Overview
What you’ll learn
- Write the word and balanced chemical equations for photosynthesis, and state where it happens.
- Trace the five fates of the glucose made in photosynthesis, and link nitrate and magnesium ions to the molecules they build.
- Describe the classic experiments that prove the need for light, chlorophyll and carbon dioxide.
- Identify and explain limiting factors of photosynthesis from graphs and data (Supplement).
- Label a dicotyledonous leaf section and explain how each layer is adapted for photosynthesis.
Syllabus checklist (0610)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 6.1Describe photosynthesis as the process by which plants synthesise carbohydrates from raw materials using energy from light.
- 6.1State the word equation for photosynthesis: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
- 6.1State that chlorophyll is a green pigment that is found in chloroplasts.
- 6.1State that chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.
- 6.1Outline the subsequent use and storage of the carbohydrates made in photosynthesis: starch as an energy store; cellulose to build cell walls; glucose used in respiration to provide energy; sucrose for transport in the phloem; nectar to attract insects for pollination.
- 6.1Explain the importance of nitrate ions for making amino acids and magnesium ions for making chlorophyll.
- 6.1Investigate the need for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls.
- 6.1Investigate and describe the effects of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis.
- 6.1Investigate and describe the effect of light and dark conditions on gas exchange in an aquatic plant using hydrogencarbonate indicator solution.
- 6.2State that most leaves have a large surface area and are thin, and explain how these features are adaptations for photosynthesis.
- 6.2Identify in diagrams and images the structures in the leaf of a dicotyledonous plant: chloroplasts, cuticle, guard cells and stomata, upper and lower epidermis, palisade mesophyll, spongy mesophyll, air spaces, vascular bundles, xylem and phloem.
- 6.2Explain how the structures of the leaf adapt it for photosynthesis.
Supplement — Extended candidates only
- 6.1State the balanced chemical equation for photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- 6.1Identify and explain the limiting factors of photosynthesis in different environmental conditions.
In 30 seconds
- 1Photosynthesis builds carbohydrates from carbon dioxide and water using light energy: carbon dioxide + water → glucose + oxygen (light + chlorophyll); balanced: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (Supplement).
- 2Chlorophyll — the green pigment in chloroplasts — transfers light energy into chemical energy in the new carbohydrate.
- 3Glucose fates: starch (store), cellulose (cell walls), respiration (energy), sucrose (phloem transport), nectar (attract pollinators).
- 4Nitrate ions make amino acids (then proteins); magnesium ions make chlorophyll — shortage turns leaves yellow.
- 5Limiting factors (Supplement): the rate is capped by whichever runs short — light intensity, carbon dioxide concentration or temperature.
- 6Leaf adaptations: large, thin, waxy cuticle, palisade layer packed with chloroplasts, air spaces for gas diffusion, stomata for entry of carbon dioxide, xylem brings water, phloem removes sucrose.
Notes · 1 of 5
Photosynthesis and the equations
The process, the word equation every candidate learns, and the balanced equation for Extended papers.
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. The raw materials are carbon dioxide (diffusing in through the stomata) and water (arriving from the roots in the xylem). The energy source is light, absorbed by chlorophyll — the green pigment inside chloroplasts. Chlorophyll transfers the light energy into energy in chemicals, which is used to build glucose from the raw materials; oxygen is released as a by-product.
| Form | Equation |
|---|---|
| Word equation (Core) | carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll) |
| Balanced equation (Supplement) | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
A labelled diagram to picture: a chloroplast as a green oval; arrows in — carbon dioxide from the air and water from the xylem; light arrows striking the chlorophyll; arrows out — glucose to the cell and oxygen diffusing out through the stomata. That single sketch answers "describe the process" questions.
Notes · 2 of 5
Five fates of glucose — and two mineral ions
What a leaf does with the sugar it makes, and why nitrates and magnesium keep a plant green and growing.
| Fate | Purpose |
|---|---|
| Starch | Energy store — insoluble, so it does not affect the cell’s water balance |
| Cellulose | Building cell walls |
| Glucose in respiration | Providing energy for the cell’s reactions |
| Sucrose | Transport in the phloem to other parts of the plant |
| Nectar | Attracting insects for pollination |
Photosynthesis alone is not plant nutrition — the plant also needs mineral ions from the soil. Nitrate ions are needed to make amino acids, which are joined into proteins for growth; without nitrate, growth stops and older leaves turn yellow. Magnesium ions are needed to make chlorophyll; without magnesium, leaves turn yellow between the veins because new chlorophyll cannot be built. Both ions are taken up from dilute soil solution by active transport in root hair cells (Topic 3).
Notes · 3 of 5
Investigating photosynthesis
The classic proof experiments, the pondweed rate experiment, and what hydrogencarbonate indicator colours mean.
- 1Need for light
Cover part of a leaf with foil for two days (keep the plant in the light). Test the leaf with iodine: covered areas stay brown (no starch); exposed areas turn blue-black. Control: the uncovered part of the same leaf.
- 2Need for chlorophyll
Use a variegated leaf (white patches have no chlorophyll). After light exposure, iodine turns blue-black only on the green patches — photosynthesis needs chlorophyll.
- 3Need for carbon dioxide
Grow two plants in sealed bell jars, one with soda-lime absorbing CO₂, one without. Only the plant with CO₂ makes starch — CO₂ is required.
Before testing a leaf for starch it must be prepared: boil in water (kill and soften it), boil in ethanol (remove chlorophyll — done in a water bath, never over a naked flame), dip in water (soften), then spread flat and add iodine. Each step has a reason that examiners ask for.
The rate experiment uses pondweed: count the oxygen bubbles per minute at different light intensities (moving a lamp closer or further), CO₂ levels (adding sodium hydrogencarbonate) or temperatures (water bath). More bubbles per minute = faster photosynthesis.
The hydrogencarbonate indicator experiment measures gas exchange in an aquatic plant. The indicator is orange-red at normal air, turns yellow when carbon dioxide is added (respiration exceeding photosynthesis, e.g. in the dark) and purple when carbon dioxide is removed (photosynthesis exceeding respiration, e.g. in bright light). Three tubes — plant in the light, plant in the dark, no plant (control) — tell the whole story.
Notes · 4 of 5
Limiting factors of photosynthesis
Which input is capping the rate — light, carbon dioxide or temperature — and how to spot it from data. (Supplement)
Supplement: a limiting factor is the input in shortest supply that is holding the rate down. Increase it and the rate rises; increase anything else and nothing happens. The three you must know are light intensity, carbon dioxide concentration and temperature.
| Graph shape | Limiting factor | Explanation |
|---|---|---|
| Rate rises steeply with light, then plateaus | Light at first; then CO₂ or temperature | Beyond the plateau another factor is the bottleneck |
| Rate rises with CO₂, then plateaus | CO₂ at first; then light or temperature | Extra CO₂ only helps until light or temperature limits |
| Rate rises with temperature, then falls | Temperature to the optimum; then enzyme denaturation | Photosynthesis is enzyme-controlled, so too much heat denatures the enzymes |
Growers exploit this in greenhouses: extra lighting raises rates on winter days, CO₂ enrichment raises them in bright conditions, and heating keeps enzymes near their optimum — each intervention only pays while that factor is the one limiting the rate.
Notes · 5 of 5
Leaf structure and adaptations
A labelled tour of a dicotyledonous leaf, layer by layer, with the function each layer serves.
A labelled diagram to picture — a vertical section through a leaf, from top to bottom: waxy cuticle (thin waterproof layer), upper epidermis (transparent, no chloroplasts, lets light through), palisade mesophyll (upright cells packed with chloroplasts), spongy mesophyll (irregular cells with large air spaces), lower epidermis with stomata — pores each guarded by two guard cells — and vascular bundles running through the mesophyll with xylem on top and phloem below.
| Structure | Adaptation for photosynthesis |
|---|---|
| Large surface area | Absorbs as much light and as much carbon dioxide as possible |
| Thin blade | Short diffusion distance for carbon dioxide to reach the mesophyll cells |
| Waxy cuticle | Limits water loss without blocking light |
| Upper epidermis | Transparent — light passes straight through to the palisade layer |
| Palisade mesophyll | Packed with chloroplasts at the top, where light is brightest |
| Spongy mesophyll and air spaces | Large internal surface area for gas exchange; carbon dioxide diffuses quickly to cells |
| Stomata with guard cells | Adjustable pores letting carbon dioxide in and oxygen out; guard cells close them to limit water loss |
| Xylem | Brings water to the mesophyll cells |
| Phloem | Carries away the sucrose made in photosynthesis |
Revise
Key terms
The words examiners expect you to use precisely.
- Photosynthesis
- The process by which plants synthesise carbohydrates from raw materials (carbon dioxide and water) using energy from light.
- Chlorophyll
- The green pigment in chloroplasts that absorbs light energy and transfers it into energy in chemicals.
- Stomata
- Pores in the leaf’s lower epidermis that allow carbon dioxide to diffuse in and oxygen and water vapour to diffuse out.Each stoma is opened and closed by two guard cells.
- Guard cells
- The pair of specialised cells surrounding each stoma that control its opening and closing.
- Palisade mesophyll
- The layer of upright, chloroplast-packed cells beneath the upper epidermis, where most photosynthesis happens.
- Spongy mesophyll
- The layer of irregular cells with air spaces below the palisade layer, adapted for gas exchange.
Show 6 more key termsShow fewer key terms
- Xylem
- The vascular tissue that transports water and mineral ions from the roots to the leaf.
- Phloem
- The vascular tissue that transports sucrose and amino acids around the plant.
- Limiting factor
- The input in shortest supply that limits the rate of photosynthesis: light intensity, carbon dioxide concentration or temperature.Supplement term.
- Variegated leaf
- A leaf with white patches lacking chlorophyll, used to show that photosynthesis needs chlorophyll.
- Hydrogencarbonate indicator
- A dye that measures carbon dioxide concentration: yellow when CO₂ is high, orange-red at normal levels, purple when CO₂ is low.
- Cuticle
- The thin waxy layer on the leaf surface that reduces water loss while letting light through.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Writing photosynthesis as "carbon dioxide + water → oxygen + glucose" but calling glucose the waste product.
Glucose is the product the plant keeps; oxygen is the by-product released.
Instead: Carbon dioxide + water → glucose + oxygen: glucose is the carbohydrate made, oxygen leaves through the stomata.
Mistake: Forgetting light and chlorophyll above the arrow.
The reaction does not run in the dark without pigment — the conditions are part of the equation.
Instead: State the word equation "in the presence of light and chlorophyll".
Show 10 more mistakesShow fewer mistakes
Mistake: Balancing 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ as 6O instead of 6O₂ on the right.
Oxygen gas exists as molecules, so the right side needs 6O₂.
Instead: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — 18 oxygen atoms on each side.
Mistake: Saying plants get nitrates "for energy".
Nitrates are used to build amino acids and proteins, not to release energy.
Instead: Nitrate ions → amino acids → proteins for growth; energy comes from respiration of glucose.
Mistake: Mixing up magnesium and nitrate deficiencies.
Magnesium builds chlorophyll (yellow leaves); nitrate builds amino acids (poor growth, general yellowing).
Instead: Magnesium → chlorophyll; nitrate → amino acids (then proteins).
Mistake: Calling a plateau on a rate graph "photosynthesis has stopped".
A plateau means the rate has stopped RISING — photosynthesis continues at that rate.
Instead: The graph plateaus because a different factor has become limiting.
Mistake: Heating ethanol over a naked flame.
Ethanol is highly flammable — this is a safety answer examiners expect.
Instead: Boil the ethanol in a water bath (or use an electric heater).
Mistake: Saying the upper epidermis photosynthesises.
Upper epidermis cells are transparent with no chloroplasts — their job is protection plus letting light through.
Instead: Photosynthesis happens in the palisade and spongy mesophyll cells.
Mistake: Drawing stomata on the upper surface.
In most dicotyledonous leaves stomata are concentrated on the LOWER epidermis, reducing water loss.
Instead: Stomata and guard cells sit in the lower epidermis.
Mistake: Claiming a yellow hydrogencarbonate indicator means lots of photosynthesis.
Yellow means MORE carbon dioxide dissolved — respiration exceeding photosynthesis, as in the dark.
Instead: Yellow = high CO₂; purple = low CO₂; orange-red = normal air.
Mistake: Saying leaves absorb water for photosynthesis from the air.
Leaves are built for gas exchange, not water uptake; their surfaces are even coated with a waterproof cuticle.
Instead: Water for photosynthesis is absorbed by **root hair cells** and carried up in the **xylem**; the leaf supplies only the carbon dioxide and receives the light.
Mistake: Assuming more light always means more photosynthesis.
Once another factor runs short, extra light changes nothing — the graph plateaus instead of rising for ever.
Instead: At high light intensity the rate levels off because **another factor (carbon dioxide or temperature) is limiting**.
Check yourself
Quick quiz
26 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Define photosynthesis as carbohydrate synthesis
Photosynthesis is the process by which plants
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Practise
Topical questions
14 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 5 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Topic 5: photosynthesis equations, uses of glucose, mineral ions, rate experiments, limiting factors, leaf structure.
Mark scheme
PROTopic 5 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
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Questions
Frequently asked questions
What are the fates of glucose made in photosynthesis?
Five: stored as starch, built into cellulose cell walls, respired for energy, converted to sucrose for transport in the phloem, and made into nectar to attract pollinators.
How do I identify the limiting factor from a graph?
Find where the line rises — that factor is limiting while the rate is rising. Where the line plateaus, something else is limiting. Temperature curves also fall after the optimum because enzymes denature.
Why does a destarched plant matter in the light experiment?
Destarching (48 hours in the dark) removes starch stored earlier, so any starch the iodine finds afterwards must have been made during the experiment — otherwise the result proves nothing.
What do the hydrogencarbonate indicator colours mean?
Yellow = more carbon dioxide (respiration > photosynthesis), orange-red = normal air, purple = less carbon dioxide (photosynthesis > respiration in the light).
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