Figure 3.49 (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
Thelarge,complex organelles inwhichaerobic cellular respiration occursineukaryotic cells arecalledmitochondria
(Figure 3.50). 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.
Chapter 3 | The Cell 125