Biology › Biological molecules, water and inorganic ions › Lipids: triglycerides, phospholipids and the ester bond
Lipids: triglycerides, phospholipids and the ester bond
Two molecules built almost the same way do two completely different jobs. Swap one fatty acid of a triglyceride for a phosphate group and you no longer have a store of energy. You have the molecule every membrane in every cell is built from.
Before this Condensation and hydrolysis in carbohydrates · Hydrogen bonding in water
COMMON MISCONCEPTION
Unsaturated means the fatty acid is shorter, with fewer carbon atoms in it.
What you should be able to do
- Name the components of a triglyceride and the bond that joins them.
- Explain how an ester bond forms, and count the water released.
- Distinguish saturated from unsaturated fatty acids, and say what the double bond does.
- Relate the structure of a triglyceride to its use as an energy store.
- Explain why a phospholipid behaves differently in water from a triglyceride, and carry out the emulsion test.
What makes a lipid a lipid
Lipids are grouped together by how they behave rather than by one shared structure. They are largely hydrocarbon, so they are non-polar: they do not dissolve in water and they do dissolve in organic solvents such as ethanol. Triglycerides, phospholipids, cholesterol and the waxes on a leaf surface are all lipids and only the first two are built the same way.
One point of vocabulary earns marks and is easy to get wrong. A triglyceride is a large molecule, but it is not a polymer. A polymer is many similar monomers joined in a repeating chain, and a triglyceride is three fatty acids and one glycerol, which are not the same unit repeated. Carbohydrates and proteins are polymers; lipids are not.
- Lipid
- A biological molecule that is largely non-polar, insoluble in water and soluble in organic solvents such as ethanol.
- Triglyceride
- One glycerol molecule joined to three fatty acids by three ester bonds.
- Ester bond
- The covalent bond formed by condensation between a hydroxyl group of glycerol and the carboxyl group of a fatty acid.
Glycerol, fatty acids and the ester bond
Every triglyceride is made from the same alcohol, glycerol, which carries three hydroxyl groups. What varies is the three fatty acids attached to it. A fatty acid is a carboxyl group with a hydrocarbon tail behind it, and the tail may be any length and may or may not contain double bonds.
The hydroxyl group of the glycerol reacts with the carboxyl group of the fatty acid. A hydrogen leaves one and a hydroxyl leaves the other, the two go as water, and an ester bond is left. That happens three times, once at each hydroxyl of the glycerol, so building one triglyceride forms three ester bonds and releases three molecules of water.
Counting again, with a different number
A cell hydrolyses 250 triglyceride molecules completely. How many molecules of water are used, and what are the products?
Show the working
Each triglyceride holds three ester bonds, and hydrolysing one bond uses one molecule of water. So 250 × 3 = 750 molecules of water are used.
The products are 250 molecules of glycerol and 750 fatty acid molecules. The glycerol count follows the number of triglycerides; the fatty acid count follows the number of bonds.
The habit worth keeping from the carbohydrate lesson is to count the bonds first and let everything else follow from that.
Saturated and unsaturated: what one double bond does
In a saturated fatty acid every carbon in the tail holds as many hydrogen atoms as it can, and there is no carbon to carbon double bond. The chain is free to rotate about every bond, so it can lie straight. In an unsaturated fatty acid at least one pair of carbons is joined by a double bond, and the chain cannot rotate there. In the cis arrangement, which is the usual one in biological fatty acids, the chain leaves the double bond on the same side it arrived and picks up a permanent kink; the trans arrangement keeps the chain nearly straight, which is why trans fats behave more like saturated ones.
A bent molecule cannot pack against its neighbours as closely as a straight one, so fewer of the attractions between neighbouring chains form and less energy is needed to separate them. That is why the melting point falls: triglycerides made largely from saturated fatty acids are solid at room temperature and are called fats, while those with unsaturated fatty acids are liquid and are called oils. Animal lipids are mostly the first, plant and fish lipids mostly the second.
One double bond in the tail makes a fatty acid monounsaturated; more than one makes it polyunsaturated.
| Saturated | Unsaturated | |
|---|---|---|
| C=C in the tail | None | One or more |
| Shape of the tail | Straight | Kinked at each cis double bond |
| Packing | Close | Less close |
| Melting point | Higher | Lower |
| At room temperature | Solid: a fat | Liquid: an oil |
| Common source | Animals | Plants and fish |
Why a triglyceride makes a good store
A gram of lipid releases roughly twice the energy of a gram of carbohydrate when it is respired, because the tails are long chains of carbon and hydrogen with very little oxygen in them, and it is the hydrogen that ends up driving the electron transport chain. For a migrating bird or a hibernating mammal, mass carried is the constraint that matters, and lipid stores twice as much energy for it.
Triglycerides are also insoluble in water, so a large store in an adipose cell has no effect on the water potential of its cytoplasm. A store of glucose of the same energy content would draw water in by osmosis; this is the same argument that makes starch and glycogen insoluble, one step further along.
Two other uses come out of the same physical properties, and both are worth naming. Adipose tissue under the skin is a poor conductor of heat, which is thermal insulation, and around the kidneys it is a physical protection. Respiring lipid also releases a good deal of water as a product, which matters to a desert animal such as a kangaroo rat that drinks very little.
Phospholipids: one fatty acid swapped for a phosphate group
Replace one of the three fatty acids of a triglyceride with a phosphate group and the molecule stops being a store and becomes the material a membrane is made of. Two fatty acid tails remain, and the phosphate group sits at the other end.
That end behaves entirely differently from the rest of the molecule. The phosphate group carries a negative charge, so it attracts water molecules and is hydrophilic. The two hydrocarbon tails are non-polar, are not attracted to water, and are hydrophobic. One molecule with both properties at opposite ends is what the whole of membrane biology rests on.
No enzyme assembles the bilayer and no covalent bond holds one phospholipid to the next. Each molecule is free to move sideways past its neighbours, which is what makes a membrane fluid. What holds the sheet together is weaker and collective: the hydrophobic effect, because any other arrangement would put more hydrophobic tails in contact with water, plus the van der Waals attractions between tails packed side by side. The membrane unit takes this much further; what belongs here is the molecule and the reason its two ends differ.
- Hydrophilic
- Attracted to water: charged or polar, so it interacts with water molecules.
- Hydrophobic
- Not attracted to water: non-polar, so it is excluded from water rather than dissolving in it.
The emulsion test
The test for a lipid uses the one thing a lipid is defined by: it dissolves in ethanol and not in water. Shake the sample with absolute ethanol so that any lipid dissolves, then pour that solution into an equal volume of water in a clean tube. A milky white emulsion forms and means a lipid is present.
The reason is worth being able to state, because questions ask for it. In ethanol the lipid is in solution. Tipped into water, it is no longer in a solvent it dissolves in, so it comes out as tiny droplets suspended through the water, and those droplets scatter light. The cloudiness is the lipid, finely divided.
TRY IT: Designing the control
A student carries out the emulsion test on a sample of milk and records a white emulsion. Her teacher asks her to repeat it with a control. Describe the control she should use, and state the result she should expect.
Check your answer
The control is the same procedure with distilled water in place of the sample: ethanol shaken with distilled water, then poured into an equal volume of water.
The mixture should stay clear, because there is no lipid to come out of solution. That shows the emulsion in the first tube came from the milk and not from the ethanol or the water.
The point of the control here is not politeness. Ethanol and water mix in all proportions and give no cloudiness on their own, and showing that is what makes the positive result mean anything.
In the exam
- Name the bond. The bond in a triglyceride is an ester bond, formed by condensation; 'a lipid bond' scores nothing.
- Three ester bonds and three molecules of water per triglyceride, every time. Hydrolysing n triglycerides completely uses 3n molecules of water.
- Unsaturated means at least one carbon to carbon double bond. If a question gives you two fatty acids of the same length, it is testing exactly that.
- For the energy store, give two reasons rather than one: about twice the energy per gram, and insoluble so the water potential of the cell is unaffected.
- In the emulsion test, name both liquids and their order: ethanol first to dissolve the lipid, then water. A white emulsion is the positive result, and a control with distilled water is expected in a full answer.
Check yourself
A triglyceride and a phospholipid are made from the same three kinds of component, and only one of them forms a bilayer in water. Explain why, and explain why a triglyceride is the better energy store of the two.
Answer
A phospholipid has two fatty acids and one phosphate group where a triglyceride has three fatty acids, so the two molecules differ at one position only.
The phosphate group is charged and therefore hydrophilic, while the fatty acid tails are non-polar and hydrophobic. A phospholipid therefore has one end attracted to water and one end not, and in water the arrangement that keeps the fewest tails in contact with it is a bilayer, heads out and tails in.
A triglyceride is hydrophobic at every point, so it forms droplets rather than a sheet.
As a store, the triglyceride carries a third hydrocarbon tail rather than a charged group, so it holds more energy per gram, and being wholly insoluble it has no effect on the water potential of the cell storing it.
Questions
Question 14 marks
Describe how you would test a sample of food for the presence of a lipid, including a suitable control.
Mark scheme
- B1 shake the sample with absolute ethanol so that any lipid dissolves
- B1 pour the ethanol into an equal volume of water in a clean tube
- B1 a white, cloudy emulsion shows that a lipid is present, because the lipid comes out of solution as droplets that scatter light
- B1 as a control, repeat the procedure using distilled water in place of the sample; the mixture should remain clear
Question 23 marks
Describe how a triglyceride is formed from its components.
Mark scheme
- B1 one molecule of glycerol reacts with three fatty acid molecules
- B1 a hydroxyl group of the glycerol reacts with the carboxyl group of each fatty acid in a condensation reaction
- B1 three ester bonds are formed and three molecules of water are released
Question 33 marks
Sunflower oil is liquid at room temperature and beef fat is solid at the same temperature. Explain this difference in terms of the fatty acids each contains.
Mark scheme
- B1 sunflower oil contains a high proportion of unsaturated fatty acids, which have one or more carbon to carbon double bonds, whereas beef fat is largely saturated with none
- B1 a double bond puts a permanent bend in the hydrocarbon tail, so the molecules cannot pack closely together
- B1 fewer attractions form between neighbouring molecules, so less energy is needed to separate them and the melting point is lower
Question 43 marks
Before migrating, a bird lays down a store of triglyceride rather than an equivalent store of glycogen. Explain the advantage of storing triglyceride.
Mark scheme
- B1 a triglyceride releases about twice as much energy per gram as a carbohydrate when respired, because its fatty acid tails are hydrocarbon and contain little oxygen
- B1 so the same energy is carried for less mass, which reduces the energy needed for flight
- B1 triglyceride is insoluble, so it has no effect on the water potential of the cell storing it, and respiring it releases water that the bird can use
Question 53 marks
Compare the behaviour of a triglyceride and a phospholipid when each is added to water.
Mark scheme
- B1 both are insoluble in water, because their fatty acid tails are non-polar and hydrophobic
- B1 a triglyceride is hydrophobic at every point, so it forms droplets, whereas a phospholipid has a hydrophilic phosphate head that interacts with water
- B1 so phospholipids arrange themselves with their heads in the water and their tails away from it, forming a bilayer, which triglycerides do not
Question 63 marks
A membrane protein is removed from a cell surface membrane and placed in pure water, where it does not dissolve. Suggest what this indicates about the amino acids on the outside of that protein, and suggest where in the membrane the protein sits.
Mark scheme
- B1 the side chains on the outside of the protein are largely non-polar, so they are hydrophobic and are not attracted to water
- B1 such a surface is compatible with the hydrocarbon tails of the phospholipids rather than with water
- B1 so the protein sits within the hydrophobic core of the bilayer rather than on the surface of the membrane
Question 72 marks
Name the two kinds of molecule from which a triglyceride is formed, and name the bond that joins them.
Mark scheme
- A1 glycerol and fatty acids
- A1 an ester bond
Question 82 marks
State one structural difference between a phospholipid and a triglyceride, and state one property of a phospholipid that results from it.
Mark scheme
- B1 a phospholipid has two fatty acids and a phosphate group, whereas a triglyceride has three fatty acids
- B1 the phosphate group is charged, so the head of a phospholipid is hydrophilic while the tails are hydrophobic
Question 92 marks
A cell hydrolyses 180 molecules of triglyceride completely. Calculate the number of molecules of water used.
Mark scheme
- M1 each triglyceride contains three ester bonds, and hydrolysing one bond uses one molecule of water
- A1 180 × 3 = 540 molecules of water
Worth remembering
- One glycerol plus three fatty acids gives three ester bonds and three molecules of water.
- Saturated means no carbon to carbon double bond, so the tail is straight and packs closely; unsaturated means at least one, so it bends.
- Lipids are macromolecules but not polymers, because the units are not all the same.
- A phospholipid is a triglyceride with one fatty acid replaced by a phosphate group, and that group is what makes the head hydrophilic.
- Emulsion test: ethanol, then water, then a white emulsion.
CHECK YOUR PROGRESS
Rate how confident you are with each objective for this lesson. Ratings are kept in this browser, on this device, and are sent nowhere.
- Name the components of a triglyceride and the bond that joins them.
- Explain how an ester bond forms, and count the water released.
- Distinguish saturated from unsaturated fatty acids, and say what the double bond does.
- Relate the structure of a triglyceride to its use as an energy store.
- Explain why a phospholipid behaves differently in water from a triglyceride, and carry out the emulsion test.
Open the full revision checklist to see every objective in the curriculum in one place.
WORKBOOK
The same questions as the player, on paper with room to work, and a separate book of mark schemes. Free to use; please do not redistribute or sell.