The C4 Pathway {Hatch and Slack Pathway}

The C4 Pathway {Hatch and Slack Pathway}

OR CO-OPERATIVE PHOTOSYNTHESIS

Most of the plants are known as C₃ plants because the first product of CO₂ fixation is a 3 C compound, phosphoglyceric acid.

But C₃ cycle (or Calvin cycle) is not the only way in plants to fix CO₂, H.P. Kortschak and C.E. Hartt (1965) found that in sugarcane (a tropical plant), leaves removed CO₂ more efficiently from the atmosphere; and the first products of photosynthesis were acids containing 4-carbon atoms (e.g., malic, oxaloacetic and aspartic acid), rather than the 3C-acid PGA.

Since then, the same has been found true for many important tropical plants including monocots (like maize, Sorghum and Eleusine) as well as dicots (like Amaranthus and Euphorbia sp.).

These plants are called C₄ plants. In 1966, two Australian scientists, Hatch and Slack showed that C₄ plants are much more efficient in CO₂ utilization than C₃ plants.

Such plants do not show photorespiration.

In 1967, Hatch and Slack explained the manner of CO₂ fixation and reduction in such plants.

The new carbon pathway in C₄ plants is called Hatch-Slack Pathway.

The leaves of C₄ plants show Kranz anatomy, means there vascular bundles are surrounded by two rings of cells:

(i) Bundle sheath cells : Thick walled, packed tightly all around the vascular bundle, plastids centripetally arranged, lack grana, contain starch grains, absence of PS II, CO₂ released by malic acid is fixed by RuBP. Calvin cycle enzymes are present in stroma.

(ii) Mesophyll cells : Concentrically arranged, form most of the leaf tissues; plastids well developed, grana large and well developed; PS II present, CO₂ accepted by PEP, CO₂ fixation occurs in cytoplasm; Calvin cycle enzymes are absent.

In C₄ plants, CO₂ is first accepted in the mesophyll cells by phosphoenol pyruvic acid (a highly energetic Cintermediate of glycolysis) using the enzyme PEPCO/PEPCase.

This forms a C₄ compound oxaloacetic acid.

It transforms into another 4 C compound malic acid.

The malic acid now diffuses to the bundle sheath cells where it gives off CO₂ It is accepted by ribulose diphosphate and refixed into hexose sugars through Calvin cycle.

The malic acid, after giving off CO₂ convert into pyruvic acid and returns to mesophyll cells. It is then converted to phosphoenolpyruvate by using ATP.

This involves formation of AMP (adenosine monophosphate) instead of ADP.

Hence, regeneration of ATP from AMP requires 2 ATP each (12 ATP for formation of 6 molecules of phosphoenolpyruvate, as the 6CO₂ molecules are to be fixed).

Therefore, C₄ pathway requires 12 additional ATP or total 30 ATP (18 ATP in C₃ cycle + 12 additional ATP).

An interesting aspect of C₄ cycle is that, it shows a division of labour between two kinds of parenchyma cells showing chloroplast dimorphism -mesophyll chloroplasts act as traps of CO₂ which is then (in the form of C₄ acids) shuttled to bundle sheath plastids where finally the CO₂ is reduced to sugars.

C₄ plants are more efficient in picking up CO₂ and tolerating drought conditions.

CO₂ compensation point C₄ is low (0-10 ppm) in C₄ plants (as compared to 25-100 ppm) for C₃ plants. Photorespiration is absent in C₄ plants, because they have a mechanism to concentrate CO₂ at enzyme site.

Some major differences between C₃ and C₄ plants

(3) CRASSULACEAN ACID METABOLISM (CAM) (Diurnal acid Cycle

(i)  The leaves of plants belonging to Crassulaceae family like Kalanchoe and Sedum undergo crassulacean acid metabolism and such plants are called CAM plants.

(ii)  All CAM plants are succulent in habit, including other plants like Orchids, pineapple, certain cactus etc. In these plants, stomata remains open in night and closed during the day (Scotoactive stomata).

(iii)  CO₂ is fixed during night (dark) to OAA using PEP carboxylase. This CO₂ comes from respiration (breakdown of starch) and also from the atmosphere. Malic acid gets stored in vacuoles.

(iv) The CAM plants also contain the enzymes of Calvin cycle. During day time, malic acid breaks into pyruvate and CO₂, While CO₂ enters the calvin cycle, pyruvate is used up to regenerate PEP.

(v)  The succulents, therefore synthesize plenty of organic acids from CO₂ during night (when stomata are open) and plenty of carbohydrates during the day (when stomata are closed)(vi)    Like Calvin cycle, CAM cycle also operates in the mesophyll ceil. None of these have shown chloroplast dimorphism as is found in C₄ plants.

(vii) t should be remembered that the slow growing desert succulents exhibiting CAM cycle have the slowest photosynthetic rate, while the species possessing C₄ pathway possess the highest rates.

(viii)  Thus CAM plants are although not as efficient as C₄ plants, they are definitely better suited to the adverse conditions (i.e., conditions of extreme desiccation).