All organisms respire in order to release energy to fuel their living processes. The respiration can be aerobic, which uses glucose and oxygen, or anaerobic which uses only glucose. Respiration must happen all of the time so that the organism can survive.
Photosynthesis is the process by which plants, and some bacteria, synthesise food molecules - which they then use, in addition to other things, for respiration. The process of photosynthesis requires energy - it is endothermic.
Don't confuse respiration with breathing, which is ventilation. Respiration happens inside cells. Breathing is the movement of air into and out of the respiratory system, which involves organs like the lungs.
All organisms need ATP to live.
Respiration is only around 40 per cent efficient. As animals respire, heat is also released.
In birds and mammals, this heat is distributed around the body by the blood. It keeps these animals warm and helps to keep a constant internal temperature.
ATP is also required: