Hatch-Slack Pathway was first discovered by M. D. Hatch and C. R. Slack in 1966. Nearly 200200 billion tons of carbon dioxide is fixed every year in biomass. The bulk of carbon is incorporated into organic compounds by the reactions associated with photosynthesis. Photosynthesis in plants and phytoplanktons has three different pathways. These three pathways are:

1. Calvin cycle (reductive pentose phosphate cycle)-\({C_3}\) species
2. \({C_4}\) photosynthetic carbon assimilation cycle
3. Crassulacean acid metabolism (CAM)
The latter two can be considered as additional mechanisms for concentrating carbon dioxide.

What is Hatch-Slack Pathway?

The Hatch–Slack pathway or \({C_4}\) dicarboxylic acid pathway is one of three known photosynthetic processes of carbon fixation in certain plants. This pathway is named after the discoverer, M. D. Hatch and C. R. Slack. Some plants, when supplied with \(^{14}C{O_2}\) incorporate the \(^{14}C\) label into four-carbon molecules first.

Calvin Cycle (\({C_3}\) Cycle) – A Brief Recap

All photosynthetic organisms, from algae to angiosperms, reduce \({CO_2}\) to carbohydrates via the same basic mechanism of the Calvin cycle. Those plants that use just the Calvin cycle to fix carbon dioxide are called \({C_3}\) plants. The first stable product in this cycle is a \(3\)-carbon compound, which is formed when a five-carbon compound acceptor molecule, ribulose-\(1,5\)-bisphosphate \({RuBP}\) accepts one mole of carbon dioxide. The Calvin cycle proceeds in three stages:

1. Carboxylation of ribulose \(1,5,\) bisphosphate to form \(2\) molecules of \(3\)-phosphoglycerate. This is the first stable compound of the cycle.
2. Reduction of \(3\)-phosphoglycerate to glyceraldehyde three phosphate, a carbohydrate.
3. Regeneration of \({CO_2}\) acceptor ribulose \(1,5,\) bisphosphate

The reactions of the Calvin cycle take place in the stroma of chloroplasts present in the mesophyll cells of the leaves.

Discovery of Hatch and Slack Pathway (\({C_4}\) Cycle)

1. H. P. Kortschack and colleagues carried out \(^{14}C{O_2}\) labelling studies in sugarcane plants.
2. Y. Karpilov and coworkers have done similar experiments in maize. 
3. When such leaves were exposed for a few seconds to \(^{14}C{O_2}\) in the light, \(70\) to \(80%\) of the radioactivity was found in the \({C_4}\) acids Malate and Aspartate.
4. This was a very different pattern from the one observed in leaves that photosynthesize solely via the Calvin cycle.
5. M. D. Hatch and C. R. Slack later elucidated the complete biochemical pathway in such plants.
6. Some plants, like sugarcane, maize, produce the \({C_4}\) acids – malate and aspartate, as the first stable intermediates of photosynthesis.
7. This pathway was found in several tropical monocotyledonous plants, like maize, sugarcane, and in some tropical dicotyledonous plants like Artiplex, Amaranthus, etc.
8. Saga and colleagues have also found this cycle to be operative in some unicellular forms like certain diatoms, algae, with dimorphic chloroplasts.
9. The primary carboxylation in these leaves is not catalyzed by \(RuBisCO\) (case in \({C_3}\) pathway) but PEP (phosphoenolpyruvate) carboxylase catalyses the step. The anatomy of \({C_3}\) and \({C_4}\) plants differ and to understand the \({C_4}\) cycle, we first need to look at the anatomical differences between them.

Anatomy of Leaf in \({C_4}\) Plants

1. \({C_4}\) plants show characteristic anatomy in their leaves, often called Kranz anatomy (from the German word meaning ‘wreath’, the vascular bundles look like a necklace).
2. The vascular bundles are surrounded by two rings of cells, the inner layer is of bundle sheath cells while the outer layer is of mesophyll cells.
3. The inner bundle sheath cells have a large number of starch-rich chloroplast which lack grana (agranal chloroplast) and is thus different from those in mesophyll cells.
4. The outer mesophyll cells have chloroplasts with grana (granal chloroplast). They lack starch grains.
5. In Kranz anatomy, the bundle sheath cells are large and no mesophyll cell is more than two or three cells away from the nearest bundle sheath cell.
6. The bundle sheath cells and mesophyll cells are connected with each other by plasmodesmata which allow the movement of metabolites between the cells.
7. For the operation of the \({C_4}\) pathway, contribution of both the cells is required. 
8. Such anatomy is absent in \({C_3}\) plants. The following diagram shows the differences between \({C_3}\) and \({C_4}\) leaf anatomy.

Fig: Difference between the anatomy of \({C_3}\) and \({C_4}\) plant leaves

Stages of Hatch-Slack Pathway (\({C_4}\) cycle)

1. The basic \({C_4}\) cycle has \(4\) steps that take place in two different types of cells. The steps are:
   a. Fixation of \({CO_2}\) by the carboxylation of phosphoenolpyruvate in the mesophyll cells to form a \({C_4}\) acid (malate and/or aspartate).
   b. Transport of the \({C_4}\) acids from mesophyll cells to the bundle sheath cells
   c. Decarboxylation of the \({C_4}\) acids within the bundle sheath cells and generation of \({CO_2}\). This \({CO_2}\) is then reduced to carbohydrate via the Calvin cycle
   d. Transport of the \({C_3}\) acid (pyruvate or alanine) that is formed by the decarboxylation step back to the mesophyll cell and regeneration of the \({CO_2}\) acceptor phosphoenolpyruvate.
2. The first carboxylation is mediated by the enzymes PEPC or Phosphoenolpyruvate carboxylase, which has more affinity for carbon dioxide than \(RuBisCO.\)
3. The cycle thus transports atmospheric \({CO_2}\) to bundle sheath cells. Such a transport maintains a high concentration of \({CO_2}\) in the bundle sheath cells.
4. The elevated \({CO_2}\) levels around the \(RuBisCO\) result in the suppression of oxygenation of ribulose \(1,5\) bisphosphate and hence photorespiration.

Variations of \({C_4}\) Metabolism

1. There are three variations among different species of \({C_4}\) plants. These variations differ from each other in terms of the \({C_4}\) acid transported to bundle sheath cells and their decarboxylation. These three variations are:
   a. In the first type, the first product (Phosphoenol Pyruvate, PEP) is converted into Oxaloacetic acid, which eventually gets converted into Malate. Malate is transported to the bundle sheath cells.
   b. In the second type, the first product (PEP) is converted into Oxaloacetic acid, which eventually gets converted into Aspartate. Aspartate is transported to the bundle sheath cells.
   c. In the third type, the first stable product (PEP) is not known to be converted into any other product and is directly diffused to the bundle sheath cells.

The total reactions of the \({C_4}\) cycle are:

	Reaction	Enzyme
1	\({\rm{Phosphoenolpyruvate }} + {\rm{ }}HC{O_3}^– \to {\rm{ Oxaloacetate }} + {\rm{ }}Pi\)	Phosphoenolpyruvate carboxylase
2	\({\rm{Oxaloacetate }} + {\rm{ }}NADPH{\rm{ }} + {\rm{ }}{H^ + } \to {\rm{ Malate }} + {\rm{ }}NADP + \)	\(NADP:\) malate dehydrogenase
3	\({\rm{\;Oxaloacetate }} + {\rm{ Glutamate }} \to {\rm{ Aspartate }} + {\rm{ }}\alpha – {\rm{ketoglutarate}}\)	Aspartate aminotransferase
4	\({\rm{Malate }} + {\rm{ }}NAD{\left( P \right)^ + } \to {\rm{ Pyruvate }} + {\rm{ }}C{O_2} + {\rm{ }}NAD\left( P \right)H{\rm{ }} + {\rm{ }}{H^ + }\)	\(NAD\left( P \right)\) malic enzyme
5	\({\rm{Oxaloacetate }} + {\rm{ }}ATP{\rm{ }} \to {\rm{ Phosphoenolpyruvate }} + {\rm{ }}C{O_2} + {\rm{ }}ADP\)	Phosphoenolpyruvate carboxykinase
6	\({\rm{Pyruvate }} + {\rm{ Glutamate }} \leftrightarrow {\rm{ Alanine }} + {\rm{ }}\alpha – {\rm{ketoglutarate}}\)	Alanine aminotransferase
7	\(AMP{\rm{ }} + {\rm{ }}ATP{\rm{ }} \to {\rm{ }}2{\rm{ }}ADP\)	Adenylate kinase
8	\({\rm{Pyruvate }} + {\rm{ }}Pi{\rm{ }} + {\rm{ }}ATP{\rm{ }} \to {\rm{ Phosphoenolpyruvate }} + {\rm{ }}AMP{\rm{ }} + {\rm{ }}PPi\)	Pyruvate–orthophosphate dikinase
9	\(\;PPi{\rm{ }} + {\rm{ }}{H_2}O{\rm{ }} \to {\rm{ }}2{\rm{ }}Pi\)	Pyrophosphatase

Fig: Basic Plan of \({C_4}\) Cycle

Fig: Hatch-Slack or \({C_4}\) Photosynthetic pathway

Advantages of \({C_4}\)Pathway

The Hatch-Slack pathway (\({C_4}\) cycle) has certain advantages.
1. \({C_4}\) pathway does not undergo photorespiration. Photorespiration reduces the efficiency of photosynthesis and hence biomass production.
2. In hot and dry summers, the \({C_4}\) photosynthesis is more efficient in terms of biomass production.
3. \({C_4}\) plants can tolerate high temperatures.
4. \({C_4}\) plants can tolerate arid and drought conditions as the water loss that takes place during carbon fixation by the \({C_4}\) pathway is very less compared to only the \({C_3}\) pathway.
5. \({C_4}\) plants can respond to high intensities of light.
6. These plants can efficiently do photosynthesis even at a lower light intensity.
7. The PEPC enzyme can pick up carbon dioxide even at a low concentration, upto \(15\) ppm.

Difference Between \({C_3}\) and \({C_4}\) Cycles

Some of the major differences between \({C_3}\) and \({C_4}\) cycles (and \({C_3}\) and \({C_4}\) plants) are like:

\({C_{\bf{3}}}\) cycle (\({C_{\bf{3}}}\) Plants)	\({C_{\bf{4}}}\) cycle (\({C_{\bf{4}}}\) Plants)
Kranz anatomy is not found in the leaves.	Kranz anatomy is present in the leaves.
Chloroplast dimorphism is absent.	Chloroplast dimorphism is present in the mesophyll (granal) and bundle sheath (agranal) cells.
The first stable product is a \(3-\)carbon compound, i.e., phosphoglyceric acid (PGA).	The first stable compound is a \(4-\)carbon compound, i.e., Oxaloacetic acid (OAA).
The first \(C{O_2}\) acceptor in the \({C_{\bf{3}}}\) cycle is \(RuBP\) (ribulose \(1,5\) bisphosphate).	The first \(C{O_2}\) acceptor in the \({C_4}\) cycle is PEP (phosphoenol pyruvate).
The enzyme needed for primary carboxylation is \(RuBisCO\) or Ribulose bisphosphate carboxylase oxygenase.	The enzyme needed for primary carboxylation is \(PEPC\) or Phosphoenolpyruvate carboxylase.
The enzyme for carbon fixation is only \(RuBisCO.\)	\(RuBisCO\) is present in bundle sheath cells and PEPC is present in mesophyll cells.
\(RuBisCO\) has a strong affinity for oxygen over carbon dioxide.	\(PEPC\) does not have an affinity for oxygen.
Photorespiration is present due to the oxygenase activity of \(RuBisCO.\)	Photorespiration is altogether absent.
Total \(18\) molecules of \(ATP\) are needed for forming one mole of glucose.	Total \(30\) molecules of \(ATP\) are needed for forming one mole of glucose.
Ratio of \(C{O_2}\,:\,NADPH.{H^ + }\,:\,ATP = 1:2:3\)	Ratio of \(C{O_2}\,:\,NADPH.{H^ + }\,:\,ATP = 1:2:5\)
The \(C{O_2}\) compensation point is \(50 – 100{\rm{ ppm}}.\)	The \(C{O_2}\) compensation point is \(0 – 25{\rm{ ppm}}.\)
All steps of the \({C_3}\) cycle take place in mesophyll cells only.	Steps of the \({C_4}\) cycle take place partly in mesophyll cells and partly in bundle sheath cells.
Plants are less efficient photosynthetically.	Photosynthetic efficiency is higher.
Water loss per gram of dry mass-produced is large \(\left( {450{\rm{ \;to\; }}950} \right).\)	Water loss per gram of dry mass-produced is large \(\left( {250{\rm{ \;to\; }}350} \right).\)
The minimum concentration of atmospheric \(C{O_2}\) is \(45{\rm{ \;to\; }}50{\rm{ ppm}}.\)	The minimum concentration of atmospheric \(C{O_2}\) is \(15{\rm{ \;to\; }}20{\rm{ ppm}}.\)
\(C{O_2}\) saturation at about \(450{\rm{ \mu l}}{{\rm{L}}^{{\rm{ – 1}}}}.\)	\(C{O_2}\) saturation at about \(360{\rm{ \mu l}}{{\rm{L}}^{{\rm{ – 1}}}}.\)
The site of hexose synthesis is mesophyll cells.	The site of hexose synthesis is bundle sheath cells.
\(30{\rm{ \;to\; }}{35^ \circ }\,{\rm{C}}\) is the most favourable temperature for these plants.	\(30{\rm{ \;to\; }}{45^ \circ }\,{\rm{C}}\) is the most favourable temperature for these plants.
Examples of \({C_3}\) plants are mango, coconut, sunflower, rice, etc.	Examples of \({C_4}\) plants are maize, sugarcane, Artiplex, Amaranthus, etc

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Summary

Let’s summarise the important features of the Hatch and Slack pathway (\({C_4}\) Cycle)
1. The (\({C_4}\)  pathway was discovered by M.D. Hatch and C.R. Slack and hence the name.
2. The first stable intermediate is \(4\)-carbon compound – oxaloacetic acid (OAA).
3. C₄ plants show Kranz anatomy in their leaves. This pathway needs both mesophyll cells and bundle sheath cells.
4. PEP (\(3\)-carbon compound) is the primary \({CO_2}\) acceptor. The enzyme PEPC is responsible for primary carbon fixation.
5. Carboxylation takes place twice.
6. The Calvin pathway is common to \({C_3}\) and \({C_4}\) plants. However, the Calvin cycle takes place in bundle sheath cells instead of mesophyll cells.
7. Photorespiration is absent in \({C_4}\) plants.
8. C₄ plants are more efficient in \({CO_2}\) fixation and tolerant to high temperatures.

FAQs

.Q.1. What is the major function of Hatch and Slack Pathway?
Ans: Major function of the Hatch and Slack pathway is the fixation of carbon dioxide in an efficient way. Photorespiration is avoided in this pathway.

Q.2. What is meant by Hatch and Slack pathway?
Ans: Hatch and Slack pathway is a cyclic process that tells about carbon dioxide fixation in a way different from Calvin Cycle. Phosphoenol pyruvate, a \(3\)-carbon compound is the primary \({CO_2}\) acceptor and oxaloacetic acid – \(4\) carbon compound is the first stable intermediate

Q.3. Why is Hatch and Slack Pathway called the \({C_4}\) cycle?
Ans: In the Hatch and Slack pathway is a cyclic process in which the first stable intermediate compound of this pathway is oxaloacetic acid-  a \(\)4 carbon compound. Hence, the Hatch and Slack pathway is called the \({C_4}\) cycle.

Q.4. Give examples of plants showing Hatch and Slack Pathway.
Ans: Plants like sugarcane, maize, sorghum, Amaranthus, switchgrass etc. show the Hatch and Slack pathway. There are \(13\) families with \(8,100\) species that use this pathway.