Flashcards · 5090 · Topic 15

Coordination and response in plants flashcards

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Phototropism and gravitropism (O Level): Phototropism: a response in which parts of a plant grow towards or away from light. Gravitropism: a response in which parts of a plant grow towards or away from gravity.
Responses of shoots and roots to gravity and light: Shoot: away from gravity (negatively gravitropic), towards light (positively phototropic). Root: towards gravity (positively gravitropic).
The four syllabus points about auxin: Made in the shoot tip; spreads through the plant from the shoot tip; unequally distributed in response to light and gravity; stimulates cell elongation.
Why does a shoot lit from one side bend towards the light?: More auxin collects on the shaded side; cells there elongate more than on the lit side, so the shaded side grows longer and the shoot bends towards the light.
Why does a horizontal shoot curve upwards?: Gravity causes more auxin on the lower side; lower-side cells elongate more, so the shoot curves upwards, away from gravity.
A shoot tip is covered with a light-proof cap and lit from one side. Result?: It grows straight up. Auxin is still made but is not unequally distributed, so all sides elongate equally.
Clinostat: A device that slowly rotates seedlings so gravity (or one-sided light) acts on all sides equally. It is the control in a gravitropism investigation.
Why is a gravitropism investigation kept in the dark?: So light cannot cause a response: gravity must be the only one-sided stimulus.