International A Level Biology / Coordination in animals and plants
Phytochrome, transcription and plant responses revision notes
Edexcel International A Level Biology (YBI11), chapter 24: Coordination in animals and plants, specification point 8.11. Part of the full course.
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Phytochrome links light to changes in gene transcription, causing seedlings to develop differently and helping some seeds germinate. Red and far-red light can switch phytochrome between forms, so the final flash can determine the response.