6.1 Photosynthesis

Cambridge O Level Biology 5090

2026 Syllabus Objectives

By the end of this subtopic, you should be able to:

  1. Understand that photosynthesis is the process by which plants make carbohydrates from raw materials using energy from light
  2. State that chlorophyll is a green pigment found in chloroplasts
  3. State that chlorophyll transfers light energy into chemical energy for the formation of glucose and other carbohydrates
  4. Outline the subsequent use and storage of carbohydrates made in photosynthesis (starch, cellulose, glucose in respiration, sucrose for transport)
  5. State the word equation and balanced chemical equation for photosynthesis
  6. Investigate the need for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls
  7. Describe and explain the effect of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis
  8. Investigate the effect of these factors using submerged aquatic plants and hydrogencarbonate indicator solution
  9. Identify and explain the limiting factors of photosynthesis in different environmental conditions

Main Content

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1. What Is Photosynthesis?

Photosynthesis is the process that green plants use to make their own food. The word "photosynthesis" comes from Greek words meaning "light" and "putting together" — and that is exactly what plants do: they use light energy to put simple substances together and make food.

During photosynthesis, plants take in two raw materials — carbon dioxide (a gas from the air) and water (absorbed through the roots) — and convert them into carbohydrates (mainly glucose, a type of sugar). At the same time, oxygen is released as a waste product.

💡 Important: The raw materials for photosynthesis are carbon dioxide and water. Although light is essential for the reaction to happen, light is a form of energy — not a substance — so it is not classed as a raw material.

Photosynthesis can be defined as:

"The process by which plants manufacture carbohydrates from raw materials using energy from light."

2. Chlorophyll — The Light-Trapping Pigment

A pigment is a coloured substance. Chlorophyll is a green pigment found inside structures called chloroplasts, which are tiny compartments found inside plant cells.

  • Chlorophyll gives plants their green colour because it reflects green light back to our eyes.
  • Chlorophyll absorbs light energy from the sun.
  • It then transfers that light energy into chemical energy, which is used to build glucose and other carbohydrates.
  • Without chlorophyll, photosynthesis cannot take place.

💡 Think of chlorophyll as a solar panel: just like a solar panel captures sunlight and converts it into electrical energy, chlorophyll captures light and converts it into chemical energy that the plant can use.

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