Cambridge O Level Chemistry · Syllabus 5070 · Chemistry of the Environment
Photosynthesis
What is Photosynthesis?
The reaction in which green plants make glucose and oxygen from carbon dioxide and water, in the presence of chlorophyll and using energy from light. Its balanced symbol equation is six carbon dioxide plus six water giving one glucose plus six oxygen. Light energy and chlorophyll are conditions written above the arrow, not reactants; the reaction removes carbon dioxide from the atmosphere.
This definition is part of the Chemistry of the Environment chapter in Cambridge O Level Chemistry.
Photosynthesis in context
Chemistry of the environment covers water, air and carbon as three linked systems. Water is tested for presence with cobalt(II) chloride or copper(II) sulfate, and for purity with a sharp melting point of about \(0\,{}^\circ\mathrm{C}\) and boiling point of about \(100\,{}^\circ\mathrm{C}\); a domestic supply is then treated by sedimentation and filtration, carbon filtration and chlorination. Clean dry air is about 78% nitrogen and 21% oxygen, and six pollutants — carbon dioxide, carbon monoxide, particulates, methane, oxides of nitrogen and sulfur dioxide — each have a distinct source and effect, such as acid rain or global warming. Carbon dioxide absorbed in photosynthesis links combustion back to plant growth.
Common mistakes with Photosynthesis
- “Photosynthesis uses oxygen and gives out carbon dioxide.” Defect The equation has been written backwards. Chemistry Photosynthesis takes in carbon dioxide and water and releases oxygen. The reverse is respiration. Repair \(\mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2}\). Transfer Which side of the arrow does glucose sit on? The right — it is a product.
- “Planting trees will solve climate change.” Defect A real strategy stated as a complete solution. Chemistry Photosynthesis removes carbon dioxide, but slowly, only while the trees grow, and only for as long as the carbon stays locked up. It does nothing about methane, sulfur dioxide or oxides of nitrogen. Repair Add the benefit and a limitation — both are credited. Transfer Name a strategy that addresses methane instead. Reduction in livestock farming.
Questions students ask about Photosynthesis
Why is "planting trees" not a complete answer to reducing air pollution?
Planting trees strengthens photosynthesis, removing carbon dioxide from the atmosphere, so it addresses global warming. But a growing forest takes decades to have an effect, needs land, and does nothing about sulfur dioxide or carbon monoxide from a chimney or exhaust. A full answer names the specific strategy that matches each specific pollutant, plus its limitation.