MB 6.6

What is the cortex?

(1) It is the outer region of cytoplasm in a eukaryotic cell, lying just under the plasma membrane, that has a more gel-like consistency than the inner regions due to the presence of multiple microfilaments. (2) In plants, ground tissue that is between the vascular tissue and dermal tissue in a root or eudicot stem.

What does interdigitated mean?

(of two or more things) interlock like the fingers of two clasped hands..

What is a basal body?

A basal body is a eukaryotic cell structure consisting of a "9+0" arrangement of microtubule triplets. The basal body may organize the microtubule assembly of a cilium or flagellum and is structurally very similar to a centriole. In many animals (including humans) the basal body of the fertilizing sperm's flagellum enters the egg and becomes a centriole.

What is a centriole?

A centrosome has a pair of centrioles within it. Each centriole is composed of nine sets of triplet microtubules that are arranged in a ring. Before an animal cell divides, the centrioles replicate. Although centrosomes with centrioles may help organize microtubule assembly in animal cells, they are not essential for this function in all eukaryotes; fungi and almost all plant cells lack centrosomes with centrioles but have well-organized microtubules. Apparently, other microtubules organizing centers play the role of centrosomes in these cells.

What is another role that cilium play aside from being the means of locomotion for a cell.

A cilium may also act as a signal-receiving "antenna" for the cell. Cilia that have this function are generally nonmotile, and there is only one per cell. (In fact, in vertebrate animals, it appears that almost all cells have a cilium, which is called a "primary cilium"). Membrane proteins on this kind of cilium transmit molecular signals from the cell's environment to its interior, triggering pathways that may lead to changes in the cell's activities. Cilium-based signaling appears to be crucial in brain function and to embryonic development. (Wow the picture of the cross sections of the flagella and cilia are beautiful.)

What are the differences in the beating patterns of flagella and cilia?

A flagellum has an undulating motion that generates force in the same direction as the flagellum's axis, like the tail of a fish. In contrast, cilia work more like oars, with alternating power and recovery strokes generating force in a direction perpendicular to the cilium's axis, much as the oars of a racing crew boat extend outward at a right angle to the boat's forward movement.

What are the mechanics of dynein based bending that are involved in a process that resembles walking?

A typical dynein protein has two "feet" that "walk" along the microtubule of the adjacent doublet, one foot maintaining contact while the other releases and reattaches one step farther along the microtubule. Without any restraints on the movement of the microtubule doublets, one doublet, would continue to "walk" along and slide past the surface of the other, elongating the cilium or flagellum rather than bending it.

Define what microtubules are and give me a description of them.

All eukaryotic cells have microtubules, which are hollow rods measuring about 25 nm in diameter and from 200 nm to 25 microns in length. (Pill shaped - longer than they are wide.) The wall of the hollow tube is constructed from globular protein called tubulin. Each tubulin is a dimer (which is a molecule made up of two subunits). A tubulin dimer consists of two slightly different polypeptides, alpa-tubulin and beta-tubulin. Microtubules grow in length by adding tubulin dimers; They can also be dissembled and their tubulin used to build microtubules elsewhere in the cell.

Describe the process of cell motility.

Cytoskeletal elements and motor proteins work together with plasma membrane molecules to allow whole cells to move along fibers outside the cell. Motor proteins bring about the bending of cilia and flagella by gripping microtubules within those organelles and sliding them against each other. A similiar mechanism involving microfilaments causes muscle cells to contract. Inside the cell, vesicles and other organelles often use motor protein "feet" to "walk" to their destinations along a track provided by the cytoskeleton. (Haha this is very similiar to traintracks [cytoskeleton] and a train [motor proteins] and the passengers [vesicles and other organelles].)

What makes intermediate filaments more permanent fixtures of cells when compared to microfilaments and microtubules, which are often disassembled and reassembled in various parts of a cell?

Even after cells die, intermediate filament networks often persist; for example, the outer layer of our skin consists of dead skin cells full of keratin proteins. Chemical treatments that remove microfilaments and microtubules from the cytoplasm of living cells leave a web of intermediate filaments that retains its original shape. Such experiments suggest that intermediate filaments are especially sturdy and that they play an important role in reinforcing the shape of a cell and fixing the position of certain organelles. For instance, the nucleus typically sits within a cage made of intermediate filaments, fixed in location by branches of the filaments that extend into the cytoplasm.

What are the mechanics of dynein based bending that involve the lateral movement of cilium or flagellum?

For lateral movement of a cilium or flagellum, the dynein "walking" must have something pull against, as when the muscles in your leg pull against your bones to move your knee. In cilia and flagella, the microtubule doublets seem to be held in place by the cross-linking proteins just inside the outer doublets and by the radial spokes and other structural elements. Thus, neighboring doublets cannot slide past each other very far. Instead, the forces exerted by dynein "walking" cause the doublets to curve, bending the cilium or flagellum.

What is a dynein?

In cilia and flagella, dynein is a large motor protein extending from one microtubule doublet to the adjacent doublet. ATP hydrolysis drives changes in dynein shape that lead to bending of cilia and flagella. Dyneins are responsible for the bending movements of the organelle. A dynein molecule performs a complex cycle of movements by changes in the shape of the protein, with ATP providing the energy for these changes.

What makes microfilaments different from microtubules?

In contrast to the compression resisting role of microtubules, the structural role of microfilaments in the cytoskeleton is to bear tension (pulling forces). A three-dimensional network is formed by microfilaments just inside the plasma membrane (cortical microfilaments) helps support the cell's shape. (Ah these guys are kind of the underlying steel bars in buildings!). This network gives the outer cytoplasmic layer of a cell, called the cortex, the semisolid consistency of a gel, in contrast with the more fluid state of the interior cytoplasm. In animal cells specialized for transporting materials across the plasma membrane, such as intestinal cells, bundles of microfilaments make up the core of microvilli, delicate projects that increase the cell's surface area.

What are some places in the human body, where intermediate filaments are found?

Intermediate filaments make up the nuclear lamina, which lines the interior of the nuclear envelope. By supporting a cell's shape, intermediate filaments help the cell carry out its specific function. For example, the long extensions (axons) of nerve cells that transmit impulses are strengthened by intermediate filaments. Thus, the various kinds of intermediate filaments may function together as the permanent framework of the entire cell.

What is a microfilament?

It is a cable composed of actin proteins in the cytoplasm of almost every eukaryotic cell. (This reminds me of a bunch of giant fiber optic cables lining the sea floor). Microfilaments make up part of the cytoskeleton and acting alone or with myosin to cause cell contraction; also know as an actin filament. Microfilaments are solid rods about 7 nm in diameter. A microfilament is a twisted double chain of actin subunits. Besides occurring as linear filaments, microfilaments can form structural networks when certain proteins bind along the side of an actin filament and allow a new filament to extend as a branch.

What is cytoplasmic streaming?

It is a circular flow of cytoplasm, involving interactions of myosin and actin filaments, that speeds the distribution of materials within cells. This movement is especially common in large plant cells, speeds the distribution of materials within the cell.

What is an intermediate filament?

It is a component of the cytoskeleton that includes filaments intermediate in size between microtubules and microfilaments. They are named for their diameter, which is 8-12 nm, larger than the diameter of microfilaments but smaller than that of microtubules. They are specialized for bearing tension (like microfilaments). They are a diverse class of cytoskeletal elements. Each type is constructed from a particular molecular subunit belong to a family of proteins whose members include keratins.

What is keratin?

It is a fibrous protein forming the main structural constituent of hair, feathers, hoofs, claws, horns, etc.

What is actin?

It is a globular protein that links into chains, two of which twist helically about each other (similiar to DNA I think) forming microfilaments (actin filaments) in muscle and other kinds of cells.

What is a flagellum (plural flagella)?

It is a long cellular appendage specialized for locomotion. Like motile cilia, eukaryotic flagella have a core with nine outer doublet microtubules (like the centriole - but not triplet) and two inner single microtubules ensheathed in an extension of the plasma membrane. Prokaryotic flagella have a different structure.

What is the cytoskeleton?

It is a network of microtubules, microfilaments, and intermediate filaments that extend throughout the cytoplasm and serve as a variety of mechanical, transport, and signaling functions. (The cytoskeleton is somewhat like a highway - which can be utilized by all the molecular machinery in the cell.)

What is a motor protein?

It is a protein that interacts with cytoskeletal elements and other cell components, producing movement of the whole cell or parts of the cell.

What is a cilium (plural cilia)?

It is a short appendage containing microtubules in eukaryotic cells. A motile cilium is specialized for locomotion or moving fluid past the cell; it is formed from a core of nine outer doublet microtubules and two inner single microtubules ensheathed in an extension of the plasma membrane. A primary cilium is usually nonmotile and plays a sensory and signaling role; it lacks the two inner microtubules.

What is a centrosome?

It is a structure present in the cytoplasm of animal cells that functions as a microtubule organizing center and is important during cell division. A centrosome has two centrioles. These microtubules function as compression resisting girders of the cytoskeleton.

What is an oviduct?

It is a tube passing from the ovary to the vagina in vertebrates or to the uterus in vertebrates, where it is also known as a fallopian tube.

What is myosin?

It is a type of motor protein that associates into filaments that interact with actin filaments to cause cell contraction. Like dynein when it interacts with microtubules, myosin acts as a motor protein by means of projections that "walk" along the actin filaments. Contraction of the muscle cell results from the actin and myosin filaments sliding past one another in this way, shortening the cell. In other kinds of cells, actin filaments are associated with myosin in miniature and less elaborate versions of the arrangement in muscle cells. These actin-myosin aggregates are responsible for localized contractions of cells. For example, a contracting belt of microfilaments form a cleavage furrow that pinches a dividing animal cell into two daughter cells.

What makes the cytoskeleton act like a "skeleton"?

Like a dome tent, the cytoskeleton is stabilized by a balance between opposing forces exerted by its elements. And just as the skeleton of an animal helps fix the positions of other body parts, the cytoskeleton provides anchorage for many organelles and even cytosolic enzyme molecules. The cytoskeleton is more dynamic than an animal skeleton, however. It can be quickly dismantled in one part of the cell and reassembled in a new location, changing the shape of the cell.

What organisms have cilia or flagella and what do the cilia or flagella function as?

Many unicellular eukaryotes are propelled through water by cilia or flagella that act as locomotor appendages, and the sperm of animals, algae, and some plants have flagella. When cilia or flagella extend from cells that are held in place as part of a tissue layer, they can move fluid over the surface of the tissue. For example, the ciliated lining of the trachea (windpipe) sweeps mucus containing trapped debris out of the lungs. In a woman's reproductive tract, the cilia lining the oviducts help move an egg toward the uterus.

Why are microfilaments important in muscle cells?

Microfilaments are well known for their role in cell motility, particularly as part of the contractile apparatus of muscle cells. Thousands of actin filaments are arranged parallel to one another along the length of a muscle cell, interdigitated with thicker filaments of made of a protein called myosin.

What are the three main types of fibers that make up the cytoskeleton?

Microtubules are the thickest of the three types of fibers Microfilaments (also called actin filaments) are the thinnest. Intermediate filaments are the fibers with diameters in the middle range.

How do amoeba's take advantage of the localized contraction brought about by actin and myosin?

Movement brought about by actin and myosin plays a role in amoeboid movement. A cell such as an amoeba crawls along a surface by extending cellular extensions called pseudopodia (from the Greek "pseudes", false, and "pod", foot), and moving toward them. Pseudopodia extend by assembly of actin subunits into microfilament networks that convert cytoplasm from a sol to a gel inside these cell projections. Cell surface proteins on the pseudopodium make strong attachments to the "road". Next the interaction of microfilaments with myosin near the cell's trailing end causes the contraction of that region, loosening its cell-surface (table! or floor) attachments and pulling it forward toward the pseudopodia. Amoebae lacking myosin can still form pseudopodia, but forward movement is greatly slowed. Amoebas are not the only cells that move by crawling; so do many cells in the animal body, including some white blood cells.

Give some specific examples of how the cytoskeleton is utilized in cells.

One example is how vesicles containing neurotransmitter molecules migrate to the tips of axons, the long extensions of nerve cells that release these molecules as chemical signals to adjacent nerve cells. The vesicles that bud off from the ER travel to the Golgi along cytoskeletal tracks. The cytoskeleton also manipulates the plasma membrane, making it bend inward to form food vacuoles or other phagocytic vesicles. And the streaming of cytoplasm that circulates materials within many large plant cells is yet another kind of cellular movement brought about by the cytoskeleton. (The cell is analogous to a city, the tissues are like the state, the organ is like the nation, and the whole body is like the world)

What is microvillus? (plural, microvilli)

One of many fine, finger-like projections of the epithelial cells in the lumen of the small intestine that increase its surface area.

What is the most obvious function of the cytoskeleton?

The most obvious function is to give mechanical support to the cell and maintain its shape. This is especially important for animal cells, which lack cell walls. This remarkable strength and resilience of the cytoskeleton as a whole is based on its architecture.

What is the uterus?

The uterus is a female organ where eggs are fertilized and/or development of the young occurs.

Why are the two ends of a microtubule slightly different?

This is because of the orientation of tubulin dimers. One end can accumulate or release tubulin dimers at a much higher rate than the other, thus growing and shrinking significantly during cellular activities. (This is called the "plus end," not because it can only add tubulin proteins but it's the end where both "on" and "off" rates are much higher.

What does the term "cell motility (movement)" encompass?

This term encompasses both changes in cell location and more limited movements of parts of the cell. Cell motility generally requires the interaction of the cytoskeleton with motor proteins.

What is sol?

it is a fluid suspension of a colloidal solid in a liquid.