Metabolism includes assimilation and alienation. Assimilation (also called anabolism) refers to the process in which organisms convert nutrients obtained from the external environment into their own components and store energy. That is, a series of metabolic pathways by which organisms use energy to synthesize small molecules into large molecules. In short, assimilation is to turn non-self into self. Biology can be divided into autotrophs and heterotrophs by assimilation. Autotrophic organisms: organisms that can synthesize their own organic matter from inorganic substances belong to autotrophs. Such as cyanobacteria, nitrifying bacteria and green plants. Heterotrophs: Organisms that can only ingest ready-made organic matter from outside belong to heterotrophs. Such as most animals and fungi.

Alienation is the catabolism of organisms. It is a process in which organisms convert macromolecules into small molecules and release energy. Breathing is an important process in alienation. Organisms can be divided into aerobic organisms, anaerobic organisms and facultative organisms according to whether they need oxygen for breathing.