When Rubisco binds oxygen instead of carbon dioxide, the resulting process is called photorespiration – an innifficent, energy-consuming process. Considering that our air is 21% Oxygen but only 0.035% Carbon dioxide,Rubisco would be considerably more likely to bind oxygen than carbon dioxide, except that it has a much higher affinity for carbon dioxide than it does for oxygen. Rubisco’s high affinity for carbon dioxide overcomes the amount and proximity of oxygen and increases the likelihood that carbon dioxide will be bound in preference to oxygen.
Only C3 plants photorespire, and the likelihood for photorespiration to occur increases with increasing temperatures and increasing light levels. Although C4 plants do have the pathways and enzymes to accommodate photorespiration, it does not occur in a measurable amount, and we assume that it is nonexistent in C4 plants.
Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3 plants. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.
The oxygenation reaction of RuBisCO is a wasteful process because 3-phosphoglycerate is created at a reduced rate and higher metabolic cost compared with RuBP carboxylase activity. While photorespiratory carbon cycling results in the formation of G3P eventually, around 25% of carbon fixed by photorespiration is re-released as CO2.
C4 plants capture carbon dioxide in their mesophyll cells (using an enzyme called phosphoenolpyruvate carboxylase which catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate (PEP)), forming oxaloacetate. This oxaloacetate is then converted to malate and is transported into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where oxygen concentration is low to avoid photorespiration. Here, carbon dioxide is removed from the malate and combined with RuBP by RuBisCO in the usual way, and the Calvin Cycle proceeds as normal. The CO2 concentrations in the Bundle Sheath are approximately 10–20 fold higher than the concentration in the mesophyll cells.
This ability to avoid photorespiration makes these plants more hardy than other plants in dry and hot environments, wherein stomata are closed and internal carbon dioxide levels are low. Under these conditions, photorespiration does occur in C4 plants, but at a much reduced level compared with C3 plants in the same conditions. C4 plants include sugar cane, corn (maize), and sorghum and nitrogen, as ammonia. Ammonia must then be detoxified at a substantial cost to the cell. Photorespiration also incurs a direct cost of one ATP and one NAD(P)H.