The shape of the lesson
By the end of the lesson, learners can
- Glycerol plus three fatty acids: Step through the saturated fatty acid, the unsaturated fatty acid, the triglyceride, the phospholipid and the bilayer.
- Structure to function: Every role of a triglyceride traces back to two facts: it is packed with C-H bonds, and it does not mix with water.
- Phospholipids: heads and tails: Return to the simulator and choose the phospholipid, then the bilayer.
- Ester bond: COOH of fatty acid + OH of glycerol, minus H2O.
- Saturated: no C=C, straight. Unsaturated: C=C, kinked.
- Triglyceride: non-polar, hydrophobic, energy-dense store.
- Phospholipid: hydrophilic phosphate head, hydrophobic tails.
One picture every idea returns to
A triglyceride is one glycerol joined to three fatty acids by three ester bonds, each formed by condensation between an OH of glycerol and the COOH of a fatty acid. Triglycerides are non-polar and hydrophobic. Saturated fatty acids have no C=C double bonds and straight tails; unsaturated ones have at least one C=C, which kinks the tail. Their many C-H bonds make triglycerides an energy-dense, insoluble store. In a phospholipid one fatty acid is replaced by a phosphate group, giving a hydrophilic (polar) head and two hydrophobic (non-polar) tails.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Fats, oils and two-faced molecules.. Triglycerides store twice as much energy per gram as carbohydrate and repel water completely. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Glycerol plus three fatty acids | Step through the saturated fatty acid, the unsaturated fatty acid, the triglyceride, the phospholipid and the bilayer. Project tab 4 of the lab (Biological molecules lab, lipids) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Structure to function | Every role of a triglyceride traces back to two facts: it is packed with C-H bonds, and it does not mix with water. | Pairs |
| 21 to 30 min | Teach and check: Phospholipids: heads and tails | Return to the simulator and choose the phospholipid, then the bilayer. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Phospholipids: heads and tails, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A triglyceride is not a polymer: glycerol and fatty acids are different molecules, not a chain of repeating monomers. | And do not call the phospholipid head "hydrophobic because it contains phosphorus": it is hydrophilic because the phosphate group is charged. |
Lipids: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the molecular structure of a triglyceride.
Explain the difference between a saturated and an unsaturated fatty acid.
Relate the structure of triglycerides to their roles in living organisms.
Describe the structure of a phospholipid and explain why phospholipids form a bilayer in water.
Glycerol has Mr 92 and palmitic acid has Mr 256. Calculate the Mr of a triglyceride made from glycerol and three palmitic acid molecules. (Mr of water = 18)
Triglyceride releases about 37 kJ g−1 and glycogen about 17 kJ g−1. A migrating bird stores 50 g of triglyceride. Calculate the mass of glycogen that would store the same energy, and suggest why birds store triglyceride.
Answer key and marking notes
- One molecule of glycerol joined to three fatty acids.
- Each fatty acid is joined by an ester bond, formed by condensation between the COOH of the fatty acid and an OH of glycerol.
- The fatty acids have long non-polar hydrocarbon tails, which may be saturated or unsaturated.
- A saturated fatty acid has no C=C double bonds in its hydrocarbon chain (it holds the maximum number of hydrogen atoms).
- An unsaturated fatty acid has one or more C=C double bonds, which put kinks in the chain.
- Many C-H bonds, so a large amount of energy released per gram: an efficient energy store.
- Non-polar and insoluble, so they do not affect water potential and can be stored in large amounts.
- Produce metabolic water when oxidised.
- Low thermal conductivity and low density: insulation under the skin and buoyancy in aquatic animals.
- Glycerol joined to two fatty acids and a phosphate group.
- The phosphate head is polar (hydrophilic); the fatty acid tails are non-polar (hydrophobic).
- In water the heads face the water on both sides and the tails point inwards away from water, forming a bilayer.
- Reactants: 92 + 3 × 256 = 860.
- Three water molecules are lost: 860 − 3 × 18 = 806.
- Energy = 50 × 37 = 1850 kJ.
- Mass of glycogen = 1850 ÷ 17 = 109 g (about twice as heavy).
- Triglyceride stores the same energy in less than half the mass, which matters for flight.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which elements are present in a triglyceride?
- Acarbon, hydrogen and oxygen only
- Bcarbon, hydrogen, oxygen and phosphorus
- Ccarbon, hydrogen, oxygen and nitrogen
- Dcarbon, hydrogen, oxygen, nitrogen and sulfur
A saturated fatty acid has the general formula CnH2n+1COOH. Which of these is a saturated fatty acid?
- AC15H31COOH
- BC17H33COOH
- CC17H31COOH
- DC15H29COOH
The table shows the melting points of four fatty acids. Which conclusion is supported by the data?
- Afor the same chain length, melting point falls as the number of C=C double bonds rises
- Bfor the same chain length, melting point rises as the number of C=C double bonds rises
- Cchain length has no effect on the melting point of a saturated fatty acid
- Dall four fatty acids would be solid at a room temperature of 20 °C
Which describes the structure of a phospholipid molecule?
- Aone glycerol joined to two fatty acids and a phosphate group
- Bone glycerol joined to three fatty acids and a phosphate group
- Ctwo glycerols joined to one fatty acid and a phosphate group
- Done glycerol joined to two phosphate groups and one fatty acid
Camels store large amounts of triglyceride in their humps. Which property of triglycerides is most useful to a camel living in a desert with little drinking water?
- Arespiring triglyceride produces a large amount of metabolic water
- Btriglyceride is a poor conductor of heat, so the hump keeps the camel warm
- Ctriglyceride is less dense than water, so it gives the camel buoyancy
- Dtriglyceride is hydrophilic, so the hump absorbs water from the blood
A triglyceride is made from one glycerol, one palmitic acid and two oleic acid molecules. Calculate its Mr. (Mr values: glycerol 92, palmitic acid 256, oleic acid 282, water 18)
- A858
- B912
- C894
- D876
Sunflower seeds store mainly oil (triglyceride), while pea seeds store mainly starch. Suggest the main advantage of storing oil for a small seed that is dispersed by wind or by animals.
- Athe same energy is stored in about half the mass, so the seed is lighter
- Boil is soluble, so it moves to the growing embryo faster than starch
- Coil releases its energy more quickly than starch when the seed germinates
- Doil is a polymer, so it can be hydrolysed one monomer at a time
Lipase is added to milk containing an indicator that is pink in alkaline conditions and colourless below about pH 8.3. Over the next few minutes the pink colour fades. What is the explanation?
- Aester bonds in the milk triglycerides are hydrolysed, releasing fatty acids that lower the pH
- Bester bonds are formed by condensation, and the water released dilutes the indicator
- Cthe glycerol released by hydrolysis is a strong acid that lowers the pH
- Dlipase hydrolyses peptide bonds in milk protein, releasing amino acids that raise the pH
Quiz key
Glycerol and fatty acids contain only carbon, hydrogen and oxygen, so a triglyceride does too, like carbohydrates but with much less oxygen. Phosphorus is found in phospholipids, where a phosphate group replaces one fatty acid. Nitrogen is a feature of amino acids and proteins (in the amine group). Sulfur is found in some proteins, in the R group of cysteine.
For n = 15, a saturated chain has 2 × 15 + 1 = 31 hydrogens, so C15H31COOH (palmitic acid) is saturated. C17H33COOH has two hydrogens fewer than the saturated C17H35COOH, so it has one C=C double bond; a longer chain does not make a fatty acid more saturated. C17H31COOH is four hydrogens short, so it has two C=C bonds. C15H29COOH is two hydrogens short of C15H31COOH, so it is monounsaturated.
Stearic, oleic and linoleic acids all have 18 carbons; as C=C bonds go from 0 to 1 to 2 the melting point falls from 70 to 13 to −5 °C, because each C=C kinks the tail so the molecules cannot pack closely. The opposite trend reverses the data. Chain length does matter: saturated palmitic (16 C) melts at 63 °C and stearic (18 C) at 70 °C. Oleic and linoleic acids melt below 20 °C, so they would be liquid at room temperature.
A phospholipid is like a triglyceride in which one of the three fatty acids is replaced by a phosphate group, leaving glycerol, two fatty acids and one phosphate. Glycerol has only three OH groups, so it cannot carry three fatty acids and a phosphate as well. A phospholipid contains one glycerol, not two. It has two hydrophobic fatty acid tails and one phosphate head, not the reverse.
Triglycerides are rich in hydrogen, so oxidising them in respiration produces a lot of water, more per gram than carbohydrate; this metabolic water helps desert animals. Fat is a good insulator, but a desert animal needs to lose heat, and concentrating fat in a hump rather than under all the skin actually helps heat loss elsewhere. Buoyancy matters to aquatic animals, not camels. Triglycerides are hydrophobic, not hydrophilic, so they do not absorb water.
Reactants: 92 + 256 + 2 × 282 = 912. Three ester bonds form, each releasing one water molecule: 912 − 3 × 18 = 858. 912 forgets that water is lost in condensation. 894 subtracts only one water, as if there were one ester bond. 876 subtracts two waters, counting only the two oleic acids.
Triglyceride releases about 37 kJ g−1 compared with about 17 kJ g−1 for carbohydrate, so a seed can carry the same energy store for the embryo with roughly half the mass, which suits dispersal. Oil is insoluble, not soluble, in water. Lipids are not mobilised faster than carbohydrate; starch is usually quicker to use. A triglyceride is not a polymer: glycerol and fatty acids are not repeating monomers.
Lipase catalyses hydrolysis of the ester bonds in triglycerides, giving glycerol and fatty acids. Fatty acids have a COOH group, so they lower the pH and the indicator loses its pink colour. Lipase catalyses hydrolysis, not condensation, in the presence of water; a few water molecules would not change an indicator colour anyway. Glycerol is an alcohol, not an acid. Lipase is specific to ester bonds in lipids; breaking peptide bonds would need a protease, and a rise in pH would not make the pink fade.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.