Hatch and Slack Cycle – Definition
The chemical process of photosynthesis which takes place independent of light is referred to as dark reaction. It occurs in the stroma of the chloroplast. This dark reaction is enzymatic purely, and compared to the light reaction, is slower. Dark reactions also take place when light is present. The sugars in the dark reactions are synthesized from carbon dioxide. The energy-deprived CO2 is fixed to energy-rich carbohydrates utilizing energy-rich compound, ATP and the assimilatory power, the NADPH2 of light reaction. The process is referred to as carbon assimilation or carbon fixation.
As Blackman illustrated the existence of a dark reaction. This reaction was referred to as Blackman’s reaction. There are two types of cyclic reactions occurring in the dark reaction –
- Calvin cycle or C3 cycle
- Hatch and Slack pathway or C4 cycle
Hatch and Slack Pathway
M. D. Hatch and C. R. Slack first outlined this metabolic pathway in depth. In this, the carbon dioxide is added first to the phosphoenolpyruvate by the action of the enzyme PEP carboxylase. Thus, producing the four-carbon compound in the mesophyll cells, which is transported later on to the bundle sheath cells to release the carbon dioxide to be used in the Calvin cycle.
In 1966, Hatch and Slack discovered the C4 cycle, hence the name. It is also referred to as the ß-carboxylation pathway and co-operative photosynthesis. The 4-carbon oxaloacetic acid is the first stable compound of the Hatch and Slack cycle, hence is called the C4 cycle.
C4 plants are plants possessing a C4 cycle. Such plants are inclusive of dicots and monocots, the C4 cycle is evident in the Chenopodiaceae, Gramineae and Cyperaceae family.
Hatch and Slack Pathway In C4 Plants
To fix carbon dioxide, this pathway is the alternate to the C3 cycle. Here, the first formed stable compound – oxaloacetic acid is a 4 carbon compound, hence the name C4 cycle. This pathway is a common sight in several grasses, maize, sugarcane, amaranthus, sorghum. The C4 plants depict a different kind of leaf anatomy (Kranz anatomy).
The chloroplasts are dimorphic and in the leaves, vascular bundles are wrapped by a bundle sheath of larger parenchymatous cells. Such bundle sheath cells possess chloroplasts, which are larger, containing starch grains, lacking grana while the chloroplasts in the mesophyll cells always possess grana and are smaller. The bundle sheath cells appear as a wreath or a ring while cells are larger. This characteristic leaf anatomy of the C4 plants is referred to as Kranz Anatomy. In German, Kranz corresponds to the wreath, hence the name Kranz Anatomy.
The C4 Cycle depicts two carboxylation reactions occurring in the chloroplasts of the mesophyll cells and others in the chloroplast of the bundle sheath cells. The Hatch and Slack Cycle involves four steps –
Occurs in the chloroplasts of the mesophyll cells. A 3-carbon compound, Phosphoenolpyruvate, collects carbon dioxide and in the presence of water, transforms to 4 carbon oxaloacetate. The enzyme phosphoenolpyruvate carboxylase catalyzes the reaction.
Readily, oxaloacetate disintegrates into 4 carbon malate and aspartate. The enzyme involved in the reaction is transaminase and malate dehydrogenase. The compounds formed diffuse into the sheath cells from the mesophyll cells.
The malate and aspartate in the sheath cells enzymatically split to produce free carbon dioxide and 3-carbon pyruvate. The carbon dioxide is made use of Calvin’s cycle in the sheath cells. The second carboxylation takes place in the chloroplasts of the bundle sheath cells. The carbon dioxide is accepted by the 5-carbon compound ribulose diphosphate with the activity of the carboxy dismutase enzyme, finally producing 3 phosphoglyceric acid. For the formation of sugars, some of the 3 phosphoglyceric acid is used and the remaining regenerates ribulose diphosphate.
The pyruvate molecules are moved to the chloroplasts of the mesophyll cells wherein, in the presence of ATP, it is phosphorylated for the regeneration of phosphoenolpyruvate. Pyruvate phosphokinase catalyzes the reaction and phosphoenolpyruvate is regenerated.
In this pathway, the C3 and C4 cycles of the carboxylation are associated as a result of the Kranz anatomy of the leaves. Compared to C3 plants, the C4 plants are more efficient in photosynthesis. Phosphoenolpyruvate carboxylase enzyme of the C4 cycle is seen to possess more affinity for carbon dioxide compared to ribulose diphosphate carboxylase of the C3 cycle when it comes to fixing molecular carbon dioxide in the organic compound at the time of carboxylation.
Different Reactions Of C4 Cycle or Hatch and Slack Cycle
Following reactions occur in the Hatch and Slack Cycle –
In Chloroplast of Mesophyll Cells
- Formation of Oxaloacetic Acid
- Formation of Malic acid and Aspartic acid
Formation of Oxaloacetic Acid
A 3-C compound phosphoenol pyruvic acid is the primary acceptor of carbon dioxide in the cycle. The atmospheric carbon dioxide in the mesophyll cells combines first with water for the formation of bicarbonate ions with the action of the carbonic anhydrase enzyme.
The PEP (phosphoenol pyruvic acid), the carbon dioxide acceptor combined with carbon dioxide forming a 4-carbon acid oxaloacetic acid with the action of the enzyme PEP carboxylase. At this juncture, a water molecule is required, a molecule of phosphoric acid is released.
Formation of Malic acid and Aspartic acid
Using light generated NADPH+H+, oxaloacetic acid is reduced to malic acid in the presence of the enzyme malic dehydrogenase.
In the presence of the enzyme aspartic transaminase, this oxaloacetic acid could also be converted into aspartic acid.
Aspartic acid and malic acid, the C4 acids are then transported to the chloroplasts of the bundle sheath.
In Bundle Sheath Chloroplast
- Formation of Pyruvic Acid
The malic acid in the bundle sheath experiences oxidative decarboxylation to produce pyruvic acid and carbon dioxide with the presence of the malic enzyme.
- The carbon dioxide and NADPH + H+ which is produced by the oxidative decarboxylation of the malic acid enters into the Calvin cycle. This carbon dioxide combines with ribulose diphosphate (RuDP) to produce 2 molecules of PGA (phosphoglyceric acid).
Formation of Phosphoenol Pyruvic Acid (PEP)
Pyruvic acid is moved back to the mesophyll cells wherein it is phosphorylated to the phosphoenol pyruvic acid with the action of the pyruvate phosphate dikinase enzyme.
Significance of C4 Pathway
The role of this pathway is to render carbon dioxide to the RPP pathway and for the refixing of any carbon dioxide which originates from photorespiration. As a result, it decreases the energy loss which occurs in the C3 plants as a result of the oxygenase action of the RuBisCO enzyme. It aids in justifying the increased growth rates seen in the C4 plants in some conditions.
Ever since the C4 pathway has been discovered, the likelihood of changing economically important plants into C4 plants has been taken into consideration. Herbicides is another significant stream to develop research with C4 plants.
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