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Photosynthesis

Every grain of wheat and rice, every mango and every breath of oxygen depends on one process. In photosynthesis, green plants use light energy to turn carbon dioxide and water into food, feeding almost every food chain on Earth.

Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D

0.10% carbon dioxide0.04% carbon dioxide (normal air)light limits the ratelight intensity →rate of photosynthesis →Temperature kept constant at 25 °C.
Figure 1. Limiting factors control how fast photosynthesis can happen.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Limiting factors
  6. Photosynthesis in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • describe photosynthesis with word and balanced symbol equations
  • explain how leaves are adapted for photosynthesis
  • describe how plants use glucose and test leaves for starch
  • explain how light, carbon dioxide and temperature limit the rate
  • analyse photosynthesis data and design fair experiments
  • discuss greenhouse farming and tree planting

Before you start

You will use these skills. If any feel shaky, review them first.

  • cells and chloroplasts (see Cell organisation)
  • enzymes and temperature (see Enzymes)
  • diffusion of gases (see Diffusion, osmosis and active transport)

Key vocabulary

Photosynthesis
The process in which plants use light energy to make glucose from carbon dioxide and water.
Chlorophyll
The green pigment in chloroplasts that absorbs light.
Chloroplast
The organelle where photosynthesis happens.
Stomata
Pores, mainly on the underside of leaves, that let gases in and out.
Limiting factor
The factor in shortest supply, which controls the rate of a process.
Starch
An insoluble store of glucose in plants.

Understanding the ideas

  1. The reaction

    In photosynthesis, chlorophyll in chloroplasts absorbs light energy, which is used to turn carbon dioxide and water into glucose, releasing oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

  2. Adapted leaves

    Leaves are adapted to do this well: they are broad to absorb light, thin so gases diffuse a short distance, full of chloroplasts in the palisade layer, have stomata for carbon dioxide to enter, and have veins to bring water.

  3. Using glucose; limiting factors

    Plants use glucose for respiration and turn it into starch for storage, cellulose for cell walls, and proteins (with nitrates). The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature.

  4. Why does it matter?

    Farmers and growers use limiting factors to increase crop yields in greenhouses, and photosynthesis by forests and oceans removes carbon dioxide from the air.

  5. What does it connect to?

    Photosynthesis links to enzymes, respiration, food chains, the carbon cycle and climate change.

Limiting factors

  • Light intensity: more light means a faster rate until another factor limits.
  • Carbon dioxide: more CO₂ means a faster rate until another factor limits.
  • Temperature: the rate rises to an optimum; above it, enzymes denature.
  • On a graph, the rising part is limited by the factor on the x-axis; the flat part is limited by something else.
  • Plants respire all the time, but photosynthesise only in the light.

Photosynthesis in the real world

Greenhouse growers add heat, light and carbon dioxide to boost yields. Wheat, rice and sugarcane, which are major crops in Pakistan, depend on sunlight and water. Tree-planting programmes aim to store carbon and restore forests and mangroves.

Worked examples

Example 1: balancing

How many carbon dioxide molecules are needed to make 3 glucose molecules?

  1. Each glucose (C₆H₁₂O₆) needs 6 CO₂.
  2. 3 × 6 = 18 molecules of CO₂.

Example 2: reading a limiting factor graph

On a graph of rate against light intensity, the line is flat at high light. What does this tell you?

  1. Light is no longer the limiting factor.
  2. Another factor, such as carbon dioxide or temperature, is limiting the rate.

Assessment tips

Photosynthesis questions often use equations, limiting factor graphs and pondweed or leaf experiments. Expect to:

  • Write word and balanced symbol equations.
  • Explain leaf adaptations and uses of glucose.
  • Interpret limiting factor graphs.
  • Design fair experiments with controlled variables.

Common mistakes: thinking plants get food from the soil; saying plants only respire at night; forgetting that the flat part of a graph is limited by a different factor; and confusing the photosynthesis and respiration equations.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: is light needed?

Partially guided investigation · two days plus 30 minutes · pairs

Research question
Does part of a leaf covered from light still make starch?
Scientific background
If light is needed for photosynthesis, a part of a leaf covered with foil should not make starch, while the uncovered part should.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
A potted plant (such as a geranium), aluminium foil, paper clips, a beaker of boiling water, a tube of ethanol in a water bath, iodine solution, a white tile, tweezers.
Method
  1. Keep the plant in the dark for 48 hours to use up its starch.
  2. Cover part of one leaf, top and bottom, with foil, and leave the plant in bright light for a day.
  3. Remove the leaf, boil it for 1 minute, then place it in hot ethanol (no flames) until it turns pale.
  4. Rinse it, spread it on a tile, add iodine and compare the covered and uncovered parts.

Safety. Ethanol is highly flammable: heat it only in a water bath with no flames nearby. Wear eye protection and take care with boiling water.

Then evaluate: why was the plant kept in the dark first, and what would you use as a control?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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