--- title: "Actin Filaments: Polarity, Treadmilling and Cell Shape" description: "Actin filaments have different plus and minus ends. During steady-state treadmilling, plus-end addition balances minus-end loss. Actin assembly near the membrane helps a crawling cell extend its leadi" canonical: https://lightmysky.com/learn/science/actin-filaments-polarity-treadmilling-and-cell-shape-mt_sJlYoA-BnA source: https://lightmysky.com/learn/science/actin-filaments-polarity-treadmilling-and-cell-shape-mt_sJlYoA-BnA.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Actin Filaments: Polarity, Treadmilling and Cell Shape Actin filaments have different plus and minus ends. During steady-state treadmilling, plus-end addition balances minus-end loss. Actin assembly near the membrane helps a crawling cell extend its leading edge. Subject: Science · Area: Organisms & Life Processes · Ages 20 to 21 Page: https://lightmysky.com/learn/science/actin-filaments-polarity-treadmilling-and-cell-shape-mt_sJlYoA-BnA ## Ready when they can - Explains treadmilling in terms of different critical concentrations at the two ends. - Links filament growth at the front of a cell to the direction it moves. - Predicts what a drug that blocks subunit addition does to a crawling cell. ## Lesson: Filaments that treadmill in place Actin filaments have plus and minus ends. Under comparable conditions, subunits add faster at the plus end. An end's critical concentration is the free-actin concentration where addition balances loss. In a basic ATP-actin model, this threshold is lower at the plus end. Between the thresholds, the plus end grows and the minus end shrinks. Below both thresholds, both ends shrink overall; above both, both grow. *(drawing: Read these as increasing concentration ranges. A shrinking end can still have individual addition events.)* During treadmilling, subunits enter mainly at the plus end and leave mainly at the minus end. At steady state, equal addition and loss rates preserve filament length. A concentration between the thresholds gives opposite net changes, but does not by itself prove equal rates. Subunits are added and removed at the ends; they do not walk along the filament. *(drawing: Turnover replaces subunits. Constant length does not mean an unchanged set of subunits.)* **Example.** In a crawling-cell model, actin assembly at the leading edge pushes the membrane outward. Adhesion to a surface and contraction help the rest of the cell follow. If the cell protrudes only at its left edge and these other steps work, it advances left. Many filaments rebuild the network; this is not one filament flowing through the cell. Actin commonly joins a filament with ATP bound. After joining, ATP is hydrolyzed to ADP and phosphate. Phosphate is released later. Older ADP-rich regions are generally less stable than regions containing ATP or ADP with phosphate still bound. These nucleotide changes help support turnover; end polarity and free-actin concentration also affect it. If a drug blocks all actin-subunit addition, existing filaments may still lose subunits. Assembly-driven protrusion cannot continue, so actin-based crawling is impaired. The cell may stop advancing or round up. Blocking addition does not instantly freeze every filament or every process in the cell. **Recap.** Steady-state treadmilling balances plus-end addition with minus-end loss. Actin assembly extends a leading edge; adhesion and contraction help the cell follow. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Apoptosis: Caspase Cascades and the Decision to Die](https://lightmysky.com/learn/science/apoptosis-caspase-cascades-and-the-decision-to-die-mt_Pi6c9yoT7w) ## Opens up - [Microtubules, Dynamic Instability and the Spindle](https://lightmysky.com/learn/science/microtubules-dynamic-instability-and-the-spindle-mt_RWhMJxOmUu)