Bio 101 Day 19

Bio 101 Day 19 - Plant Growth and Development (Growth of a...

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Plant Growth and Development (Growth of a plant in response to a stimulus is called a tropism) Chapter 37 Phototropism Growth in response to the direction of light --Light (<500 nm) activates photoreceptors called phototropins; subsequent phosphorylation of these receptors leads to signal cascade --This photochemical signal is then translated into a chemical (hormonal) signal --Shoot bends to the light --Similar mechanism for stomata opening in response to light Gravitropism Growth in response to gravity --Most stem tips grow away from earth’s center (negative gravitropism) --Most root tips show positive gravitropism (NB: root cap removal)—amyloplasts in specialized cells collect at the bottom of the root cap --So that regardless of orientation—stem up, root down (root cap) --Left: normal Right: seedling Bending=auxin (auxein=to increase) Thigmotropism --Growth in response to contact with a solid object --Mechanical stimulus helping a climbing plant twine or curl --Production of tendrils --e.g. vine Plant Hormones --Organic compounds produced by plants --:<10 (-6) mol/l (uM)
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--Act as highly specific chemical signals --Small molecules with paracrine activity --Elicit responses that regulate growth and development: --Stimulate at a particular concentration --Inhibit at another concentration --e.g. auxin MECHANISM FOR AUXIN Darwins’ Experiments (1870s) Phototropism experiments on grass seedlings --Results: --Coleoptide exposed to light, bends toward light --Covered coleoptile tip, plant didn’t bend --Cut off coleoptiles tip, plant didn’t bend --Bottom of coleoptiles shielded from light, only coleoptiles tip bent --Conclusion:
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--From these experiments, it seemed clear that the stimulus (light) was being detected at one location (the coleoptiles tip), while the response (bending) was being carried out at another and that the tip was in some way, communicating with cells in the region of elongation Fritz Went’s Experiments (1920s) (Elucidated the hormonal mechanism involved) --Isolated the phototropic hormone (in Darwin’s expt) from oat coleoptiles --Firstly, he removed coleoptiles tip, placed it on agar blocks
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Bio 101 Day 19 - Plant Growth and Development (Growth of a...

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