Photosynthesis
Photosynthetic Pathways
C3 Photosynthetic Pathway
The first photosynthetic pathway that was biochemically characterized is called the C3 pathway because the initial products of carbon fixation are 3-carbon compounds (phosphoglyceric acid, PGA) (Figure 2). CO2 is combined with a 5-carbon sugar, ribulose bisphosphate (RuBP), using the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco):[1]
C4 Photosynthetic Pathway
The C4 photosynthetic pathway differs from the C3 pathway in its initial carboxylating enzyme and initial products (Figure 4). The primary carboxylating enzyme in C4 plants is phosphoenolpyruvate (PEP) carboxylase. This enzyme has a higher affinity for CO2 than rubisco and has no oxygenase activity. The enzyme catalyzes the reaction of CO2 with PEP, forming a 4-carbon acid. C4 plants still have C3 biochemistry; however, there is a physical separation of enzymes. The outer, mesophyll cells have PEP carboxylase and few chloroplasts while rubisco and many chloroplasts are located in bundle sheath cells, a jacket of cells surrounding the xylem and phloem (Figure 5).[2]