What photosynthesis is and why it matters: WASSCE Biology

Plants make their own food. That single fact is the foundation of almost all life on Earth. The process by which they do it is photosynthesis. It feeds you, it gives you oxygen to breathe, and it shapes the climate. WAEC tests this topic heavily — usually 8-15 marks across objective and essay questions.

Definition

Photosynthesis — the process by which green plants make their own food (glucose) using carbon dioxide and water, in the presence of sunlight and chlorophyll.

Autotroph — an organism that makes its own food. Green plants are autotrophs.

Heterotroph — an organism that cannot make its own food; it must eat other things. All animals are heterotrophs.

Chlorophyll — the green pigment in plants that captures light energy.

Glucose — a simple sugar (C₆H₁₂O₆). The food that photosynthesis produces.

The equation

Photosynthesis can be summed up in one balanced equation. Memorise this — it appears in nearly every WAEC paper.

Word equation:

carbon dioxide + water → glucose + oxygen (in the presence of sunlight and chlorophyll)

Chemical equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Note the conditions written above the arrow: sunlight provides the energy; chlorophyll is the substance that captures that energy. Without both, the reaction does not happen.

What this equation actually means

Think of photosynthesis as a tiny chemical factory inside the leaf. The factory needs:

Raw materials: carbon dioxide (from the air) and water (from the soil, through the roots)
Energy: sunlight
A worker: chlorophyll

It produces:

Useful product: glucose (the plant's food)
Waste product: oxygen

The oxygen is released into the air through the leaves. The glucose is either used immediately or stored — usually as starch.

Common mistake

Students often say plants "get their food from the soil." They do not. Plants make their food from carbon dioxide and water using sunlight. The soil gives them water and minerals (like nitrates for proteins), but the actual food — the glucose — is made inside the leaf. This is a common WAEC mark loss.

Why photosynthesis matters

This is the part that earns higher-level marks. WAEC essay questions often ask for the importance of photosynthesis. Four reasons:

1. It is the source of nearly all food on Earth. You eat plants (rice, cassava, vegetables). The animals you eat (chicken, fish, cow) also eat plants or eat other animals that ate plants. Trace any meal back far enough, and it started with a plant making glucose by photosynthesis.

2. It produces almost all the oxygen in the atmosphere. Every breath you take is possible because plants released oxygen as a waste product of photosynthesis. Without plants, there would be no breathable air.

3. It removes carbon dioxide from the atmosphere. This helps regulate Earth's temperature and is why deforestation is a concern — fewer trees mean more CO₂ stays in the air.

4. It is the start of every food chain. A goat eats grass. A leopard eats the goat. Both depend on the grass having photosynthesised. The plant is always at the bottom — it is called the producer. Everything else is a consumer.

A quick note on respiration

Respiration is the opposite of photosynthesis. In respiration, glucose and oxygen react to release energy, producing carbon dioxide and water. Plants do both — they photosynthesise during the day (when there is light) and respire all the time (day and night). We will compare them properly in a later lesson.

Where it happens

Photosynthesis happens inside chloroplasts — tiny green organelles in plant cells. Chloroplasts contain chlorophyll. Most chloroplasts are found in leaves, especially in the palisade layer just under the upper surface. This is why leaves are typically green and flat — green to capture light, flat to expose lots of surface area to the sun.

The next lesson covers the leaf in detail.

Worked example

Question: State the word equation for photosynthesis and name the conditions necessary for it to take place.

Solution:

Word equation:

carbon dioxide + water → glucose + oxygen

Necessary conditions:

Sunlight (provides the energy)
Chlorophyll (the green pigment that absorbs light energy)

(Some answers also accept: a suitable temperature. But sunlight and chlorophyll are the two that WAEC marks most reliably.)

WAEC tests this

State the equation for photosynthesis — almost guaranteed. Both word and chemical forms can be asked.
What are the raw materials of photosynthesis? — carbon dioxide and water.
What are the products? — glucose and oxygen.
What are the necessary conditions? — sunlight and chlorophyll.
Why is photosynthesis important? — common essay. Answer: food source, oxygen production, removes CO₂, base of food chains.
What is the difference between an autotroph and a heterotroph? — short definition question.

Summary

Photosynthesis is how green plants make their own food.
Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, needing sunlight and chlorophyll.
Raw materials: CO₂ (from air) and water (from soil).
Products: glucose (food) and oxygen (released into the air).
Photosynthesis happens in chloroplasts, mostly in leaves.
Why it matters: feeds nearly all life, produces oxygen, removes CO₂, starts every food chain.
Plants are autotrophs (make their food); animals are heterotrophs (eat others).

What photosynthesis is and why it matters