Final answer:
Photorespiration is considered a "mistake" because it results in a less efficient form of photosynthesis that produces a toxic byproduct and uses additional energy for its metabolism. Despite this inefficiency, it helped early plants survive fluctuating oxygen levels after the oxygen catastrophe.
Step-by-step explanation:
Why Photorespiration Is Considered a "Mistake"
Photorespiration is often referred to as a cellular "mistake" because it reduces the efficiency of photosynthesis.
During photorespiration, the enzyme Rubisco catalyzes the reaction of RuBP with O2 instead of CO2, leading to the production of a 3-carbon molecule (3-phosphoglyceric acid) and a 2-carbon molecule (phosphoglycolate), the latter being cytotoxic and less energetically useful.
This reaction occurs when CO2 concentration is low inside leaves, which is often under drought conditions when plants close their stomata to conserve water, restricting CO2 intake and causing O2 to accumulate.
The 2-carbon molecule produced is not only wasteful because it contains less stored energy compared to the two 3-carbon carbohydrate molecules produced in the normal Calvin Cycle but also because it is toxic and needs to be metabolized at an additional energy cost to the plant.
Despite this, photorespiration might be seen as an evolutionary compromise, allowing plants to survive in environments with fluctuating levels of CO2 and O2.
It could be considered an evolutionary relic from a time when atmospheric oxygen levels increased, and many organisms that could not effectively deal with the high levels of oxygen were selected against, favoring organisms that could tolerate and eventually use oxygen through cellular respiration and detoxifying mechanisms.
Consequently, photorespiration may persist as a byproduct of this adaptation—putatively, a necessary 'mistake' from our plants' ancestors living during the oxygen catastrophe, enabling them to survive until the present day.