page_title: Photorespiration
section_title: C4
hierarchical_section_title: Photorespiration / Biological adaptation to minimize photorespiration / Biochemical carbon concentrating mechanisms / C4
caption_attribution_description: English: A corn field in Liechtenstein. Keywords: Field, corn, Liechtenstein, Mountains, Alps, Vaduz, sky, clouds, landscape. Español: Una plantación de maíz en Liechtenstein.
context_page_description: Photorespiration 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, a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP, 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 CO
  2 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 NADH.
context_section_description: 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 CO
  ₂ 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.
caption: Maize uses the C4 pathway, minimizing photorespiration.
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