One role of the skeletal system is to produce

The skeletal system in the human body provides protection to the soft tissues, helps in the process of mineral homeostasis, and also does a production of blood cells. The skeletal system comprises connective tissues including cartilage, bone, ligaments, and tendons.  

The skeletal system stores fats, minerals, and generates blood cells in the bone marrow, present in the long bones. Another prime function of the skeletal system is to help with mobility.  

So, the given blank can be filled with blood cells.  

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.