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A-Level Biology OCR A

Tip Side Response Evidence Shoot Stimulus Mica Pass

Experiments on phototropism show the shoot tip detects light and produces a chemical messenger; removing or covering the tip blocks the response, and a physical barrier (mica) on the illuminated side allows hormone passage to the shaded side, causing bending.

Les expériences sur le phototropisme montrent que la pointe de la pousse détecte la lumière et produit un messager chimique ; l'élimination ou la couverture de la pointe empêche la réponse, et une barrière physique (mica) sur le côté éclairé permet le passage de l'hormone côté ombragé, provoquant la courbure.

Front Describe the evidence showing how a plant grows towards light.
Back Evidence for phototropism:
  • Shoot tip removed - no response - tip must either detect stimulus or produce messenger or both
  • Lightproof cover placed over intact tip - no response - light stimulus must be detected by tip
  • Mica on illuminated side of shoot allows hormone to pass only down the shaded side where it increases growth and causes bending
  • Mica on unlluminated side - movement of chemical down shaded side is prevented by it - no response
  • Gelatin block inserted beneath tip - since gelatin allows chemicals to pass through it but not the electrical messages, the bending which occurs must be due to chemical from tip

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