Living organisms are typically classified according to two groups, namely prokaryotes and eukaryotes. A separating feature of these two is that eukaryotes have what is called a nucleus, in which they keep their DNA. Prokaryotes do not have this same distinct, intracellular compartment. Characteristic examples of eukaryotes are plants, animals, and fungi. A well-known example of a prokaryote is bacteria. Archaea are also prokaryotes, and constitute a separate class from bacteria (Alberts et al. 2014).
Some defining features of prokaryotic cells are that they typically live as independent individuals and can also form loosely organized communities. They have a few different shapes, including spherical and rod-shaped, and in linear dimension usually measure a few micrometers. Prokaryotes are also defined by their cell wall, which is a tough, protective coat, under which there is a plasma membrane, which itself encloses a cytoplasmic compartment containing DNA, RNA, and proteins, along with other small molecules required for life (Alberts et al. 2014).
Prokaryotes have a wide range of biochemical capabilities, which exceeds those of eukaryotes by far. Prokaryotes can be of organotrophic, phototrophic, and lithotrophic types, as discussed in detail a few posts back (Alberts et al. 2014). Below is a diagram that gives some information about prokaryotes that it would be useful for you to understand.
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