The primary acceptor of carbon di-oxide in Hatch-Slack cycle is:

Question

Accepted Answer

The correct option is 2:
The correct answer is **Phosphoenol Pyruvic acid (PEP)**. The Hatch-Slack cycle, also known as the C4 pathway, is a sophisticated photosynthetic adaptation that allows plants like maize, sugarcane, and millet to efficiently fix carbon dioxide in hot, arid environments. This adaptation is critical because it acts as a mechanism to concentrate CO2 around the primary carbon-fixing enzyme of the Calvin cycle, RuBisCO, thereby drastically reducing the wasteful process of photorespiration. The entire process begins in the peripheral mesophyll cells of the leaf, where the three-carbon compound, Phosphoenol Pyruvic acid (PEP), serves as the primary acceptor molecule for atmospheric CO2. This carboxylation step is catalyzed by the enzyme PEP Carboxylase (PEPCase). PEPCase is remarkable because of its extremely high affinity for carbon dioxide, which allows it to scavenge CO2 even when its concentration is low, and, crucially, it does not bind to oxygen. The immediate result of this initial fixation is the synthesis of a four-carbon compound, Oxaloacetic acid, which is where the name "C4" cycle originates. This two-part process—initial fixation by PEP in the mesophyll and secondary fixation by RuBP in the bundle sheath—is fundamental to the C4 strategy.

The incorrect option is 1: Oxalic acid
This option is incorrect because **Oxaloacetic acid (OAA)** is the **first stable product** of carbon fixation in the C4 pathway, not the primary acceptor. The entire purpose of the first reaction in the mesophyll cell is for Phosphoenol Pyruvic acid (PEP) to accept CO2 and immediately form the four-carbon compound, Oxaloacetic acid. OAA is the result of the CO2 being added to the three-carbon PEP molecule, making it the product of the reaction. Furthermore, OAA is very quickly converted into other four-carbon acids, such as malate or aspartate, which are then transported into the bundle sheath cells for the next stage of the C4 process.

The incorrect option is 3: Ribulose biphosphate
Ribulose biphosphate, more accurately **Ribulose-1,5-bisphosphate (RuBP)**, is the universally recognized primary acceptor of carbon dioxide, but it is specific to the **Calvin cycle (C3 pathway)**. In C3 plants, the enzyme RuBisCO catalyzes the fixation reaction between the five-carbon RuBP and CO2. While C4 plants perform the Calvin cycle (the C3 stage) in their bundle sheath cells, this is considered the *secondary* fixation stage, as the CO2 has already been accepted and concentrated by a different molecule. Therefore, to correctly identify the *primary* acceptor in the *Hatch-Slack* cycle, we must refer to the molecule involved in the very first step, which is PEP.

The incorrect option is 4: Phosphoglyceric acid
This option is incorrect because **Phosphoglyceric acid (PGA)**, specifically 3-Phosphoglyceric acid, is the **first stable product** of carbon fixation in the **C3 pathway**. When RuBP accepts CO2 in the Calvin cycle, the unstable six-carbon intermediate immediately breaks down to form two molecules of the three-carbon compound, 3-PGA. PGA is a metabolite *downstream* of CO2 acceptance and is not the acceptor itself. In the C4 cycle, PGA is only formed much later in the bundle sheath cells after the concentrated CO2 is released and finally enters the standard Calvin cycle for sugar synthesis.