Photo respiration occurs when RuBisCO (Ribulose-1, 5-bisphosphate carboxylase) combines RuBP (Ribulose 1, 5-bisphosphate) with O2. RuBP is a colourless anion that is involved in the process of photosynthesis, RuBisCO is a large protein in plants and the efficiency of photosynthesis is lowered by Photorespiration.