ABSTRACT
Carbohydrates can be used later to supply the energy needs to the cell. Therefore, photosynthesis is a process in which solar energy is stored in the bonds of carbohydrates.
The amount of energy trapped by photosynthesis is immense, approximately 100 terawatts (1 TW = 1012 W), which is about six times larger than the power consumption of humanity (Figure 2.1). [4, 5] The energy produced
by photosynthesis forms the basis of virtually all terrestrial and aquatic food chains. Consequently, photosynthesis is the ultimate source of the carbon in the organic molecules found in most organisms. [6] Photosynthetic organisms convert about 100 billion tonnes of carbon into biomass per year. [7]
Photosynthesis evolved early in the evolutionary history of life, when all forms of life on Earth were microorganisms and the atmosphere had much more CO2. [7] The first photosynthetic organisms probably evolved about 3,500 million years ago, and used hydrogen or hydrogen sulfide as sources of electrons, rather than water. Cyanobacteria appeared later, around 3,000 million years ago, and changed the Earth for ever when they began to oxygenate the atmosphere, beginning about 2,400 million years ago. This new atmosphere allowed the evolution of complex life such as protists. Eventually, no later than a billion years ago, one of these protists formed a symbiotic relationship with a cyanobacterium, producing the ancestor of the plants and algae. The chloroplasts in modern plants are the descendants of these ancient symbiotic cyanobacteria.
