Types of Photosynthesis: I. C3 Photosynthesis

C3 Photosynthesis. This type of photosynthesis employs solely the C3 Cycle by which carbon dioxide go through chemical reactions to form carbohydrates. The C3 cyclic reactions also include the regeneration of the CO2-acceptor chemical substrate. Plants exhibiting this mechanism of photosynthesis are called C3 plants.

The C3 cycle is also called the photosynthetic carbon reduction (PCR) cycle and Calvin Cycle, after Melvin Calvin, an American biochemist. Calvin headed the research effort at the University of California which, with the use of radiolabeled 14CO2, finally detailed the sequential reactions leading to the conversion of inorganic, atmospheric CO2 into carbohydrates and other organic compounds. For this work, he won the Nobel Prize in Chemistry in 1961.

The C3 photosynthesis is only the first to be understood. Other mechanisms, referred to as C4 photosynthesis and CAM photosynthesis, were later discovered.

The C3 cycle can be divided into three stages: CO2 Fixation, Reduction, and Regeneration.

1. In CO2 Fixation, the CO2 that diffuses into the stroma of the chloroplast in mesophyll cells is added (covalently bonded) to the five-carbon acceptor ribulose-1,5-bisphosphate (RuBP, C5H12O11P2), also called ribulose-1,5-diphosphate (RuDP), yielding a six-carbon intermediate product.

This intermediate is hydrated and then cleaved,  producing two molecules of three-carbon 3-phosphoglycerate or phosphoglyceric acid (3-PGA or simply PGA, C3H5O6P) as the first stable product. The reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo).

The summary reaction is shown below: 

Rubisco

CO2 + RuBP ----------------------------------> 2 3-PGA

This first stable product (3-PGA), being a three-carbon compound, is the reason why this mechanism of photosynthesis is called C3 Cycle.

2. Carbon Reduction stage follows.  Each of the two molecules of 3-PGA undergoes further reactions to produce the three-carbon triose phosphate sugar glyceraldehyde-3-phosphate (G3P, C3H7O6P), also called phosphoglyceraldehyde (PGAL). A molecule of G3P is first phosphorylated by ATP, producing 1,3-bisphosphoglycerate which is in turn reduced to G3P with NADPH as the reducing agent.

Glyceraldehyde-3-phosphate or G3P is the carbohydrate product of the C3 cycle and is the precursor of glucose and other products of metabolism. This is so in all mechanisms of photosynthesis including in C4 and CAM photosynthesis.

3. The Regeneration stage completes the cycle. Some molecules of G3P go through further reactions which result to the reformation of RuBP, the CO2 acceptor in the C3 cycle.

To complete the process of photosynthesis, the other molecules of G3P leave the cycle and proceed to a series of reactions to form glucose and other sugars, starch, and other organic compounds. It takes six turns of the cycle, or a total of six molecules of CO2, to produce one molecule of glucose (C6H12O6) (Mathews and Van Holde 1990; Simpson 2010).

But the efficiency of C3 photosynthesis may be lessened because of the affinity of the CO2-fixing enzyme Rubisco to both CO2 and O2. Under light when the concentration of CO2 is low and O2 is high within the plant, Rubisco may behave as an oxygenase and fix O2 instead. This occurs when the stomata close at high light intensity and under water stress, preventing the entry of CO2. This will result to photorespiration which is a wasteful process. Ribulose-1,5-bisphosphate (RuBP), the CO2 acceptor in the Calvin cycle, will be lost; the fixation of CO2 via the C3 pathway is stopped; and instead CO2 that is already fixed is released.

(Ben G. Bareja Aug. 2013)

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