Lesson 1 of 12 · 7 min

What do we know about photosynthesis

NCERT §11.1

Every monsoon Kavya watches her family's paddy turn from a knee-high green mat into grain heavy enough to bow the stem, without anyone feeding it a single meal.

The story this chapter follows: Kavya's paddy field at dawn

Every morning before school, Kavya wades into her family's flooded paddy field to check the water level, the rice blades catching the first sunlight. A short walk up the bund, on the same farm's drier upper ground, her family also grows a patch of sugarcane. Both fields sit under the same light and the same air, but run on not quite the same chemistry, and this chapter follows both of them from a single photon to a grain of starch.

The lesson in notes

In short

Photosynthesis is the physico-chemical process by which green plants harness light energy to build organic compounds; every other organism ultimately relies on it for food.

Two things make it important: all food on earth comes primarily from it, and it releases oxygen into the atmosphere.

Simple school experiments show what is needed: a variegated leaf, or a leaf partly covered with black paper, forms starch only in the green and lit parts, showing chlorophyll and light are required.

In the half-leaf experiment, one part of a leaf is kept in a test tube containing KOH-soaked cotton, which absorbs CO₂; that part does not form starch, while the part exposed to air does, showing CO₂ is required.