Lipids
AQA
A-Level
Lipids contain carbon, hydrogen and oxygen, but with proportionally less oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents like ethanol. The specification names two groups: triglycerides and phospholipids. Triglycerides consist of one glycerol joined to three fatty acids. They are non-polar, hydrophobic molecules, making them ideal energy storage lipids in adipose tissue. Phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group. This structure makes them amphipathic: they have a hydrophilic phosphate head and hydrophobic fatty acid tails. This property is essential for forming the bilayer of cell membranes.
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Objectives
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Exam Tips
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Pitfalls
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Key Terms
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Mark Points
Subtopics in this area
Triglycerides and phospholipids are two groups of lipid.
Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid.
A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond.
The R-group of a fatty acid may be saturated or unsaturated.
In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group.
The different properties of triglycerides and phospholipids related to their different structures.
The emulsion test for lipids.
Students should be able to: recognise, from diagrams, saturated and unsaturated fatty acids explain the different properties of triglycerides and phospholipids.
Lipids Revision Guide
Learning Objectives
What you need to know and understand
- Name the two groups of lipid in the specification and state the components of each.
- Explain why lipids are not classified as polymers even though they are large organic molecules.
- State the roles of triglycerides and phospholipids in a cell and link each role to where the molecule is found.
- Describe the formation of a triglyceride, stating the number of condensation reactions, ester bonds and water molecules involved.
- Draw or label a triglyceride to show glycerol, the three fatty acids and the position of the ester bonds.
- Explain how a triglyceride is broken down again, naming the enzyme, the reaction and the products.
- Name the bond formed between glycerol and a fatty acid and identify the two functional groups that react.
- Describe how an ester bond is formed and broken, stating what happens to a water molecule in each case.
- Explain why the R group of a fatty acid is not involved in the ester bond, and what it determines instead.
- State the difference between a saturated and an unsaturated fatty acid in terms of carbon-carbon double bonds.
- Explain why a triglyceride containing unsaturated fatty acids is liquid at room temperature while a saturated one is solid.
- Suggest how the proportion of unsaturated fatty acid tails affects the fluidity of a cell-surface membrane.
- Describe how the structure of a phospholipid differs from that of a triglyceride.
- Explain why phospholipids arrange themselves into a bilayer when placed in water.
- Suggest how the hydrophobic and hydrophilic regions of a protein determine its position in a phospholipid bilayer.
- Explain how the three hydrophobic tails of a triglyceride make it suitable as an energy store in a cell.
- Explain why a phospholipid, but not a triglyceride, can form a bilayer in an aqueous environment.
- Compare and contrast triglycerides and phospholipids in terms of structure and the role each plays in a cell.
- Describe the emulsion test for lipids, giving the reagents in the correct order and the positive result.
- Explain why a cloudy white emulsion forms when the ethanol solution is added to water.
- Suggest a suitable control for the emulsion test and explain why the apparatus must be clean and dry.
- Identify from a diagram whether a fatty acid is saturated or unsaturated and justify the choice.
- Explain the properties of a triglyceride and a phospholipid by reference to their different structures.
- Suggest how the hydrophobic and hydrophilic parts of a phospholipid determine its arrangement in a bilayer.
Marking Points
Key points examiners look for in your answers
- State that triglycerides consist of one glycerol joined to three fatty acids.
- State that phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group.
- Explain that triglycerides are non-polar and hydrophobic, acting as energy storage molecules.
- Explain that phospholipids are amphipathic, allowing them to form the bilayer of cell membranes.
- Identify that lipids contain carbon, hydrogen and oxygen, with proportionally less oxygen than carbohydrates.
- one mark for one glycerol joined to three fatty acids
- one mark for stating that the bonds are formed by condensation reactions
- one mark for naming the ester bond and stating that three are formed
- one mark for three molecules of water being released, one per ester bond
- one mark for stating that lipase hydrolyses the ester bonds, using water, to release glycerol and fatty acids
- one mark for identifying the reaction between glycerol and a fatty acid as a condensation reaction
- one mark for naming the bond formed as an ester bond
- one mark for stating that one molecule of water is released per ester bond formed
- one mark for identifying the groups involved as the carboxyl group of the fatty acid and a hydroxyl group of glycerol
- one mark for stating that the ester bond is broken by hydrolysis, catalysed by lipase
- one mark for saturated meaning no carbon-carbon double bonds and the maximum number of hydrogen atoms
- one mark for unsaturated meaning one or more carbon-carbon double bonds present
- one mark for the double bond causing a kink or bend in the hydrocarbon chain
- one mark for kinked chains packing less closely, so the melting point is lower and the lipid is liquid at room temperature
- one mark for linking unsaturated fatty acid tails to greater membrane fluidity
- one mark for one fatty acid of the triglyceride being replaced by a phosphate-containing group
- one mark for the phosphate head being polar or charged, and therefore hydrophilic
- one mark for the two fatty acid tails being non-polar and therefore hydrophobic
- one mark for the molecules forming a bilayer with hydrophilic heads facing the water and hydrophobic tails facing inwards
- one mark for the hydrophobic core preventing water-soluble, polar or charged substances from crossing the membrane
- One mark for the triglyceride having three fatty acid tails and no polar group, so it is hydrophobic and insoluble in water.
- One mark for the triglyceride releasing about twice as much energy per gram as carbohydrate, because of the high proportion of carbon-hydrogen bonds.
- One mark for insolubility meaning no effect on water potential, so stored fat does not cause water to enter the cell by osmosis.
- One mark for the phospholipid being amphipathic, with a hydrophilic phosphate-containing head and hydrophobic tails.
- One mark for the phospholipid forming a bilayer and so acting as the basis of membranes rather than as a store.
- One mark for explaining that the hydrophobic tails face inwards and the hydrophilic heads face outwards in a bilayer in an aqueous environment.
- one mark for adding ethanol to the sample and shaking to dissolve any lipid present
- one mark for then adding or pouring the mixture into an equal volume of water
- one mark for a positive result described as a cloudy, milky-white emulsion rather than simply 'it turns white'
- one mark for explaining that the lipid is insoluble in water so comes out of solution as droplets that scatter light
- one mark for a negative control using distilled water in place of the sample, which remains clear
- One mark for identifying a fatty acid with no carbon-carbon double bonds in the R group as saturated.
- One mark for identifying a double bond or kink in the chain as showing an unsaturated fatty acid.
- One mark for the triglyceride being hydrophobic and insoluble because it has three non-polar hydrocarbon tails.
- One mark for the phospholipid having a hydrophilic phosphate-containing head and hydrophobic tails, so it forms a bilayer.
- One mark for explaining that in a bilayer the hydrophobic tails face inwards and the hydrophilic heads face outwards towards water.
Examiner Tips
Expert advice for maximising your marks
- 💡Fix the two structures by counting fatty acids: three means a triglyceride (storage), while two plus a phosphate-containing group means a phospholipid (membrane).
- 💡Never use the words monomer or polymer when describing lipids, as examiners frequently penalise this error.
- 💡Draw the glycerol as a vertical backbone with three attachment points; it makes the count of bonds and water molecules obvious.
- 💡Write 'three condensation reactions' explicitly rather than 'condensation', because the number is often the mark.
- 💡Mention that the three fatty acids may differ - it sets up the saturated and unsaturated question that usually follows.
- 💡Name the two functional groups involved, carboxyl and hydroxyl, rather than describing them as 'the ends' of the molecules.
- 💡Set out condensation and hydrolysis as a pair in your revision notes so you can answer either direction with the same three facts.
- 💡If the question supplies RCOOH, use that notation in your answer to show which part of the fatty acid reacts.
- 💡Look only for C=C double bonds when identifying saturation from a diagram; ignore the C=O in the carboxyl group.
- 💡Always convert the structural point into a consequence - kink, less close packing, lower melting point, liquid at room temperature.
- 💡Carry this idea forward to membranes, where unsaturated tails are the standard answer for maintaining fluidity in the cold.
- 💡Use the terms hydrophilic and hydrophobic explicitly, then say why - polar or charged versus non-polar.
- 💡When explaining how a protein sits in a membrane, mirror the phospholipid logic: hydrophobic R groups face the tails, hydrophilic R groups face the water or line a channel.
- 💡Say 'phosphate-containing group' rather than just 'phosphate' when describing the structure, matching the specification wording.
- 💡Structure the answer as a contrast in each sentence: three tails versus two tails and a phosphate-containing group, storage versus membranes.
- 💡Metabolic water and thermal insulation are creditable extra properties of triglycerides; keep them ready for suggest questions about desert or arctic animals, but present them as possible advantages rather than proven causes of fat storage.
- 💡When explaining insolubility, use the words non-polar and hydrophobic, since 'does not dissolve' alone is rarely enough.
- 💡Write the method as numbered steps in the correct order; sequence marks are commonly lost here.
- 💡Always name the solvent as ethanol, and state that the tube and the sample must be clean and dry.
- 💡Include a control for any practical description - distilled water in place of the sample is the simple, creditable one.
- 💡In recognition questions, circle the double bond on the diagram before writing; it prevents describing the wrong molecule.
- 💡Answer property questions in the form structure, therefore property, therefore use - each link can carry a mark.
- 💡When a diagram shows a phospholipid bilayer, describe the arrangement by referring to the hydrophobic tails facing inwards and the hydrophilic heads facing the aqueous environment.
Common Mistakes
Pitfalls to avoid in your exam answers
- Describing a triglyceride as a polymer of fatty acids; Correction: Lipids are macromolecules but not polymers, as they are not made of repeating monomer units.
- Stating that all lipids contain phosphorus; Correction: Only phospholipids contain a phosphate-containing group; triglycerides contain only C, H, and O.
- Saying lipids are insoluble in all solvents; Correction: Lipids are insoluble in water but soluble in organic solvents such as ethanol.
- saying one molecule of water is released when a triglyceride forms, instead of three
- reversing the components and writing three glycerols with one fatty acid
- describing the bond as a glycosidic or peptide bond
- stating that the fatty acids join to each other rather than each to the glycerol
- claiming a triglyceride is a polymer of three fatty acid monomers
- calling the bond a glycosidic or peptide bond in a lipid question
- saying the ester bond forms between two fatty acids rather than between glycerol and a fatty acid
- stating that the R group of the fatty acid forms the bond with glycerol
- writing that water is added when the ester bond forms
- spelling or describing the carboxyl group as a carbonyl group
- defining unsaturated as having fewer carbon atoms rather than fewer hydrogen atoms
- saying that double bonds break the fatty acid chain rather than bending it
- confusing saturation of a fatty acid with a saturated solution
- stating that saturated fats melt at lower temperatures than unsaturated oils
- answering about health effects of saturated fat when the question asks about structure and properties
- saying the phosphate group replaces the glycerol rather than one fatty acid
- describing the fatty acid tails as hydrophilic and the phosphate head as hydrophobic
- stating that phospholipids form a single layer in a cell-surface membrane
- explaining hydrophobic as 'hates water' without linking it to being non-polar or repelled by water
- forgetting that the heads face the watery cytoplasm on the inside as well as the tissue fluid on the outside
- Stating that triglycerides form the cell-surface membrane bilayer; correction: triglycerides are storage lipids, while phospholipids form bilayers.
- Offering phospholipids as the main energy store of the cell; correction: triglycerides are the main energy store.
- Saying triglycerides are insoluble 'because they are large'; correction: they are insoluble because they are non-polar and hydrophobic.
- Describing a phospholipid as entirely hydrophobic and then trying to explain bilayer formation; correction: phospholipids are amphipathic, with a hydrophilic head and hydrophobic tails.
- Claiming lipids give twice as much energy as carbohydrate because they contain more oxygen; correction: they contain proportionally less oxygen and more hydrogen, and it is the high proportion of carbon-hydrogen bonds that releases more energy on oxidation.
- adding water before ethanol, so the lipid never dissolves and no emulsion forms
- explaining the dry-tube rule as ethanol and water turning cloudy together, when they mix clear - the real problem is diluted ethanol leaving lipid undissolved
- describing the positive result as a colour change rather than the formation of an emulsion
- stating that the degree of cloudiness measures the lipid concentration, when the test is qualitative
- confusing this test with the biuret test and adding copper sulfate
- Counting hydrogen atoms on a skeletal diagram instead of looking for a carbon-carbon double bond; correction: the presence or absence of a C=C in the hydrocarbon chain determines saturation.
- Mistaking the C=O of the carboxyl group for evidence of an unsaturated R group; correction: the carboxyl group is present in all fatty acids, so look for C=C within the hydrocarbon chain.
- Describing structure without giving a consequence, so an explain question is answered as a describe question; correction: link each structural feature to a property or role.
- Saying triglycerides form membranes because they contain fatty acids; correction: triglycerides are storage lipids, while phospholipids form membranes.
- Stating that a phospholipid is hydrophobic overall, then trying to explain why it faces water; correction: phospholipids are amphipathic, with a hydrophilic head and hydrophobic tails.