Among all the living organisms, those belonging to prokaryotes (bacteria and archaea) enjoy living on all the three types of energy sources, viz. light, inorganic molecules, and organic molecules. When a leaf, a site of photosynthesis, is in equilibrium, energy input equals to energy output. Short-wave and long-wave radiation are the inputs, while long-wave radiation, conduction and convection, transpiration, transmission, reflectance, and energy in carbon compounds (sugars) are the outputs in the energy budget of a leaf. The “water cost–carbon gain” is an inevitable play the photosynthesis plays. The three different biochemical pathways of photosynthesis broadly determined, according to ecological differences, are C3, C4, and crassulacean-acid metabolism (CAM) photosynthetic pathways discussed in detail in this chapter. Cphotosynthesis is an alternative pathway in which carbon fixation and light-dependent reactions of photosynthesis take place in separate cells while CAM is a sort of strategic photosynthesis adapted to extremely hot and dry environments. Although CAM plants do not normally perform photosynthesis at very high rates, their water-use efficiency (mass of COfixed per kg of water used) exceeds that of Cand Cplants. Einstein’s E = mcis not valid for ecosystems. Just five constituents, viz., carbon, hydrogen, oxygen, nitrogen, and phosphorus—or CHONP—make up to 93%–97% of the biomass of all the organisms, plants being chemically the most distinctive, often containing lower contents of nitrogen and phosphorus. Organisms follow what is called optimal foraging to fulfill their nutrient and energy needs. Biological systems sustain in a state far from equilibrium, which happens due to continuous energy enrichment of biomolecules through energy flux from sun to the plants and consumers, an essential condition for the biosphere to reverberate with life.