Is There Cytoskeleton In the Arms Of Actinosphaerium

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What is a Cytoskelton and How is Cytoskeleton Made?

Every life form is made up of cells, or at least one cell. Most cells, such as those of plants, animals, humans and even fungi and algae are eurokaryotic. Eurokaryotic cells are complex. They carry out many cell functions and make use of many organelles in order to carry out those cell functions. Bacterial cells are prokaryotic. Prokaryotic cells are much simpler and smaller and therefore require less structure because they have fewer cell functions. A cytoskeleton, made out of protein, is found in the cytoplasm of cells and provides support, shape and protection for the cell, contributes to the cell’s motility and also takes part in intracellular transport and the division of the cell. Both plant and animal cells contain cytoskeleton, which consists of microfilaments, microtubules and intermediate filaments. The microfilaments are strongly concentrated beneath the cell membrane, and part of their function is to maintain the shape of the cell. They also aid in the creation of the cleavage furrow during cytokinesis, the splitting of a parent cell into two daughter cells. Microtubules are the “support beams” of the cytoskeleton and hold an important role in the mitotic spindle during the process of mitosis, in the formation of the cell wall in plants and in intracellular transportation. The intermediate filaments are larger in diameter than the microfilaments but smaller in diameter than the microtubules. They are thought of as the cables of the cytoskeleton, managing cell tension. Aside from their part in organizing the internal structure of the cell, they also provide the structure for the fibrillar network in the nucleus and muscle sarcomeres.

The cytoskeleton is a three dimensional structure within a cell that is both a muscle and a skeleton. This means that it provides structure and stability but it is also dynamic and flexible. Its stable flexibility is created through tension binding so that if it is pushed, it can bounce back and maintain its structure, yet it is also flexible. The cytoskeleton is made up of protein fibers, or protein filaments. There are three types of protein filaments that make up the cytoskeleton: microfilaments, intermediate filaments and microtubules.

The cytoskeleton is responsible for a number of cell functions. It gives shape to cells which lack a cell wall. It facilitates intercellular as well as intracellular movement. The cytoskeleton plays a role in cell division. The movement of white blood cells, and muscle contraction are other examples of cell movement facilitated by the cytoskeleton. The cytoskeleton plays a key role in the movement of chromosomes during cell division. In animal cells, the cytoskeleton has an additional function. It performs the contraction or pinching of the animal cells during cytokinesis.

Microfilaments are also known as actin filaments, because they are associated with the protein called actin. These are the thinnest filaments in the cytoskeleton structure. They are tubular in shape and are solid. They are composed of the protein called actin and they provide support to the cytoskeleton much like wires or rope provide tension to a tent. Their role is primarily played in phagocytosis, a process which causes the cell to change shape and absorb a foreign particle from outside the cell, into its own cell. Other roles of the microfilaments are to propel organelles and cells, as well as to participate in mitosis, or cell division.

Intermediate filaments are slightly larger in diameter to the microfilaments. They are tough fibers, made up of polypeptides. They are instrumental in stabilizing the shape of a cell and supporting the frameworks within the cell. Not all intermediate filaments are the same. There are different types and each is made of a different protein. Keratins are found in the cells that form hair and nails. Vimentins are for muscle cells. Neurofilaments strengthen neurons. Nuclear lamins work to create a web of inter-meshing ties which stabilize the nuclear membrane.

Microtubules are twice the diameter of intermediate filaments and vary in length. They can grow relatively long in comparison to the other protein filaments: up to 1000 times longer than their diameter. They are comprised of sub-units of the protein tublin, tiny balls of matter which are generated by hydrolysis of GTP. Microtubules can grow at each end by the polymerization of tublin dimers and by the same token can shrink at each end by a reverse process called depolymerization. They are needed to provide structural support for they play a significant role in cell division, cell movement and propulsion of organelle.

Microtubules, when bound together in specific ways, make up centrioles. Centrioles are also cylinder-shaped structural units. They are made up of a ring of nine bundles of three microtubules each. They facilitate the formation of cilia and flagella. Cilia and flagella are responsible for movement of matter along the endometrium and for muscle contraction for the same purpose. For example, they line the esophagus and help push food down to the stomach with the help of muscular contractions of the flagella and rhythmic swaying of the cilia.

Is There Cytoskeleton in the Arms of an Actinosphaerium?

Formerly, it was assumed that the cytoskeleton existed only in eukaryotic cells, those possessing a nucleus, but recently, similar proteins have been discovered in prokaryotic cells, cells whose DNA is not bound inside a nucleus. Some of the proteins found in the prokaryotic cytoskeleton are FtsZ (filamenting temperature-sensitive mutant Z), MreB and ParM.

The actinosphaerium is a round protozoan belonging to the Phylum. Resembling a sea urchin, the actinosphaerium has many straight, stiff “arms”, sticking out in all directions. It is found all over the world, preferring concentrations of fresh water containing decaying plants, and motility occurs when protoplasm flows into the arms of the creature. The arms also increase the surface of the actinosphaerium, allowing for more contact with potential prey. Numerous adhesive capture organelles are found on the arms, and their purpose is to hold onto food. Also known as a heliozoan, due to its sun-like shape, the actinosphaerium has a number of small nuclei. Its exoplasm contains large vacuoles, while the endoplasm has small vacuoles. It dines on small protists and can grow to be a millimeter wide.

Microtubules, found in the cytoskeleton, have been persistently identified in the arms of the actinosphaerium, making up a large porting of the cytoplasm. Each arm possesses several hundred microtubules, which are axially aligned. As the arms grow, the microtubules become geometrically lined up into two spiral shapes, as seen on a cross section of the arm. The arms are calculated to contain 10 percent of the cytoplasm of the entire organism. The cytoplasm flows back and forth to and from the arms, bringing back to the cell body whatever bacteria and other food that sticks to the arms. When exposed to cold temperatures, the arms retract, and their core disappears. The cytoplasm also withdraws from the arms into the body of the actinosphaerium. When the chilled actinosphaerium is returned to room temperature it starts to recover after a few minutes, but takes a half-hour or more for the microtubules to re-form and for the arms to grow back to their original length. These microtubules not only provide the form and rigidity of the arms but are also responsible for their growth. Therefore, the answer to the above question is yes: The arms of the actinosphaerium do have cytoskeleton and its quite active.

Resources about the Cytoskeleton

Arizona.edu tutorial about the Cytoskeleton

Kenyon.edu basic information about the Cytoskeleton

Purdue.edu This material was originally published in the Purdue Cytometry CD-ROM Series,volume 4. The ability of eucaryotic cells to adopt a variety of shapes and to carry out coordinated and directed movements depends on the cytoskeleton, a complex network of protein filaments that extends throughout the cytoplasm…

Resource by L.Fuller

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