HATCH-SLACK (C4) PATHWAY OF CO2 FIXATION

The discovery of C4 cycle in monocots such as sugarcane, maize and sorghum has indicated that these plants have solved the problem of photorespiration. The CO2 is fixed in the mesophyll cells. The initial product being a-4 carbon compound, the process is called C4pathway of carbon dioxide fixation.

HATCH-SLACK PATHWAY

2 Australian botanists Hatch and Slack (1966) discovered that there are two types of chloroplasts in sugarcane. 1 type restricted to bundle sheath cells have the normal grana. These chloroplasts carry on Hatch-Slack cycle. Hence, Hatch-Slack cycle has been found in most monocots and some dicots. The plants having C3 cycle are known as C3 plants, and the plants C4 cycle are  C4  plants.

Photorespiration occurs in C3 plants, which leads to a 25 percent loss of the fixed CO2.

In C3 plants, photosynthesis occurs in mesophyll cells. Photosynthesis has two types of reactions, i.e, light reactions and dark reactions.

In light reactions, NADPH2 and ATP are produced, and as a result of photolysis of water Oxygen is released.

During dark reactions, Carbon dioxide is assimilated and carbohydrates are produced.

C4 pathway requires the presence of 2 types of photosynthetic cells, i.e., mesophyll cells and bundle sheath cells. The bundle sheath cells are arranged in a wreath( circle) like manner. This kind of cells arrangement is called Kranz anatomy. In Kranz anatomy, the mesophyll and bundle sheath cells are connected by cytoplasmic bridges or plasmodesmata

The C4 plants contain dimorphic chloroplasts.
The chloroplasts in mesophyll cells are granal, whereas in bundle sheath cells they are agranal.

The granal chloroplasts contain thylakoids which are stacked to form grana, as formed in C3 plants. However, in agranal chloroplasts of bundle sheath cells grana are absent and thylakoids are present only as stroma lamellae.

The presence of two types of cells (granal and agranal) allows occurrence of light and carbon (dark) reactions separately in each type.

Here, release of oxygen takes place in one type, while fixation of Carbon dioxide catalysed by Rubisco enzyme occurs in another type of cells.

In maize, sugarcane, etc. light reactions occur in mesophyll cells, whereas Carbon dioxide assimilation takes place in bundle sheath cells. Such arrangement of cells does not allow oxygen released in mesophyll cells to enter in bundle-sheath cells.

Hence, Rubisco enzyme, which is only present  in bundle-sheath cells, does not come into contact with oxygen, and thus, oxygenation of RuBP is completely avoided.

In C4 plants, a Carbon dioxide concentrating mechanism is present which helps in reducing the occurrence of photorespiration. This type of Carbon dioxide  concentrating mechanism is called C4 pathway.

In C4 pathway, both mesophyll and bundle-sheath cells are required. The main objective of C4 pathway is to build up high concentration of Carbon dioxide  near Rubisco enzyme in bundle- sheath cells. High concentration of Carbon dioxide near Rubisco enhances carboxylation and reduces photorespiration.

C4 photosynthetic Carbon Cycle:

In C4 cycle Carbon dioxide from the atmosphere enters through stomata into the mesophyll cells and combines with phosphoenol pyruvate. This reaction catalysed by an enzyme known as phosphoenol pyruvate carboxylase. With the result, 4 carbon acid, oxaloacetic acid is formed.

The above-mentioned reaction occurs in cytosol of the mesophyll cells and is called fixation of Carboxylation.

The next step of reaction is transport of oxalo acetic acid (OAA – 4 C compounds) from cytosol of mesophyll cells to chloroplasts of bundle-sheath cells, where it is decarboxylated to release fixed Carbon dioxide and high concentration of Carbon dioxide is generated near Rubisco.

The other product of decarboxylation reaction is 3-carbon compound called pyruvic acid.
Now, this is transported back to mesophyll cells, where if regenerates phosphoenol pyruvate to its own for continuation of C4 pathway.

However, Due to absence of photorespiration in C4 plants C4 pathway is more efficient than C3 pathway