Photosynthesis: reaction, reactants and products

What you'll learn

Photosynthesis is the fundamental process by which plants manufacture glucose from simple inorganic molecules. This revision guide covers the chemical equation for photosynthesis, the role of each reactant and product, and how to balance and interpret the equation in exam questions. You'll learn exactly what AQA GCSE Biology examiners expect when testing this essential topic.

Key terms and definitions

Photosynthesis — the process by which plants synthesise glucose from carbon dioxide and water using light energy, which is absorbed by chlorophyll

Chlorophyll — the green pigment found in chloroplasts that absorbs light energy for photosynthesis

Reactants — the substances that are used up during a chemical reaction (carbon dioxide and water in photosynthesis)

Products — the substances that are made during a chemical reaction (glucose and oxygen in photosynthesis)

Endothermic reaction — a reaction that requires energy to be transferred from the surroundings (photosynthesis requires light energy)

Glucose — a simple sugar (C₆H₁₂O₆) produced during photosynthesis, used for respiration or converted to other substances

Limiting factor — a variable that restricts the rate of photosynthesis when in short supply (carbon dioxide concentration, light intensity or temperature)

Chloroplast — the organelle in plant cells where photosynthesis occurs, containing chlorophyll in thylakoid membranes

Core concepts

The word equation for photosynthesis

The word equation summarises the overall process of photosynthesis:

carbon dioxide + water → glucose + oxygen

You must be able to write this equation from memory in the exam. Notice that:

• The reactants (substances used up) appear on the left side before the arrow
• The products (substances made) appear on the right side after the arrow
• Light energy is required for the reaction to occur
• The arrow means "reacts to produce" or "yields"

Some exam questions ask you to add additional information about where the energy comes from. The complete version includes:

carbon dioxide + water --(light energy)--> glucose + oxygen (chlorophyll)

This shows that light energy is required and that chlorophyll is necessary, though chlorophyll itself is not used up (it acts as a catalyst).

The symbol equation for photosynthesis

At GCSE level, you must also know the balanced symbol equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Key points about this equation:

• The numbers before each formula (coefficients) show the ratio of molecules involved
• 6 molecules of carbon dioxide react with 6 molecules of water
• This produces 1 molecule of glucose and 6 molecules of oxygen
• The equation must be balanced — the same number of each type of atom appears on both sides
• You may be asked to balance this equation or explain why the numbers are necessary

Checking the balance:

• Carbon atoms: 6 on left (from 6CO₂), 6 on right (in C₆H₁₂O₆) ✓
• Hydrogen atoms: 12 on left (from 6H₂O), 12 on right (in C₆H₁₂O₆) ✓
• Oxygen atoms: 18 on left (12 from 6CO₂ + 6 from 6H₂O), 18 on right (6 from C₆H₁₂O₆ + 12 from 6O₂) ✓

Carbon dioxide as a reactant

Carbon dioxide enters the plant through stomata (tiny pores mainly on the underside of leaves). Understanding the journey of CO₂ is important:

• CO₂ diffuses from the air (approximately 0.04% concentration) into air spaces in the leaf
• It then diffuses through cell walls and membranes into mesophyll cells
• Finally, CO₂ enters chloroplasts where photosynthesis occurs
• Each carbon dioxide molecule provides one carbon atom for glucose synthesis

The carbon from CO₂ is incorporated into glucose molecules. Since glucose has the formula C₆H₁₂O₆, six carbon atoms are needed, which is why six CO₂ molecules are required.

Low carbon dioxide concentration limits the rate of photosynthesis. This is why gardeners sometimes add CO₂ to greenhouses to increase crop yields.

Water as a reactant

Water is absorbed by plant roots from the soil and transported to leaves through xylem vessels. Key facts about water in photosynthesis:

• Water molecules are split during photosynthesis (a process called photolysis in later studies)
• This occurs in the light-dependent reactions within chloroplasts
• Each water molecule provides hydrogen atoms that combine with carbon to form glucose
• Oxygen from water molecules contributes to both glucose and oxygen gas produced

The 6H₂O in the equation shows that six water molecules are needed for each glucose molecule produced. Water is essential — without it, photosynthesis stops immediately, even if light and CO₂ are available.

Students often ask: "If plants need water for photosynthesis, why do they wilt when overwatered?" This is because waterlogged soil lacks oxygen for root respiration, not because of photosynthesis itself.

Glucose as a product

Glucose (C₆H₁₂O₆) is the immediate product of photosynthesis and serves multiple purposes in the plant:

Immediate use:

• Converted to sucrose for transport in phloem to other parts of the plant
• Used in respiration to release energy for cell processes

Storage:

• Converted to starch (insoluble, compact storage molecule) in chloroplasts and other organs
• Stored in roots (e.g. carrots), stems (e.g. sugar cane) or seeds

Synthesis of other molecules:

• Converted to cellulose for cell walls
• Combined with nitrate ions (from soil) to make amino acids, then proteins
• Used to make lipids (fats and oils) for storage in seeds

The versatility of glucose explains why photosynthesis is so crucial — it provides both energy and raw materials for the entire plant.

Oxygen as a product

Oxygen gas (O₂) is the other product of photosynthesis. Important points:

• Oxygen is released as a waste product — plants don't need all the oxygen they produce
• It diffuses out of leaves through stomata into the atmosphere
• Some oxygen is used by the plant for respiration, especially at night
• The oxygen we breathe comes primarily from photosynthesising organisms (plants and algae)

Testing for oxygen production proves photosynthesis is occurring:

• Aquatic plants like Elodea release visible oxygen bubbles when photosynthesising
• The gas can be collected and tested with a glowing splint, which relights
• Rate of bubble production indicates the rate of photosynthesis

Nearly all oxygen atoms in the O₂ released come from water molecules, not from carbon dioxide, though this detail is beyond GCSE requirements.

Energy transfers in photosynthesis

Photosynthesis is an endothermic reaction — energy is transferred from the surroundings to make it happen. Specifically:

• Light energy is absorbed by chlorophyll in chloroplasts
• This energy is transferred to chemical energy stored in glucose molecules
• The glucose molecule contains more chemical energy than the reactants
• This stored energy can later be released during respiration

The source of light is typically the Sun, though artificial lights work too (important for growing plants indoors or in greenhouses).

Energy transfer can be shown in the equation:

light energy + carbon dioxide + water → glucose (chemical energy stored) + oxygen

This energy transfer is why photosynthesis is so important globally — it converts light energy into a form (chemical energy in glucose) that can be used by living organisms through food chains.

Worked examples

Example 1: Balancing the photosynthesis equation (2 marks)

Question: Complete the balanced symbol equation for photosynthesis by adding the correct numbers:

__CO₂ + __H₂O → __C₆H₁₂O₆ + __O₂

Mark scheme answer:

6CO₂ + 6H₂O → 1C₆H₁₂O₆ + 6O₂ (2 marks)

• Award 1 mark for any three correct coefficients
• Award 2 marks for all four correct coefficients
• Accept the '1' being omitted before C₆H₁₂O₆

Examiner tip: Count atoms systematically. Start with carbon (6 needed for glucose), then hydrogen (12 needed), then calculate oxygen last. Show your working if space permits.

Example 2: Identifying reactants and products (4 marks)

Question: A student investigated photosynthesis in pondweed. The pondweed was placed in water containing an indicator that changes colour when carbon dioxide concentration changes.

(a) Name two reactants needed for photosynthesis. (2 marks)

(b) Name the product of photosynthesis that the plant uses in respiration. (1 mark)

(c) Name the product of photosynthesis that can be detected as bubbles from pondweed. (1 mark)

Mark scheme answer:

(a) Carbon dioxide (1 mark) Water (1 mark)

(b) Glucose (1 mark) Accept: sugar

(c) Oxygen (1 mark)

Examiner tip: Read the question carefully. Part (a) asks for reactants (plural), so you must give two answers. Don't waste time giving products when reactants are asked for.

Example 3: Application question (6 marks)

Question: A farmer grows tomatoes in a greenhouse. He adds extra carbon dioxide to the air inside the greenhouse during daylight hours.

(a) Explain why adding carbon dioxide increases the rate of photosynthesis in the tomato plants. (3 marks)

(b) Explain why the farmer only adds carbon dioxide during daylight hours. (3 marks)

Mark scheme answer:

(a)

• Carbon dioxide is a reactant in photosynthesis (1 mark)
• Increasing CO₂ concentration means more frequent collisions/more CO₂ available (1 mark)
• So more glucose can be produced per unit time / rate increases (1 mark)

Accept: CO₂ is a limiting factor / without enough CO₂ the rate is limited

(b)

• Photosynthesis requires light (1 mark)
• At night there is no light / in darkness photosynthesis does not occur (1 mark)
• So the plants would not use the carbon dioxide / it would be wasted (1 mark)

Accept: plants respire at night, releasing CO₂, so don't need extra

Examiner tip: 'Explain' questions require reasoning, not just facts. Link your points with 'because', 'therefore' or 'so' to show causation. Three marks suggests three linked points are needed.

Common mistakes and how to avoid them

• Writing "sunlight" or "light" as a reactant — Light is the energy source, not a reactant. Reactants are substances (carbon dioxide and water only). Energy is shown above the arrow or stated separately.

• Confusing photosynthesis with respiration — Photosynthesis makes glucose using CO₂ and water; respiration breaks down glucose using oxygen. They are opposite processes. Make sure you know which direction each equation goes.

• Forgetting to balance the symbol equation — The equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ must have these specific numbers. CO₂ + H₂O → C₆H₁₂O₆ + O₂ is incorrect and will lose marks.

• Stating that plants only photosynthesise during the day — While true, be careful with wording. Plants photosynthesise when light is available. In an exam, saying "plants photosynthesise in the light" is more accurate than "during the day."

• Mixing up the uses of glucose — Glucose is used for respiration (releases energy), converted to starch (storage), made into cellulose (cell walls), or used to make amino acids/proteins. Don't say glucose is stored directly — it's converted to starch first.

• Not checking which form of equation is required — Questions may ask for a word equation OR symbol equation. Read carefully. If it asks for "the equation," the word equation is usually acceptable unless symbols are specifically requested.

Exam technique for "Photosynthesis: reaction, reactants and products"

• Command words matter: "State" needs brief answers (e.g. "carbon dioxide, water"). "Explain" requires reasoning (e.g. "Carbon dioxide is needed because it provides carbon atoms for glucose"). "Describe" needs observations or steps without explanation.

• Check mark allocations: If a question is worth 2 marks asking for reactants, you must name both carbon dioxide and water. One answer = maximum 1 mark. Count the marks and provide that many valid points.

• Use correct scientific terminology: Write "carbon dioxide" not "CO₂" in word equations. Write "C₆H₁₂O₆" not "glucose" in symbol equations. Write "glucose" not "sugar" unless the mark scheme accepts alternatives.

• Link ideas in longer answers: For 4-6 mark questions about photosynthesis, structure your answer logically: reactants → process → products → uses. Use connectives like "therefore," "this means," and "as a result" to show understanding.

Quick revision summary

Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy absorbed by chlorophyll. The word equation is: carbon dioxide + water → glucose + oxygen. The balanced symbol equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Photosynthesis is an endothermic reaction occurring in chloroplasts. Glucose is used for respiration, converted to starch for storage, or made into other molecules like cellulose and proteins. Oxygen is released as a waste product through stomata.