The shape of the lesson
By the end of the lesson, learners can
- Light into chemical energy: In the sim, carbon dioxide molecules drift in from the air and water molecules arrive from the xylem.
- Where the glucose goes: Glucose is made in the leaves, but most of it does not stay as glucose.
- Minerals from the soil: Photosynthesis only makes carbohydrates, which contain carbon, hydrogen and oxygen.
- Word equation: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
- Extended: 6CO2 + 6H2O → C6H12O6 + 6O2.
- Glucose becomes: starch (store), cellulose (walls), sucrose (phloem), nectar (insects), or is respired.
- Nitrate for amino acids; magnesium for chlorophyll.
One picture every idea returns to
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. Chlorophyll is a green pigment found in chloroplasts; it transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.
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, Sugar from air, water and light.. A green plant builds its own food. 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: Light into chemical energy | In the sim, carbon dioxide molecules drift in from the air and water molecules arrive from the xylem. Project tab 1 of the lab (Inside a chloroplast) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Where the glucose goes | Glucose is made in the leaves, but most of it does not stay as glucose. Project tab 2 of the lab (Following glucose around the plant) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Minerals from the soil | Photosynthesis only makes carbohydrates, which contain carbon, hydrogen and oxygen. Project tab 3 of the lab (Nitrate and magnesium deficiency) and let a student drive it while the class predicts. | Anticipation / Reaction Guide |
| 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, 20 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 Minerals from the soil, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| Light and chlorophyll are written over the arrow, not as reactants. | Chlorophyll is not used up and light is not a substance. Also, plants do not "make energy": chlorophyll transfers light energy into chemical energy. |
| Glucose is not transported in the phloem: it is converted to sucrose first. | And it is starch, not glucose, that is stored, because starch is insoluble. |
| Nitrate is for amino acids (proteins), not for chlorophyll directly; magnesium is for chlorophyll. | Do not swap them, and do not say minerals give the plant energy. |
Photosynthesis: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
(a) State the word equation for photosynthesis. (b) State where chlorophyll is found in a leaf cell and describe its role.
Extended. Write the balanced chemical equation for photosynthesis.
Outline how a plant uses and stores the carbohydrates it makes in photosynthesis. Give five uses.
Suggest why sucrose, not starch, is the form in which carbohydrate is carried in the phloem.
Seedlings were grown for four weeks in three solutions. The table shows the results.
| Solution | Mean height / cm | Leaf colour |
|---|---|---|
| all mineral ions | 18 | dark green |
| no nitrate | 7 | small leaves, older leaves yellow |
| no magnesium | 14 | yellow between the veins |
(a) Calculate the percentage reduction in height caused by the lack of nitrate. (b) Explain the results for the two deficient solutions.
A potato plant photosynthesises in its leaves, yet starch is found in the tubers underground. Explain how the starch gets there.
Answer key and marking notes
- (a) carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
- (b) Chlorophyll is a green pigment found in chloroplasts.
- It absorbs light and transfers the energy from light into energy in chemicals, for the synthesis of carbohydrates.
- 6CO2 + 6H2O → C6H12O6 + 6O2
- One mark for correct formulae, one mark for correct balancing.
- Converted to starch as an energy store, in leaves, roots, tubers and seeds.
- Converted to cellulose to build cell walls.
- Glucose is used in respiration to provide energy.
- Converted to sucrose for transport in the phloem.
- Converted to nectar to attract insects for pollination.
- Sucrose is soluble, so it can be carried dissolved in the phloem sap.
- Starch is insoluble, so it cannot be carried in solution.
- Starch is therefore used for storage, where it does not affect osmosis, and sucrose for transport.
- (a) (18 − 7) ÷ 18 × 100 = 61% (to 2 significant figures).
- (b) Nitrate ions are needed to make amino acids and so proteins; without them few new cells can be made, so growth is stunted.
- Magnesium ions are needed to make chlorophyll; without them leaves turn yellow between the veins.
- With less chlorophyll, less light is absorbed, so less photosynthesis happens and growth is reduced, though less severely.
- Glucose made in the leaves is converted into sucrose.
- Sucrose is carried down to the tubers in the phloem.
- In the tuber cells sucrose is converted back to glucose and then into starch for storage.
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 two substances are the raw materials for photosynthesis?
- Acarbon dioxide and water
- Bglucose and oxygen
- Ccarbon dioxide and oxygen
- Dwater and glucose
In which part of a leaf cell is chlorophyll found?
- Achloroplasts
- Bmitochondria
- Cthe vacuole
- Dthe nucleus
Which energy transfer does chlorophyll carry out during photosynthesis?
- Alight energy into chemical energy
- Bchemical energy into light energy
- Cthermal energy into chemical energy
- Dlight energy into thermal energy
Which carbohydrate made from glucose is used to build plant cell walls?
- Acellulose
- Bstarch
- Csucrose
- Dglycogen
Some of the sugar made in photosynthesis ends up in the nectar of flowers. What is the advantage to the plant?
- ANectar attracts insects that pollinate the flowers
- BNectar stores energy for the seeds to use later
- CNectar is carried to the roots in the phloem
- DNectar protects the flower from water loss
Extended: according to the balanced equation, how many molecules of carbon dioxide are needed to make one molecule of glucose?
- A6
- B1
- C12
- D3
Extended: a leaf makes 4 molecules of glucose by photosynthesis. Using the balanced equation, how many molecules of oxygen are released?
- A24
- B4
- C6
- D12
A plant is grown in soil lacking magnesium ions. Why does it photosynthesise less than a plant given all mineral ions?
- AIt makes less chlorophyll, so absorbs less light
- BIt makes fewer amino acids, so has no enzymes
- CIts stomata cannot open to let in carbon dioxide
- DIts xylem cannot carry water to the leaves
Quiz key
Carbon dioxide + water → glucose + oxygen. Glucose and oxygen are the products, not the raw materials. Oxygen is released, not taken in, and glucose is made, not used.
Chlorophyll is a green pigment found in chloroplasts. Mitochondria are the site of aerobic respiration, the vacuole holds cell sap, and the nucleus holds the genetic material; none contains chlorophyll.
Chlorophyll absorbs light and transfers the energy into energy in chemicals (glucose). Chemical into light is what a glow-worm does. Thermal energy is not the source, and converting light to thermal energy would waste it rather than store it in carbohydrate.
Glucose molecules are joined into cellulose chains to build cell walls. Starch is the energy store, sucrose is the transport sugar in the phloem, and glycogen is the storage carbohydrate of animals and fungi, not plants.
Insects visit flowers to feed on nectar and carry pollen from flower to flower, so the flowers are pollinated. Seeds store energy as starch, not nectar. Sucrose, not nectar, is carried in the phloem, and nectar does nothing to reduce water loss.
6CO2 + 6H2O → C6H12O6 + 6O2. Glucose has six carbon atoms and each carbon dioxide supplies one, so six are needed. 1 ignores the balancing, 12 is the number of hydrogen atoms in glucose, and 3 halves the correct value.
Each glucose made releases 6O2, so 4 × 6 = 24. 4 assumes one oxygen per glucose, 6 is the value for a single glucose, and 12 counts oxygen atoms per glucose rather than molecules for four glucose.
Magnesium ions are needed to make chlorophyll, so the leaves turn yellow and absorb less light energy. Nitrate, not magnesium, is needed for amino acids. Magnesium deficiency does not stop stomata opening or water moving in the xylem.
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.