As we all know, photosynthesis is a biochemical process of producing carbohydrates using light energy. The whole process is carried in two phases.

1. Photochemical phase – In the photochemical phase, ATP and NADPH are produced
2. Biosynthetic phase – In this phase, the final product glucose is formed.

Based on, how plants proceed in the biosynthetic phase, plants are further classified as C₃ and C₄ plants. Another factor which differentiates a C₄ plant from C₃ is photorespiration.

Photorespiration

Photorespiration is a respiratory process in many higher plants. This is also known as the oxidative photosynthetic,  or C₂ photosynthesis or carbon cycle.

For a better understanding of photorespiration, let’s recall the Calvin cycle, the first step of the biosynthetic phase in C₃ plants- Plants which use only the Calvin cycle for fixing the carbon dioxide.

The reaction in which carbon dioxide and water combine to give carbohydrates is termed as carbon fixation. Calvin cycle is the first step of carbon fixation where CO­₂ combines with Ribulose-1 and 5-bisphosphate (RuBP) to form two molecules of 3 carbon acid called 3-phosphoglycericacid (PGA). The reaction is catalyzed by the most abundant enzyme in the world called RuBisCO (RuBP carboxylase-oxygenase). RuBisCO is the enzyme that has an affinity for both CO₂ and O₂ but has more affinity for CO₂. Hence, the binding of CO₂ and O₂ are competitive in nature and the concentration of the molecules in the atmosphere determines the winner. This is the basis for photorespiration in plants.

Sometimes in C₃ plants, RuBisCO binds to oxygen molecules and the reaction deviates from the regular metabolic pathway. The combination of RuBP and oxygen molecules leads to the formation of one molecule of phosphoglycerate and phosphoglycolate. This pathway is called photorespiration. During photorespiration, no sugar or ATP molecules are synthesized, but just CO₂ is released at the expense of ATP and the whole process is futile.

However, C₄ plants do not undergo photorespiration due to their special mechanism to increase the CO₂ level for enzyme binding. During the Hatch and Slack Pathway, the C₄ acid, oxaloacetic acid (OAA) breaks down to release CO₂. This ensures the high concentration of intercellular CO₂. Thus, in C₄ plants, RuBisCO is more active as a carboxylase enzyme rather than as oxygenase. This is why C₄ plants have better productivity.

For more detailed information about photorespiration, visit BYJU’S.