Hatch and Slack Cycle - Definition

The Hatch and Slack Cycle is a process of alternating between periods of intense work (hatch) and periods of rest and relaxation (slack). This cycle helps to ensure productivity and maintain a healthy work-life balance.

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.

Blackman illustrated the existence of a dark reaction, referred to as Blackman’s reaction. This reaction consists of two types of cyclic reactions occurring in the dark.

Calvin Cycle (C3 Cycle)

Hatch and Slack Pathway or C4 Cycle

Hatch and Slack Pathway

M. D. Hatch and C. R. Slack were the first to provide an in-depth outline of this metabolic pathway. Through the action of the enzyme PEP carboxylase, carbon dioxide is added to the phosphoenolpyruvate in the mesophyll cells, forming a four-carbon compound which is then transported to the bundle sheath cells to release the carbon dioxide for use in the Calvin cycle.

In 1966, Hatch and Slack discovered the C4 cycle, thus giving it its name. It is also known 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, which is why it is referred to as the C4 cycle.

C4 plants possess a C4 cycle and are inclusive of both dicots and monocots. This cycle is seen in the families of Chenopodiaceae, Gramineae and Cyperaceae.

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Hatch and Slack Pathway in C4 Plants

The C4 cycle is an alternate pathway to the C3 cycle for fixing carbon dioxide. It is so-named because the first stable compound formed, oxaloacetic acid, is a 4-carbon compound. This pathway is commonly seen in grasses, maize, sugarcane, amaranthus, and sorghum. Additionally, C4 plants show a distinct type of leaf anatomy known as Kranz anatomy.

The chloroplasts of C4 plants are dimorphic; in the leaves, vascular bundles are surrounded by a bundle sheath of larger parenchymatous cells. These bundle sheath cells contain larger chloroplasts with starch grains but lacking grana, while the chloroplasts in the mesophyll cells possess grana and are smaller. This characteristic leaf anatomy is referred to as Kranz Anatomy, which in German corresponds to the word for wreath.

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 –

Carboxylation

Breakdown

Splitting

Phosphorylation

Carboxylation

The enzyme phosphoenolpyruvate carboxylase catalyzes the reaction of a 3-carbon compound, Phosphoenolpyruvate, which collects carbon dioxide and in the presence of water, transforms to 4 carbon oxaloacetate. This reaction occurs in the chloroplasts of the mesophyll cells.

Breakdown:

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The enzyme transaminase and malate dehydrogenase catalyze the reaction of oxaloacetate into 4 carbon malate and aspartate, which then diffuse into the sheath cells from the mesophyll cells.

Combining

The malate and aspartate in the sheath cells enzymatically split to produce free carbon dioxide and 3-carbon pyruvate. The carbon dioxide is then used in Calvin’s cycle in the sheath cells. The second carboxylation takes place in the chloroplasts of the bundle sheath cells, where the carbon dioxide is accepted by the 5-carbon compound ribulose diphosphate with the activity of the carboxy dismutase enzyme, eventually producing 3 phosphoglyceric acid. For the formation of sugars, some of the 3 phosphoglyceric acid is used and the remaining is used to regenerate ribulose diphosphate.

Phosphorylation

Pyruvate molecules are moved to the chloroplasts of the mesophyll cells, where, in the presence of ATP, they are phosphorylated by Pyruvate phosphokinase, resulting in the regeneration of phosphoenolpyruvate.

The Kranz anatomy of leaves is associated with the C3 and C4 cycles of carboxylation. C4 plants are more efficient in photosynthesis than C3 plants. The C4 cycle’s phosphoenolpyruvate carboxylase enzyme has a greater affinity for carbon dioxide than the C3 cycle’s ribulose diphosphate carboxylase enzyme when it comes to fixing molecular carbon dioxide in an organic compound during carboxylation.

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Different Reactions of the C4 Cycle and the Hatch-Slack Cycle

Following reactions occur in the Hatch and Slack Cycle:

In Mesophyll Cells’ Chloroplasts

Formation of Oxaloacetic Acid

Formation of Malic acid and Aspartic acid

Formation of Oxaloacetic Acid

The atmospheric carbon dioxide in the mesophyll cells combines with water, catalyzed by the carbonic anhydrase enzyme, to form bicarbonate ions which then act as the primary acceptor of carbon dioxide in the cycle, ultimately leading to the formation of 3-C compound phosphoenol pyruvic acid.

![Hatch and Slack Pathway image 5]()

The enzyme PEP carboxylase combines Phosphoenol Pyruvic Acid (PEP) with Carbon Dioxide to form a 4-carbon acid, Oxaloacetic Acid. In this process, a Water Molecule is needed and a Molecule of Phosphoric Acid is released.

Formation of Malic Acid and Aspartic Acid

The enzyme malic dehydrogenase reduces oxaloacetic acid to malic acid in the presence of light generated NADPH+H+.

In the presence of the enzyme aspartic transaminase, this oxaloacetic acid could also be converted into aspartic acid.

Aspartic Transaminase

Oxaloacetic Acid

Aspartic acid

Aspartic acid and malic acid, the C4 acids, are transported to the chloroplasts of the bundle sheath.

In Bundle Sheath Chloroplasts

Formation of Pyruvic Acid

The malic enzyme facilitates the oxidative decarboxylation of malic acid in the bundle sheath, resulting in the production of pyruvic acid and carbon dioxide.

The carbon dioxide and NADPH + H+ produced by the oxidative decarboxylation of malic acid enter the Calvin cycle. This carbon dioxide combines with ribulose diphosphate (RuDP) to yield two molecules of PGA (phosphoglyceric acid).

![Hatch and Slack Pathway image 9a]()

##Mesophyll Cells

Formation of Phosphoenolpyruvic Acid (PEP)

Pyruvic acid is transported back to the mesophyll cells, where it is phosphorylated to form phosphoenol pyruvic acid by the action of the pyruvate phosphate dikinase enzyme.

Significance of C4 Pathway

The role of this pathway is to transfer carbon dioxide to the RPP pathway and refix any carbon dioxide which originates from photorespiration. This decreases the energy loss which occurs in the C3 plants due to the oxygenase action of the RuBisCO enzyme, thus justifying the increased growth rates seen in the C4 plants in some conditions.

Since the discovery of the C4 pathway, the potential for altering economically important plants to become C4 plants has been explored, as well as the use of herbicides for further research on C4 plants.