A cytostome (from cyto-, cell and stome-, mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuoles.
Only certain groups of protozoa, such as the Ciliophora and Excavata, have cytostomes. An example is Balantidium coli, a ciliate. In other protozoa, and in cells from multicellular organisms, phagocytosis takes place at any point on the cell or feeding takes place by absorption.
The cytostome forms an invagination on the cell surface and is typically directed towards the nucleus of the cell. The cytostome is often labeled as the entire invagination, but in fact the cytostome only constitutes the opening of the invagination at the surface of the cell. The rest of the invagination is classified as the cytopharynx. The cytopharynx works in conjunction with the cytostome in order to import macromolecules into the cell. This strong association between the cytostome and cytopharynx is often called the cytostome-cytopharynx complex or the cytopharyngeal apparatus. However, in a small number of cases the cytostome works independently in order to import macromolecules. In these instances, the cytostome imports macromolecules by directly forming vesicles that are imported into the interior of the cell.
The cytostome is associated with microtubules that function in maintaining its form. One set of microtubules is arranged in a triplet formation and is located directly beneath the cytostome membrane. A second set of microtubules is positioned directly beneath the flagellar pocket membrane and forms a quartet.
Equally as important in the function of endocytosis is the structure known as the cytopharynx. The cytopharynx is a long, tube-like structure that forms the invagination associated with the cytostome. While the shape of the cytopharynx is not constant, it is typically directed towards the posterior of the cell, often hooking around a central nucleus.
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The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of their life cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.
Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically, protozoans were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.
An amoeba (əˈmiːbə; less commonly spelled ameba or amœba; : am(o)ebas or am(o)ebae əˈmiːbi), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopods. Amoebae do not form a single taxonomic group; instead, they are found in every major lineage of eukaryotic organisms. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals.
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