Ecological selection pressures for C4 photosynthesis in the grasses.

Sage RF, Sage TL and Kocacinar F (2012) Photorespiration and the evolution of C4 photosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 63: 19–47.

Below is a diagram of the photosynthetic process in grass. (Source: Wikimedia Commons)

Trees first appeared during a plant diversity crisis, and the arrival of seed plants and ferns ended the dominance of the first trees, so the plant crises may have been more about evolutionary experiments than environmental conditions, although a carbon dioxide crash and ice age conditions would have impacted photosynthesizers. The that gave rise to trees and seed plants largely went extinct at the Devonian’s end. But what might have been the most dramatic extinction, as far as humans are concerned, was the impact on land vertebrates. During the about 20% of all families, 50% of all genera, and 70% of all species disappeared forever.

of organisms, ecological selection pressures sometimes lead …

Westhoff P and Gowik U (2010) Evolution of C4 photosynthesis‐looking for the master switch. Plant Physiology 154: 598–601.

The dates are controversial, but it appears that after hundreds of millions of years of using various molecules as electron donors for photosynthesis, began to split water to get the donor electron, and oxygen was the waste byproduct. Cyanobacterial colonies are dated to as early as 2.8 bya, and it is speculated that may have appeared as early as 3.5 bya and then spread throughout the oceans. Those cyanobacterial colonies formed the first fossils in the geologic record, called . At Shark Bay in Australia and some other places the water is too saline to support animals that can eat cyanobacteria, and give us a glimpse into early life on Earth.

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Although most plants exhibit kranz anatomy, there are, however, a few species that operate a limited cycle without any distinct bundle sheath tissue. , and (all ) are terrestrial plants that inhabit dry, salty depressions in the deserts of the . These plants have been shown to operate single-cell -concentrating mechanisms, which are unique among the known mechanisms. Although the cytology of both genera differs slightly, the basic principle is that fluid-filled vacuoles are employed to divide the cell into two separate areas. Carboxylation enzymes in the cytosol can, therefore, be kept separate from decarboxylase enzymes and RuBisCO in the chloroplasts, and a diffusive barrier can be established between the chloroplasts (which contain RuBisCO) and the cytosol. This enables a bundle-sheath-type area and a mesophyll-type area to be established within a single cell. Although this does allow a limited cycle to operate, it is relatively inefficient, with the occurrence of much leakage of from around RuBisCO. There is also evidence for the exhibiting of inducible photosynthesis by non-kranz aquatic under warm conditions, although the mechanism by which leakage from around RuBisCO is minimised is currently uncertain.

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Since every molecule has to be fixed twice, first by 4-carbon organic acid and second by RuBisCO, the pathway uses more energy than the pathway. The pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the pathway requires 30 molecules of ATP. This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in as occurs in some tropical plants, making it an adaptive mechanism for minimizing the loss.

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Kani R and Edwards GE (1999) The biochemistry of C4 photosynthesis. In: Sage RF and Monson RK (eds) C4 Plant Biology, pp. 49–87. San Diego: Academic Press.