Explain why animal and plant cells cannot exist in the absence of carbon, hydrogen, oxygen.

The answer here may be one reason – it is the source of energy. A person cannot exist not only without air, but also without water, without food. A car will not run without fuel, and a flower will not grow without sunlight. Can something exist without air? An example of this is the studied part of the cosmos. Nothing but dust and stones. Different sources of livelihood are needed for animal and plant cells. Oxygen, hydrogen, carbon are their main constituents. For each his own, irreplaceable.