Cytoskeleton

How does the microtubule change length?

A microtubule changes in length by adding or removing tubulin dimers.

What does a nonmotile cilium do?

A single nonmotile cilium may also act as a kind of "antenna" for a cell. ○ Membrane proteins on this primary cilium transmit molecular signals from the environment outside the cell to the cell's nucleus, resulting in changes in the cell's activities. ○ Such pathways involving cilia-based signaling appear to be crucial to brain function and to embryonic development.

What are the cilia and flagella?

A specialized arrangement of microtubules is responsible for the beating of cilia and flagella. ○ Many unicellular eukaryotic organisms are propelled through water by cilia and flagella. ○ The sperm of animals, algae, and some plants have flagella.

What causes the contraction in filaments near the cell's trailing edge?

According to a widely accepted model, filaments near the cell's trailing edge interact with myosin, causing contraction. ○ The contraction forces the interior fluid into the pseudopodium, where the actin network has been weakened. ○ The pseudopodium extends until the actin reassembles into a network.

Are centrosomes with centrioles absolutely needed?

Although centrosomes with centrioles may help organize microtubule assembly in animal cells, they are not essential for this function. o Fungi and most plant cells lack centrosomes with centrioles but have well-organized microtubules.

What do the centrioles do before replicating?

Before an animal cell divides, the centrioles replicate.

Distinguish the cilia and flagellas beating patterns

Cilia and flagella differ in their beating patterns. ○ A flagellum has an undulatory movement that generates force in the same direction as the flagellum's axis, like the tail of a fish. ○ Cilia move more like oars, with alternating power and recovery strokes that generate force perpendicular to the cilium's axis.

how long are cilia and flagella?

Cilia usually occur in large numbers on the cell surface. ○ Cilia are about 0.25 µm in diameter and 2-20 µm long. Flagella are the same diameter as cilia, but are 10-200 µm long. ○ There are usually just one or a few flagella per cell.

What are cortical microfilaments?

Cortical microfilaments form a three-dimensional network just inside the plasma membrane to help support the cell's shape, giving the cell cortex the semisolid consistency of a gel. ○ This consistency contrasts with the more fluid (sol) state of the interior cytoplasm.

What is the cytoplasmic streaming caused by in plant cells?

Cytoplasmic streaming in plant cells is caused by the cytoskeleton.

How do motor proteins bring about movement of cilia and flagella?

Motor proteins bring about movements of cilia and flagella by gripping cytoskeletal components such as microtubules and moving them past each other. ○ A similar mechanism causes muscle cells to contract.

What are microfilaments built by?

Each microfilament is built as a twisted double chain of actin subunits. Microfilaments can form structural networks because of their ability to branch.

What is the structure of cilia and flagella?

Each motile cilium and flagellum has a group of microtubules sheathed by an extension of the plasma membrane. ○ Nine doublets of microtubules are arranged in a ring around a pair at the center. This "9 + 2" pattern is found in nearly all eukaryotic flagella and motile cilia. ○ Nonmotile primary cilia have a "9 + 0" pattern, lacking the central pair of microtubules.

What are dyneins?

Each outer doublet also has pairs of large protruding motor proteins called dyneins spaced along its length and reaching toward the neighboring doublet. ○ Dyneins are responsible for the bending movements of the organelle, as they perform a complex cycle of movements caused by changes in the shape of the protein. ○ These changes in shape are powered by ATP.

What connects the outer doublets to each other?

Flexible cross-linking proteins connect outer doublets to each other and to the two central microtubules.

What makes up the core of microvilli?

In animal cells specialized for transporting materials across the plasma membrane, such as intestinal cells, bundles of microfilaments make up the core of microvilli.

What are the microtubule doublets held in place by?

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 does the microtubule grow out of?

In many animal cells, microtubules grow out from a centrosome near the nucleus.

What causes localized contraction?

In other cells, actin-myosin aggregates are less organized but still cause localized contraction. ○ A contracting belt of microfilaments forms a cleavage furrow that divides the cytoplasm of animal cells during cell division.

What causes cytoplasmic streaming?

In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming, which creates a circular flow of cytoplasm in the cell, speeding the distribution of materials within the cell.

How do vesicles travel along the cytoskeleton?

Inside the cell, vesicles use motor protein "feet" to "walk" to destinations along a track provided by the cytoskeleton. ○ This is how vesicles containing neurotransmitter molecules migrate to the tips of axons.

What are intermediate filaments?

Intermediate filaments are a diverse class of cytoskeletal units, built from a family of proteins that includes the keratins.

How permanent is intermediate filaments?

Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes. ○ Even after cells die, intermediate filament networks often persist. ○ The networks reinforce cell shape and fix organelle location.

How big are intermediate filaments?

Intermediate filaments range in diameter from 8 to 12 nm, larger than microfilaments but smaller than microtubules.

Do intermediate filamentse compress or bear tension?

Like microfilaments, intermediate filaments are specialized for bearing tension.

How does actin and mysosin drive amoeboid movement?

Localized contraction brought about by actin and myosin also drives amoeboid movement. ○ Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments. Microfilaments assemble into networks that convert sol to gel.

What cells in the animal body move by crawling?

Many cells in the animal body, including some white blood cells, also move by crawling.

What are microfilaments important in?

Microfilaments are important in cell motility, especially as part of the contractile apparatus of muscle cells. ○ In muscle cells, thousands of actin filaments are arranged parallel to one another. ○ Thicker filaments composed of myosin interdigitate with the thinner actin fibers. ○ Like dynein with microtubules, myosin acts as a motor protein, walking along the actin filaments to shorten the cell.

How long are microfilaments

Microfilaments or actin filaments are solid rods about 7 nm in diameter, present in all eukaryotic cells.

What protein are microtubules constructed of?

Microtubule fibers are constructed of the globular protein tubulin. Each tubulin molecule is a dimer consisting of two subunits. ○ A tubulin dimer consists of two slightly different polypeptides: -tubulin and -tubulin.

How long are micortubules?

Microtubules are hollow rods about 25 nm in diameter and 200 nm to 25 µm in length.

What are microtubles?

Microtubules are the thickest of the three types of fibers; microfilaments (or actin filaments) are the thinnest; and intermediate filaments are fibers with diameters in a middle range.

What is the main function of the microtubule?

Microtubules shape and support the cell and serve as tracks to guide motor proteins carrying organelles to their destination. ○ Microtubules guide secretory vesicles from the Golgi apparatus to the plasma membrane. ○ Microtubules are also responsible for the separation of chromosomes during cell division.

What is the basal body?

The cilium or flagellum is anchored in the cell by a basal body, whose structure is identical to that of a centriole. ○ In many animals (including humans), the basal body of the fertilizing sperm's flagellum enters the egg and becomes a centriole.

How does the cytoskeleton play a major role in cell motility?

The cytoskeleton also plays a major role in cell motility, including changes in cell location and limited movements of parts of the cell.

How does the cytoskeleton play a role in biochemical activities?

The cytoskeleton also plays a role in the regulation of biochemical activities in the cell in response to mechanical stimulation. ○ Cytoskeletal elements transmit forces exerted by extracellular molecules on cell surface proteins into the cell—and even into the nucleus. ○ In an experiment, investigators used a micromanipulation device to pull on certain plasma membrane proteins attached to the cytoskeleton, leading to almost instantaneous rearrangements of nucleoli and other structures in the nucleus. ○ In this way, cytoskeletal transmission of naturally occurring mechanical signals may help regulate and coordinate the cell's response.

What does the cytoskeleton interact with to produce motility?

The cytoskeleton interacts with motor proteins to produce motility. Cytoskeleton elements and motor proteins work together with plasma membrane molecules to move the whole cell along fibers outside the cell.

What is the cytoskeleton?

The cytoskeleton is a network of fibers extending through the cytoplasm that provides mechanical support and maintains the cell's shape. The cytoskeleton provides support, motility, and regulation.

Is the cytoskeleton static or dynamic?

The cytoskeleton is dynamic and can be dismantled in one part and reassembled in another to change the shape of the cell.

Why does the cytoskeleton manipulate the plasma membrane?

The cytoskeleton manipulates the plasma membrane to form food vacuoles during phagocytosis.

What does the cytoskeleton provide?

The cytoskeleton provides anchorage for many organelles and cytosolic enzymes.

What are the mechanics of dynein-based bending in cillia and flagella?

The mechanics of dynein-based bending involve a process that resembles walking. ○ A typical dynein protein has two "feet" that "walk" along the microtubule of the adjacent doublet; one foot maintains contact while the other releases and reattaches one step farther along the microtubule. ○ Without restraint on the movement of the microtubule doublets, one doublet continues to "walk" along and slide past the surface of the other, elongating the cilium or flagellum rather than bending it. ○ For lateral movement of a cilium or flagellum, the dynein "walking" must have something to pull against, as when the muscles in your leg pull against your bones to move your knee.

What does the intermediate filament make up inteh cell?

The nucleus sits within a cage made of intermediate filaments, fixed in location by branches of the filaments that extend into the cytoplasm. ○ Other intermediate filaments make up the nuclear lamina that lines the interior of the nuclear envelope. When the shape of the entire cell is correlated with function, intermediate filaments support that shape. ○ The axons of nerve cells are strengthened by one class of intermediate filament. Various kinds of intermediate filaments function as the framework of the entire cytoskeleton.

What is the structural role of microfiliaments?

The structural role of microfilaments in the cytoskeleton is to bear tension, resisting pulling forces within the cell.

How are the two ends of the microtubule different?

The two ends of a microtubule are slightly different. ○ The "plus" end can accumulate or release tubulin dimers at a much higher rate than the other end.

How do the vesicles that bud off from the ER go to the golgi?

The vesicles that bud off from the ER travel to the Golgi apparatus along tracks built of cytoskeletal elements.

In what cell is the cytoskeleton especially important?

This function is especially important in animal cells, which lack walls.

Where are the cilia and flagella met?

When cilia or flagella extend from cells within a tissue layer, they can beat to move fluid over the surface of the tissue. ○ For example, cilia lining the trachea sweep mucus carrying trapped debris out of the lungs. ○ In the reproductive tract, cilia lining the oviducts help move an egg toward the uterus.

What are within the centrisome?

Within the centrosome is a pair of centrioles, each with nine triplets of microtubules arranged in a ring.