The Chemistry of Photosynthesis
Plants perform one of the most important chemical reactions on Earth: they build sugar out of thin air and water, powered by sunlight. This single process feeds almost every food chain, maintains the oxygen in our atmosphere, and locks away enormous amounts of carbon. Its chemistry — energy in, sugar and oxygen out — is worth understanding in detail.
The overall equation
Photosynthesis converts carbon dioxide and water into glucose and oxygen: 6 CO2 + 6 H2O → C6H12O6 + 6 O2 (using light energy). Every atom is accounted for — the oxygen we breathe is the byproduct of plants building sugar from air and water.
An endothermic process
Building glucose from CO2 and water requires a large input of energy, making photosynthesis strongly endothermic (ΔH positive). Sunlight supplies that energy, and the plant stores it in the chemical bonds of sugar — which is why nearly all life ultimately runs on captured solar energy.
Chlorophyll as a light harvester
Chlorophyll, the green pigment in leaves, absorbs red and blue light and reflects green — which is why plants look green. The absorbed light energizes electrons that drive the reactions splitting water and fixing carbon dioxide, effectively turning the leaf into a solar-powered chemical factory.
Limiting reactants in nature
Just like in the lab, photosynthesis is limited by whichever reactant runs out first. On a cloudy day light is limiting; in a crowded greenhouse CO2 can be limiting (growers sometimes add it); in dry soil water is limiting. Farmers and ecologists think in exactly these limiting-reactant terms.
Why it matters for the planet
Photosynthesis removes roughly 120 billion tons of carbon from the atmosphere each year, balancing the carbon cycle. The oxygen in every breath and the carbon in every bite of food trace back to this single reaction — it is the chemical foundation of nearly all ecosystems.
Try this yourself
- With an adult's supervision, place one potted plant (or one branch in a clear bag) in sunlight and another in a dark closet for a day. Observe and compare the leaves' appearance and discuss what the light provided.
- Hold a white paper behind a leaf with an adult and observe the green color. Discuss which colors of light chlorophyll absorbs and which it reflects.
- With an adult, research why greenhouse growers sometimes add extra CO2, and explain it using the idea of limiting reactants.