原核生物

Prokaryotes include archaea and bacteria, which are simple, unicellular organisms that lack membrane-bound organelles.

These cells are surrounded by a selectively permeable plasma membrane that encloses the cellular contents within a gel-like cytoplasm.

An additional protective layer called the cell wall surrounds the plasma membrane. In bacteria that possess a cell wall, it is made of peptidoglycan, a polymer of amino acids and sugars, and helps maintain cell shape and internal osmotic balance. In archaea, the cell wall does not contain peptidoglycan and is composed of other materials, such as proteins or polysaccharides.

Some bacteria also have a capsule, which is a polysaccharide layer that promotes adhesion to surfaces and provides protection.

Inside the cell, typically a single circular DNA molecule is located in a region called the nucleoid. This DNA is not enclosed within a membrane.

Many prokaryotes also contain smaller circular DNA molecules called plasmids, which replicate independently and can be transferred between cells. These plasmids can also carry genes that provide adaptive advantages, such as antibiotic resistance.

Although prokaryotes lack membrane-bound organelles, some species contain specialized internal structures. For example, cyanobacteria have carboxysomes and thylakoids that enable photosynthesis.

Prokaryotic cells have limited internal compartmentalization and can survive in diverse and sometimes extreme environments.