CO2 Fixation Pathways—C4 Cycle | Photosynthesis

In this article we will discuss about the CO2 Fixation Pathways—C4 Cycle.

In 1954, in addition to Calvin cycle, an alternative pathway for CO₂ fixation in photosynthesis was discovered by Kortschak who reported the formation of C₄dicarboxylic acids as primary products of photosynthesis in sugarcane. When ¹⁴CO₂ was supplied to sugarcane leaves for a short period, 3 four carbon dicarboxylic acids (oxaloacetate, malate and aspartate) were produced initially.

When the period of exposure was extended, label appeared in 3-phosphoglycerate. M.D. Hatch and C.R. Slack (1966) extended these studies to several plants and then proposed an alternative pathway.

The cycle begins with the fixation of CO₂ by PEP-carboxylase (Phosphoenolpyruvate carboxylase) (Fig 13-29). PEP is a CO₂-acceptor molecule which is carboxylated by the addition of CO₂ and as a result 4-carbon product oxaloacetate is produced.

This product being unstable breaks down readily to malate (MDH enzyme) and aspartate through transamination (GOT enzyme). These C₄acids are enzymatically degraded to yield free CO₂and pyruvate. The latter 3-carbon intermediate compound is phosphorylated to produce PEP. Thus C₄ cycle can continue in this way as the new CO₂ enters the system.

It is believed that CO₂ released from C₄ cycle is utilised by RuBP carboxylase system in the Calvin C₃ cycle. The CO₂ is reduced to carbohydrates. In summary, C₄ plants have an additional CO₂– fixing sequence which is coordinated with the C₃-cycle and RuBP-carboxylase. Thus CO₂ is fixed by PEP carboxylase and is finally handed over to RuBP carboxylase within the leaf.

Downton (1975) Raghavendra and Das (1978) has listed over 1500 species 120 genera of plants belonging to 13 families possessing C₄ mechanism of CO₂ fixation. Out of the 18 families, 15 were dicotyledons while the remaining 3 were monocotyledons. At least 11 genera had both C₃ and C₄ species.

All these plants have some features in common: similar anatomical structure (“Kranz” type of leaf anatomy) and capability to grow successfully in environments with low levels of CO₂ in the air. C₄ are generally of tropical or subtropical origin.

Fig. 13-30. Shows a cross section of a leaf from C₃ and C₄ plant respectively. Anatomically the two types look quite different. For instance, mesophyll cells of C₄ plants contain a few chloroplast. Secondly, the vascular bundles are surrounded by concentric cylinders called vascular bundle sheath.

Vascular bundle sheath (VBS) cells are packed with chloroplast and starch and the sheath is surrounded by mesophyll cells which are rich in chloroplast as well. Both, mesophyll and vascular bundle sheath cells take part in photosynthesis.

In recent years, very exciting studies have been carried out on these two categories of plants and these involve EM and biochemistry. In C₄ plants, the C₄ acids and their precursors are seen in the mesophyll cells chloroplast whereas bundle sheath cells abound in Calvin cycle compounds (Fig.’13-31, 32).

One additional difference merits attention. Chloroplast in the cells of VBS contain ribulose-1, 5-bisphosphate carboxylase (causes addition of CO₂ to C₅ sugars) while mesophyll chloroplast has PEP-carboxylase (CO₂ + PEP). W.M. Laetch and C.C. Black have described EM of C₄ and C₃ plants leaves and compared their structural peculiarities. In general, compared with VBS, mesophyll chloroplast of C₄ plants has well developed grana.

The significance of two photosynthetic carbon cycles operating in the same individual appears to assist the mechanism by which CO₂ in the environment is utilized most efficiently.

On the basis of photosynthetic carbon fixation pathway some workers recognize three groups within C₄ plants (Fig. 13-31, 32, 33).

These are:

(i) Maize, sugar-cane-NADP-ME [Malate Former]

(ii) Panicum maximum and Sporobolus-PEP-etc., sp.

(iii) Portulaca, Cyndodon-NAD-ME (ii) and (iii) are Aspartate formers.

The enzyme PEP—carboxylase involved in carbon metabolism has also been reported to occur at high levels in the pollen and pollen tubes and in the guard cells in the leaf epidermal tissues. Thus, besides C₄ plants, many other systems also have an enzyme which could rapidly utilize CO₂.

The two types of cells (mesophyll and vascular bundle sheath) are associated with each other by a complicated transport process (Fig. 13-30). Consequently malate is exported from the mesophyll and pyruvate is imported in turn. At present not much is known about the process precisely. In fact Figs. 13-30, 34 are over simplification of the whole process and do not attempt to deal with this problem.

In all the land plants CO₂ is ultimately assimilated by the reductive pentose phosphate pathways as shown in Figures 13-36, 37. In recent years two groups of plants have been discovered, CAM and C₄ plants, where modifications of the primary CO₂ trapping systems have been noticed. Both these secondary mechanisms are associated with distinctive anatomical characteristics and seem to have arisen independently several times in different angiosperm families during evolution.

In general PEP-carboxylase has strong affinity to combine with CO₂ than RuBP carboxylase. The CO₂ concentration for maximum photosynthesis in vivo is lower for PEP carboxylase. This explains why the rate of photosynthesis is higher and CO₂ compensation point is lower in C₄ compared with C3 plants.

Thus C₄ plants can concentrate CO₂ from weak concentration with the help of PEP-carboxylase. Obviously C₄ plants are more efficient for increased photosynthesis under low CO₂ concentration. The role of C₄ plants appears to concentrate CO₂ for RuBP carboxylase of C₃ cycle.

C₄ plants are only encountered in the angiosperms. This group originated in the Cretaceous period when O₂ in the atmosphere increased from 2-21%. The origin of C₄ plants could be by chance or mutation. However, it is fairly recent that these plants attained widespread distribution.

Plant breeders are now engaged in incorporating C₄ properties (Table 13-5) to the C₃ plants and thereby augmenting the amount of metabolites they form. Possibly very few genes are involved which need to be brought together.

Incidentally, crosses between C₃ and C₄Atriplex species have failed to yield progeny with integrated C₄ characters. In view of the non-availability of plants with C₄ characteristics closely related to cereals, the attempt is being made to screen different seed plants for C₄ types. The possibility of breeding entirely new plants from the genetic pool of C₄ types is not ruled out.

Three variations of C₄ cycle are shown in the figure 13-30a. A persual of the above figure shall indicate that the main differences are in regard to the following: form of C₄ acid translocated into the bundle sheath cell; nature and location of the decarboxylating enzyme (cytosol; mitochondrion, or chloroplast), and the form of the C₃ acid put back to the mesophyll cells.

In summary, the principal effect of the C₄ cycle appears to concentrate CO₂ in the bundle-sheath cells where the enzymes of the PCR cycle are located. In this way it is possible to build much high CO₂ concentrations in the bundle-sheath cells.

Some studies have indicated that the concentration of CO₂ in bundle-sheath cells may reach 10-fold higher than that in C₃ plants, and this would suppress photorespiration and promote high rates of photosynthesis. On the contrary there is an energy cost to build high CO₂ concentrations.

For every CO₂ assimilated two ATP must be spent in the regeneration of PEP. This is additional to the ATP and NDPH needed during the PCR cycle. Thus the net energy expenditure for assimilation of CO₂ by the C4 cycle is five ATP and two NADPH.

A few additional points need to be stated: first, C₄ plants may not be competitive in all situations, and some C₃ plants may even equal or exceed C₄ plants in productivity given the optimal combination of temperature, light and less water. Second, several of weeds are C₄ species e.g. Amaranthus, Digitaria, Saisola. Following two equations tend to compare the bioenergetics of C₃ and C₄ pathways.

C₃ 6 CO₂ + 18 ATP + 12NADPH+ 12 H⁺ → C₆H₁₂O₆ + 18ADP+ 18Pi + 12 NADP⁺ + 6H₂O

C₄ 6 CO₂ + 30 ATP + 12NADPH+ 12 H⁺ → C₆H₁₂O₆ + 30 ADP + 30P i + 12 NADP⁺ + 6H₂O