Photosynthesis captures about eight times the power human civilisation uses
Globally, photosynthesis traps energy at roughly 130 terawatts and turns around 100 billion tons of carbon into living matter each year. It also filled the air with oxygen, making complex life possible. Yet the very first photosynthesisers may have been purple microbes that made no oxygen at all.
Plants, algae and cyanobacteria use pigments to turn light, usually sunlight, into chemical energy, stored in the bonds of carbohydrates such as glucose, sucrose, starch and cellulose. Cells later release that energy through respiration. The familiar oxygenic form splits water and gives off oxygen as a by-product, sustaining the atmosphere and supplying most of the energy that complex life runs on.
It happens in two stages. First, light strikes reaction centres, proteins holding pigments; in plants these are chlorophylls inside chloroplasts, which absorb red and blue light and reflect green, while cyanobacteria carry them in their cell membrane. That energy strips electrons from water, releasing oxygen, and the freed hydrogen helps build the energy carriers ATP and NADPH. Second, in the light-independent Calvin cycle, carbon dioxide from the air is attached to existing molecules such as ribulose bisphosphate and, powered by that ATP and NADPH, reduced into sugars. Some bacteria use other routes, like the reverse Krebs cycle.
Not all photosynthesis makes oxygen. Purple bacteria use bacteriochlorophyll to split hydrogen sulfide and release sulfur instead, a form that dominated the sulfide-rich oceans of the Boring Billion. Archaea like Halobacterium use a simple pigment, retinal, to absorb green light and pump protons that drive ATP production directly, without fixing carbon. This may have been Earth's earliest photosynthesis, dating back to the Paleoarchean, an idea known as the Purple Earth hypothesis.
Early photosynthesisers probably drew electrons from hydrogen or hydrogen sulfide rather than water. Cyanobacteria came later, and their surplus oxygen transformed the planet. Jan Ingenhousz uncovered the process in 1779, demonstrating that soil and water alone were not enough: plants also required light.
Source: Photosynthesis
