Biology Chapter 39 Objectives

Biology Chapter 39 Objectives - Chapter 39 2. The light...

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Chapter 39 2. The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways. One pathway uses cGMP as a second messenger that activates a specific protein kinase. The other pathway involves an increase in cytoplasmic Ca2+ that activates another specific protein kinase. 3. Receptors are sensitive to weak environmental and chemical signals. Some de- etiolation responses are triggered by extremely low levels of light. Second messengers which are small, internally produced chemicals that transfer and amplify the signal from the receptor to other proteins that cause the response. 4. The first mechanism is Transcriptional regulation which is the stimulation of the transcription of mRNA for the enzyme. Transcription factors bind directly to specific regions of DNA and control the transcription of specific genes. The second mechanism is post-translational modification which involves activating existing enzyme molecules. 5. Researchers have learned that the activity of hormones is affected by proteins that are either newly transcribed or activated by by phosphorolation during de-etiolation. For example, auxin and brassinosteriods, that enhance the stem elongation decreace following phytochrome activation. 6. Charles and Francis Darwin hypothesized that only if the tip of the coleoptile was present could the seedling bend towards the light. In their experiment, they removed the tip and the seedling did not bend. They also covered the tip with an opaque cap which resulted in no bending toward the light. They concluded that the tip of the coleoptile was responsible for sensing light. Peter Boysen-Jensen postulated that some signal was transmitted downward from the tip of the elongating region to the coleoptile. In his experiment, he separated the tip from the remainder of the coleoptile by a block of gelatin which would prevent cellular contact but allow chemicals to pass. These seedlings responded normally and bent towards the light. But if the tip was experimentally segregated from the lower coeloptile by an impermeable barrier no phototropic response occurred. He concluded that his hypothesis was correct and that a chemical signal is transmitted down the root. Frits Went believed in the same hypothesis as Boysen-Jenson but wanted to know what the chemical messenger for phototropism was. He hypothesized that asymmetrical distribution of a growth-promoting chemical causes a coleoptile to grow toward the light. In his experiment modeled after Boysen-Jenson's, he placed coleoptiles in the dark and removed their tips putting some tips on agar blocks. The control coleoptile had a block with no chemicals. The others had chemicals on the block either centered to have even distribution or offset to increase the concentration on one side. Went concluded that a coleoptile curved toward light because its dark side ha a higher concentration of he growth-promoting chemical, which he named auxin. 7. Auxin: produce in the embryo of seed, meristems of apical buds and young leaves;
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This note was uploaded on 08/29/2009 for the course BIOL 101 taught by Professor Sakji during the Fall '08 term at Linn Tech.

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Biology Chapter 39 Objectives - Chapter 39 2. The light...

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