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

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

  1. In the basic ATP-actin model, free actin is between the two end critical concentrations. What are the net changes?

    • Plus grows; minus shrinks
    • Both ends grow; neither shrinks
    • Both ends shrink; neither grows

  2. Under comparable conditions, which actin-filament end adds subunits faster?

    • The minus end of the filament
    • The plus end of the filament
    • Both ends always add equally

  3. Actin assembly near a crawling cell's leading edge can push its membrane outward.

    Circle one:   True   False

  4. Which sequence of nucleotide changes helps produce older, less stable ADP-rich regions after ATP-actin joins a filament?

    • Phosphate release before ATP hydrolysis
    • ATP hydrolysis, then phosphate release
    • ADP conversion to ATP without energy input

  5. A drug blocks all actin-subunit addition in a crawling cell. What follows for its actin network?

    • Addition continues; protrusion remains unchanged
    • Both filament ends immediately stop all subunit loss
    • Addition stops; subunit loss can continue

  6. A cell protrudes only at its right edge. Adhesion and contraction work normally. In this model, which way does it advance?

    • Toward its upper edge
    • Toward its left edge
    • Toward its right edge

  7. In the basic ATP-actin model, free actin falls below both end critical concentrations. What is the net result?

    • Net subunit gain at both ends
    • Net subunit loss at both ends
    • Net subunit gain at the plus end

  8. Three subunits join a filament's plus end while three leave its minus end. Its length stays constant. What does this show?

    • Constant length proves every subunit stays in place
    • The two ends must always have identical reactions
    • Balanced turnover can keep length unchanged
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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. Plus grows; minus shrinks · The concentration is above the lower plus-end threshold and below the higher minus-end threshold.
  2. The plus end of the filament · Addition is faster at the plus end under comparable conditions. This does not mean that end grows at every concentration.
  3. True · Assembly can produce local protrusion. Adhesion and contraction help the rest of the cell follow.
  4. ATP hydrolysis, then phosphate release · ATP hydrolysis produces ADP and phosphate. Later phosphate release leaves ADP-rich regions that are generally less stable.
  5. Addition stops; subunit loss can continue · Blocking addition prevents assembly-driven protrusion, but existing filaments may continue losing subunits.
  6. Toward its right edge · The right edge is the protruding front. Working adhesion and contraction allow the rest of the cell to follow that front.
  7. Net subunit loss at both ends · Below an end's critical concentration, loss exceeds addition. Both ends therefore shrink overall.
  8. Balanced turnover can keep length unchanged · Three additions balance three losses. Constant length does not prove that the filament contains an unchanged set of subunits.
Worksheet · LightMySky