Editing Cell (biology) (section)

== Structures outside the cell membrane ==

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

=== Cell wall ===

{{further|Cell wall}}

Many types of prokaryotic and eukaryotic cells have a [[cell wall]]. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of [[cellulose]], fungi cell walls are made up of [[chitin]] and bacteria cell walls are made up of [[peptidoglycan]].

=== Prokaryotic ===

==== Capsule ====

A gelatinous [[bacterial capsule|capsule]] is present in some bacteria outside the cell membrane and cell wall. The capsule may be [[polysaccharide]] as in [[pneumococci]], [[meningococci]] or [[polypeptide]] as ''[[Bacillus anthracis]]'' or [[hyaluronic acid]] as in [[streptococci]].
Capsules are not marked by normal staining protocols and can be detected by [[India ink#Uses other than writing|India ink]] or [[methyl blue]], which allows for higher contrast between the cells for observation.<ref>{{cite book |url=https://books.google.com/books?id=N2GU-DYKkk0C&q=Prokaryotic+india+ink&pg=PA87 |title=Prokaryotes |publisher=Newnes |date=1996 |isbn=978-0080984735 |access-date=November 9, 2020 |archive-date=April 14, 2021 |archive-url=https://web.archive.org/web/20210414134256/https://books.google.com/books?id=N2GU-DYKkk0C&q=Prokaryotic+india+ink&pg=PA87 |url-status=live }}</ref>{{rp|87}}

==== Flagella ====

[[Flagella]] are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.

==== Fimbriae ====

A [[fimbria (bacteriology)|fimbria]] (plural fimbriae also known as a [[pilus]], plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called [[pilin]] ([[antigenic]]) and are responsible for the attachment of bacteria to specific receptors on human cells ([[cell adhesion]]). There are special types of pili involved in [[bacterial conjugation]].