What photosynthesis is and how we learnt it

Photosynthesis is the process by which green plants use light energy to make food (organic compounds) from simple materials. It is a physico-chemical process: both physics (light) and chemistry are involved. It is the main source of all food on Earth, and it releases oxygen. Plants make their own food, so they are autotrophs.

Simple experiments show what it needs. Take a variegated leaf (partly green, partly white), or a leaf partly covered with black paper, keep it in light, then test it for starch: only the green parts that got light make starch. So chlorophyll and light are needed. Put half a leaf inside a test tube with KOH, which absorbs CO₂: that half makes no starch. So CO₂ is needed too.

How scientists worked it out:

• Joseph Priestley (1770) found that a burning candle or a mouse in a closed jar soon "damaged" the air, but a mint plant restored it, so the candle kept burning and the mouse lived.
• Jan Ingenhousz showed that this happens only in sunlight, and that only the green parts of a plant give off oxygen bubbles.
• Julius von Sachs (1854) showed that glucose is made in the green parts (in bodies later named chloroplasts) and stored as starch.
• T.W. Engelmann split light into colours with a prism and shone it on a green alga, Cladophora, placed with bacteria that need oxygen. The bacteria crowded in blue and red light, where most oxygen was given off. This was the first action spectrum of photosynthesis.

Cornelis van Niel, studying purple and green bacteria, showed that photosynthesis uses hydrogen from some compound to reduce CO₂. In green plants, that hydrogen comes from water, so the oxygen released comes from water, not from CO₂. Later experiments with radioactive tracers proved it. The full equation is: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂.