The Elements of Evolution (49-5) Photosynthesis


Photosynthesis is an efficient process, albeit not in absolute terms of converting available sunlight into usable chemical energy. The different photosynthesis pathways illustrate that ecological trade-offs define what optimality really means.

There are 3 metabolic pathways for carbon fixation in photosynthesis: C3, C4, and CAM. The C3 pathway consumes 18 ATP molecules to synthesize 1 glucose molecule, whereas the later-evolved C4 pathway requires 20 ATP. Despite lower ATP efficiency, C4 is an evolutionary advancement, adapted to locales with high levels of light, where the reduced ATP efficiency is more than offset by soaking up more light. The ability of C4 plants to thrive despite a restricted water supply maximizes the ability to use available light. C3 plants cannot tolerate the heat or aridity that C4 plants can. Whereas C3 is well-suited for cool, wet environments, C4 plants do well in hot, sunny climes.

C4 independently arose at least 62 times in 18 different plant families. Many evolutionary pathways were available, but these many distinct plants converged to the same chemical reaction series 7–6 million years ago, all adapting to a more arid climate.

CAM ATP usage is comparable to C4, but the CAM pathway is further optimized for minimal water usage, allowing CAM plants to live in scorching, dry environments. Like C4, CAM convergently evolved many times in the over 40 plant families which employ it. ~7% of plant species use CAM, including ancient ones which updated their photosynthetic technique.