Photorespiration is a process which involves oxidation of organic compounds in plants by oxygen in the presence of light. Like ordinary respiration, this process also releases carbon from organic compound in the form of carbon dioxide but does not produce ATP. Thus, it seems to be a wasteful process. Photorespiration occurs only in C3 plant during daytime usually when there is high concentration of oxygen. RuBP carboxylase (or RuBisCO), the enzyme that joins carbon dioxide to RuBP now, functions as oxygynase. As a result, oxygen instead of carbon dioxide, gets attached to the binding site of the enzyme and RuBP is oxidised. RuBP releases one molecule of 3 carbon compound phosphoglyceric acid, which enters C3 cycle and one molecule of a 2 carbon compound phosphoglycolate.
Photorespiration was first demonstrated by Dicker and Tio (1959) in tobacco and the term, photorespiration, was given by Krotkov in the year 1963. The process of photorespiration takes place in chloroplast, peroxisome and mitochondria.
The steps involved in photorespiration in C3 plant are as follows:
- When carbon dioxide concentration in the atmosphere becomes less and oxygen concentration inside the plant increases ribulose 1-5 diphosphate combines with oxygen to form one molecule each of 3 phosphoglyceric acid and 2 phosphoglycolic acid (2 carbon compound) in the presence of enzyme RuBP oxygenase.
- 2 phosphoglycolic acid loses its phosphate group in the presence of enzyme phosphatase and convert it into glycolic acid.
- The glycolic acid synthesized in chloroplast is then transported to peroxisome, inside the peroxisome, it reacts with oxysome to form glyoxylic acid and H2O2 in the presence of enzyme, glycolic acid oxidase. H2O2 is converted into water and oxygen in the presence of enzyme, catalase.
- Glyoxylic acid is then converted into an amino acid, glycine by transamination reaction with glutamic acid.
- Glycine enters into mitochondria where 2 molecules of glycine interacts to form 1 molecule each of serine, carbon dioxide, and ammonia. NH3 is transported to cytoplasm where it is synthesized into glutamic acid.
- Serine returns to peroxisome where it is deaminated and reduced to hydroxy pyruvic acid and finally to glyceric acid.
- Glyceric acid finally enters into chloroplast where it is phosphorylated to 3 phosphoglyceric acid, which enters into C3 cycle.
C4 PATHWAY (HATCH AND SLACK PATHWAY)
In 1967, M.D. Hatch and C. R. Slack demonstrated an alternate pathway of carbon dioxide fixation, in higher plants found in tropical region. They found that in certain plants, the first product of photosynthesis is a 4 carbon acid, oxaloacetic acid (OAA), instead of 3 carbon compound. This type of carbon dioxide fixation was first demonstrated in some plants of family Poaceae like sorghum, maize, sugarcane (monocots) and in some dicots, Atriplex, Amaranthus, Euphorbia.
The C4 appears to be better equipped to withstand drought and are able to maintain active photosynthesis under condition of water stress. Water stress would lead to stomatal closure in C3 plants and consequent reduction in carbon dioxide uptake, whereas in C4 plants, carbon dioxide concentration is higher resulting in the suppression of photorespiratory carbon dioxide loss.
In C4 plants, initial fixation of carbondioxide occurs in mesophyll cells . The primary acceptor of carbondioxide is phosphoenolpyruvate. It combines with carbondioxide in the presence of phosphoenol pyruvate carboxylase to form oxaloacetic acid. Oxaloacetic acid is reduced to malic acid. Inside the bundle sheath cells malic acid is decarboxylated to form pyruvate and carbondioxide. Carbondioxide is again fixed inside the bundle sheath cells through calvin cycle. RuBP is called secondary or final acceptor of carbondioxide of C4 plants. Therefore, C4 plants have 2 carboxylation reaction.
Another basic feature of C4 plants is the occurrence of Kranz anatomy in the leaves. The chloroplasts present in bundle sheath cells are of abnormal type. They are large in size, centripetally arranged and lack well-organized grana. They contain starch grain. The chloroplast of mesophyll cells are normal. Hence, in C4 plants, chloroplasts are dimorphic in nature.
C4 leaves are also characterized by the presence of tightly packed, thick-walled bundle sheath cells all around the vascular bundle. Because of the wreath-like configuration of these bundle sheath cells, this arrangement is known as Kranz anatomy .Bundle sheath cells are well protected from oxygen being released from mesophyll cells.
The steps involved in Hatch and Slack pathway are as follows:
- Phosphoenolpyruvic acid accepts carbon dioxide and forms oxaloacetic acid inside mesophyll cells.in the presence of enzyme, phosphoenolpyruvate carboxylase.
- Oxaloacetic acid is reduced by NADPH2 to form malic acid in the presence of enzyme, malate dehydrogenase.
- Oxaloacetic acid also produces aspartic acid by a transamination reaction in the presence of enzyme, transaminase.
- Malic acid is transported to bundle sheath cells, where it is decarboxylated by NADP and specific malic enzyme to produce pyruvic acid and carbon dioxide. Carbondioxide is again fixed inside the bundle sheath cells through calvin cycle.
- Pyruvic acid is transported back to mesophyll cells, where it is converted to phosphoenolpyruvate by using ATP. This results in the formation of AMP (adenosine monophosphate) instead of ADP (adenosine diphosphate).Hence, regeneration of ATP from AMP requires 2 ATP each or 12 ATP for formation of 6 molecules of phosphoenol pyruvate . Therefore ,C4 pathway requires 12 additional ATP or total 30 ATP(18 ATP in C3 cycle+12 additional ATP).