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Life on earth is carbon based and is sustained by photosynthetic use of energy from sunlight to fix atmospheric CO2. With evolution of life, primitive photoautotrophs gave rise to vascular plants which in turn adapted to changing aquatic and terrestrial environments via distinctive modes of photosynthesis.

Features of C3, C4 CAM and SAM photosynthesis (Section 2.1) highlight the central significance of Rubisco and outline metabolic extensions to that basic process that have enabled adaptation to arid regions (CAM) and aquatic environments (SAM), or have led to spectacular gains in photosynthetic capacity of C4 plants (C4 subgroups in Section 2.2).

Paradoxically, oxygenic photosynthesis in C3 plants is actually inhibited by oxygen! This outcome has major implications for global productivity and holds special relevance for comparative performance of C3 and C4 plants. Recognition of such O2-dependent photorespiratory loss of carbon from C3 plants but not from C4 plants represented a major paradigm shift in plant science and is recounted in Section 2.3.

Finally, a significant fraction of the carbon gained via photosynthesis becomes a respiratory substrate for generation of energy to sustain vital processes, as well as providing carbon skeletons for a multitude of synthetic events (Section 2.4).