In sugarcane (monocot)formation of malic acid occur by C4 cycle . By carbon fixation with PEP( phospho enol pyruvate) formation of oxalic acid acid occur which further convert into malic acid or aspartic acid.
this happen in mesophyll cell of sugarcane leaf. carbon fixation require PEPcase or PEP carboxylase .

Sugarcane and CO2 fixation in malic acid

Introduction
Sugarcane is a C4 plant that utilizes a specialized pathway known as the C4 pathway for carbon dioxide (CO2) fixation. This pathway allows the plant to efficiently capture and concentrate CO2, enhancing photosynthetic efficiency and water use efficiency.

The role of malic acid
Malic acid plays a crucial role in the C4 pathway of sugarcane. It serves as a temporary storage molecule for CO2 before it is used in the Calvin cycle for sugar synthesis. Malic acid acts as a shuttle molecule, transferring the fixed CO2 from the mesophyll cells to the bundle sheath cells where the Calvin cycle takes place.

The enzyme involved
The enzyme responsible for fixing CO2 in malic acid is phosphoenolpyruvate (PEP) carboxylase. PEP carboxylase is an enzyme found in the mesophyll cells of C4 plants. It catalyzes the carboxylation of phosphoenolpyruvate (PEP) to form oxaloacetate (OAA), a four-carbon compound.

The mechanism
1. CO2 enters the mesophyll cells through stomata and is initially fixed by PEP carboxylase. PEP carboxylase has a high affinity for CO2, allowing it to efficiently capture and fix the CO2 in the form of OAA.
2. OAA is then reduced to malate using NADPH and ATP. Malate is a more stable compound and can be easily transported to the bundle sheath cells.
3. In the bundle sheath cells, malate is decarboxylated to release CO2, which is then used in the Calvin cycle for sugar synthesis. The released CO2 replenishes the pool of CO2 available for fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the enzyme responsible for CO2 fixation in the Calvin cycle.

Advantages of the C4 pathway
The C4 pathway provides several advantages to sugarcane and other C4 plants:
1. Increased photosynthetic efficiency: The C4 pathway allows for the efficient capture and concentration of CO2, enhancing photosynthetic efficiency even in conditions of high temperature and limited water availability.
2. Reduced photorespiration: By concentrating CO2 around RuBisCO in the bundle sheath cells, the C4 pathway minimizes the wasteful process of photorespiration, which occurs when RuBisCO binds to oxygen instead of CO2.
3. Enhanced water use efficiency: The spatial separation of CO2 fixation and sugar synthesis in C4 plants reduces the rate of transpiration, resulting in improved water use efficiency.

Conclusion
In sugarcane, CO2 fixation occurs in malic acid with the help of the enzyme PEP carboxylase. This process is part of the C4 pathway, which provides several advantages to sugarcane and other C4 plants. Understanding the mechanisms and enzymes involved in CO2 fixation is crucial for improving crop productivity and developing strategies to mitigate the effects of climate change.