Bioenergetics · GCSE Biology
Photosynthesis equation
Write the GCSE Biology photosynthesis equations from memory: carbon dioxide and water to glucose and oxygen, endothermic in chloroplasts, and why this is the reverse skeleton of respiration.
Light in, glucose and oxygen out. Word: carbon dioxide + water → glucose + oxygen. Symbol: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The factory is the chloroplast; the reaction is endothermic.
The important bits
What you need to know
- 1
Photosynthesis is an endothermic reaction: energy is transferred from the environment (light) to the chloroplasts and stored in glucose.
- 2
Word equation: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
- 3
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Balance the 6s; glucose is C₆H₁₂O₆, not C₆H₆O₆.
- 4
Chlorophyll in chloroplasts absorbs light, mainly red and blue wavelengths. Green light is mostly reflected, which is why leaves look green.
- 5
Carbon dioxide enters through stomata. Water arrives from the roots in the xylem. Oxygen is a by-product and leaves through stomata (or is used in respiration).
- 6
The reaction occurs in palisade mesophyll cells packed with chloroplasts, and also in spongy mesophyll and guard cells, not in root cells.
- 7
Respiration is the exothermic counterpart: glucose + oxygen → carbon dioxide + water. Plants photosynthesise in the light and respire all the time. Do not swap the equations.
- 8
Glucose is soluble; plants often convert it quickly to starch for storage (iodine test), cellulose, lipids or amino acids (with nitrate).
Quotations worth analysing
Short evidence. Real method.
“carbon dioxide + water → glucose + oxygen”
State that light and chlorophyll are required. Endothermic means energy is taken in from light, not released.
“6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂”
The 6s balance carbon, hydrogen and oxygen. Mixing this with the respiration equation is the most expensive error in bioenergetics.
“Photosynthesis is endothermic and occurs in chloroplasts.”
Location plus energy type. Mitochondria are for aerobic respiration, not for this reaction.
Go deeper
How do I stop swapping photosynthesis and respiration?
Learn them as a pair with opposite arrows. Photosynthesis builds glucose and oxygen from carbon dioxide and water using light in chloroplasts; it is endothermic. Respiration breaks glucose (with oxygen, if aerobic) into carbon dioxide and water in mitochondria (mainly) and is exothermic. In the light a leaf may have a net output of oxygen because photosynthesis is faster than respiration. In the dark only respiration continues, so the leaf takes in oxygen and releases carbon dioxide. A common practical uses hydrogen-carbonate indicator: purple in low CO₂ (photosynthesis dominating), yellow in high CO₂ (respiration dominating). If you cannot write both equations without hesitation, stop and drill them before graphs or limiting factors. Examiners recycle the swap across both papers.
Go deeper
What do the numbers in 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ actually mean?
Six carbon dioxide molecules supply the six carbons in glucose. Twelve hydrogens in glucose come from six water molecules. Six oxygen molecules are released. You do not need the light-dependent and light-independent biochemistry at Combined level; Triple may mention that light is used to split water and that carbon dioxide is fixed into sugar, but the balanced equation is the scoring core. If a question gives masses, conservation of mass still applies: the mass of CO₂ and water used equals the mass of glucose and oxygen produced. Growers care because extra CO₂ can mean extra glucose if light and temperature allow. Write the equation first, then attach chloroplast and chlorophyll. “Plants breathe in carbon dioxide” is everyday talk, not a mark.
Go deeper
Where in the leaf do the reactants actually meet?
Water travels xylem → mesophyll cell → chloroplast. Carbon dioxide diffuses: air → stoma → air space → cell membrane → chloroplast. Light hits chlorophyll. Glucose is made in the chloroplast then used in the same cell’s mitochondria, moved as sucrose in phloem, or stored as starch. Oxygen diffuses out the reverse path, which is why pondweed bubbles in bright light. If stomata close to save water, CO₂ entry slows and photosynthesis may be limited even on a sunny day — that is already a limiting-factor story. Root cells have water but no chloroplasts, so they cannot run this equation; they import sugar. Always name chloroplast as the site, not “the leaf” alone, when the command word is explain.
See the idea in action
Write the symbol equation and state where the reaction happens. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in chloroplasts, using light absorbed by chlorophyll. It is endothermic. A student who writes glucose + oxygen → carbon dioxide + water has given aerobic respiration and will lose the equation mark and often the endothermic mark. A pondweed sprig producing bubbles in the light is releasing the O₂ from this equation; counting bubbles is a proxy for rate, not a different reaction.
Exam technique
Turn knowledge into marks
Write both word and symbol forms until they are automatic. Always add chloroplasts and chlorophyll. State endothermic. Then, if asked, name stomata for CO₂ and xylem for water. Never place photosynthesis in mitochondria.
Common mistakes
Do not give these marks away
- 01
Writing the respiration equation instead, or omitting that the reaction happens in chloroplasts.
- 02
Forgetting the 6s in the symbol equation, or writing the wrong glucose formula.
- 03
Saying plants only photosynthesise and do not respire, or calling photosynthesis exothermic.
Which is the correct symbol equation for photosynthesis?
AC₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
B6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Cglucose → lactic acid
D6CO₂ + 6O₂ → C₆H₁₂O₆ + 6H₂O
Show the answer
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon dioxide and water are the reactants; glucose and oxygen are the products. The first option is aerobic respiration. Light and chlorophyll are also required.
Quick questions
If this is the bit you searched
What is the word equation for photosynthesis?
Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
What is the symbol equation for photosynthesis?
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Is photosynthesis endothermic or exothermic?
Endothermic. Energy is transferred from light into the chloroplasts and stored in glucose. Respiration is exothermic.
Where does photosynthesis take place in a plant cell?
In chloroplasts, which contain chlorophyll to absorb light. Root cells usually have no chloroplasts.