4.8.1 Study Notes Cytoskeleton 

A. Cytoskeleton

• network of connected filaments and tubules

• extends from the nucleus to the plasma membrane in eukaryotes

• function: 

  • maintain the cell shape 

  • allow cells and organelles to move

• make up various other parts: 

  • actin filaments

  • intermediate filaments

  • microtubules.

B. Actin Filaments

• small fibers 

  • long, thin fibers (about 7 nm in diameter)

  • occur in bundles or mesh-like networks

  • consists of two chains of globular actin monomers twisted to form a helix

• play a structural role

  • forming a dense complex web just under the plasma membrane

  • seen as the formation of pseudopods in amoeboid movement

• found in microvilli of intestinal cells likely shorten or extend cell into intestine

• plant cells form tracks along which chloroplasts circulate, called cytoplasmic streaming

• move by interacting with motor molecules

  •  which are proteins that can attach, detach, and reattach farther along an actin filament

  • example: 

    • myosin combines with and splits ATP, binding to actin and changing configuration to pull actin filament forward

  • Similar action accounts for pinching off cells during cell division.

C. Intermediate Filaments

• Intermediate filaments are 8–11 nm in diameter

  • between actin filaments and microtubules in size

• rope-like assemblies of fibrous polypeptides

• some support the nuclear envelope

• some support plasma membrane and form cell-to-cell junctions

D. Microtubules

• small hollow cylinders (25 nm in diameter and from 0.2–25 μm in length)

• composed of a globular protein tubulin that occurs as α tubulin and β tubulin

• Assembly brings these two together as dimers and the dimers arrange themselves in rows

• Regulation of microtubule assembly is under control of a microtubule organizing center (MTOC)

  • the main MTOC is called a centrosome.

  • helping maintain the shape of cells 

  • acting as tracks along which organelles move

• similar to actin myosin, the motor molecules kinesin and dynein are associated with microtubules

  • Different kinds of kinesin proteins specialize to move one kind of vesicle or cell organelle

  • Cytoplasmic dynein is similar to the molecule dynein found in flagella.

• During cellular division, microtubules disassemble and reassemble to form spindle fibers

E. Centrioles

• short cylinders with a ring pattern (9 + 0) of microtubule triplets

• In animal cells and most protists, centrosome contains two centrioles lying at right angles to each other

• During cellular division:

  • centrioles replicate and become part of a centrosome 

  • function to organize the mitotic spindle

• Plant and fungal cells have the equivalent of a centrosome, but they do not contain centrioles

• Centrioles serve as basal bodies for cilia and flagella

E. Cilia and Flagella

• cillia: 

  • short, usually numerous hairlike projections that can move in an undulating fashion (e.g., Paramecium)

• flagella:

  • are longer, usually fewer, projections that move in whip-like fashion (e.g., sperm cells)

    • differ from prokaryotic flagella

• both locomation projections:

  • move when the microtubules slide past one another

  • have a basal body at base with the same arrangement of microtubule triples as centrioles

  • grow by the addition of tubulin dimers to their tips