Photosynthesis Explained – IGCSE & O Level Biology (Video Lesson)

How do plants turn sunlight into food? Photosynthesis is one of the most important reactions on Earth – it feeds almost every living thing and puts the oxygen into the air we breathe. This is a full, exam-focused guide for IGCSE & O Level Biology. Watch the video above, then use the notes below to revise.

Overview of photosynthesis
The big picture: light, water and carbon dioxide go in; glucose and oxygen come out. Image: Wikimedia Commons.

1. What is photosynthesis?

Photosynthesis is the process by which green plants, algae and some bacteria use light energy (absorbed by chlorophyll) to make glucose from carbon dioxide and water. Oxygen is released as a by-product. In short, light energy is transformed into chemical energy stored in glucose.

Word equation

carbon dioxide + water  —(light energy / chlorophyll)→  glucose + oxygen

Balanced symbol equation

6CO2 + 6H2O  →  C6H12O6 + 6O2

(in the presence of light energy and chlorophyll)

2. The raw materials (and how they reach the leaf)

  • Light – usually from the Sun; provides the energy for the reaction.
  • Water – absorbed from the soil by the root hair cells and carried up to the leaves in the xylem.
  • Carbon dioxide – diffuses into the leaf from the air through tiny pores called stomata on the underside of the leaf.

Chlorophyll is not a raw material, but it is essential: it is the green pigment that absorbs light (mainly the red and blue parts of the spectrum) and reflects green – which is why leaves look green.

3. Where it happens: the leaf and the chloroplast

Photosynthesis takes place inside chloroplasts, found mainly in the palisade mesophyll cells near the top of the leaf. Each chloroplast has two important regions:

  • Grana (stacks of disc-shaped thylakoids) – contain chlorophyll and are where light is captured (Stage 1).
  • Stroma – the fluid that surrounds the grana, where glucose is built (Stage 2).
Structure of a chloroplast
Inside a chloroplast: grana (stacks of thylakoids) sit in the stroma. Image: Wikimedia Commons.

4. The two stages of photosynthesis

Stage 1 – Light-dependent reactions (in the grana)

  • Chlorophyll absorbs light energy.
  • The energy is used to split water molecules apart – a step called photolysis.
  • Oxygen is released as a waste product (the oxygen we breathe).
  • The energy is stored temporarily in two carrier molecules, ATP and NADPH, which are passed to Stage 2.

Stage 2 – The Calvin cycle / light-independent reactions (in the stroma)

  • This stage does not need light directly.
  • The ATP and NADPH from Stage 1 provide the energy to combine carbon dioxide with hydrogen.
  • Carbon dioxide is “fixed” and reduced to form glucose – the plant’s food.

Remember: Stage 1 captures the energy; Stage 2 uses it to build sugar.

5. What happens to the glucose?

Glucose is a small, soluble sugar. Plants use it in several ways:

  • Respiration – broken down to release energy for the plant’s cells.
  • Starch – converted to insoluble starch for storage in leaves, roots and stems.
  • Cellulose – used to build strong cell walls.
  • Sucrose – the form in which sugar is transported around the plant in the phloem.
  • Proteins – combined with nitrate ions from the soil to make amino acids, then proteins.
  • Lipids / oils – stored energy, especially in seeds.

6. Limiting factors (a favourite exam topic)

The rate of photosynthesis is controlled by whichever factor is in shortest supply – the limiting factor. The three main ones are:

  • Light intensity – as light increases the rate increases, then levels off when another factor becomes limiting.
  • Carbon dioxide concentration – increasing CO2 increases the rate up to a point, then it plateaus.
  • Temperature – the rate rises as temperature increases (enzymes work faster), reaches an optimum, then falls sharply as the enzymes denature at high temperatures.

Exam tip: when a graph “flattens out”, always say which factor has become the limiting factor.

7. How the leaf is adapted for photosynthesis

  • Large, flat surface area – absorbs as much light as possible.
  • Thin – short diffusion distance for gases.
  • Many chloroplasts in the palisade cells near the top surface.
  • Network of veins (xylem & phloem) – brings water and removes sugars.
  • Stomata and guard cells – let carbon dioxide in and oxygen out.
  • Waxy cuticle – transparent so light passes through, and reduces water loss.

8. Classic experiments you should know

Testing a leaf for starch (iodine test): boil the leaf in water (to stop reactions and soften it), then in ethanol in a water bath to remove the chlorophyll (never heat ethanol over a naked flame – it is flammable), rinse, then add iodine solution. Starch turns iodine from orange-brown to blue-black.

  • Is light needed? Destarch a plant (leave it in the dark for 24–48 hours), cover part of a leaf with foil, place it in light, then test for starch. Only the exposed part goes blue-black.
  • Is chlorophyll needed? Test a variegated leaf (partly green, partly white). Only the green parts turn blue-black.
  • Is carbon dioxide needed? Enclose a leaf with soda lime (which absorbs CO2) – no starch forms – and compare with a control that still has CO2.

9. Why photosynthesis matters

  • Food – plants are producers; glucose is the base of nearly every food chain.
  • Oxygen – the oxygen released supports respiration in almost all living things.
  • Climate – photosynthesis removes carbon dioxide (a greenhouse gas) from the air and locks carbon into plants and forests.

10. Quick self-check

  1. Write the balanced symbol equation for photosynthesis.
  2. Where exactly in the chloroplast do the light-dependent reactions happen?
  3. Name the gas released when water is split.
  4. Give three limiting factors of photosynthesis.
  5. What colour does iodine turn when starch is present?

Answers: 1) 6CO2 + 6H2O → C6H12O6 + 6O2.   2) In the grana (thylakoids).   3) Oxygen.   4) Light intensity, carbon dioxide concentration, temperature.   5) Blue-black.

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