Introduction

Before the numbered sections begin, the NCERT chapter opens with a short profile of the biochemist Melvin Calvin (born 1911), who earned a Ph.D. in Chemistry from the University of Minnesota and later became a Professor of Chemistry at the University of California, Berkeley. In the years after the Second World War, Calvin and his co-worker J.A. Bassham put radioactivity to constructive use: by labelling carbon dioxide with the radioactive isotope C14, they traced how green plants convert simple raw materials — carbon dioxide, water and minerals — into sugars and other organic substances. Calvin proposed that plants convert light energy into chemical energy by passing an electron through an organised array of pigment molecules and other components. For mapping this pathway of carbon assimilation during photosynthesis — the sequence now known as the Calvin cycle — he was awarded the Nobel Prize in 1961. The same principles he established continue to inform present-day research into renewable energy, solar energy and related materials.

The chapter itself opens by pointing out a fact most students already take for granted: all animals, including human beings, ultimately depend on plants for their food. Have you ever wondered how plants themselves get the food they need? Green plants do not depend on any external source of food — they synthesise it themselves, and every other living organism depends on them, directly or indirectly, for its nutrition. Because they can manufacture their own organic food, green plants are called autotrophs, while organisms that must obtain food from green plants (or from other organisms) are called heterotrophs — a distinction you have already met earlier. The process by which green plants make this food is photosynthesis, a physico-chemical process in which plants use light energy to drive the synthesis of organic compounds. Since virtually every living form on Earth ultimately depends on sunlight for energy, the capture of that sunlight by photosynthesising plants is the very basis of life on Earth.

Photosynthesis matters for two central reasons: it is the primary source of all food on Earth, and it is responsible for the release of oxygen into the atmosphere — the same oxygen every breathing organism depends on. This chapter's focus, going forward, is the structure of the photosynthetic machinery inside the plant cell and the sequence of reactions by which it converts light energy into chemical energy.