The C4 plants are generally found in the tropical and subtropical regions and they have evolved C4 pathway as a mechanism to avoid the occurrence of photorespiration, which is considered a wasteful process. All those plants which exhibit C4 pathway of photosynthesis are called C4 plants, e.g. Zea maize (maize), Saccharum (sugar cane), Amaranthus, Digitalis, etc. Photorespiration (oxidation of RuBP in presence of O2) is absent in these plants. So the photosynthetic rate is high.
The bundle sheath cells are large Chlorenchyma/parenchyma cells, arranged in a single layer around the vascular bundles like a wreath; this type of arrangement is known as Kranz anatomy. These are; they have dimorphic chloroplasts, light reactions and the carbon cycle occur separately in two different types of cells, the former taking place in mesophyll cells and the latter occurring in bundle sheath cells. This type of cellular arrangement does not allow the oxygen to come in contact with the Rubisco in the bundle sheath cells, hence oxygenation of RuBP is completely avoided. Added to this, they have evolved a carbon dioxide concentrating mechanism, called C4 pathway.
The CO2 acceptor in C4 plants is phosphoenolpyruvate (PEP). PEP reacts with CO2 to form oxaloacetic acid which is reduced by NADPH to form malic acid. The malic acid then reacts with RuBP to form pyruvic acid and PGA. The pyruvic acid is then phosphorylated by ATP to regenerate PEP while PGA is converted to triose phosphate as far as C3 plants. It combines with CO2 in presence of an enzyme Phosphoenolpyruvate carboxylase (PEPCase) and forms a C4 acid, oxaloacetic acid (OAA). OAA fixation of CO2 occurs in the cytosol of the mesophyll cells of the leaf. OAA is the first stable product of this cycle and hence the name C4 pathway. OAA then travels from mesophyll cells to the chloroplasts of bundle sheath cell where it releases the fixed CO2. C3 cycle operates within these cells and this CO2 immediately combines with RuBP and C3 cycle continues producing sugars.
C3 Plants | C4 Plants | |
Carbon dioxide fixation | Occurs once | Occurs twice, first in mesophyll cells, then in bundle sheath cells |
Carbon dioxide acceptor | RuBP, a 5-C-compound | Mesophyll cells. PEP (phosphoenolPyruvic acid), a 5C- compound in the bundle sheath cells-RuBP |
Carbon dioxide fixing enzymes | RuBP carboxylase, which is inefficient | PEP carboxylase which is very efficient. RuBP carboxylase, works efficiently because carbon dioxide concentration is high |
First product of photosynthesis | A C3 acid, PGA | A C4 acid, e.g. oxaloacetic acid |
Leaf anatomy | Only one type of chloroplast | Kranz anatomy, i.e. two types of cell, each with its own type of chloroplast |
Photorespiration | Occurs; oxygen is an inhibitor of photosynthesis | Is inhibited by high carbon dioxide concentration. Therefore atmosphere oxygen is not an inhibitor of photosynthesis |
Efficiency | Less efficient photosynthesis than C4 plants. Yields usually much lower | More efficient photosynthesis than C3 plants. Yields usually much higher. |
Chloroplasts | Monomorphic-one type | Dimorphic-two types |
Examples | Bean, Rice, potato etc. | Maize, sugar cane, sorghum, etc. |