Topic 14.5 · Supplement
Auxin and the Chemical Control of Shoot Growth
Phototropism and gravitropism in shoots are both examples of chemical control of plant growth, and for this syllabus that chemical is a single named hormone: auxin. The whole mechanism reduces to one idea — auxin ends up unevenly spread across the shoot, and the side with more of it grows faster.
The required auxin model
Auxin is made in the shoot tip, and it diffuses through the plant from there. Light and gravity can each cause auxin to become unequally distributed across the shoot, and in shoots specifically, auxin stimulates cell elongation. That last point is worth holding onto precisely: auxin causes cells to get longer, never to divide more — describing tropic bending as “more cells” forming on one side is describing the wrong process. Unequal auxin distribution produces unequal cell elongation on the two sides of the shoot, one side grows longer than the other, and that difference is what makes the whole shoot bend.
Auxin in phototropism
If light reaches a shoot mainly from one side, auxin becomes more concentrated on the shaded side. Because auxin stimulates cell elongation, cells on the shaded side elongate more than cells on the lit side, so the shaded side grows faster than the lit side, and the shoot bends towards the light — positive phototropism, in the terms introduced on the tropic responses page.
Auxin in shoot gravitropism
When a shoot is placed horizontally, gravity causes auxin to become unequally distributed with more accumulating on the lower side. Auxin stimulates greater cell elongation there, so the lower side becomes longer than the upper side, and the shoot curves upwards, away from gravity — negative gravitropism.
It’s worth being deliberate about which side gets the extra auxin in each case, since the two mechanisms are easy to swap under pressure: shaded side for phototropism, lower side for gravitropism. Both follow the same underlying rule — more auxin, more elongation, more growth on that side — just triggered by different stimuli.
What this mechanism does and doesn’t explain
Unequal auxin distribution is what turns a directional stimulus — light from one side, or gravity acting on a shoot lying horizontally — into a directional growth response, and it does so without any muscles or nervous control at all: cell elongation alone is enough to reorient the whole structure. Auxin doesn’t “pull” the shoot in any mechanical sense; the bending is entirely a consequence of one side elongating faster than the other.
For this syllabus, the required auxin mechanism is specifically limited to its role in shoot growth — a detailed root-auxin mechanism isn’t required, even though the Core description of tropic responses on the tropic responses page still covers both roots and shoots in general terms. The required transport wording is diffusion from the shoot tip; active transport or phloem transport aren’t the terms Cambridge is looking for here, so it’s worth keeping your answer to the diffusion model this page describes.