualitative test: it tells you a lipid is present, not how much.

Worked examples

Model answer 1: "Explain how the structure of a triglyceride makes it a good energy storage molecule." (3 marks)

1. A triglyceride is made of glycerol and three fatty acids, whose long hydrocarbon tails contain a high proportion of carbon-hydrogen bonds.
2. When the triglyceride is respired, these bonds release a large amount of energy, more energy per gram than carbohydrate, so a lot of energy is stored in a small mass.
3. Triglycerides are hydrophobic and insoluble in water, so they can be stored without affecting the cell's water potential or drawing in water by osmosis.

Each numbered point is one linked idea, and the third point (the insolubility argument) is the one students most often leave out.

Model answer 2: "Compare and contrast the structure of a triglyceride and a phospholipid." (4 marks)

For a "compare and contrast" question every point must be comparative: state the similarity or difference in one sentence. The examiner will not join two separate descriptions together for you.

1. Both contain glycerol joined to fatty acids by ester bonds. (similarity)
2. Both are insoluble in water. (similarity)
3. A triglyceride has three fatty acids, whereas a phospholipid has two fatty acids and a phosphate group. (difference)
4. A triglyceride is entirely hydrophobic, whereas a phospholipid has a hydrophilic head and hydrophobic tails. (difference)

Common exam mistakes

• Writing "double bond" for an unsaturated fatty acid without saying it is a carbon-to-carbon (C=C) double bond in the hydrocarbon chain. The location and the "between carbons" wording carry the mark.
• Getting saturated and unsaturated the wrong way round: saturated has no C=C, unsaturated has at least one C=C.
• Calling the bonds in a fatty acid chain hydrogen bonds. The chain has carbon-to-carbon bonds; the bond joining a fatty acid to glycerol is an ester bond.
• Writing "phosphorus" or just "P" for the phospholipid head. The mark needs phosphate (a phosphate-containing group).
• Saying a phospholipid is "a triglyceride with a phosphate added". One fatty acid is replaced by the phosphate group, so a phospholipid has two fatty acids, not three.
• Saying a triglyceride contains three glycerol molecules. It is one glycerol and three fatty acids. Also do not confuse glycerol with glycogen.
• In the emulsion test: adding water before ethanol, heating the mixture, calling the result a "precipitate", or failing to say the emulsion is white.
• Reaching for the wrong biochemical test (Benedict's, biuret or iodine) instead of the emulsion test for lipids.
• Writing that respiring lipids "makes" or "creates" energy. Energy is released, not made.
• In a "compare and contrast" answer, describing a triglyceride and a phospholipid separately instead of writing comparative statements. Unlinked descriptions are not credited.

Key definitions

• Triglyceride: a lipid formed by the condensation of one molecule of glycerol and three molecules of fatty acid.
• Condensation reaction: a reaction that joins two molecules together with the formation of a chemical bond and the elimination of a molecule of water.
• Ester bond: the bond formed between glycerol and a fatty acid in a condensation reaction.
• Hydrolysis: the breaking of a chemical bond between two molecules involving the use of a water molecule.
• Saturated fatty acid: a fatty acid whose hydrocarbon chain contains no carbon-to-carbon double bonds (every carbon is bonded to the maximum number of hydrogen atoms).
• Unsaturated fatty acid: a fatty acid whose hydrocarbon chain contains at least one carbon-to-carbon (C=C) double bond.
• Phospholipid: a lipid in which one of the three fatty acids of a triglyceride is replaced by a phosphate-containing group.
• Hydrophilic: attracted to, or interacting with, water (polar).
• Hydrophobic: repelled by, or not interacting with, water (non-polar).
• Amphipathic: having both a hydrophilic region and a hydrophobic region in the same molecule.
• Emulsion test (positive result): a white (milky) emulsion indicates the presence of a lipid.

Specification

• I can state that triglycerides and phospholipids are two groups of lipid.
• I can describe how a triglyceride is formed by the condensation of one molecule of glycerol and three molecules of fatty acid, forming three ester bonds and releasing three molecules of water.
• I can state that a condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond, and that hydrolysis reverses this.
• I can recognise, from diagrams, saturated and unsaturated fatty acids (unsaturated has one or more C=C bonds in the hydrocarbon chain).
• I can describe how a phospholipid differs from a triglyceride (one fatty acid is replaced by a phosphate-containing group).
• I can explain the different properties of triglycerides and phospholipids in terms of their different structures.
• I can carry out and interpret the emulsion test for lipids.

Related notes

• Monomers and polymers
• Carbohydrates
• Proteins
• Transport across cell membranes