This chapter focuses on key biochemistry and processes from a mass perspective. The mass percentages of key elements such as carbon, oxygen, hydrogen, and nitrogen are examined in core nutrition components, such a carbohydrates, proteins, fats, water, carbon dioxide, and others, and how these become wastes and inputs for other processes. Key reactions are examined at a high engineering level, to estimate how organisms move masses of elements from input to output, and especially how aerobic metabolism feeds photosynthesis and the reverse.