Four Way Carbon Traffic:
The different ways by which green plants fix carbon dioxide and produce different sugar molecules is called Four Way Carbon Traffic. They are C2, C3, C4, and CAM (Crassulacean Acid metabolism) and their functions is given below:
C2 Pathway:
This process is also called photorespiration, it occurs in any green plant except C4 or CAm plants. It also occurs in the presence of a higher concentration of oxygen and due to the dual nature of the RuDP carboxylase as an oxygenase.
It produces the first sugar which is two carbon called phospho-glycolic acid which produces a series of compounds that delays the formation of PGA and thus the sugar concentration is reduced. Though apparently, it is a wasteful process, it is somewhat useful because it generates different essential amino acids like glycine and serine, and also it maintains coordination between organelles like chloroplastid, microbody, and mitochondria.
C3 Pathway:
The C3 pathway is also called the Calvin cycle. It is operating in normal photosynthetic green plants. It occurs in the stroma of chloroplastid and the first photosynthetic product is a three-carbon compound called PGA (Phospho Glyceric Acid). The fixation of CO2 is bought about by RuDP carboxylase.
C4 Pathway:
This pathway operates in certain temperate grasses like Panicum. These leaves have typical anatomy called Kranz Anatomy, where tye chloroplastids are broadly classified into two types:
- Mesophyll Chloroplast
- Bundle Sheath Chloroplast
The initial carbon dioxide is fixed with the help of PEP carboxylase (Phospho Enolpyruvate Carboxylase) and the first photosynthetic product is OAA (Oxalo Acetic Acid). OAA is readily converted to malic acid, it undergoes decarboxylation to produce CO2, which is accepted by RuDp carboxylase to produce the normal photosynthetic 3 carbon product called PGA. Thus, these grasses are photosynthetically more productive than other plants.
CAM Pathway:
The CAM pathway occurs in plants belonging to plants of Crassulaceae. These plants are exhibiting a reverse circadian rhythm, where the stomata remain open during the night and closed during the day. This involves night accumulation of malic acid, which gets decarboxylated during the daytime, producing CO2 inside the plant body. Thus photosynthesis can operate keeping the stomata closed during the daytime.