Photosynthesis Explained: KS3 Biology Guide

Photosynthesis is the process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. The word equation is: carbon dioxide + water → glucose + oxygen. It takes place mainly in the leaves and requires chlorophyll to absorb light. Photosynthesis is one of the most important biological processes on Earth and a core topic in KS3 biology.

What is the word equation for photosynthesis?

The KS3 word equation you must know is:

carbon dioxide + water → glucose + oxygen

The arrow means "produces" or "yields." Light energy (from the Sun) drives this reaction — it does not appear in the equation as a reactant, but is written above the arrow to show it is required.

In symbol form (which you may encounter in Year 9 and GCSE):

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

At KS3, you are expected to recall the word equation. The symbol equation is a useful extension but is not required for most Year 7–9 assessments.

Where does photosynthesis take place?

Photosynthesis takes place inside chloroplasts, which are organelles found mainly in the cells of leaves. Chloroplasts contain a green pigment called chlorophyll. Chlorophyll absorbs light energy — it absorbs red and blue wavelengths particularly well and reflects green wavelengths, which is why leaves look green.

Leaves are adapted in several ways to maximise photosynthesis:

Broad and flat: large surface area to capture more sunlight

Thin: short distance for carbon dioxide and light to travel to chloroplasts

Stomata on the underside: allow carbon dioxide to diffuse in and oxygen to diffuse out

Veins (vascular bundles): transport water from roots to leaves (via xylem) and carry glucose away (via phloem)

Palisade cells near the top of the leaf: packed with chloroplasts to catch the most light

What does a plant need for photosynthesis?

A plant needs three things:

Light — the energy source. Without light, the reaction cannot proceed.

Carbon dioxide — taken in through stomata by diffusion from the air.

Water — absorbed by roots and transported up through xylem vessels to the leaves.

Chlorophyll is not consumed — it is a pigment that absorbs and transfers light energy to drive the chemical reactions. Without chlorophyll, the plant cannot capture light energy and photosynthesis cannot occur.

What does photosynthesis produce?

Photosynthesis produces two things:

Glucose — the plant's food. Glucose is used for respiration (to release energy), for growth (converted into cellulose for cell walls), for storing as starch, or for making other biological molecules such as proteins and fats.

Oxygen — released as a by-product through the stomata. The oxygen produced by photosynthesis by plants and algae is the source of almost all atmospheric oxygen on Earth.

A worked example: testing a leaf for starch

One of the most common KS3 practical tasks is testing whether a leaf has been photosynthesising by checking for starch (into which plants convert glucose for storage).

Procedure (standard school method):

Take a leaf that has been in light for several hours and a leaf kept in the dark for 24 hours.

Kill the cells by placing each leaf in boiling water for one minute (this stops chemical reactions and softens the leaf).

Remove the chlorophyll by placing the leaf in ethanol (industrial methylated spirit) heated in a water bath — never heat ethanol directly over a flame, as it is flammable. This decolourises the leaf.

Rinse with cold water to rehydrate.

Spread the leaf flat on a white tile and add a few drops of iodine solution.

Results:

The leaf kept in light turns blue-black: starch is present, confirming photosynthesis occurred.

The leaf kept in the dark remains yellow-brown: no starch, confirming photosynthesis did not occur without light.

This experiment demonstrates that light is a necessary condition for photosynthesis.

What affects the rate of photosynthesis?

Three main factors are examined at KS3 and GCSE:

Light intensity: More light means faster photosynthesis — up to a point. Beyond a certain level, other factors become limiting.

Carbon dioxide concentration: More CO₂ speeds up photosynthesis — again up to a limiting point.

Temperature: Photosynthesis involves enzymes. Increasing temperature up to about 25–35 °C generally speeds up enzyme activity and therefore the rate. Above this range, enzymes begin to denature (change shape permanently) and the rate falls sharply.

The concept of a limiting factor — the factor that is currently preventing the rate from going faster — is key for GCSE and is worth introducing at KS3.

How does photosynthesis relate to respiration?

Photosynthesis and respiration are often described as opposite processes, and in some ways they are:

| Photosynthesis | Respiration
Energy | Stores energy (from light) in glucose | Releases energy from glucose
Gas in | Carbon dioxide | Oxygen
Gas out | Oxygen | Carbon dioxide
Where | Chloroplasts (plant cells with chlorophyll) | Mitochondria (all living cells)
Time | Daylight hours | All the time, day and night

During the day, a plant photosynthesises faster than it respires, so it takes in net carbon dioxide and releases net oxygen. At night, with no light, photosynthesis stops but respiration continues, so the plant takes in oxygen and releases carbon dioxide — just like animals.

What is the national curriculum requirement for photosynthesis at KS3?

The Department for Education's Science Programmes of Study for Key Stage 3 specifies that pupils should understand "the reactants in, and products of, photosynthesis, and a word summary for photosynthesis" and "the dependence of almost all life on Earth on the ability of photosynthetic organisms to use sunlight in photosynthesis to build organic molecules that are an essential energy store." BBC Bitesize KS3 biology aligns this directly to the word equation and leaf adaptation content.

Frequently asked questions

What is the word equation for photosynthesis at KS3?

Carbon dioxide + water → glucose + oxygen. Light energy drives the reaction and chlorophyll absorbs it. You must be able to write this from memory for Year 8/9 assessments and GCSE.

Where does photosynthesis happen in the leaf?

Photosynthesis happens in the chloroplasts, which are found mainly in the palisade mesophyll cells in the upper part of the leaf. Chloroplasts contain chlorophyll, which absorbs the light energy needed to drive the reaction.

Why do plants need photosynthesis?

Plants use photosynthesis to make glucose, which they need for energy (via respiration), for growth (building cell walls from cellulose), and for making other biological molecules. Without photosynthesis, plants could not grow or survive.

What happens to photosynthesis at night?

At night there is no light, so photosynthesis stops. Plants continue to respire around the clock, however, taking in oxygen and releasing carbon dioxide. During the day, photosynthesis outpaces respiration, so the net effect is carbon dioxide in and oxygen out.

How can you test whether a leaf has photosynthesised?

Test the leaf for starch using iodine solution. If starch is present, the leaf turns blue-black, indicating that photosynthesis occurred and glucose was converted into starch for storage. A leaf that was kept in the dark will not contain starch and will stay yellow-brown.

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