Photosynthesis
Evolution of Photosynthesis
Photosynthesis originated not long after the origin of life. We know very little about the earliest origins of photosynthesis. There have been numerous suggestions as to where and how the process originated, but there is no direct evidence to support any of the possible origins. Overwhelming evidence indicates that eukaryotic photosynthesis originated from endosymbiosis of cyanobacterial-like organisms, which ultimately became chloroplasts. Therefore, the evolutionary origin of photosynthesis is to be found in the bacterial domain. The complex evolution of photosynthesis is responsible for the creation of multiple types of photosynthetic organisms that exist today. There are three variations of the Calvin Cycle in photosynthesis. The C3 pathway evolved earlier than the other two, the C4 and CAM pathways.
C3 Plants
The C3 pathway evolved earlier than the C4 and CAM pathways. In C3 plants, only the Calvin Cycle is used to fix CO2. C3 plants can succesfully fix CO2 in the absense of photorespiration.The majority of plants are C3 plants, for example, rice and wheat.
Rice
Wheat
C4 Plants
Plants have developed alternatice strategies to overcome photorespiration and maximize CO2 fixation using the C4 plathway. In this pathway, the enzyme PEP carboxylase (phosphoenolpyruvate carboxylase) fixed CO2 into 4-C sugars. This enzyme has no affinity for O2, so CO2 fixation still continues during hot sunny weather (when CO2 levels decrease), and photorespiration is minimized. C4 plants use both C3 and C4 pathways to fix CO2, but the C3 pathway is limited to bundle sheath cells and the C4 pathway is limited to mesophyll cells. C4 plants are efficient in CO2 fixation and using water effectively. Two examples of C4 plants are corn and sugarcane.
Corn
Sugarcane
CAM Plants
CAM (Crassulacean Acid Metabolism) plants were first discovered in the family Crassulaceae. These plants operate both C3 and C4 cycles to fix CO2 into 4-C sugars first and thereafter into 3-C sugars. Unlike C4 plants, CAM plants do not have separate cells for C3 and C4 pathways. Cam plants carry out C3 reactions during the day when stomata are closed due to hot and sunny weather and ATP and NADPH are available, and C4 reactions during nighttime when the stomata are open in cooler weather and CO2 is available. Many succulent desert plants use this strategy to fix CO2, for example, cacti.
Cacti
Photorespiration
Photorespiration is the process of fixing O2 into RuBP (ribulose biphosphate). In a situation with low CO2 levels, rubisco utilizes the process of photorespiration to fix O2 into RuBP in a series of reactions, and then finally releases CO2. This results in the use of RuBP without making any sugars or ATP, and this process occurs until normal CO2 levels are restored.
