The shape of the lesson
By the end of the lesson, learners can
- The elements inside: Turn the glucose molecule in the sim and press each element chip.
- Big molecules from small ones: Each large molecule is a chain of many copies of a small molecule.
- Why the building blocks matter: These facts are the foundation for several later topics, so it is worth seeing where they lead.
- Carbohydrates and fats: carbon, hydrogen and oxygen only.
- Proteins: carbon, hydrogen, oxygen and nitrogen; some contain sulfur as well.
- From glucose: starch, glycogen and cellulose. From amino acids: proteins. From fatty acids and glycerol: fats and oils.
- Name both parts of a fat and never say a large molecule is made from another large molecule.
One picture every idea returns to
Carbohydrates and fats contain carbon, hydrogen and oxygen only. Proteins contain carbon, hydrogen, oxygen and nitrogen (some also contain sulfur). Large biological molecules are made by joining many small molecules together: starch, glycogen and cellulose are made from glucose; proteins from amino acids; fats and oils from fatty acids and glycerol.
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, Few elements, many molecules.. Almost everything in a living cell is built from a handful of elements. 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: The elements inside | Turn the glucose molecule in the sim and press each element chip. Project tab 1 of the lab (The elements in glucose, a fat and two amino acids) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Big molecules from small ones | Each large molecule is a chain of many copies of a small molecule. Project tab 2 of the lab (Building starch, glycogen, cellulose, protein and fat) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Why the building blocks matter | These facts are the foundation for several later topics, so it is worth seeing where they lead. | 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, 19 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 Why the building blocks matter, 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 |
|---|---|
| Nitrogen is the element that separates proteins from the other two groups. "Carbon, hydrogen and oxygen" is a correct answer for carbohydrates and for fats, but it loses the mark for proteins unless you add nitrogen. | Return to The elements inside: ask a pair to state the correct version and show it on the lab before moving on. |
| "Fats are made from fatty acids" is only half an answer. | Always name both parts: fatty acids and glycerol. Likewise, cellulose is made from glucose, not from starch. |
Large molecules from small ones: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
List the chemical elements found in (a) carbohydrates, (b) fats and (c) proteins.
Complete the table by naming the small molecules from which each large molecule is made.
| Large molecule | Made from |
|---|---|
| cellulose | |
| protein | |
| oil |
A student analysed four pure substances found in a seed. The table shows the elements present in each.
| Substance | Elements present | Small molecules produced when digested |
|---|---|---|
| P | C, H, O | glucose |
| Q | C, H, O, N, S | amino acids |
| R | C, H, O | fatty acids and glycerol |
| S | C, H, O | cannot be digested by humans |
Identify substances P, Q, R and S, giving a reason for each.
Starch, glycogen and cellulose are all made from glucose, yet starch is a food store while cellulose forms strong cell walls. Suggest how molecules made from the same unit can have such different properties.
A plant makes glucose by photosynthesis. Explain why the plant also needs to absorb nitrate ions from the soil in order to grow.
A fat molecule and a protein molecule are each broken down into their small molecules. State how many small molecules a single fat molecule gives, and name them. Then explain why the same question cannot be answered with a single number for a protein.
Answer key and marking notes
- (a) Carbohydrates: carbon, hydrogen and oxygen.
- (b) Fats: carbon, hydrogen and oxygen.
- (c) Proteins: carbon, hydrogen, oxygen and nitrogen (and sometimes sulfur).
- Cellulose: glucose.
- Protein: amino acids.
- Oil: fatty acids and glycerol (both needed).
- P is starch: made of C, H and O only and digested to glucose (a plant's store, so starch rather than glycogen).
- Q is a protein: it contains nitrogen (and sulfur) and is digested to amino acids.
- R is a fat or oil: C, H and O only, digested to fatty acids and glycerol.
- S is cellulose: a carbohydrate made of glucose units that humans cannot digest (fibre).
- The glucose units are joined in different arrangements: starch chains are coiled into a compact spiral, glycogen chains are branched, cellulose chains are straight.
- In cellulose the straight chains lie side by side and are held together, forming strong fibres suited to a cell wall.
- The compact starch and highly branched glycogen are easily broken down to release glucose, which suits a store.
- Glucose contains only carbon, hydrogen and oxygen, so it can be turned into starch and cellulose but not into proteins.
- Proteins (and the amino acids they are made from) contain nitrogen.
- The plant obtains that nitrogen from nitrate ions absorbed by the roots, so it can make amino acids and then proteins for new cells, enzymes and growth.
- A fat gives four small molecules: one glycerol and three fatty acids.
- A protein is a long chain of amino acids, so it gives many amino acids.
- The number is different for every protein because proteins differ in length (and in the order of the roughly 20 kinds of amino acid).
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 element is found in all proteins but not in carbohydrates or fats?
- Anitrogen
- Boxygen
- Ccarbon
- Dhydrogen
Glycogen, stored in the liver and muscles, is made by joining together many molecules of:
- Aglucose
- Bamino acids
- Cfatty acids and glycerol
- Dstarch
Chemical analysis shows that a substance from a seed contains carbon, hydrogen, oxygen, nitrogen and sulfur. The substance is most likely:
- Aa protein
- Bglycogen
- Can oil
- Dcellulose
Which pair of large molecules are both made from glucose?
- Astarch and cellulose
- Bstarch and protein
- Cglycogen and fat
- Dcellulose and oil
Lipase completely digests 20 fat molecules. How many small molecules of each kind are produced?
- A60 fatty acids and 20 glycerol
- B20 fatty acids and 60 glycerol
- C60 fatty acids and 60 glycerol
- D20 fatty acids and 20 glycerol
A plant is grown in a soil that contains no nitrate ions but has plenty of water, carbon dioxide and light. Which substance will it be least able to make?
- Aprotein
- Bstarch
- Ccellulose
- Doil
The table shows the percentage by mass of four elements in four pure substances W, X, Y and Z. Which substance is most likely to be a fat?
- AX
- BW
- CY
- DZ
Starch and cellulose are both made only of glucose, yet starch is a compact food store and cellulose forms strong cell walls. Which statement best explains the difference?
- AThe glucose units are joined in different arrangements: coiled chains in starch, straight chains side by side in cellulose
- BCellulose contains nitrogen, which makes its chains stronger than those of starch
- CStarch is made of short glucose chains and cellulose of single glucose molecules
- DCellulose is made by joining many starch molecules end to end
Quiz key
Carbohydrates and fats contain only carbon, hydrogen and oxygen. Every amino acid, and so every protein, also contains nitrogen. Oxygen, carbon and hydrogen are in all three groups, so none of them can tell proteins apart.
Glycogen is a carbohydrate built from many glucose units in branched chains. Amino acids build proteins and fatty acids with glycerol build fats. Starch is itself a large molecule; glycogen is not made from starch, though both are made from glucose.
Nitrogen is present in all proteins, and some amino acids also contain sulfur. Glycogen and cellulose are carbohydrates and an oil is a fat: all three contain only carbon, hydrogen and oxygen, so none of them could contain nitrogen or sulfur.
Starch, glycogen and cellulose are all made from glucose. Proteins are made from amino acids, and fats and oils from fatty acids and glycerol, so every other pair contains a molecule that is not built from glucose.
Each fat molecule is one glycerol joined to three fatty acids, so 20 fat molecules give 20 × 3 = 60 fatty acids and 20 glycerol. Swapping the numbers reverses the structure, 60 of each counts three glycerols per fat, and 20 of each counts only one fatty acid per fat.
Nitrate ions supply the nitrogen needed to make amino acids and so proteins. Starch and cellulose are made from glucose, and oils from fatty acids and glycerol: all contain only carbon, hydrogen and oxygen, which the plant gets from carbon dioxide and water through photosynthesis.
X contains no nitrogen and only 11% oxygen: fats contain carbon, hydrogen and oxygen but far less oxygen than carbohydrates. W and Z have about 50% oxygen, typical of carbohydrates such as starch and glucose. Y contains nitrogen, so it is a protein.
Both are long chains of glucose, but starch chains coil into a compact spiral while cellulose chains are straight and lie side by side, held together into strong fibres. Cellulose contains no nitrogen; it is a carbohydrate. Single glucose molecules would not form a wall, and cellulose is made from glucose, not from starch.
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.