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The photosynthesis equation is carbon dioxide + water → glucose + oxygen, or 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Free notes with key terms and 14 questions answered.
The Photosynthesis Equation and Chemical Symbols is part of Photosynthesis in GCSE AQA Higher Biology.
4 min
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GCSE AQA Higher
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The photosynthesis equation is carbon dioxide + water → glucose + oxygen, or 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Free notes with key terms and 14 questions answered.
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Use this lesson to stay inside Photosynthesis while connecting the detail back to the wider Bioenergetics topic.
Read the core explanations from The Photosynthesis Equation and Chemical Symbols before testing yourself from memory.
Every leaf on every plant is running the same chemical reaction right now. It's taking two simple ingredients from the environment and building something complex - glucose. This process keeps every food chain on Earth running, but how does it actually work at the molecular level?
The secret lies in understanding the chemical equation. Once you can read the symbols, you'll see exactly what goes in, what comes out, and why plants need sunlight to make it happen.
Scientists write chemical reactions in two ways: word equations (clear to understand) and symbol equations (precise and compact). Let's start with the version that makes sense in plain English.
Required equation: carbon dioxide + water → glucose + oxygen, with light energy needed for the reaction. In symbols, this is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ with light above the arrow.
This tells us the story: plants take carbon dioxide from the air and water from their roots. With light energy, they build glucose (sugar) and release oxygen as a waste product. But examiners want precision, which means chemical symbols.
Notice the numbers in front - these balance the equation. You need 6 molecules of carbon dioxide and 6 molecules of water to make 1 molecule of glucose and 6 molecules of oxygen. The arrow shows light energy is needed to make this reaction happen.
Chemical symbols look intimidating, but they follow simple rules. Each symbol tells you exactly which atoms are present and how many of each. Let's decode the four key symbols you must recognise for photosynthesis.
Trap: Many students think the small numbers (subscripts) can be ignored. They can't - H₂O is completely different from HO. The subscript tells you exactly how many atoms of each element are in the molecule.
Not all chemical reactions are the same. Some release energy (like burning), others need energy input to happen. Photosynthesis belongs to the second group - it's endothermic.
Think about it logically: carbon dioxide and water are simple, low-energy molecules. Glucose is complex and energy-rich - that's why we eat it for fuel. To build something complex from simple parts requires energy input, just like building a house requires work.
Plants get this energy from light, which is absorbed by chloroplasts in leaf cells. The chloroplasts contain chlorophyll, which captures light energy and transfers it to the chemical reaction. Without light, photosynthesis stops.
Now you can read the full story that the photosynthesis equation tells. Plants take low-energy reactants (CO₂ and H₂O) and use light energy to build glucose, which stores chemical energy. Oxygen is released as a by-product. This is why plants need sunlight and why they are the foundation of food chains.
Worked example: Writing the Balanced Photosynthesis Equation
Write the balanced symbol equation for photosynthesis, including the energy requirement. [3 marks]
Answer: $6CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2$
Examiners have specific mark schemes for photosynthesis questions. Here's how to hit every mark point and avoid the common traps that cost students marks.
Command word alert: If asked to 'describe' photosynthesis, state what happens. If asked to 'explain' why it's endothermic, give the reason (energy is taken in from surroundings). The command word tells you how much detail to include.
A balanced equation is essential because atoms are conserved in chemical reactions. In photosynthesis, six carbon dioxide molecules and six water molecules provide enough atoms to form one glucose molecule and six oxygen molecules. Mentioning both balancing and energy input from light usually secures full marks on explain questions.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The equation for photosynthesis is carbon dioxide + water → glucose + oxygen, in the presence of light. In symbols this is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂: plants take carbon dioxide from the air and water from their roots, and use light energy to build glucose, releasing oxygen.
The word equation for photosynthesis is carbon dioxide + water → glucose + oxygen, with light energy needed for the reaction. Plants take carbon dioxide from the air and water from their roots, build glucose and release oxygen as a waste product.
The chemical symbol for glucose is C₆H₁₂O₆: six carbon, twelve hydrogen and six oxygen atoms. Glucose is the sugar molecule plants make in photosynthesis and store as starch.
The symbol equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light shown over the arrow. Six molecules of carbon dioxide and six of water make one molecule of glucose and six molecules of oxygen.
The balanced symbol equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy shown on the arrow. Six molecules of carbon dioxide and six of water make one molecule of glucose and six of oxygen, because atoms are conserved in chemical reactions.
Photosynthesis is endothermic because it takes in energy from the surroundings: the products have more energy than the reactants. Carbon dioxide and water are simple, low-energy molecules, while glucose is complex and energy-rich, so building it requires energy input from light.
The word equation for photosynthesis is carbon dioxide + water → glucose + oxygen, with light energy needed. The symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light shown over the arrow.
Photosynthesis is shown in symbols by putting the reactants CO₂ + H₂O, adding light over the arrow, adding the products C₆H₁₂O₆ + O₂, then balancing: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Six of each reactant are needed to make one glucose.
The energy for photosynthesis comes from light, which is absorbed by chloroplasts in leaf cells. The chloroplasts contain chlorophyll, which captures light energy and transfers it to the chemical reaction, so without light photosynthesis stops.
In the photosynthesis equation, CO₂ is carbon dioxide (one carbon and two oxygen atoms), H₂O is water (two hydrogen and one oxygen atom), C₆H₁₂O₆ is glucose (six carbon, twelve hydrogen and six oxygen atoms) and O₂ is oxygen (two oxygen atoms).
The chemical symbol equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light over the arrow. The numbers in front balance the equation, because atoms are conserved in chemical reactions.
The photosynthesis equation is balanced with sixes because atoms are conserved in chemical reactions. Six carbon dioxide molecules and six water molecules provide enough atoms to form one glucose molecule (C₆H₁₂O₆) and six oxygen molecules.
The small numbers (subscripts) in chemical symbols matter because they show exactly how many atoms of each element are in the molecule. H₂O is completely different from HO, so subscripts cannot be ignored.
An endothermic reaction is a chemical reaction that takes in energy from the surroundings, so the products have more energy than the reactants. Photosynthesis is endothermic because building energy-rich glucose from simple, low-energy carbon dioxide and water requires light energy.