Photosynthesis-Carbon
Fixation-1 
The C3 Cycle
It
has been estimated that 200 Billion tons of Carbon is fixed as Carbohydrates each
year. Marine
Plankton accounts for 40% of this.
The
biochemical steps for this occur in the Chloroplast Stroma. Consequently, these can be called Stroma Reactions. Melvin Calvin elucidated the basic steps in this process and they are often referred to as the Calvin Cycle. The first product of CO2 fixation is a 3-Carbon unit. Consequently, this can be called C3 Photosynthesis. There are three basic events which characterize this process.They are Carboxylation, Reduction & Regeneration.
 The first step involves the addition of CO2 to a 5-Carbon acceptor [RUBP].This leads to the production of two 3-Carbon molecules.The next step involves the reduction of the 3-Carbon molecule into a Carbohydrate.The final step is a complex series of reactions, which regenerates the5-Carbon Acceptor (Ribulose-1-5-bisphosphate [RUBP]). One CO2 is gained with each cycle. Six cycles lead to the formation of a 6-Carbon sugar. |
The Enzyme that adds CO2 to RUBP is called RUBISCO (Ribulose Bisphosphate Carboxylase/Oxygenase). It is the most abundant protein on the planet. It can represent 40% of the soluble protein in a leaf. I wonder why!
Photorespiration results in the loss of CO2 which negates CO2 fixation!
Both of these reactions involve the same active site, and the substrates (CO2 & O2) directly compete with one another.
Photorespiration reduces the efficiency of photosynthesis by as much as 50%. It is interesting to note that the dual nature of RUBISCO is universal from orchids to photosynthetic bacteria.
The Concentrations of CO2 & O2, plus the Leaf Temperature regulate the Balance between Carboxylation (Carbon Gain) & Oxygenation (Carbon Loss).
Carbon Dioxide concentrations
are always much lower than Oxygen levels inside leaves and in the atmosphere.Leaf Temperature rises during the day when photosynthesis occurs. This is especially true under dry, sunny conditions. This favors Oxygenation (Photorespiration).
The kinetic properties of RUBISCO also favor Oxygenation at high temperatures.
Elevated Temperatures
favor Photorespiration
One way to favor Carboxylation over Oxygenation would be the Elevation of CO2 levels in the vicinity of RUBISCO!
We will see two ways in which plants achieve this later!
Algae & Cyanobacteria
CO2 Concentration in Cyanobacteria and Algae occurs via CO2 Pumps which are located in the Plasmalemma.
These proteins are produced at low CO2 levels, and they use ATP from the light harvesting reactions to concentrate CO2 which then enters the Calvin (C3) Cycle.
