(1) The process by which in the presence of light plant consumes oxygen and releases carbon dioxide (in stead of fixing carbon dioxide) during photosynthesis, resulting in a decrease in photosynthetic output since no ATP is produced and carbon (as well as nitrogen in the form of ammonia) is lost inevitably.
(2) An alternative counter-productive metabolic pathway wherein oxygen molecule is fixed rather than carbon dioxide to produce glyceraldehyde 3-phosphate (G3P).
Plants, especially C3 plants, face the problem of photorespiration. In hot dry days, these plants tend to close their stomata to prevent excessive loss of water (from transpiration). Inevitably, the carbon dioxide cannot enter the leaves (via the stomata) resulting in the levels of carbon dioxide within the leaves to become low. Since there are few carbon dioxide molecules to fix, the oxygen molecules are used as a substitute to produce G3P. Because of photorespiration, in stead of about 2 molecules of G3P, only one G3P is produced and a toxic phosphoglycolate (which the plant must get rid of) is also formed. Some plants such as CAM plants and C4 plants have evolved mechanisms to avoid photorespiration.
Word origin: photo (light) + respiration