In respiration, CO₂ is released as a by-product of breaking down glucose. In photosynthesis, O₂ is released as a by-product when water molecules are split during the process.
The examiner expects one clear contrasting line naming the gas released in each process. Respiration → releases CO₂; Photosynthesis → releases O₂. Avoid writing lengthy descriptions; just the gas and the process are enough for 1 mark.