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Material Type: Notes; Professor: Brand; Class: INTRODUCTORY BIOLOGY I; Subject: Biology; University: University of Texas - Austin; Term: Spring 2009;
Typology: Study notes
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Although we learn in textbooks that the cytoskeleton is a feature only ofeukaryotic cells, in fact prokaryotic cells have components that
are very
similar to components of a eukaryotic cytoskeleton. The following article isfrom the January 23, 2009 issue of Science Magazine. “Protein Filaments Caught in the Act” www.sciencemag.org
Previous issues Volume 323
page 472
.
flagellated
unicellular eukaryote
flagellum
basal body
flagellar
root
nucleus
In the cytoplasmic
matrix of eukaryotic cells there is a region called
the centrosome
or microtubule organizing center
(MTOC).
Microtubules radiate from this region. In animal cells there are
two
structures called centrioles
in the centrosome.
animal cell
From textbook Fig. 6.9, p. 100
centrosome
The MTOC provides a reference point fororganizing the locations of various structureswithin the cell with respect to a framework ofmicrotubules.
The two centrioles
in a centrosome
lie at right angles to each other.
Each centriole
in the centrosome
appears identical to a basal body.
See textbook Fig. 6.22, p. 114
pairs of centrioles
The animal cell illustratedhere is just beginning theprocess of mitosis. The pairof centrioles
has recently
been duplicated and thereare now two centrosomes in the MTOC.
The cell is now in a later stage ofmitosis. One centrosome
has
migrated to the opposite side ofthe cell, forming a second MTOC.
The centrosome
(MTOC) plays a central role in mitosis and other
events associated with eukaryotic cell division.
stages of mitosis
duplicated pair of centrioles
nucleus
Relationship Between Basal Bodies and Centrioles
in a
Unicellular Flagellated Eukaryote
flagella
basal bodies
flagellar
roots
nucleus
centrosome
(containing centrioles)
These illustrations represent two stages in the life cycle of the same cell. (1) During mostof its lifetime, the cell has flagella with basal bodies as well
as flagellar
roots
that radiate
deep into the cell interior, helping to organize the cell contents. (2) The cell loses it'sflagella when it prepares to undergo mitosis and divide. The basal bodies then move to asite near the center of the cell where they become centrioles
as a centrosome
forms.
Microtubules are produced by the cell and radiate from the centrosome, where theyprovide a framework for re-organizing cellular components in preparation for mitosis andcell division. After the cell divides, the centrioles
of each new cell will again
migrate to the front part of the cell to become basal bodies and provide a framework for the formation of new flagella.
1
2
Microfilaments are important for maintaining
the shapes of animal cells.
microvilli
= small protrusions on the surface of
these epithelial cells, used to increase surface area.
direction of
food transport
Epithelial cells that line the small intestine
epithelial cell
Examples of Microfilament Function
See textbook Fig. 6.27, p. 117
(cyclosis)
A Few of the Many Types of Intermediate
Filaments that Occur in Higher Animal Cells
Keratin: is the major component of skin cell cytoplasmic
matrix;
is the major component of hair and nails; is the main component of bird feathers.
Neurofilaments: occur in the cytoplasmic
matrix of nerve cells.
Lamins: occur in nucleoplasm.
Keratin intermediate filaments align in parallel within the cytoplasmic matrix of skin cells. They are concentrated at points where the cell isconnected tightly to adjacent cells through anchoring junctions calleddesmosomes.
keratin filament
desmosome
Skin cells continuously die and arereplaced by new cells. The dead cellsare completely destroyed, except forthe keratin microfilaments, whichremain as a surface coating on theouter layer of skin. Keratin is highlyprotective, since it is impenetrable tomost solvents and inert to mostchemical agents.
skin cells
See textbook Fig. 6.32, p. 121
Globular polypeptide
chains (proteins)
Fibrous polypeptide
chains (proteins)
Globular vs
Fibrous Polypeptide Chains
t,
protect the cell from being eaten by other cells or organisms,
make the cell stronger and/or more rigid,
protect the cell from drying out,
anchor the cell to a surface or to another cell,
allow the cell to swim and/or float in water,
serve as an array of sensors, allowing the cell to detect andreact to its environment, and/or allowing other cells in theenvironment to detect and react to
it.
External Coatings of Cells
Nearly all kinds of cells (prokaryotic and eukaryotic) have a coatingexternal to the plasma of membrane. Depending on the kind of celland its location, this coating may be used to:
