Cell Biology Chapter 17 Quiz

Cytoskeleton Overview

• The cytoskeleton is essential for eukaryotic cells to maintain shape, organize cellular components, and enable movement.
• Comprised of three types of protein filaments: intermediate filaments, microtubules, and actin filaments.

Intermediate Filaments

• Diameter: approximately 10 nm; made of fibrous intermediate filament proteins.
• Provides mechanical strength and distributes stress across epithelial tissues.
• Forms the nuclear lamina beneath the inner nuclear membrane.
• Very flexible and withstands deformation without rupturing.
• Assembles from monomers into dimers, then tetramers, and finally into ropelike structures.

Microtubules

• Hollow cylinders made from tubulin, typically 25 nm in diameter.
• More rigid than actin filaments and intermediate filaments; can rupture when stretched.
• One end anchored at centrosomes; involved in various cellular structures like cilia and flagella.
• Built from tubulin dimers (α-tubulin and β-tubulin) linked by noncovalent interactions to form protofilaments (13 per microtubule).

Actin Filaments

• Also called microfilaments; consist of helical polymers of actin protein with a diameter of about 7 nm.
• Highly flexible and organized into bundles and networks, primarily concentrated in the cell cortex.
• Provides structural support and facilitates movements, including cytoplasmic streaming and cell crawling.

Motor Proteins

• Kinesins: Move toward the plus end of microtubules, transporting cellular cargo.
• Dyneins: Move toward the minus end, involved in organelle positioning.
• Both motor proteins convert ATP hydrolysis to mechanical work for movement along filaments.

Cilia and Flagella

• Cilia are hairlike structures, about 0.25 µm in diameter, containing microtubules arranged in a 9+2 structure.
• Cilia function to either move substances over cell surfaces or propel single cells through fluids.
• Flagella are longer than cilia and allow for movement of cells like sperm.
• Movement results from microtubule bending due to dynein activity.

Actin Dynamics and Cell Movement

• Cell crawling involves processes driven by cortical actin.
• Key processes include protrusion of cell membrane, adhesion to surfaces, and rear contraction.
• Actin polymerization at the leading edge initiates movement, forming structures like lamellipodia and filopodia.
• Treadmilling refers to dynamic turnover, where actin monomers add to the plus end while losing from the minus end, maintaining a constant length.

Myosin Protein Types

• Myosin-I: Simplest motor protein, involved in vesicle transport and shaping plasma membrane.
• Myosin-II: Forms bipolar filaments for muscle contraction, slides actin filaments past each other.
• Muscle contraction is mediated by interactions between actin and myosin-II bundles, essential for sarcomere function.

Sarcomeres

• The functional units of muscle contraction, composed of actin and myosin filaments.
• Z discs serve as attachment points for actin filaments, while myosin filaments occupy the center.
• Contraction occurs through sliding of myosin filaments relative to actin, powered by ATP hydrolysis.