Auxin: the message from the tip — answer key

1. AQ1 · Practice

Model answer. Auxin is made in the shoot tip. It diffuses down from the tip into the region below, where it acts.

A full answer includes

• I chose the shoot tip.

Also accept

• in the shoot tip

Watch for

• “In the roots.” — The auxin that controls shoot growth is made in the shoot tip and diffuses down from it.
• “In the leaves, by photosynthesis.” — Auxin is not a product of photosynthesis. In a shoot it is made in the tip.
• “In the region below the tip, where the curve forms.” — The curve forms below the tip, but the auxin is made in the tip and diffuses down to that region.

2. AQ2 · Practice

Model answer. Auxin stimulates cell elongation: the young cells below the tip take in water and grow longer. The more auxin a cell receives, the more it elongates.

A full answer includes

• I chose: it stimulates them to elongate.

Also accept

• it stimulates cell elongation

Watch for

• “It makes them divide faster.” — The curve forms where cells are getting longer. Auxin stimulates cell elongation, not division.
• “It makes them photosynthesise faster.” — Auxin controls growth: it makes the cells longer, not faster at making food.
• “It makes the cells on the lit side shrink.” — The cells on the lit side still grow, just less than those on the shaded side. Auxin stimulates elongation.

3. AQ3 · Practice

Model answer. The south side. Light from the hole in the north wall makes the auxin unequally distributed: more of it moves to the shaded, south side as it diffuses down. That side elongates more, so the shoot curves north, towards the hole.

A full answer includes

• I chose the south side, away from the hole: the shaded side.

Also accept

• the south side, away from the light

Watch for

• “The north side, facing the hole.” — That is the lit side. Auxin moves to the shaded side.
• “Evenly on every side.” — Only even light leaves the auxin even. Light from one side makes it unequally distributed.
• “In the tip only; none of it moves down.” — Auxin diffuses down from the tip in every case. Light from one side makes more of it go down the shaded side.

4. AQ4 · Practice

Options as numbered on the pupil sheet: 1 — The shaded cells elongate more than the lit cells.; 2 — The shoot curves towards the light.; 3 — Light reaches the tip from one side, and the tip detects its direction.; 4 — The cells on the shaded side receive more auxin than those on the lit side.; 5 — Auxin made in the tip moves to the shaded side as it diffuses down..

Model answer. The tip detects the direction of the light; auxin made in the tip moves to the shaded side as it diffuses down; the cells on the shaded side receive more auxin; they elongate more than the lit cells; so the shoot curves towards the light.

A full answer includes

• I put the tip detecting the light first and the curve last.
• I put "auxin moves to the shaded side" before "the shaded cells elongate more".

Also accept

• tip detects; auxin to the shaded side; more auxin there; more elongation; curve towards the light

Watch for

• “The lit cells elongate more and push the shoot over.” — The shaded cells elongate more. The longer side ends up on the outside of the curve, so the shoot turns towards the light.

5. AQ5 · Practice

Model answer. Both responses are controlled by a hormone, auxin: it is made in the shoot tip, travels down by diffusion, and changes how much the cells below elongate. That is chemical control of growth.

A full answer includes

• I chose: a hormone made in one part travels to another part and changes how much its cells grow.

Also accept

• a hormone made in one part travels to another and controls growth there

Watch for

• “Chemicals from the soil steer the shoot.” — Mineral ions from the soil are needed for growth, but they do not control its direction. A hormone made in the shoot tip does.
• “Nerve impulses carry the message from the tip.” — Plants have no nerves. The message is a chemical, auxin, that diffuses from the tip.
• “Light and gravity are the chemicals.” — Light and gravity are the stimuli. The chemical is auxin, a hormone made in the tip.

6. AQ6 · Practice

Model answer. B. Gravity makes the auxin collect on the lower side. The cells there receive more auxin and elongate more, so the lower side grows longer and the shoot curves upwards, away from gravity.

A full answer includes

• I chose row B.
• I did not choose row C: in a shoot, the side with more auxin elongates more.

Also accept

• B

Watch for

• “Row A: auxin collects on the upper side, away from gravity.” — Gravity makes the auxin collect on the lower side.
• “Row C: the side with more auxin grows less.” — In a shoot, auxin stimulates cell elongation: the side with more auxin elongates more.
• “Row D: the upper side gets more auxin, but the lower side grows more.” — The side that elongates more is the side that receives more auxin, the lower side.

7. AQ7 · Practice

Model answer. It grows straight up. With light from every side, the auxin from the tip stays evenly distributed as it diffuses down, so the cells on every side elongate by the same amount and the shoot does not curve.

A full answer includes

• I predicted that it grows straight up.
• I wrote that the auxin is spread evenly because no side is shaded more than another.
• I wrote that every side elongates by the same amount.

Also accept

• straight up: auxin even, equal elongation

Watch for

• “It does not grow at all, because there is no one-sided light.” — Auxin still diffuses down from the tip and stimulates elongation; the shoot grows taller, just straight.
• “It curves towards the brightest lamp.” — The room is lit evenly, so no side is shaded and the auxin is not moved to one side.

8. AQ8 · Practice

Options as numbered on the pupil sheet: 1 — grows very little and does not curve; 2 — curves to the left; 3 — curves to the right; 4 — grows straight up, getting longer.

Model answer. X: auxin from its block diffuses into the right side, so the right side elongates more and the stump curves to the left. Y: auxin reaches every side equally, so the stump grows straight and longer. Z: plain agar carries no auxin, and the stump has no tip to make any, so it grows very little and does not curve.

A full answer includes

• I matched X with curving to the left.
• I matched Y with growing straight up, getting longer.
• I matched Z with growing very little and not curving.

Also accept

• X curves left; Y straight and longer; Z very little, no curve

Watch for

• “X curves to the right, towards the block.” — The side under the block elongates more, so it ends up on the outside of the curve: the stump curves away from the block.
• “Z curves to the left like X.” — Plain agar has no auxin in it, so nothing makes the right side grow more.
• “Y does not grow, because its tip has been cut off.” — The block over Y carries auxin from the tips, so its cells still elongate, evenly.

9. AQ9 · Practice

Model answer. Correct: auxin is made in the shoot tip, it diffuses from the tip into the region below, and light from one side makes more of it move to the shaded side. It stimulates cell elongation, not division, and in a shoot on its side it collects on the lower side.

A full answer includes

• I chose: made in the shoot tip; diffuses from the tip into the region below; more moves to the shaded side.
• I did not choose "divide faster": auxin stimulates cell elongation.
• I did not choose "upper side": gravity makes it collect on the lower side.

Also accept

• made in the tip; diffuses down; more moves to the shaded side

Watch for

• “It makes the cells on one side divide faster.” — Auxin stimulates cell elongation: the curve forms where cells get longer.
• “Gravity makes it collect on the upper side of a shoot on its side.” — It collects on the lower side, which is why the lower side grows longer and the shoot curves up.

10. AT1 · Apply

Model answer. The message from the tip crossed the gelatin, which chemicals can diffuse through, but not the mica, which they cannot. So the message is a chemical, auxin, that diffuses from the tip into the region below. With mica in the way, no auxin reached the cells below, so the two sides could not elongate by different amounts and the shoot did not bend.

A full answer includes

• I wrote that the message from the tip passes through gelatin but not through mica.
• I wrote that the message is a chemical, auxin, that diffuses down from the tip.
• I wrote that with mica no auxin reached the cells below, so the two sides could not elongate by different amounts.

Also accept

• the message is a chemical that diffuses from the tip: auxin

Watch for

• “The mica stopped the light reaching the tip.” — The mica was between the tip and the stump; the tip was still lit. The mica blocked a chemical moving down.

11. AT2 · Apply

Model answer. The light is detected at the tip, where auxin is made, and more of the auxin moves to the shaded, left side. The auxin diffuses down into the region below, where the cells respond to how much auxin they receive, not to light. The left side elongates more, so the shoot curves to the right even though that region is covered.

A full answer includes

• I wrote that the tip detects the light and more auxin moves to the shaded, left side.
• I wrote that the auxin diffuses down into the region below.
• I wrote that the cells there respond to the amount of auxin, not to light, so the left side elongates more.

Also accept

• the tip detects; auxin diffuses down; the cells respond to auxin, not light

Watch for

• “It curves because some light gets under the band.” — The band is light-proof. The region below does not need light, because it responds to the auxin coming from the tip.