Photorespiration

The video introduces photorespiration as a diversion from the normal Calvin cycle, primarily due to a quirk in the enzyme rubisco. It begins with a quick overview of the normal Calvin cycle to provide context for understanding this deviation. The normal cycle involves fixing carbon dioxide with a five-carbon molecule called ribulose one, five, bisphosphate (RuBP), facilitated by rubisco acting as a carboxylase.

In the normal Calvin cycle, rubisco (short for ribulose one, five, bisphosphate oxygenase-carboxylase) acts as a carboxylase, fixing carbon dioxide. This process converts six five-carbon RuBP molecules and six carbon dioxide molecules into twelve three-carbon molecules (three phosphoglycerate). These three-carbon molecules eventually lead to the production of glucose and the regeneration of RuBP, serving as the plant's energy storage.

The quirk of rubisco is its ability to also act as an oxygenase, meaning it can fix oxygen instead of carbon dioxide, a process known as photorespiration. This is generally considered an inefficiency, possibly a legacy from ancient evolutionary conditions with low atmospheric oxygen. Photorespiration is detrimental to the plant because it reduces the efficiency of carbon fixation and energy production.

Photorespiration is more likely to occur in C3 plants under hot conditions. In high temperatures, rubisco's affinity for oxygen increases, and plants close their stomata to conserve water. Stomata closure prevents CO2 from diffusing in and O2 from diffusing out, leading to a higher O2 to CO2 ratio within the plant. This combination of factors makes rubisco more likely to bind with oxygen.

When photorespiration occurs, rubisco fixes oxygen with RuBP, resulting in fewer three-carbon molecules (phosphoglycerate) and the production of a two-carbon molecule called phosphoglycolate. This is an inefficient process as it doesn't yield the carbons needed for glucose production. Plants have evolved a salvage pathway to recover some of these carbons, involving complex steps through peroxisomes and mitochondria. However, this pathway is energy-intensive and releases carbon dioxide, mimicking respiration, hence the name photorespiration.

Photorespiration makes the Calvin cycle less efficient. While some theories suggest it might have an unknown benefit, it's widely seen as a biological quirk. Rubisco is a crucial enzyme, making up about 20% of the protein mass in plant leaves, highlighting the importance of understanding this inefficiency in plant metabolism. This quirk diverts the plant from its primary goal of efficiently producing glucose.