Photosynthesis

Do plants only take things in while making food, or do they give something out too? Activity 10.5 sets up two water plants, each under a funnel with an upside-down test tube on top. Set-up A is in sunlight; set-up B is in the dark.

In A, little bubbles of gas rise and collect in the test tube. B gives no bubbles.

Which gas? When enough has collected, Barkha didi puts her thumb over the test tube, lifts it out, and quickly pushes in a lit matchstick. It burns with a much brighter flame.

A brighter flame means the gas is rich in oxygen. So plants release oxygen during photosynthesis — the oxygen we breathe. And since B in the dark gave no bubbles, photosynthesis happens only in sunlight.

Now we can write the whole process. Water, sunlight, carbon dioxide and chlorophyll are needed, and photosynthesis makes carbohydrates:

• Carbon dioxide + Water → (sunlight, chlorophyll) → Glucose + Oxygen

The food actually made is glucose, a simple carbohydrate. Glucose gives quick energy, and the plant later turns some of it into starch to store.

Many scientists helped us understand photosynthesis. In India, Rustom Hormusji Dastur (1896–1961), head of Botany at the Royal Institute of Science, Bombay, from 1921 to 1935, studied how the amount of water, the temperature and the colour of light affect it.