(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.
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.
