Plant adaptations against photorespiration

Question

The enzyme RuBisCO is responsible for fixing carbon dioxide in the Calvin Cycle, but it can also bind to oxygen, leading to a process called photorespiration. Under which environmental conditions is photorespiration most likely to occur, and what adaptations have evolved in plants to minimize its effects?

Answer 1

Photorespiration is the counterproductive process plants can undergo when RuBisCO binds with oxygen when it is disproportionately high within the plant. Instead of fixing carbon dioxide to produce glucose, photorespiration produces low-value sugars, releases needed carbon dioxide, and increases oxygen levels even more, perpetuating the cycle. Heat and excess light also favor photorespiration. Three types of plants evolved, unique to their environments, to mitigate the occurrence of photorespiration.

The first type of plant is found in cooler temperate climates. They are known as C3 plants. They open their stomata when more carbon dioxide is needed within the plant. During this time, oxygen is released, and carbon dioxide enters via passive diffusion. This adaptation depends on a cool, moist environment, otherwise, the plant will release water from the stomata and dehydrate.

The second type of plant is known as C4 plants and includes many angiosperms. These plants utilize a slightly different biochemical approach from C3 plants, as well as compartmentalizing the reactions. C4 plant first takes in carbon dioxide through the stomata and fixes it into a 4-carbon sugar (hence the name) within the mesophyll cells close to the surface of the leaf. RuBisCO also happens to be absent from the mesophyll cells, preventing photorespiration when the leaves are exposed to high levels of oxygen. Next, the 4-carbon sugar is transported to the bundle sheath cells located deep within the leaf. The 4-carbon sugar is broken down to release carbon dioxide, allowing it to participate in the Calvin cycle. C4 plants also tend to have fewer photosystem II, which decreases the total amount of O₂ produced by the Calvin Cycle. C4 Plants thrive in high heat and high light intensity environments.

The third type of plant is known as CAM (crassulacean acid metabolism). These plants utilize the C4 pathway. Since these plants grow in high heat and low humidity environments, CAM plants separate the initial carbon fixing step and the Calvin cycle into time segments. CAM plants open their stomata at night only to take in carbon dioxide to prevent excess water loss. CAM plants fix carbon into 4-carbon sugars and sequester them until daylight. During the day, CAM plants only perform the Calvin cycle. These actions prevent O₂ from interacting with RuBisCO.