Products of photosynthesis: oxygen, CO2 uptake and measuring rate

The big idea: Photosynthesis turns carbon dioxide and water into glucose and oxygen, using light energy trapped by chlorophyll.

So the reaction has two products: glucose (which stores the chemical energy) and oxygen (which is released as a waste gas).

At the same time, the plant takes in carbon dioxide as a raw material. These two gas changes — oxygen released, carbon dioxide absorbed — are how we can tell photosynthesis is happening and how fast it is going.

Product

A substance that is made by a reaction. The products of photosynthesis are glucose and oxygen.

Raw material (reactant)

A substance that is used up by a reaction. The raw materials of photosynthesis are carbon dioxide and water.

Oxygen (O₂)

A gas released as a waste product of photosynthesis. It comes from the splitting of water.

Carbon dioxide (CO₂)

A gas taken in (absorbed) during photosynthesis. Its carbon is built into glucose.

Rate of photosynthesis

How fast photosynthesis is happening — for example, how much oxygen is produced (or carbon dioxide used) each minute.

Two gases, opposite directions: Photosynthesis gives off oxygen and takes in carbon dioxide.

These are opposite to respiration (which uses oxygen and gives off carbon dioxide) — keep the two reactions clearly separate in your mind.

Because photosynthesis releases oxygen and uses up carbon dioxide, we can measure how fast it is going by tracking either gas.

There are three common ways: measure the oxygen produced, measure the carbon dioxide taken up, or measure the pH of the surrounding water (which changes as carbon dioxide levels change).

1. Count the oxygen bubbles: An aquatic plant (such as pondweed) in bright light releases bubbles of oxygen from its cut stem.

The faster it photosynthesises, the more bubbles are given off each minute. Counting bubbles (or collecting and measuring the volume of gas) is a simple measure of the rate.

2. Use a carbon dioxide indicator: A CO₂ (hydrogencarbonate) indicator changes colour depending on how much carbon dioxide is dissolved in the water.

As a plant photosynthesises it removes carbon dioxide, so the indicator's colour shifts — showing that CO₂ has been used up. The bigger the colour change, the more photosynthesis has happened.

This is a favourite data question: you are shown the colour (or a result) and have to work out which tube had the least carbon dioxide left in the water.

3. Track the pH: Dissolved carbon dioxide makes water slightly acidic (it forms a weak carbonic acid).

When a plant photosynthesises and removes CO₂, there is less acid, so the pH rises (goes up).

Measuring the pH is therefore an indirect way to measure the rate — a rising pH means carbon dioxide is being taken in faster.

Why pH goes UP, not down: Removing carbon dioxide removes acid from the water, so the pH rises.

A common slip is to think more activity makes water more acidic — but for photosynthesis it is the opposite: CO₂ out → less acid → higher pH.

How this is tested: A 1-mark Paper 1 question often asks you to explain why an illuminated aquatic plant produces gas bubbles — the answer links the bubbles to oxygen released by photosynthesis.

A Paper 1A / 1B data question may give the colour of a CO₂ indicator in different light conditions and ask which tube's water ends up free of carbon dioxide.

On Paper 3 a 2-mark Suggest question can ask why faster photosynthesis raises the pH of the surrounding water — you are expected to reason from CO₂ removal to a rise in pH.

✓ Why this scores full marks: It names the gas (oxygen), explains where it comes from (a product of photosynthesis in the light), and connects it to the rate (more bubbles = faster).

Just saying 'the plant gives off gas' would not score — you must state it is oxygen and that it is a product of photosynthesis.