Evolution of photosynthetic pathways

The C4 metabolic pathway is a valuable recent innovation in plants. Photosynthesis is not quite as simple as adding water to CO2 to produce sugars and oxygen. A complex chemical pathway is involved, facilitated along the way by a range of enzymes and co-enzymes. The enzyme RuBisCO is responsible for "fixing" CO2 that is, it attaches it to a carbon-based molecule to form a sugar, which can be used by the plant, releasing an oxygen molecule along the way. However, the enzyme is notoriously inefficient, and just as effectively will also fix oxygen instead of CO2 in a process called photorespiration. This is energetically costly as the plant has to use energy to turn the products of photorepsiration back into a form that can react with CO2. [edit]Concentrating carbon C4 plants evolved carbon concentrating mechanisms. These work by increasing the concentration of CO2 around RuBisCO, thereby facilitating photosynthesis and decreasing photorespiration. The process of concentrating CO2 around RuBisCO requires more energy than allowing gases to diffuse, but under certain conditions i.e. warm temperatures (>25C), low CO2 concentrations, or high oxygen concentrations pays off in terms of the decreased loss of sugars through photorespiration. One type of C4 metabolism employs a so-called Kranz anatomy. This transports CO2 through an outer mesophyll layer, via a range of organic molecules, to the central bundle sheath cells, where the CO2 is released. In this way, CO2 is concentrated near the site of RuBisCO operation. Because RuBisCO is operating in an environment with much more CO2 than it otherwise would be, it performs more efficiently. A second mechanism, CAM photosynthesis, temporally separates photosynthesis from the action of RuBisCO. RuBisCO only operates during the day, when stomata are sealed and CO2 is provided by the breakdown of the chemical malate. More CO2 is then harvested from the atmosphere when stomata open, during the cool, moist nights, reducing water loss. [edit]Evolutionary record These two pathways, with the same effect on RuBisCO, evolved a number of times independently indeed, C4 alone arose in 18 different plant families. The C4 construction is most famously used by a subset of grasses, while CAM is employed by many succulents and cacti. The trait appears to have emerged during the Oligocene, around 25 to 32 million years ago;[105] however, they did not become ecologically significant until the Miocene, 6 to 7 million years ago.[106] Remarkably, some charcoalified fossils preserve tissue organised into the Kranz anatomy, with intact bundle sheath cells,[107] allowing the presence C4 metabolism to be identified without doubt at this time. Isotopic markers are used to deduce their distribution and significance. C3 plants preferentially use the lighter of two isotopes of carbon in the atmosphere, 12C, which is more readily involved in the chemical pathways involved in its fixation. Because C4 metabolism involves a further chemical step, this effect is accentuated. Plant material can be analysed to deduce the ratio of the heavier 13C to 12C. This ratio is denoted ?13C. C3 plants are on average around 14 (parts per thousand) lighter than the atmospheric ratio, while C4 plants are about 28 lighter. The ?13C of CAM plants depends on the percentage of carbon fixed at night relative to what is fixed in the day, being closer to C3 plants if they fix most carbon in the day and closer to C4 plants if they fix all their carbon at night.[108]