C4 Cycle

C₄ Cycle or Hatch and Slack cycle

Discovery

During 1950s H. P. Kortschack and Y. Karpilov while experimenting with sugarcane observed that ¹⁴CO₂ labelled was detected initially in the form of four-carbon acid: malate and aspartate. This finding was unexpected, because earlier ¹⁴CO₂ label was detected in three-carbon acid: 3, phosphoglycerate in the process, what we know as C₃ cycle or Calvin-Benson cycle. Later, M. D. Hatch and C. R. Slack explained the detailed process of four-carbon acid metabolism in maize. This pathway is now known as C₄ cycle or Hatch and Slack cycle or C₄ photosynthetic carbon cycle.

Kranz anatomy

Initially discovered in the tropical monocot grasses, C₄ cycle now reported from 19 different families of the angiosperms including Chenopodiaceae and Amaranthaceae. Leaves of the monocot plants have parallel venation and the vascular bundles are surrounded by a continuous row of bundle sheath cells, which in turn are surrounded by mesophyll cells. This gives the appearance of ‘wreath’ (kranz in German), hence, this is also called as ‘kranz anatomy’.
Beside these plants, C4 cycle is also know to occur in two chilling tolerant species Miscanthus and Spartina found in moderately colder region.

In C₄ plants CO₂ uptake and fixation are spatially separated

C₄ plants with kranz anatomy initially fix CO₂ available in dissolved form (HCO₃^-) to the oxalic acid in the mesophyll cells by the enzyme, phosphoenolpyruvate carboxylase (PEPCase). Oxalic acid is then converted into malate or aspartate, which is diffuse to the bundle sheath cell through plasmodesmata where decarboxylation of fixed 4-carbon organic acid takes place concentrating CO₂ in the vicinity of rubisco. In bundle sheath CO₂ is fixed in the sugars via Calvin-Benson cycle and efficiently transported to the vascular tissues.
Initial carboxylation enzyme is found in the chloroplast of mesophyll cells, whereas, rubisco is found the chloroplast of bundle sheath cells. Chloroplasts in the mesophyll cell contain stalked thylakoids, where only ATP is required, whereas, chloroplasts of bundle sheath cell contain mostly unstacked thylakoids, both NADPH and ATP are needed.

C₄ cycle also occurs in single cell

C₄ cycle also occurs even in single cell. Initial carboxylation takes place in the chloroplast facing outer atmosphere, while CO₂ through Calvin-Benson cycle takes place in the chloroplasts of inner side. The middle phase of cell provides the diffusion barrier for the 4-carbon organic acid and primary acceptor of CO₂. Suaeda aralocaspica (formerly Borszczowia aralocaspica) and Bienertia cycloptera (both of family Chenopodiaceae) perform complete C₄ cycle in single chlorenchyma cell. Two diatoms viz., Thalassiosira pseudonana and Phaeodactylum tricornutum have been also reported to contains the genes for the enzymes of C₄ cycle.

C₄ cycle has high energy demands than C₃ cycle

In order to regenerate the one molecule of phosphoenolpyruvate (initial acceptor of CO₂) need 2 ATP. Hence the total energy needed to fix single molecule of CO₂ via C₄ cycle is 2 NADPH and 5 ATP.

C₄ cycle reduces the photorespiration up to level of 2 %

The C₄ cycle evolved in plants as a major carbon concentrating mechanism in order to overcome the loss of carbon in photorespiration associated with the oxygenation property of rubisco. In fact, some of high yielding crops including maize and sugarcane use this carbon concentration mechanism to reduce the oxygenase property of rubisco. C₄ plants are more efficient than C₃ plants they have reduced photorespiration because:

• Carbon is initially fixed by PEPCase. The substrate for this enzyme is HCO3- rather than CO₂, hence there no competition for the oxygenation.
• Up to ten times higher CO₂ is concentrated around the rubisco in the bundle sheath cells’ chloroplast, which again reduces the oxygenase activity of rubisco.
• Carbon is fixed in the form of sugars in the bundle sheath cell, which are in close contact with the vascular tissue. This helps in the removal of photosynthetic product very efficiently.

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First published on 13-04-2021
Last updated on 06-12-2021