A lipid molecule primarily is used as a storage mechanism for the cell as ATP doesn't last very long before degenerating.
The ultimate source of energy is sunlight, and living things must break the sunlight down using their own methods to use it.
Not entire sure because I haven't taken human anatomy but I'd guess it would have to be the digestive enzymes breaking it down
When the rates of two processes photosynthesis and respiration becomes equal. Phototrophic organisms wouldn't be able to compensate for the lack of carbon dioxide levels. Phototrophs wouldn't be able to produce their food in the form of glucose and hence wouldn't be able to respire too. That would result in lack of overall available food from phototrophs and lack of available oxygen for respiration.
Photosynthesis also uses carbon dioxide faster than respiration produces it. Oxygen surplus is is released into the air and unused glucose is stored in the plant for later. That's why plants are so important to human and animal survival. Without photosynthesis we wouldn't have oxygen for food to stay alive and we would end up like the movie The Lorax
