How does the law of conservation of matter relate to biology?
All matter, living and non-living, in the universe are made up of atoms.
According to the the law of Conservation of Matter, matter is neither created nor destroyed. All matter is made up of atoms. When matter changes from one form to another, e.g., a portion of beef steak (beef protein) is eaten and digested, the macro-structure of the beef is broken by our digestive enzymes into its component amino acids.
These amino acids are now used by our body to create new proteins that our body needs, e.g., our body muscles. This is the reason why body builders need high-amino acid/protein diets in the course of their training.
Biology calls this cycling of matter as nutrient cycling. Another common example would be:
grass ---> eaten by herbivores---> matter in grass used to create herbivore body ---> herbivore body eaten by carnivore ---> matter in herbivore body used to create carnivore's body ---> carnivore dies and decays in soil ---> detritivore (e.g., insects/worms) consume decaying carnivore or makes soil nutrient rich thus making the grass healthy... And the cycle continous.