The shape of the lesson
By the end of the lesson, learners can
- Monomers, polymers and ring sugars: Rotate α-glucose, β-glucose and ribose in the simulator, with the carbon numbers switched on.
- Condensation and hydrolysis: Choose the glycosidic reaction and press Condense, then Hydrolyse.
- Starch, glycogen and cellulose: Switch between the four polysaccharides and compare the monomer, the bonds and the overall shape.
- α-glucose: C1 OH below the ring. β: above.
- Condensation removes water; hydrolysis adds it.
- Starch and glycogen: α-glucose, compact, insoluble stores.
- Cellulose: β-glucose, straight chains, H-bonded microfibrils.
One picture every idea returns to
Carbohydrates are built from monosaccharides such as glucose (a hexose) and ribose (a pentose). Glucose exists as α-glucose (OH on carbon 1 below the ring) and β-glucose (OH on carbon 1 above). Two monosaccharides join by condensation, losing water and forming a covalent glycosidic bond; hydrolysis adds water to break it. Chains of α-glucose coil into starch and glycogen, compact insoluble energy stores; chains of β-glucose, with alternate units inverted, lie straight and hydrogen-bond into the microfibrils of cellulose.
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, One sugar, three polymers.. Starch, glycogen and cellulose are all made of nothing but glucose, yet one is a plant energy store, one is an animal energy store and one holds up a tree. 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: Monomers, polymers and ring sugars | Rotate α-glucose, β-glucose and ribose in the simulator, with the carbon numbers switched on. Project tab 1 of the lab (Biological molecules lab, monosaccharides in 3D) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Condensation and hydrolysis | Choose the glycosidic reaction and press Condense, then Hydrolyse. Project tab 2 of the lab (Biological molecules lab, condensation and hydrolysis) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Starch, glycogen and cellulose | Switch between the four polysaccharides and compare the monomer, the bonds and the overall shape. Project tab 3 of the lab (Biological molecules lab, starch, glycogen and cellulose) and let a student drive it while the class predicts. | Corners |
| 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, 18 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 Starch, glycogen and cellulose, 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.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
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 |
|---|---|
| Each hydrogen bond in cellulose is weak; the strength comes from the very large number of them between many parallel chains. | And cellulose is not coiled: only α-glucose chains form helices. |
Carbohydrates: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Define the terms monomer, polymer and macromolecule.
Describe the difference in structure between α-glucose and β-glucose.
Describe how a molecule of sucrose is formed.
Three glucose molecules (C6H12O6) are joined in a chain by condensation. Give the molecular formula of the product.
Explain how the molecular structure of glycogen makes it suitable as an energy store in liver and muscle cells.
Outline how the arrangement of cellulose molecules gives a plant cell wall its strength.
Answer key and marking notes
- Monomer: a small molecule that can be joined to many others of the same kind to make a polymer.
- Polymer: a large molecule made of many repeating monomer units joined by covalent bonds.
- Macromolecule: a very large molecule (for example a polysaccharide, protein or nucleic acid).
- Both are six-membered rings with the formula C6H12O6.
- In α-glucose the OH group on carbon 1 is below the plane of the ring; in β-glucose it is above (with H and OH swapped).
- An α-glucose and a fructose molecule join in a condensation reaction.
- A molecule of water is removed, from an OH on each monosaccharide.
- A (1,2) glycosidic bond forms, a covalent bond via an oxygen atom.
- Two glycosidic bonds form, so two H2O are removed.
- 3 × C6H12O6 = C18H36O18; minus 2 H2O gives C18H32O16.
- Made of α-glucose, which can be released and respired.
- Insoluble, so it does not affect the water potential of the cell or diffuse out.
- Highly branched (1,6 bonds) and coiled, so compact: a lot of glucose stored in a small space.
- Many chain ends, so enzymes can hydrolyse it to glucose rapidly.
- Cellulose is made of β-glucose with alternate units inverted, forming long straight unbranched chains.
- Many hydrogen bonds form between OH groups of neighbouring parallel chains.
- Chains group into microfibrils, which form fibres, laid down in layers at different angles.
- This gives high tensile strength, so the wall resists the pressure of a turgid cell without bursting.
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 row correctly matches a disaccharide with the two monosaccharides that form it?
- Alactose: glucose and galactose
- Bsucrose: glucose and glucose
- Cmaltose: glucose and fructose
- Dlactose: glucose and fructose
Which molecule is a polymer?
- Aamylose
- Bsucrose
- Ca triglyceride
- Dfructose
The diagram shows the ring form of a monosaccharide. Which statement identifies it correctly?
- Aβ-glucose, because the OH on carbon 1 is above the ring, on the same side as CH2OH
- Bα-glucose, because the OH on carbon 1 is above the ring, on the same side as CH2OH
- Cβ-glucose, because the OH on carbon 4 is below the ring
- Dribose, because the ring contains one oxygen atom
Which describes the bonds that join the monomers in a polysaccharide and how they are formed?
- Acovalent glycosidic bonds, formed by condensation
- Bcovalent glycosidic bonds, formed by hydrolysis
- Chydrogen bonds, formed by condensation
- Dionic bonds, formed between the OH groups
How does the structure of amylopectin differ from that of amylose?
- Aamylopectin has 1,6 glycosidic bonds at branch points as well as 1,4 bonds
- Bamylopectin contains β-glucose units at each branch point
- Camylopectin chains are linked to each other by hydrogen bonds into microfibrils
- Damylopectin has only 1,6 glycosidic bonds, so it coils into a helix
An unbranched amylose molecule contains 500 glucose units. What is its Mr? (Mr of glucose = 180, Mr of water = 18)
- A81018
- B81000
- C90000
- D89982
Trehalose, a sugar found in insects and fungi, is a disaccharide of two α-glucose molecules joined by an α-1,1 glycosidic bond. Predict the result of heating trehalose with Benedict's solution.
- Anegative, because carbon 1 of both glucose units is used in the bond, leaving no free reducing group
- Bpositive, because trehalose is made only of glucose, which is a reducing sugar
- Cpositive, because heating with Benedict's solution breaks the 1,1 glycosidic bond
- Dnegative, because α-glucose is not a reducing sugar until it becomes β-glucose
Both amylose and cellulose contain 1,4 glycosidic bonds between glucose units, but amylase can hydrolyse only amylose. What is the best explanation?
- Aalternate β-glucose units in cellulose are inverted, so the chain's shape does not fit the active site of amylase
- Bcellulose has no glycosidic bonds; its glucose units are held together by hydrogen bonds
- Ccellulose contains 1,6 glycosidic bonds, which amylase cannot break
- Dthe hydrogen bonds between cellulose chains are covalent and too strong to break
Quiz key
Lactose, the sugar in milk, is formed from glucose and galactose. Sucrose is glucose and fructose, not two glucoses. Maltose is two α-glucose molecules, not glucose and fructose; mixing up maltose and sucrose is a common error. Lactose does not contain fructose; that pairing belongs to sucrose.
Amylose is a chain of hundreds or thousands of α-glucose monomers joined by 1,4 glycosidic bonds, so it is a polymer. Sucrose contains only two monosaccharide units, so it is a disaccharide, not a polymer. A triglyceride is a fairly large molecule, but it is made of glycerol and three fatty acids, which are not repeating monomers. Fructose is a single monosaccharide, a monomer.
The OH on carbon 1 points up, on the same side of the ring as the CH2OH on carbon 5, so this is β-glucose. In α-glucose the OH on carbon 1 is below the ring, so the second option has the rule reversed. The OH on carbon 4 is below the ring in both α- and β-glucose, so it cannot tell them apart. Ribose also has a ring oxygen, but its ring has only four carbons and five ring atoms; this ring has six atoms, five of them carbon.
Monomers in a polymer are joined by covalent bonds; in polysaccharides these are glycosidic bonds, each formed by a condensation reaction that releases one water molecule. Hydrolysis is the reverse reaction, which uses water to break glycosidic bonds. Hydrogen bonds hold separate cellulose chains together but do not link the monomers within a chain. OH groups are polar, not charged, so ionic bonds do not form between them.
Amylose is an unbranched helix of α-glucose joined only by 1,4 glycosidic bonds. Amylopectin has the same 1,4 chains but also 1,6 glycosidic bonds where branches join. Both contain only α-glucose; β-glucose is found in cellulose. Hydrogen-bonded microfibrils are a feature of cellulose. Amylopectin's main chains are still 1,4 linked; the 1,6 bonds are only at the branch points, and the helix is a feature of 1,4-linked chains.
A chain of 500 units has 499 glycosidic bonds, each releasing one water molecule. Mr = 500 × 180 − 499 × 18 = 90000 − 8982 = 81018. 81000 subtracts 500 waters (one per glucose) instead of 499. 90000 forgets that water is lost at all. 89982 subtracts only a single water molecule, as if the whole chain were formed by one condensation.
The reducing group of glucose is at carbon 1. In maltose (1,4 bond) one glucose still has a free carbon 1, so maltose is reducing. In trehalose both carbon 1 atoms are in the glycosidic bond, just as in sucrose both reducing ends are used, so trehalose is a non-reducing sugar. Being made of glucose is not enough; it depends on whether a reducing group is free. Benedict's solution is alkaline and does not hydrolyse glycosidic bonds; acid is needed for that. Both α- and β-glucose are reducing sugars.
Cellulose is made of β-glucose, and every other unit is turned 180° to form the 1,4 bond, giving a straight chain with a different shape around each bond. Amylase is specific to the α-1,4 linkages of starch, so cellulose does not fit its active site. Cellulose glucose units are joined by covalent glycosidic bonds; hydrogen bonds link neighbouring chains, not the units in a chain. Cellulose is unbranched, with no 1,6 bonds. Hydrogen bonds are not covalent, and each is weak; amylase would act on single chains anyway.
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.