Cambridge O Level Biology · Syllabus 5090 · Coordination and Response in Plants
Auxin
What is Auxin?
A plant growth substance produced in the shoot tip. Auxin moves from the tip through the plant and stimulates cell elongation in shoots. When a directional stimulus such as one-sided light or gravity makes its distribution unequal, the two sides of the shoot elongate at different rates and the shoot bends.
This definition is part of the Coordination and Response in Plants chapter in Cambridge O Level Biology.
Auxin 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.
Common mistakes with Auxin
- 7. “Light destroys all the auxin on the illuminated side.” Why it is wrong: it is an absolute claim that the syllabus does not make and that no experiment described in this chapter tests. SayAuxin becomes unequally distributed, with more auxin present on the shaded side.
- 8. “Auxin pulls the shoot toward the light.” Why it is wrong: auxin exerts no force. Its stated action is to stimulate cells to elongate. SayAuxin stimulates cell elongation, so the side with more auxin grows longer and the shoot curves.
- 11. “Auxin is an enzyme.” Why it is wrong: it is neither a catalyst nor food nor energy nor a nerve impulse. It is a plant growth substance. SayAuxin is a plant growth substance produced in the shoot tip that stimulates cell elongation in shoots.
- 12. “The root must work the same way as the shoot.” Why it is wrong: applying “more auxin on the lower side, so it elongates more” to a root predicts the root curving upward, which is the opposite of what is observed. SayThe root grows toward gravity — positive gravitropism — which anchors the plant and reaches water and mineral ions. State the observable response and its advantage, and stop there.
- 16. “Cutting off the tip proves that auxin causes the bending.” Why it is wrong: no substance was detected, identified or measured. Removing a tip is also major damage, which may reduce growth for unrelated reasons. SayThe result supports the conclusion that the tip is involved in detecting or controlling the response. It does not identify the substance responsible.
Questions students ask about Auxin
Where is auxin made, and what does it do?
Auxin is a plant growth substance produced in the shoot tip. It moves from the tip through the shoot and stimulates cell elongation in shoots, so the side of a shoot with more auxin elongates more.
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.
Does light destroy auxin on the illuminated side?
No. The statement required is that auxin becomes unequally distributed, with more present on the shaded side. Claiming that light destroys the auxin is an absolute claim that the syllabus does not make and that the standard experiments do not test.
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.
What does removing a shoot tip actually prove?
It supports the conclusion that the tip is involved in detecting the direction of the light or in controlling the response. It does not identify auxin chemically, does not show where any substance moved, and cannot exclude the possibility that removing the tip reduced growth for other reasons.