As previously mentioned, the enzyme responsible for the whole shebang is Rubisco - it captures CO2 in the first place, and allows the chloroplasts to do their thing. The problem with this system is that Rubisco also has an affinity to Oxygen (particularly at higher temperatures or low CO2 levels), meaning when the temperature of a leaf increases less photosynthesis is achieved. The process of Rubisco fixing O2, rather than CO2, is called photorespiration and can slow plant growth by reducing the rate of photosynthesis by a significant amount.
Photorespiration is a particular problem in the tropics, as the temperature is usually high (and this also causes problems with waterloss). This has lead to the evolution of a different form of photosynthesis, that allows the plant to separate the absorption of CO2 either temporally or spatially, hence reducing photorespiration by limiting Rubisco's access to O2.
The first alternative to C3 photosynthesis is called C4. This is because it involves the CO2 being fixed into a 4-Carbon sugar at the first stage. For the initial absorption of CO2, C4 plants use a different enzyme called PEP Carboxylase. This then delivers the CO2 to the photosynthetic part of the leaf by the 4-C sugar breaking down and releasing the CO2 directly to the Rubisco.
The whole process works well because C4 plants have a slightly different leaf anatomy to C3 - they have specialised cells that surround the veins in the leaves, called bundle-sheath cells (this is known as Kranz anatomy). These are packed with chloroplasts, and photosynthesis takes place here rather than in the mesophyll. So by spatially separating the initial capture of CO2 and the Rubisco, photorespiration is significantly reduced. However this form of photosynthesis is also more energetically costly, requiring more ATP than C3. This means it is less efficient that C3, and only occurs in areas where the benefit of reducing photoresipration and waterloss outweigh the larger energy cost, such as the tropics and some arid zones.
The second alternative to C3 photosynthesis is CAM photosynthesis. CAM photosynthesis (Crassulacean Acid Metabolism photosynthesis) was named after the family of plants in which it was first discovered, Crassulaceae (a family of succulents). It tends to occur in arid plants, like cacti, and works by separating the CO2 capture and photosynthesis temporally. This again reduces photorspiration, however perhaps more substantially reduces waterloss. It works in two stages, at night when it is cooler and waterloss is less severe, the plant opens its stomata. It stores the captured CO2 as a 4-Carbon acid, Malate, and then breaks this down back to CO2 during the day to feed back to the Rubisco so photosynthesis can occur. This allows the plant to keep its stomata closed during the day, when it is hot and dry, helping it avoid waterloss and photorespiration.
I also have a healthy admiration for CAM plants, as they can also use this system to do what other plants cannot. During times of extreme stress, such as a severe drought, they can 'CAM-idle'. This means they can keep their stomata closed during the day and the night, and use the CO2 released through respiration at night for photosynthesis during the day and the O2 released from the photosynthesis during the day for respiration at night. Which is pretty cool. But they can't do this indefinitely, and will eventually need to open their stomata to start the photosynthetic process up again. It does, however, allow them to survive long periods of water stress, and also to recover quickly when water again becomes available.
So that, in 3 long posts, is how photosynthesis works. Perhaps next time I'll discuss talking trees, or some other thing that isn't physiology related ;)
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