10.3Chemistry of the Environment
Photosynthesis and the carbon dioxide balance in air — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 6 parts Lesson 4 of 4 Unit quiz
Lesson summary
Photosynthesis and the carbon dioxide balance in air — IGCSE CHEMISTRY notes: State the word and balanced equations for photosynthesis, explain why it is endothermic and describe how it removes carbon dioxide from the air.Green plants use light energy to convert carbon dioxide an…
- carbon dioxide + water → glucose + oxygen
- 6CO2 + 6H2O → C6H12O6 + 6O2
- Chlorophyll
- Carbon cycle
- Reforestation
- Green plants use light energy to convert carbon dioxide and water into glucose and oxygen
Part 1 of 4Photosynthesis: reactants, products and conditions
1
Green plants use light energy to convert carbon dioxide and water into glucose and oxygen.
2
Chlorophyll in chloroplasts absorbs the light energy needed.
3
Carbon dioxide enters the leaves from the air; water is taken up through the roots.
4
Word equation: carbon dioxide + water → glucose + oxygen (light, chlorophyll).
5
Extended tier balanced equation: 6CO2 + 6H2O → C6H12O6 + 6O2, with light energy and chlorophyll shown over the arrow.
- Six CO2 molecules supply the six carbon atoms of glucose.
- Checkeach side has 6 C, 12 H and 18 O atoms.
- Light and chlorophyll are needed but are not reactants consumed in the equation.
Part 2 of 4Energy change and the fate of glucose
Endothermic
Photosynthesis is endothermic: energy is taken in from sunlight by chlorophyll/chloroplasts.
Glucose stores some of that energy in its chemical bonds.
Plants use glucose for respiration, convert it to starch for storage, or make other substances for growth.
Part 3 of 4Link to air quality and climate
1
Photosynthesis removes carbon dioxide from the air and fixes its carbon in plant biomass.
2
Respiration and combustion return carbon dioxide to the atmosphere, so photosynthesis is one part of the carbon cycle.
3
Planting more trees (reforestation) and reducing deforestation helps reduce atmospheric carbon dioxide through photosynthesis.
4
A faster rate of photosynthesis removes CO2 faster only if light, water and a suitable temperature are available.
5
Plant uptake alone does not cancel all emissions from burning fossil fuels; the net change depends on the balance of all processes.
| Process | Effect on atmospheric CO2 |
|---|---|
| Photosynthesis | Removes |
| Respiration | Adds |
| Combustion of fuels | Adds |
Part 4 of 4More from the specification
1Photosynthesis and Carbon Balance
- Photosynthesis by green plants and phytoplankton absorbs atmospheric carbon dioxide and releases oxygen,
- (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂)
- regulating the global carbon cycle.
2Combustion and Greenhouse Gases
- Complete combustion of fossil fuels produces carbon dioxide and water vapour,
- while anaerobic decomposition generates methane; both act as greenhouse gases that trap infrared radiation.
3Oxides of Nitrogen and Sulfur
- Combustion of fossil fuels containing sulfur impurities produces sulfur dioxide,
- while high temperatures in car engines react nitrogen with oxygen to produce toxic oxides of nitrogen (NOx).
4Acid Rain and Climate Impact
- Acidic emissions of sulfur and nitrogen oxides dissolve in rainwater to form acid rain, damaging forests, aquatic ecosystems,
- and building stonework across the Earth.
5Climate change
- A long-term shift in worldwide weather patterns and conditions
6Sulfur dioxide
- An acidic gas made when sulfur-containing fuels burn.
7Incomplete combustion
- Burning in limited oxygen to give CO, soot and water.
8Global warming
- Rise in the Earth's average temperature caused by extra greenhouse gases.
9Carbon monoxide
- A toxic gas that binds to haemoglobin.
10Complete combustion
- Burning in plenty of oxygen to give carbon dioxide and water.
11Greenhouse effect
- Absorption and re-emission of thermal energy by atmospheric gases, which reduces energy lost to space.
12Catalytic converter
- A device that converts toxic exhaust gases into less harmful gases.
13Symbol equation
- Reaction written with formulae.
14Oxide
- A compound of oxygen with one other element.
Key terms
| Term | Meaning |
|---|---|
| Photosynthesis | Process in green plants that uses light energy to make glucose and oxygen from carbon dioxide and water. |
| Chlorophyll | Green pigment in chloroplasts that absorbs light for photosynthesis. |
| Endothermic | A reaction that takes in energy from the surroundings. |
| Carbon cycle | Movement of carbon between the air, living things and fuels. |
| Reforestation | Planting trees to replace those removed, increasing CO2 uptake. |
Avoid these mistakes
1
Writing light or chlorophyll as reactants in the equation – they are conditions shown over the arrow.
2
Calling photosynthesis exothermic – it takes in energy from sunlight.
3
Saying plants take in oxygen for photosynthesis – they take in CO2 and release O2.
4
Assuming planting trees fully cancels fossil-fuel emissions – the net effect depends on the balance of processes.
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