Photosynthesis

Photosynthesis is how green plants make their own food. The word comes from two Greek words: photo meaning "light" and synthesis meaning "putting together". So photosynthesis literally means "putting together with light". It is the most important chemical reaction on Earth — without it, there would be no plants, no animals (because they all eat plants or eat things that eat plants), and almost no oxygen in the air.

The four things a plant needs for photosynthesis:

1. Sunlight — the energy source. Plants capture sunlight using a green pigment called chlorophyll, which lives inside tiny structures called chloroplasts in the leaf cells.
2. Carbon dioxide (CO₂) — taken in from the air through tiny holes on the underside of the leaf called stomata.
3. Water (H₂O) — absorbed from the soil through the roots and carried up to the leaves through the stem in tubes called xylem.
4. Chlorophyll — the green chemical that traps the light energy. It is found in the chloroplasts of the palisade cells just under the upper surface of the leaf.

The photosynthesis word equation:

Carbon dioxide + Water → Glucose + Oxygen (in the presence of sunlight and chlorophyll)

The chemical equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

What the plant gains:

• Glucose — a sugar that the plant uses as food. It gives the plant energy to grow, make seeds and repair damage. Some glucose is stored as starch in the leaves, roots or fruits — this is what we eat in maize, potatoes, sweet potatoes and bananas.
• Oxygen — a "waste" product released into the air through the stomata. This oxygen is what humans and animals breathe.

Where photosynthesis happens:

Photosynthesis takes place mainly in the leaves. A leaf is the perfect organ for the job: it is flat and wide (large surface area to catch sunlight), thin (so light reaches all cells), green (because of chlorophyll), and has stomata for gas exchange. The veins inside the leaf carry water in and glucose out.

Day and night:

Photosynthesis only happens in daylight, because it needs sunlight. At night, when there is no light, plants do not photosynthesise — but they still respire (breathe in oxygen and release carbon dioxide), just like animals do. So at night a plant is a NET producer of CO₂; during the day it produces more oxygen than it uses up.

Testing a leaf for starch:

A common CBC and KPSEA experiment is to test whether a leaf has been photosynthesising. The steps are:

1. Boil the leaf in water to soften it.
2. Boil it in methylated spirit to remove the green chlorophyll.
3. Dip in warm water to soften again.
4. Add iodine solution. If starch is present, the leaf turns blue-black. No starch → it stays brown/yellow.

A plant kept in the dark for two days fails the iodine test — proof that sunlight is needed for photosynthesis.

Common student mistakes to avoid:

• Saying plants "eat soil". Plants make their own food (glucose); they only take water and minerals from soil.
• Mixing up photosynthesis with respiration. Photosynthesis takes in CO₂ and releases O₂; respiration is the opposite.
• Forgetting that ALL living plant cells respire — including at night. Photosynthesis is only during the day.
• Confusing chlorophyll (the green chemical) with chloroplast (the structure that contains the chemical).

Why photosynthesis matters:

Every food chain on Earth starts with a green plant or algae. Plants are called producers because they produce food from raw materials. Animals are consumers because they consume food made by plants. Photosynthesis is also the main source of the oxygen we breathe — about half from forests on land, the other half from algae in the oceans. Cutting down forests reduces photosynthesis worldwide, raising CO₂ and contributing to climate change.

CBC Grade 5 introduces plant parts and their functions; Grade 6 covers photosynthesis as a process and the leaf as an organ; Grade 7–9 Integrated Science extends to mineral nutrition, transpiration, and the testing-for-starch experiment as part of KPSEA / KJSEA practical work.