Photorespiration

Photosynthetic Carbon Oxidation or PCO Cycle or C₂ cycle

It was discovered by Dicker and Tio (1959) in Tobacco leaves. Subsequently, photorespiration was found to be universal in C3 plants. It involves three cell organelles - chloroplasts, peroxisomes and mitochondria. The various evidences in support of occurrence of photorespiration are

• Decrease in the rate of photosynthesis with the increase in light intensity.
• Decrease in the rate of photosynthesis when O2 concentration is raised from 2.5% to 21%.

Fig. Mechanism of photorespiration

Mechanism

RUBP carboxylase functions as RUBP oxygenase with the decrease in CO₂ : O₂ ratio

In photorespiration two molecules of phosphoglycolate formed by oxygenation of RUBP is changed to one molecule of RUBP is changed to one molecule of phosphoglycerate (PGA) and one molecule of CO₂.  Thus, 75% of carbon lost in oxygenation of RUBP is recovered by photorespiratory carbon oxygenation.

Photo respiration does not give any energy or reducing power.  It consumes energy.  So it is a wasteful process, especially in C₃ plants but negligible or absent in C₄ plants.

Photo respiration also increases with temperature and age of the leaf.

Glycine and serine are two amino acids formed in photorespiration or C₂ cycle.

Hatch and Slack Cycle or C₄ cycle

Kortschak, Hartt and Burr (1965) reported that rapidly photosynthesizing sugarcane leaves produced a 4-C compound like aspartic acid and malic acid as a result of CO₂-fixation.  This was later supported by M.D. Hatch and C.R. Slack (1966) and they reported that a 4-C compound oxaloacetic acid (OAA) is the first product in CO₂ reduction process. 

This led to an alternative pathway of CO₂ fixation, which is known as Hatch and Slack’s cycle or C₄ cycle (as 4-C compound is first stable product).

This pathway was first reported in members of family Gramineae (grasses) like sugarcane, maize, sorghum, etc., (Tropical grasses) but later on in other sub-tropical plants also like Atriplex and Amaranthus.

Genera like (Asteraceae) Flaveria panicum, Alternanthera and Atriplex contain both C₃ and C₄ species. These C₄ plants have a characteristic leaf anatomy called Kranz anatomy (German word meaning : Wreath, Ring or Halo)

Fig. Leaf showing kranz anatomy  Fig. Hatch and Slack’s cycle (C₄ cycle)

Here vascular bundles are surrounded by sheath of large parenchymatous cells called bundle sheaths which are surrounded by mesophyll cells. Here two types of chloroplasts are present (Chloroplast Dimorphism)

(i) Bundle sheath chloroplasts : Larger in size, lack grana (Agranal choroplasts) and contain starch grains.

(ii) Mesophyll chloroplasts : Similar in size, contain grana (Granal chloroplasts) and lack starch grains. Bundle sheath cells and mesophyll cells are connected by plasmodesmata.

CO₂ acceptor molecule here is PEP  (Phospho Enol Pyruvate) and not RuBP.  Further, PEP-carboxylase (PEPCO) is the key enzyme (RuBP-carboxylase enzyme is negligible or absent in mesophyll chloroplast, but present in bundle sheath chloroplast)

In C₄ plants, for formation of one mole of hexose (glucose), 30 ATP and 12 NADPH₂ are required. 

Significance of C4 Cycle

This C₄ cycle is helpful to plants growing in dense tropical forests, where there is poor supply of CO₂.  Because here there is internal supply of CO₂, so these plants can survive in poor CO2 conditions.  Photorespiration is negligible or absent in C₄ plants and present only in C₃ plants.