生物学的五界分类体系

　　我记得我在读初中的时候学植物学和动物学时，生物的分类基本上六界体系：动物界、植物界、真菌界、病毒界、原核生物界、原生生物界。但是较新的生物分界是五界系统:原核生物界，原生生物界，真菌界，植物界，动物界。

　　Cecie Starr and Ralph Taggart said in their textbook,biology,which is published by Wadsworth Publishing Company,whose ISBN is 0-534-21060-0, that life on earth is diversified, and life's diversity is further classified into five kingdoms.

    The classification levels in biology are, from broad to narrow: kingdom, phylum, class, order, family, genus, species. Every division level can have some subdivision such as subphylum, subclass, suborder etc.

    Kingdom is the highest unit of classification, all the lives can be assigned to one of the following five kingdoms:

    1.Kingdom Monera: Monerans are bacteria, single cells, all prokaryotic( their DNA is not enclosed in a membrane-bound compartment called a nucleus). Producers, consumers, decomposers. Kingdom of greatest metabolic diversity.

    2.Kingdom Protista: Protistans are mostly single cells, larger than bacteria. Eukaryotic(DNA enclosed in a nucleus).Producers, consumers. Diverse life-styles.

    3.Kingdom Fungi: Mostly multicelled. Eukaryotic. Decomposers, consumers. Their secretions digest food outside the fungal body;their cells absorb the breakdown products.

    4.Kingdom Plantae: Plants are mostly multicelled. Eukaryotic. Nearly all producers that rely on photosynthesis.

    5.Kingdom Animalia: Animals are melticelled. Eukaryotic. Consumers, typically motile, with deiverse life-style.