[Heterotrophication] Humans and animals cannot carry out photosynthesis like green plants, nor can they carry out chemical synthesis like nitrifying bacteria. They can only rely on the ready-made organic matter in the external environment to maintain their own life activities. This metabolic type belongs to heterotrophy. In addition, the metabolic types of fungi and most saprophytic or parasitic bacteria are also heterotrophic. In a word, in the process of assimilation, organisms convert the ready-made organic matter absorbed from the external environment into their own components and store energy. This metabolic type is called heterotrophy.