Last reviewed: 25 October 2026
Photosynthesis is how green plants make their own food, and almost every food chain on Earth depends on it. It is a key topic in junior school Integrated Science, with questions on the equation, the leaf and the classic starch test experiment. This guide explains each part in plain language. Nutrition in plants, which covers photosynthesis, is a sub-strand in KICD’s Grade 9 Integrated Science curriculum design.
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What Photosynthesis Is
Photosynthesis is the process in which green plants use light energy to make glucose (a sugar) from carbon dioxide and water, releasing oxygen as a by-product. It takes place mainly in the leaves, inside tiny structures called chloroplasts.
The Word Equation
carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
Learners should be able to write this from memory and name the raw materials (carbon dioxide and water), the products (glucose and oxygen) and the conditions (light and chlorophyll).
What Each Ingredient Does
- Light: provides the energy. Sunlight is the main source.
- Chlorophyll: the green pigment in chloroplasts that traps light energy.
- Carbon dioxide: enters the leaf from the air through tiny pores called stomata.
- Water: absorbed from the soil by root hairs and carried up the stem in xylem vessels to the leaves.
What Happens to the Glucose
Plants use glucose for energy through respiration. Extra glucose is changed into starch for storage in leaves, roots, stems and seeds. This is why foods such as cassava, potatoes, maize and rice are rich in starch. Glucose is also used to make cellulose for cell walls and, with minerals from the soil, proteins for growth.
How Leaves Are Adapted
- Broad and flat: a large surface area to absorb light.
- Thin: gases do not have far to travel.
- Many chloroplasts: especially in the palisade layer near the top surface.
- Stomata: mostly on the underside, letting carbon dioxide in and oxygen out.
- Veins: bring water and carry away food.
The Starch Test Experiment
Because plants store food as starch, testing a leaf for starch shows whether photosynthesis has taken place. The usual steps are:
- Boil the leaf in water for about a minute to kill it and soften the cells.
- Place it in warm alcohol (methylated spirit) in a test tube standing in hot water, never over a flame, to remove the chlorophyll. The leaf turns pale.
- Dip it in warm water to soften it again.
- Add a few drops of iodine solution. A blue-black colour shows starch is present; brown means no starch.
To show light is needed, cover part of a leaf with foil on a plant kept in the dark beforehand (destarched), then expose it to light for a day. After the test, only the uncovered part turns blue-black. A variegated leaf, with green and white parts, can show that chlorophyll is needed, as only the green parts store starch.
Factors Affecting the Rate
Photosynthesis speeds up with more light, more carbon dioxide and a suitable warm temperature, up to a limit. Too much heat slows it down. Water shortage also reduces it, which is one reason crops suffer during drought.
Why Photosynthesis Matters
It produces the food that feeds plants, animals and people through food chains, and it releases the oxygen that living things breathe. It also removes carbon dioxide from the air, which is one reason tree planting is encouraged in Kenya and around the world.
Revise and Practise
Write the equation daily until it is automatic, and draw the starch test set-up with labels. Our guides to cells, tissues and organs and flowers and pollination link closely to this topic. Try the Grade 8 Integrated Science papers, and use the exam countdown and What to Revise report to target weak areas. Full papers with answers come with the Monthly Pass (KSh 500 by M-Pesa).