Photorespiration is a light-dependent metabolic process that competes with photosynthesis, the main pathway where plants convert carbon dioxide and water into sugars. Photorespiration reduces the plant’s efficiency in producing these sugars, essentially undercutting its own food-making process. This alternative pathway results in a net loss of carbon that the plant has already invested energy into fixing, making it counterproductive to growth.
The RuBisCO Dilemma
The root cause of photorespiration lies with the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase, shortened to RuBisCO. RuBisCO is the most abundant protein on Earth. It catalyzes the first step of the Calvin Cycle, fixing atmospheric carbon dioxide (\(\text{CO}_2\)) onto the five-carbon sugar, ribulose-1,5-bisphosphate (\(\text{RuBP}\)). This carboxylation reaction is the gateway to producing sugars.
The dilemma arises because RuBisCO is an imperfect enzyme with a dual nature; it can bind to and react with both \(\text{CO}_2\) and oxygen (\(\text{O}_2\)). When the enzyme mistakenly binds \(\text{O}_2\) instead of \(\text{CO}_2\), it initiates photorespiration. This oxygenation reaction breaks down the \(\text{RuBP}\) molecule, generating one useful three-carbon compound and one molecule of phosphoglycolate. Phosphoglycolate is a metabolic dead-end and is considered toxic if allowed to accumulate, forcing the plant to spend energy to process it.
The Wasteful Cycle: Energy Drain and Carbon Release
Photorespiration is detrimental due to the significant metabolic cost incurred to clean up the byproduct of RuBisCO’s mistake. The plant must expend previously fixed energy, specifically adenosine triphosphate (\(\text{ATP}\)) and nicotinamide adenine dinucleotide phosphate (\(\text{NADPH}\)), to metabolize the toxic phosphoglycolate. This complex recycling process involves chemical reactions across three different organelles—the chloroplast, peroxisome, and mitochondrion.
The recycling of phosphoglycolate is highly inefficient and creates a substantial energy drain. For every oxygenation event, the pathway consumes approximately 3.5 molecules of \(\text{ATP}\) and 2 molecules of \(\text{NADPH}\) equivalents just to recover 75% of the carbon. Compounding this energy loss, the cleanup process releases \(\text{CO}_2\) back into the atmosphere in the mitochondrion. This represents a net loss of captured carbon, directly reducing photosynthetic efficiency. For plants relying solely on the standard carbon fixation pathway, this process can reduce photosynthetic output by 25% or more.
Conditions That Magnify Photorespiration
Photorespiration is heavily influenced by environmental conditions that change the ratio of \(\text{O}_2\) to \(\text{CO}_2\) inside the leaf. High temperatures are a major factor because RuBisCO’s affinity for \(\text{CO}_2\) decreases more rapidly than its affinity for \(\text{O}_2\) as temperature rises. This shift means the enzyme is more likely to bind oxygen, even if the \(\text{CO}_2\) concentration remains constant.
When plants experience hot or dry conditions, they close their stomata (small pores on the leaf surface) to conserve water. Closing the stomata prevents water vapor from escaping, but it also restricts the entry of fresh atmospheric \(\text{CO}_2\). As photosynthesis continues, the internal \(\text{CO}_2\) concentration drops, while the \(\text{O}_2\) concentration (produced by the light reactions) builds up. This combination of low \(\text{CO}_2\) and high \(\text{O}_2\) strongly favors the oxygenase activity of RuBisCO, magnifying photorespiration.
How Plants Bypass the Problem
Some plant groups have evolved specialized mechanisms to suppress photorespiration and improve efficiency in hot, dry environments. The \(\text{C}_4\) pathway, used by plants like corn and sugarcane, achieves this by physically separating the initial carbon fixation step from the Calvin Cycle. \(\text{C}_4\) plants use the enzyme PEP carboxylase to fix \(\text{CO}_2\) into a four-carbon molecule in the outer mesophyll cells.
This four-carbon compound is then transported to specialized inner bundle sheath cells, where it is broken down to release a high concentration of \(\text{CO}_2\) directly around RuBisCO. This spatial separation creates a \(\text{CO}_2\)-rich microenvironment that saturates RuBisCO, forcing it to primarily perform the desirable carboxylation reaction and nearly eliminating photorespiration. Crassulacean Acid Metabolism (\(\text{CAM}\)), used by cacti and pineapples, separates these processes temporally.
\(\text{CAM}\) plants open their stomata only at night, fixing \(\text{CO}_2\) into a four-carbon acid stored in vacuoles. During the day, the stomata close to conserve water, and the stored acid is broken down to release a high concentration of \(\text{CO}_2\) to feed the Calvin Cycle. Both \(\text{C}_4\) and \(\text{CAM}\) systems function as carbon-concentrating mechanisms, ensuring RuBisCO encounters high levels of \(\text{CO}_2\) despite environmental stress, thus bypassing photorespiration.