CH 17: Cytoskeleton: Actin Filaments and Muscle Contraction

What are actin filaments?

Thin, flexible cytoskeletal fibers made of actin protein subunits twisted into a helix

What are the two ends of an actin filament

The positive end grows faster, the negative end grows slower

How do actin and tubulin polymerize similarly?

Both assemble from protein subunits, have polarity, and use nucleotide hydrolysis to regulate filament dynamics.

What is tubulin?

A protein that assembles into microtubules, which are are hollow tubes that help support cells and transport materialsP

What is polymerization?

Process of joining small protein subunits together to form a larger structure, such as a filament

What is nucleotide hydrolysis

The breakdown of nucleotide (such as ATP or GTP), which helps regulate filament behavior

What nucleotide does actin use?

ATP, broken down after actin subunits join a filament

What nucleotide does tubulin use?

GTP, broken down after tubulin subunits join a microtubule

What do actin binding proteins do?

Proteins that attach to actin and regulate its assembly, disassembly, stabilization, bundling, and organization

What are myosins?

Motor proteins that bind actin and use ATP energy to produce movement

What does the myosin head do?

The part of myosin that binds actin and ATP to generate movement

What does the myosin tail do?

The part of myosin that attaches to cargo or helps assemble myosin molecules into filaments

What is the cell cortex?

A network of actin filaments directly beneath the plasma membrane that supports cell shape and strength

What is the plasma membrane?

The outer boundary of a cell that controls what enters and leaves

What does cortical actin do?

Supports the plasma membrane, maintains cell shape, and allows cells to change shape

What is cell crawling?

A type of cell movement where actin pushes the front of the cell forward and myosin helps pull the rest along

What are lamellipodia

Broad, flat extensions of a cell made of branched actin filaments that help the cell crawl

What are filopodia

Thin, finger like extensions made of parallel actin filaments that help cells explore their surroundings

What are actin binding proteins roles in cell protrusions?

They organize actin into different arrangements, producing structures such as lamellipodia and filopodia

How do extracellular signals affect actin filaments

Signals from outside the cell activate pathways that change actin organization, cell shape, and movement

What is cell polarity?

The organization of a cell into distinct regions with different structures or functions

How do actin filaments and microtubules establish cell polarity?

They work together to organize cell structures and direct materials to specific regions

What is actin treadmilling?

A process where actin subunits are added to the positive end and removed from the negative end, allowing continuous filament turnover

What causes muscle contraction?

Interactions between actin (thin protein filaments) and myosin (motor proteins that form thick filaments) generate force and movement

What is myosin II?

A motor protein that assembles into thick filaments and pulls actin filaments during muscle contraction

How do myosin II filaments interact with actin?

Myosin heads attach to actin and pull thin filaments toward the center of the sarcomere

What is a myosin heavy chain?

A large protein that makes up much of a myosin II molecules, including its head and tail regions

What does the myosin heavy chain head do?

Binds actin and ATP and changes shape to generate movement

What does the myosin heavy chain tail do?

Helps myosin molecules assemble into thick filaments

What is a sarcomere?

The basic contractile unit of skeletal muscle, extending from one Z disc to another

What are Z discs?

Structures that mark the boundaries of each sarcomere and anchor actin filaments

Where are actin filaments located in a sarcomere?

Thin filaments are attached to the Z discs and extend toward the center

Where are myosin filaments located in a sarcomere?

Thick filaments are positioned in the center of the sarcomere

What does skeletal muscle have a striated appearance?

Actin and myosin filaments are arranged in repeating patterns that create alternating light and dark bands

What is the sliding filament mechanism?

The process where actin filaments slide past myosin filaments, shortening the sarcomere

What happens to Z discs during muscle contraction?

They move closer together as the sarcomere shortens

Do actin and myosin filaments shorten during contraction

No, the filaments maintain their lengths and slide past one another

What is the role of myosin head movement during contraction

Myosin heads pivot and pull actin toward the center of the sarcomere

What happens when ATP binds to myosin?

ATP binding causes myosin to detach from actin

What is ATP hydrolysis?

The breakdown of ATP into ADP and phosphate, releasing energy that repositions the myosin head

What happens during the power stroke?

The myosin head pivots and pulls the actin filament toward the center of the sarcomere

What is the cross bridge cycle

The repeating process of myosin attaching to actin, pulling it, detaching, and resetting using ATP

Why is ATP necessary for muscle contraction?

ATP allows myosin to detach from actin and provides energy to reset the myosin head

What triggers skeletal muscle contraction

A sudden increase in cytosolic CA 2 + (calcium ions inside the cell)

What are T tubules?

Extensions of the muscle cell’s plasma membrane that carry electrical signals deep into the cell

What is the sarcoplasmic reticulum (SR)?

A specialized membrane network inside muscle cells that stores and releases calcium ions (Ca 2+)

How do T tubules and the SR work together?

Electrical signals travel through T tubules, triggering the SR to release Ca 2+ into the cytosol

What is the cytosol?

The fluid inside a cell where organelles and other cell structures are suspended

What happens when CA 2+ is released into the cytosol?

Calcium binds to troponin, allowing myosin to access actin’s binding sites

What is troponin?

A protein complex attached to actins regulatory system that binds Ca 2+ and controls tropomyosin

What is tropomyosin?

A long protein that lies along actin filaments and blocks myosin binding sites when muscle is relaxed

How does Ca+ allow actin and myosin to interact?

Ca 2+ binds to troponin, changing its shape and moving tropomyosin away from actin’s binding sites

What happens when Ca 2+ levels decrease?

Calcium returns to the SR, tropomyosin blocks actin’s binding sites again, and the muscle relaxes

What is the difference between muscle contraction and relaxation?

Contraction happens when Ca 2+ exposes actins binding sites; relaxation happens when those sites become blocked again