Download Chapter 6: Concept 6.6

Objectives
Describe the role of the cytoskeleton in cell movement.
Compare and contrast the functions of flagella and cilia.
Explain why a cell can be described as a coordinated unit.
Key Terms
microtubule
microfilament
flagella
cilia
Some cells are capable of moving by extending parts of themselves and
"oozing" from one place to another. Just as you have an internal
skeleton that serves several functions in your body, a cell has its own
kind of internal support system that enables it to move, support
organelles, and maintain shape.
The Cytoskeleton
Biologists once thought that the organelles of a cell drifted about freely
in the cytoplasm. However, improvements in microscopes and research
techniques revealed a cytoskeleton (cyto means "cell"), a network of
fibers extending throughout the cytoplasm. Unlike your body's
skeleton, the skeleton of most cells does not keep the same structural
pattern all the time. It is always changing, with new extensions building
at the same time that others are breaking apart.
Different kinds of fibers make up the cytoskeleton. Straight, hollow
tubes of proteins that give rigidity, shape, and organization to a cell are
called microtubules. As protein subunits are added or subtracted from
the microtubules, these structures lengthen or shorten. One function of
microtubules is to provide "tracks" along which other organelles can
move. For example, a lysosome might reach a food vacuole by moving
along a microtubule. Thinner, solid rods of protein called
microfilaments enable the cell to move or change shape when protein
subunits slide past one another. This process contributes to the oozing
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Wednesday, October 19, 2011 9:15:48 AM CT
movements of an amoeba and some white blood cells.
Flagella and Cilia
Unlike an amoeba that moves as changes occur to microfilaments in its
cytoplasm, many other kinds of cells move as a result of the action of
specialized structures that project from the cell. Flagella (singular,
flagellum) are long, thin, whip-like structures, with a core of
microtubules, that enable some cells to move. A flagellum usually
waves with an "S"-shaped motion that propels the cell. Cilia (singular,
cilium) are generally shorter and more numerous than flagella. Like
flagella, cilia also contain bundles of microtubules, but cilia have a
back-and-forth motion—something like the oars of a rowboat—that
moves a cell through its surroundings.
Cilia or flagella can also extend out from stationary cells that are held
in place as part of a layer of tissue in a multicellular organism. Here,
their motion moves fluid over the surface of the tissue. For example, the
cells lining your windpipe have cilia that sweep mucus with trapped
debris out of your lungs. This sweeping action helps keep your
respiratory system clean and allows air to flow through it smoothly.
The Cell as a Coordinated Unit
From the overview of a cell's organization to a close-up inspection of
each organelle's architecture, this tour of the cell has provided many
opportunities to connect structure with function. As you study the parts
of a cell, remember that none of its organelles works alone.
Consider the white blood cell's role in helping defend the body against
infections by ingesting bacteria. The cell moves toward the bacteria
using thin cytoplasmic extensions created by the interaction of parts of
the cytoskeleton. After the cell engulfs the bacteria, they are destroyed
by lysosomes that were produced by the ER and Golgi apparatus.
Ribosomes made the proteins of the cytoskeleton and the enzymes
within the lysosomes. And the production of these proteins was
programmed by messages dispatched from the DNA in the nucleus. All
these processes require energy, which mitochondria supply in the form
of ATP. The cooperation of cellular organelles makes a cell a living
unit that is greater than the sum of its parts.
Concept Check 6.6
1. How do microfilaments function in the cytoskeleton of a cell?
2. How do flagella differ in structure and function from cilia?
3. Give an example of coordination within a cell.
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Wednesday, October 19, 2011 9:15:48 AM CT
Copyright © 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights
reserved.
Staff
Wednesday, October 19, 2011 9:15:48 AM CT