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a) A micrograph of a cross section of a flagellum showing a ring of 9 sets of structures that are made of smaller rings. In the center are two more complete smaller rings. B)  A micrograph showing a flagellum. This shows a star shaped structure in the cell attached to the long lines that make up the filament of the flagellum. A diagram shows the triplet centriole in the cell as part of the basal body that attaches the filament to the cell. The diagram also shows a cross section of the filament. The outer ring is made of 9 sets of the following: a ring labeled subfiber A, a ring labeled subfiber B, a projection labeled radial spoke with a small end labeled spoke head, a projection towards the center labeled inner dynein, and a projection towards the outside labeled outer dynein. Each of these 9 sets are connected to the ones next to it via a line called nexin. These 9 sets form a ring; in the center of this ring are 2 small circles labeled central singlet microtubule. These two are attached to each other by a line labeled central bridge. C) A cell with flagella on either end. D) A cell with many small cilia along the outside and an indentation labeled mouth.
(a) Eukaryotic flagella and cilia are composed of a 9+2 array of microtubules, as seen in this transmission electron micrograph cross-section. (b) The sliding of these microtubules relative to each other causes a flagellum to bend. (c) An illustration of Trichomonas vaginalis , a flagellated protozoan parasite that causes vaginitis. (d) Many protozoans, like this Paramecium , have numerous cilia that aid in locomotion as well as in feeding. Note the mouth opening shown here. (credit d: modification of work by University of Vermont/National Institutes of Health)
  • Explain how the cellular envelope of eukaryotic cells compares to that of prokaryotic cells.
  • Explain the difference between eukaryotic and prokaryotic flagella.

Resolution

Since amoxicillin has not resolved Barbara’s case of pneumonia, the PA prescribes another antibiotic, azithromycin, which targets bacterial ribosomes rather than peptidoglycan. After taking the azithromycin as directed, Barbara’s symptoms resolve and she finally begins to feel like herself again. Presuming no drug resistance to amoxicillin was involved, and given the effectiveness of azithromycin, the causative agent of Barbara’s pneumonia is most likely Mycoplasma pneumoniae . Even though this bacterium is a prokaryotic cell, it is not inhibited by amoxicillin because it does not have a cell wall and, therefore, does not make peptidoglycan.

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Key concepts and summary

  • Eukaryotic cells are defined by the presence of a nucleus containing the DNA genome and bound by a nuclear membrane (or nuclear envelope ) composed of two lipid bilayers that regulate transport of materials into and out of the nucleus through nuclear pores.
  • Eukaryotic cell morphologies vary greatly and may be maintained by various structures, including the cytoskeleton, the cell membrane, and/or the cell wall
  • The nucleolus , located in the nucleus of eukaryotic cells, is the site of ribosomal synthesis and the first stages of ribosome assembly.
  • Eukaryotic cells contain 80S ribosomes in the rough endoplasmic reticulum ( membrane bound-ribosomes ) and cytoplasm ( free ribosomes ). They contain 70s ribosomes in mitochondria and chloroplasts.
  • Eukaryotic cells have evolved an endomembrane system, containing membrane-bound organelles involved in transport. These include vesicles, the endoplasmic reticulum, and the Golgi apparatus.
  • The smooth endoplasmic reticulum plays a role in lipid biosynthesis, carbohydrate metabolism, and detoxification of toxic compounds. The rough endoplasmic reticulum contains membrane-bound 80S ribosomes that synthesize proteins destined for the cell membrane
  • The Golgi apparatus processes proteins and lipids, typically through the addition of sugar molecules, producing glycoproteins or glycolipids, components of the plasma membrane that are used in cell-to-cell communication.
  • Lysosomes contain digestive enzymes that break down small particles ingested by endocytosis , large particles or cells ingested by phagocytosis , and damaged intracellular components.
  • The cytoskeleton , composed of microfilaments , intermediate filaments , and microtubules , provides structural support in eukaryotic cells and serves as a network for transport of intracellular materials.
  • Centrosomes are microtubule-organizing centers important in the formation of the mitotic spindle in mitosis.
  • Mitochondria are the site of cellular respiration. They have two membranes: an outer membrane and an inner membrane with cristae. The mitochondrial matrix, within the inner membrane, contains the mitochondrial DNA, 70S ribosomes, and metabolic enzymes.
  • The plasma membrane of eukaryotic cells is structurally similar to that found in prokaryotic cells, and membrane components move according to the fluid mosaic model. However, eukaryotic membranes contain sterols, which alter membrane fluidity, as well as glycoproteins and glycolipids, which help the cell recognize other cells and infectious particles.
  • In addition to active transport and passive transport, eukaryotic cell membranes can take material into the cell via endocytosis , or expel matter from the cell via exocytosis.
  • Cells of fungi, algae, plants, and some protists have a cell wall, whereas cells of animals and some protozoans have a sticky extracellular matrix that provides structural support and mediates cellular signaling.
  • Eukaryotic flagella are structurally distinct from prokaryotic flagella but serve a similar purpose (locomotion). Cilia are structurally similar to eukaryotic flagella, but shorter; they may be used for locomotion, feeding, or movement of extracellular particles.

True/false

Mitochondria in eukaryotic cells contain ribosomes that are structurally similar to those found in prokaryotic cells.

True

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Fill in the blank

Peroxisomes typically produce _____________, a harsh chemical that helps break down molecules.

hydrogen peroxide

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Microfilaments are composed of _____________ monomers.

actin

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Short answer

What existing evidence supports the theory that mitochondria are of prokaryotic origin?

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Why do eukaryotic cells require an endomembrane system?

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Name at least two ways that prokaryotic flagella are different from eukaryotic flagella.

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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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