1. In photorespiration, oxygen is consumed in the peroxisomes, and by Rubisco in the chloroplasts; in true repiration, it is consumed in the mitochondria. True respiration occurs in both the light and the dark, and is largely independent of oxygen availability. Photorespiration occurs only in the light (and immediately after turning off the light), and it magnitude increases almost linearly with increasing oxygen availability. True respiration is a useful process, generating NADH and ATP. Photorespiration is a wasteful mistake of no evolutionary benefit.
2. Photorespiration degrades RuBP without fixing CO₂. Recovery of the phosphoglycolate wastes ATP (converting glycerate to PGA) and generates peroxide, which requires catalase to destroy it. The conversion of PGA back to RuBP wastes further NADPH and ATP. However, under some circumstances, this may be beneficial, as it provides a safety valve to prevent the formation of reactive oxygen species through 'backing-up' of the photophosphorylation electron transport chain.
3. Barley is a temperate grass, so is not exposed to very high temperatures or very high light levels. It does not loose enough water, nor generate enough ATP for the advantages of C4 metabolism to outweigh C4's energetic disadvantages. Maize is a tropical grass, so it benefits from the C4 metabolism, as it has freer access to ATP and more pressing water-loss problems.
4. CAM uses ATP to regenerate PEP and does not fix carbon during the day. C3 plants would fix carbon for longer, and although they would photorespire more than the CAM plants, these losses would be less than those suffered by the CAM plants.