Hatch and Slack's Cycle/C-4 Cycle
Hatch and Slack (1966) reported that a 4-C compound oxaloacetic acid (OAA) is the first stable product in CO2 reduction process and this pathway is known as Hatch and Slack's cycle or C-4 cycle.
This pathway was first reported in members of family gramineae (grasses) like sugar cane, maize, sorghum, etc., tropical grasses but later on in the sub-tropical plants also like Artiplesand Amaranthus. In C4 cycle PEP accepts CO2 and oxaloacetic acid (4C compound) is formed. The enzyme catalyzing this reaction is PEP carboxylase (PEPcase). All known C4 plants are angiosperms and these are common in bright, hot and open ecosystem. There are no known C4 gymnosperms, bryophytes or algae. C4 plants have a characteristic leaf anatomy called Kranzanatomy .In Kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles having large sheath cells. C4 plants process dimorphic chloroplasts. The mesophyll cells possess chloroplasts with well developed grana whereas the bundles sheath chloroplasts lack the organization of grana. Chloroplasts of mesophyll cell are grana rich they do not produce starch. Chloroplasts of bundle sheath cells lack grana but they are starch rich.
In C3 plants, mesophyll cells carry out calvin cycle or C3 cycle whereas in C4 plants mesophyll cells carry out Hatch Slack pathways or C4 cycle. In C4 plants, there are two carboxylation reactions : First in mesophyll chloroplast and second in bundle sheath chloroplast (both C3 and D4 cycle are found.) First stable product in C4 plants is OAA (oxaloacetic acid). The CO2 released in the conversion of malic acid to pyruvic acid in C4 plants is fixed by Rubisco in the clavin cycle operating in bundle sheath cells. In C4 plants for formation of one mole of hexose (glucose) 30 ATP are required .Significance of C-4 cycle is that it is helpful to plants growing in dense tropical forests, where there is poor supply of CO2. Because here, there is internal supply of CO2, so these plants can survive in poor CO2 conditions.
RuBisCO that is the most abundant enzyme in the world is characterized by the fact that its active site can bind to both CO2 and O2-hence the name. RuBisCO has a much greater affinity for CO2 than for O2. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.
In C3 plants some O2 does bind to RuBisCO and hence CO2 fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate in a pathway called photorespiration. In the photorespiratory pathway, there is neither synthesis of sugars nor of ATP. Rather it results in the release of CO2 with the utilization of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH.
Photorespiration is also called, C2- cycle because glycolate and some other metabolites of the cycle are 2C compounds. The cell organelles involved in photorespiration are chloroplast, peroxisomes and mitochondria. Photorespiration is negligible or absent in C-4 plnats and present only in C-3 plants. So, C-4 plants are photosynthetically more efficient.