★★☆ Moderate

🧬 Cytoskeleton & Cell Motility

Actin filaments polymerize at the cell's leading edge and depolymerize at the rear — a process called treadmilling. Myosin II motors slide anti-parallel filaments, generating contractile force. Together they power cell crawling at speeds of 0.1–1 µm/min.

ATP level: 70% Signal gradient: 60% Myosin activity: 50%

Velocity: — µm/min

Filaments: —

Protrusion: — µm/min

Retraction: — µm/min

v_cell = v_protrusion − v_retraction │ v_prot ≈ k_on·[ATP] │ v_retr ≈ k_myo·[Myosin II]

How Cells Move

Cell motility depends on the dynamic actin cytoskeleton. At the leading edge (lamellipodia), the Arp2/3 complex nucleates new branched actin filaments that push the membrane forward. Filaments grow by adding ATP-actin monomers at their barbed end (k_on ≈ 11.6 µM⁻¹s⁻¹) and shrink at their pointed end.

Meanwhile, myosin II bipolar filaments bind actin stress fibers and slide anti-parallel filaments past each other, contracting the cell body and pulling the rear forward. Focal adhesions (yellow dots) anchor the cell transiently to the substrate, providing traction.