Final answer:
Photorespiration produces less carbohydrate and a toxic molecule compared to the Calvin Cycle, representing a less efficient plant process. It is considered an evolutionary relic due to its association with fluctuating early Earth atmospheric conditions and the enzyme Rubisco's dual functions which are less beneficial in current high oxygen conditions.
Step-by-step explanation:
The major consequences of photorespiration include the production of a 3-carbon carbohydrate and a 2-carbon molecule called phosphoglycolate, which is toxic to cells. In contrast to the Calvin Cycle, which produces two 3-carbon carbohydrates, photorespiration results in a less efficient use of the plant's energy resources. This innefficiency is believed to be an evolutionary relic because its occurrence is linked to the rise of atmospheric oxygen after photosynthesis evolved, which fundamentally changed the Earth's atmosphere and the biochemical pathways of life forms. Photorespiration emerged as an intermediate mechanism before plants evolved more oxygen-tolerant forms of photosynthesis, such as the C4 and CAM pathways.
Photorespiration is thought to be an evolutionary relic because it likely arose during a time when atmospheric oxygen levels were much lower than today. Plants had to deal with fluctuating levels of carbon dioxide and oxygen, which led to the evolution of an enzyme, Rubisco, with both carboxylase (for photosynthesis) and oxygenase (leading to photorespiration) activities. Over time, as oxygen levels increased and conditions on Earth changed, this metabolic pathway became less advantageous.