Hatch-Slack Pathway

When the chemical process of photosynthesis is carried out without the presence of light, it is referred to as the dark reaction. The chloroplast’s stroma is where it can be found. This dark reaction is entirely enzymatic, and it is significantly slower than the light reaction. When light is present, dark reactions can occur as well as light reactions. The sugars used in the dark reactions are created by converting carbon dioxide into glucose. The energy-depleted CO2 is converted to energy-dense carbohydrates by utilising an energy-dense compound, ATP, as well as the assimilatory power of the NADPH2 produced by the photochemical reaction. Carbon assimilation of carbon fixation is the term used to describe the process.

As Blackman demonstrated, there is such a thing as a dark reaction. The term “Blackman’s reaction” was used to describe this type of reaction. During the dark reaction, there are two types of cyclic reactions that can take place:

• Calvin cycle (also known as C3 cycle)
• Hatch and Slack pathway (also known as C4 cycle)

Hatch and Slack Pathway

1. D. Hatch and C. R. Slack were the first to describe in detail this metabolic pathway. The carbon dioxide is added to the phosphoenolpyruvate first, thanks to the action of the enzyme PEP carboxylase, before the rest of the ingredients are combined. As a result, the four-carbon compound is produced in the mesophyll cells, which is then transported to the bundle sheath cells, where it is converted into carbon dioxide and used in the Calvin cycle to produce energy.

The C4 cycle was discovered by Hatch and Slack in 1966, hence the name “C4 cycle.” There are several names for this pathway, including the ß-carboxylation pathway and co-operative photosynthesis. The C4 cycle is named after the 4-carbon oxaloacetic acid which is the first stable compound of the Hatch and Slack cycle, which is also known as the C4 cycle.

C4 plants are those that have a C4 cycle in their life. Such plants include both dicots and monocots, and the C4 cycle can be observed in the Chenopodiaceae, Gramineae, and Cyperaceae families, among other groups.

Hatch and Slack Pathway In C4 Plants

This pathway, which is an alternative to the C3 cycle, is used to fix carbon dioxide. In this case, the first stable compound formed – oxaloacetic acid – is a four-carbon compound, thus the name “C4 cycle” was given. This pathway can be found in a variety of grasses, including maize, sugarcane, amaranthus, and sorghum. The C4 plants have a different type of leaf anatomy than the other plants (Kranz anatomy).

The chloroplasts are dimorphic, and in the leaves, vascular bundles are surrounded by a sheath of larger parenchymatous cells, which acts as a protective sheath. Bundle sheath cells have larger chloroplasts, contain starch grains, and are devoid of grana, whereas the chloroplasts in mesophyll cells are always smaller and contain grana, and are surrounded by starch grains. Cells in the bundle sheath appear as a wreath or a ring when they are larger than the surrounding cells. The Kranz Anatomy of the C4 plants refers to the distinctive leaf anatomy that they have developed. Kranz is the German word for the wreath, which is how the name Kranz Anatomy came to be.

The C4 Cycle depicts two carboxylation reactions that take place in the chloroplasts of mesophyll cells, as well as other carboxylation reactions that take place in the chloroplasts of bundle sheath cells. To complete the Hatch and Slack Cycle, there are four steps:

• Carboxylation 
• Breakdown 
• Splitting 
• Phosphorylation

Carboxylation

This compound is found in the chloroplasts of mesophyll cells. Phosphoenolpyruvate is a 3-carbon compound that collects carbon dioxide and, in the presence of water, transforms into the 4-carbon compound oxaloacetate. In this case, the reaction is catalysed by the enzyme phosphoenolpyruvate carboxylase.

Breakdown

Oxaloacetate readily decomposes into malate and aspartate, both of which contain four carbons. Transaminase and malate dehydrogenase are the enzymes that are involved in the reactions. The compounds formed diffuse into the sheath cells from the mesophyll cells, where they bind to other compounds.

Splitting

Carbon dioxide is liberated and 3-carbon pyruvate is produced by an enzyme reaction in the sheath cells’ malate and aspartate molecules. Calvin’s cycle is utilised in the sheath cells to make use of the carbon dioxide. Coxylation occurs in the chloroplasts of the bundle sheath cells during the second step of this process. Carboxy dismutase enzyme activity allows the carbon dioxide to be accepted by the 5-carbon compound ribulose diphosphate, which results in the formation of 3 phosphoglyceric acids. It is necessary to use some of the 3 phosphoglyceric acids for the formation of sugars, while the remainder is used to regenerate ribulose diphosphate.

Phosphorylation

The pyruvate molecules are transported to the chloroplasts of the mesophyll cells, where they are phosphorylated in the presence of ATP to produce phosphoenolpyruvate, which is then transported back to the cell nucleus. The enzyme pyruvate phosphokinase catalyses the reaction, which results in the regeneration of phosphoenolpyruvate.

As a result of the Kranz anatomy of the leaves, the C3 and C4 carboxylation cycles are linked together in this pathway, resulting in increased efficiency. When compared to C3 plants, C4 plants are more efficient at photosynthesis, according to the USDA. When it comes to fixing molecular carbon dioxide in the organic compound at the time of carboxylation, the phosphoenolpyruvate carboxylase enzyme of the C4 cycle is found to have a greater affinity for carbon dioxide than the ribulose diphosphate carboxylase enzyme of the C3 cycle.

Different Reactions Of C4 Cycle or Hatch and Slack Cycle

The following reactions take place during the Hatch and Slack Cycle: 

• In the chloroplast of Mesophyll Cells, this process results in the formation of Oxaloacetic Acid, Malic Acid, and Aspartic Acid.
• Oxaloacetic Acid is formed during the process.
• The primary acceptor of carbon dioxide in the cycle is phosphoenol pyruvic acid, which is a 3-carbon compound. In mesophyll cells, atmospheric carbon dioxide reacts first with water to form bicarbonate ions, which are then broken down further by the carbonic anhydrase enzyme to form carbon dioxide and water again.
• With the action of the enzyme PEP carboxylase, the carbon dioxide acceptor PEP (phosphoenol pyruvic acid) combines with carbon dioxide to form a 4-carbon acid oxaloacetic acid, which is then converted to acetic acid. 
• The release of a molecule of phosphoric acid is required at this point, as is the presence of a water molecule.

Conclusion:

The purpose of this pathway is to transfer carbon dioxide to the RPP pathway and to refix any carbon dioxide that has been produced by photorespiration in the atmosphere. A consequence of this is that the energy loss that occurs in C3 plants as a result of the oxygenase action of the RuBisCO enzyme is reduced by as much as 50%. It contributes to the explanation of the increased growth rates observed in C4 plants under certain conditions.

Almost from the moment that the C4 pathway was discovered, the possibility of converting economically important plant species to C4 species has been taken into consideration. Herbicides are another important area of study for C4 plants that are currently being investigated.