---
title: "Actin Filaments: Polarity, Treadmilling and Cell Shape"
description: "Actin filaments have two different ends, so subunits can add at one end and leave at the other while the filament length stays the same. Regulating that turnover lets a cell push out a leading edge an"
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: 2026-09-12
---

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# Actin Filaments: Polarity, Treadmilling and Cell Shape

Actin filaments have two different ends, so subunits can add at one end and leave at the other while the filament length stays the same. Regulating that turnover lets a cell push out a leading edge and crawl.

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

An actin filament has two ends with different manners. The plus end adds subunits fast while the minus end loses them. Each end has its own critical concentration, so at a subunit level between the two, the plus end grows while the minus end shrinks at the same pace.

**Example.** Picture a crawling cell reaching forward. It builds fresh filament at the front edge near the membrane and takes old filament apart at the back. The filament keeps about the same length while its subunits stream forward, so growth at the front sets the direction of travel.

Energy keeps the cycle turning one way. Fresh subunits carry ATP and add on readily, then break the ATP down and leave more easily as ADP actin. A drug that blocks new additions freezes the front edge, treadmilling stalls, and the crawling cell stops and may round up.

**Tip.** When you explain treadmilling, name both ends, state what each end does at your chosen concentration, and finish by linking front growth to the way the cell moves.

**Recap.** Treadmilling is balanced growth at the plus end with matched loss at the minus end, and that turnover pushes the cell forward.

## 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)