Actin Filaments: Polarity, Treadmilling and Cell Shape · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Filaments that treadmill in place

Science · Organisms & Life Processes · ages 20-21
Name ______________________   Date ____________

  1. At a subunit level between the two critical values, what happens?

    • The plus end grows while the minus end shrinks
    • Both ends grow longer
    • Both ends shrink away

  2. Which end of an actin filament adds subunits fast?

    • The slow end near the back
    • The plus end
    • Both ends at equal speed

  3. Growth of fresh filament at the front edge sets the direction a crawling cell moves.

    Circle one:   True   False

  4. Why does ATP use keep treadmilling running one way?

    • ATP glues both ends shut
    • ATP actin adds readily but ADP actin leaves easily
    • ATP dissolves the whole filament

  5. A drug blocks all new subunit addition. What happens to the crawling cell?

    • It keeps crawling unchanged
    • It crawls twice as fast
    • It stops advancing and may round up

  6. A cell builds new filament at its left edge only. Which way does it crawl?

    • Straight upward
    • Staying in place
    • Toward the left edge

  7. The subunit pool drops below both critical values. What does the filament do?

    • It keeps treadmilling as before
    • It shrinks from both ends
    • It grows from both ends

  8. A filament holds constant length for a minute. A student says nothing is moving. What is the error?

    • Length proves all motion stopped
    • Filaments cannot hold constant length
    • Subunits can stream through while length stays fixed
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Filaments that treadmill in place W1-mt_sJlYoA-BnA-s1

  1. The plus end grows while the minus end shrinks · Between the two thresholds, one end builds while the other sheds.
  2. The plus end · The plus end is the fast growing end, and the minus end is slow.
  3. True · The cell flows into the new space that front growth opens.
  4. ATP actin adds readily but ADP actin leaves easily · The gap between easy adding and easy leaving drives the one way cycle.
  5. It stops advancing and may round up · With no front growth the push vanishes and treadmilling stalls.
  6. Toward the left edge · The cell moves toward the edge where fresh filament grows.
  7. It shrinks from both ends · Below both thresholds neither end can add, so both ends lose.
  8. Subunits can stream through while length stays fixed · Matched adding and loss hold length steady while subunits flow.
Worksheet · LightMySky