Cambridge O Level Biology · Syllabus 5090 · Coordination and Response in Plants
Cell Elongation
What is Cell Elongation?
The increase in length of an individual plant cell after it has been formed. Auxin stimulates cell elongation in shoots, so a side of a shoot receiving more auxin elongates more than the opposite side; the difference in length between the two sides bends the shoot.
This definition is part of the Coordination and Response in Plants chapter in Cambridge O Level Biology.
Cell Elongation in context
A tropism is a directional growth response: a plant organ grows unequally so that it ends up pointing toward or away from a stimulus that has a direction. Shoots are usually positively phototropic and negatively gravitropic; roots are usually positively gravitropic. In shoots the response is explained by auxin: it is produced in the shoot tip, it moves from the tip through the shoot, a one-sided stimulus makes its distribution unequal, and it stimulates greater cell elongation on the side where more of it collects. That side grows longer, so the shoot bends.
Questions students ask about Cell Elongation
Why does a shoot bend toward the light?
Light arriving from one side causes auxin to become unequally distributed, with more auxin on the shaded side. Auxin stimulates cell elongation, so cells on the shaded side elongate more and that side becomes longer than the illuminated side. The unequal growth curves the shoot so that its tip points toward the light. This is positive phototropism.
Why does a horizontal shoot grow upward?
Gravity acts as a directional stimulus, and auxin becomes more concentrated on the lower side of the shoot. Auxin stimulates cell elongation, so the lower side elongates more and becomes longer than the upper side, curving the shoot upward. This is negative gravitropism.
Why does the shoot bend away from the side with more auxin?
Because more auxin means more cell elongation, which makes that side of the shoot longer. A structure that is longer along one edge must curve, with the longer edge on the outside of the curve, so the tip ends up pointing away from that side.
Can the auxin explanation be used for roots?
Not unless the question supplies the root mechanism. The syllabus states that auxin stimulates cell elongation in shoots. For a root, describe the observable response — it grows toward gravity — classify it as positive gravitropism and give its advantage: anchorage and the uptake of water and mineral ions.