How do living systems follow the law of conservation of mass and energy?

Because all living systems state that matter is neither created or destroyed hope this helps! :D

Living systems use the law of conservation of mass when a living organism dies. When the organism decomposes carbon is released and enters back into the earth. It follows the law because the mass is not being created or destroyed, but reused.
Living systems use the law of conversation of energy when an organism decomposes. Thermal energy is recycled back into the atmosphere or absorbed by other living things.
Hope this helps.