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Tetrahymena phagocytosis: Food vacuoles filled with India ink.

Tetrahymena

NOTE:  You cannot use information from this box of text as a cited source in a paper.  It is here to give you an overview and context.  All its facts come from articles in the left-hand sidebar of this page.

Tetrahymena are one-celled freshwater organisms.  Two species commonly used in research biology are Tetrahymena pyriformis, and Tetrahymena thermophila.

All Tetrahymena species share these characteristics:

• Single-celled; the cells are roughly pear-shaped.
• The cell's surface is covered with cilia (microscopic hairs), which beat in synchronous waves to allow the cell to move and change direction rapidly, and to sweep food particles into the cell's gullet (see below).
• The cell has a tube-shaped "gullet" that leads from a pore at one end of the cell, into the cell's interior.
• Food particles are swept by the cell's cilia into the bottom of the gullet, which then pinches off to form a membrane-enclosed "bubble" of food (a food vacuole).
• The food vacuoles fuse with small sacs of digestive enzymes (the sacs are called lysosomes), and the food particles are broken down.
• Tetrahymena cells have two nuclei.  One is the "active" nucleus (called the "macro-nucleus" or "Mac" because it is the larger of the two).  The macronucleus is the "working memory" of the cell; the genes in this nucleus actively control the cell's behavior, growth, and metabolism.  Eventually, the genes in this nucleus will degrade from constant use.  Like computer software, the genes accumulate slight errors (mutations).
• The second nucleus (micro-nucleus or Mic) remains inactive--and well-protected from degradation--until cell division or sexual reproduction.
• When a Tetrahymena cells divide, the macro-nucleus is discarded.  The micro-nuclei replicate and divide, and each new cell gets two copies.  One copy will be used as the new macro-nucleus; the other will remain as a protected micronucleus.  In the image, left, the nuclei are stained with a fluorescent blue dye--note the macro- and micro-nuclei visible in most of the cells.
• When Tetrahymena cells mate, they fuse their membranes at one end of each cell.  The micronuclei of each cell divide, and genes from the micronuclei are exchanged between the two cells.  Then the two cells separate, each now carrying some new genes from the mating partner.  In the image below: two Tetrahymena, fused and mating.
• Tetrahymena pyriformis has seven sexes (called "mating types").  Each sex will only mate with a different sex, never with its own.
• Each Tetrahymena cell has the ability to be any of the seven mating types.  When ready to mate, each of the cells "rolls the genetic dice" to determine mating type it will be for this mating event.
• Thanks in part to the two nuclei, with one protected from mutations, Tetrahymena cells do not age.  They can be eaten, poisoned, or meet with other mishap--but they do not die of old-age.

NOTE: You can use the information in this box as a cited source.

From: Leick, V. (1988). Gliding Tetrahymena thermophila: oriented chemokinesis in a ciliate. European journal of protistology, 23(4), 354-360.

Starving Tetrahymena thermophila transform into a gliding elongated form when migrating from a liquid medium into gels containing 5-10% gelatine. The gliding cells are elongated up to three times when compared with the normal ellipsoid form, probably due to a mechanical squeezing of the actively migrating cells. Migration occurs in linear tracks perpendicular to the surface of the gel. The rate of migration is several hundred times slower (25–100 μm per min. at 23 °C) compared to swimming rates in liquid medium. Elongated cells can also transform into an amoebic state moving by creeping using the wedge-shaped extended anterior end to guide the cell flow.

The gliding movement of elongated cells can be directed by chemo-attractants like proteose peptone and platelet-derived growth factor. Gliding cells will return to the liquid phase when a chemoattractant is added to this outer medium.

Cells embedded in solid gelatine where they are unable to glide respond to controlled chemical gradients by orienting themselves with their long axis along such gradients indicating that the gliding movement can be oriented.

The described behaviour of Tetrahymena burrowing in soft semi-solid substrates may reflect aspects of the ecology of this ciliate and may be part of an important feeding mechanism in nature.

• Plasma membrane: The outer membrane that surrounds all cells.  The plasma membrane is composed mainly of lipid molecules (fats and oils), with lots of proteins embedded among the lipids.  "Plasma" means "flexible" (the word "plastic" comes from the same root); the plasma membrane is highly flexible and fluid, yet is able to form a stable boundary for the cell.
  
• Vacuole:  A membrane-surrounded "bubble" inside the cell, which may store various substances:
• Food vacuole: A "bubble" of food particles that has been ingested by Tetrahymena
• Water vacuole: A bubble containing water; Tetrahymena's "water bottle."
• Contractile vacuole: A bubble, connected to the surface membrane, that can contract and squeeze out excess water.
• Vesicle: An organelle that carries a substance with a specific purpose, such as...
•  Lysosome: A vesicle filled with digestive enzymes.  Lysosomes fuse with food vacuoles, so that the food particles can be broken down and absorbed into the cell.
• Phagocytosis: Literally, "a cell eating."  The process of ingesting food particles.  Cilia (tiny hairs) sweep food particles into the cytostome ("cell mouth"), and down into the oral groove or gullet (a tube leading from the cytostome into the middle of the cell).  As the end of the gullet fills with food particles, the tip pinches off to form a food vacuole.  The gullet membrane, therefore, constantly has to grow to replace pinched-off vacuoles.
• Chemotaxis: Deliberate movement toward a substance ("positive chemotaxix") or away from a substance ("negative chemotaxis").