chap 39 - 1. the hormone reaches a receptor on the outside...

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1. the hormone reaches a receptor on the outside of the thyroid cells and binds to it. This causes a signal transduction pathway to start. The second messenger is Ca2+ which then binds to another enzyme that controls the release of thyroxine. Injecting the hormone would not necessarily work because the receptor is on the outside of the thyroid cell. The pathway can only be turned on by the hormone binding to the receptors, triggering a response. 2. Summarize the experiments of the Darwin’s, Boysen-Jensen, and Went. Describe how the experiments were set up, and what the results and conclusions of each were. The Darwin’s observed that a grass seedling would not bend toward the light it its tip were removed or covered by an opaque cap. They postulated that a signal must be transmitted from the top to the elongating region of the coleoptile. They came upon these conclusions with their different mechanisms to test the grass seedlings communication and responses. The four different way were, with the tip removed, tip covered by opaque cap, tip covered by transparent cap, and base covered by opaque shield. They concluded that the tip was responsible for sensing the light. The growth response, however, occurred downward from the tip. Boysen and Jensen demonstrated that a signal was a mobile substance, capable of being transmitted through a block of gelatin separating the tip from the rest of the coleoptile. A gelatin block was inserted into one grass seedling separating the tip, and a piece of mica was inserted into another. The gelatin allowed for passage of chemicals but prevented cell-cell recognition. Went placed coleoptile tips on blocks of agar to extract the chemical messenger. A block that was centered on top of the coleoptile causes the stem to grow straight upward. If the block was placed off center the coleoptile began to grow away from the side with the agar, as though growing toward the light (even though this was conducted in the dark). He concluded that the chemical produced in the tip promoted growth and that it was in higher concentration on the side away from the light. 3.
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chap 39 - 1. the hormone reaches a receptor on the outside...

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