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A) a computer simulation shows micrtububles as spheres forming a rube structure. This can be drawn as spheres forming a ring; stacks of these rings form the tube. Each ring is 13 polymerized dimers of alpha-tubulin and beta-tubulin. C) The long tubes that are formed create a structure similar to a railroad track; motor proteins move along the microtubule track to carry vesicles throughout the cell.
(a) Microtubules are hollow structures composed of polymerized tubulin dimers. (b) They are involved in several cellular processes, including the movement of organelles throughout the cytoplasm. Motor proteins carry organelles along microtubule tracks that crisscross the entire cell. (credit b: modification of work by National Institute on Aging)

In addition, microtubules are involved in cell division, forming the mitotic spindle that serves to separate chromosomes during mitosis and meiosis. The mitotic spindle is produced by two centrosomes , which are essentially microtubule-organizing centers, at opposite ends of the cell. Each centrosome is composed of a pair of centrioles positioned at right angles to each other, and each centriole is an array of nine parallel microtubules arranged in triplets ( [link] ).

a) Centrosomes are shown as short tubes. The outside of these tubes is made of 9 sets of microtubule triplets. These sets are held together by lines labeled centrioles. B) Centrosomes are shown on the two poles of a cell. Lines connect the centrosomes to chromosomes in the center of the cell.
(a) A centrosome is composed of two centrioles positioned at right angles to each other. Each centriole is composed of nine triplets of microtubules held together by accessory proteins. (b) In animal cells, the centrosomes (arrows) serve as microtubule-organizing centers of the mitotic spindle during mitosis.
  • Compare and contrast the three types of cytoskeletal structures described in this section.

Mitochondria

The large, complex organelles in which aerobic cellular respiration occurs in eukaryotic cells are called mitochondria ( [link] ). The term “mitochondrion” was first coined by German microbiologist Carl Benda in 1898 and was later connected with the process of respiration by Otto Warburg in 1913. Scientists during the 1960s discovered that mitochondria have their own genome and 70S ribosomes. The mitochondrial genome was found to be bacterial, when it was sequenced in 1976. These findings ultimately supported the endosymbiotic theory proposed by Lynn Margulis , which states that mitochondria originally arose through an endosymbiotic event in which a bacterium capable of aerobic cellular respiration was taken up by phagocytosis into a host cell and remained as a viable intracellular component.

Each mitochondrion has two lipid membranes. The outer membrane is a remnant of the original host cell’s membrane structures. The inner membrane was derived from the bacterial plasma membrane. The electron transport chain for aerobic respiration uses integral proteins embedded in the inner membrane. The mitochondrial matrix , corresponding to the location of the original bacterium’s cytoplasm, is the current location of many metabolic enzymes. It also contains mitochondrial DNA and 70S ribosomes . Invaginations of the inner membrane, called cristae, evolved to increase surface area for the location of biochemical reactions. The folding patterns of the cristae differ among various types of eukaryotic cells and are used to distinguish different eukaryotic organisms from each other.

The mitochondria is shown as a long oval structure. The outside of the structure is an outer membrane. Inside of that is an inner membrane that folds back and forth filling most of the inner part of the mitochondrion. The folds of the inner membrane are labeled cristae and the fluid inside the inner membrane is the mitochondrial matrix. The space between the inner and outer membranes is the inner membrane space. Inside the mitochondrial matrix are found DNA, ribosomes and granules.
Each mitochondrion is surrounded by two membranes, the inner of which is extensively folded into cristae and is the site of the intermembrane space. The mitochondrial matrix contains the mitochondrial DNA, ribosomes, and metabolic enzymes. The transmission electron micrograph of a mitochondrion, on the right, shows both membranes, including cristae and the mitochondrial matrix. (credit “micrograph”: modification of work by Matthew Britton; scale-bar data from Matt Russell)

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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