Introduction

Every organic compound on Earth ultimately traces back to plants - directly or indirectly, all life depends on them as the primary source of the carbon-based molecules that build carbohydrates, lipids, proteins, nucleic acids and every other biomolecule. Despite all of humanity's technological achievements, no artificial process yet matches the plant's own metabolic machinery for turning sunlight into usable energy and biological material. Plants capture this energy through photosynthesis, converting solar (radiant) energy into stored chemical energy - a process that acts as the driving force behind both the living and non-living world. Its scale is enormous: photosynthesis generates roughly 1,700 million tonnes of dry organic matter every year by fixing about 75 x 10^12 kg of carbon, even though photosynthetic organisms capture only about 0.2% of the sunlight reaching Earth. The carbohydrates it produces are the basic raw material for respiration and for building countless other organic compounds, and the process is what keeps atmospheric oxygen and carbon dioxide in balance - continually taking up the CO2 that respiration keeps adding back. Because it stores far more energy than it costs to run, photosynthesis is described as a strongly endergonic reaction. This chapter traces that energy-capturing process in detail, along with the different pathways plants use to convert it into carbohydrates.