Lecture 18 BIO 311D 2nd EditionOutline of Last Lecture I. Plant signals (within a plant, for now)II. Seed germination Outline of Current Lecture I. ReviewII. Plant tropismsIII. Signaling IV. Design an experiment Current LectureI. Review (Iclicker questions)1. After dormancy is broken by a change in environment, which of these happens first?- Water enters seed 2. The hypothesis for particular experiment stats that plants with greater density of stomata loses more water in transpiration. Which of these is the best statement of the null hypothesis?- The density of stomata has no effect on the rate of transpiration3. Water will tend to move into a cell with what combinations of conditions?- Higher solute concentration and lower fluid pressure II. Plant tropisms- Growth of young seedlings uses energy and nutrients from endosperm - How does the young plant moves after being knocked down - How could your plant sense gravity? By what mechanism could the plant re-orient itself?Growth hormones (mechanism)Grains of starch-like things (heavier density) statolith help plants detect the pull of gravity Sedimentation of specialized plastids that contain starch, called amyloplasts accumulates on the downside of the plant - How could your plant sense light? By what mechanism could the plant re-orient itself?Photoreceptors (receptor molecules that can collect light) Plant starts growing towards the light through cell elongation Phototropism- movement toward or away from light III. Signaling- Auxin is a hormone that can trigger cell elongation - How could the same input (auxin) result in different output (direction of growth)?Shoot: cells are elongated toward the lightRoot: cells are elongated away from the light- Control of cell responses – signal transduction pathway - Think of a pathway by which cells could elongate in response to auxin binding to a signal receptor protein. Name some steps that would be necessary… (Include a step n which the cell wall loosens)Auxin binds receptor (signal transduction)proton pump loosen cell wall or increase in solute water enters the cell cell elongates - Acid-Growth hypothesis - Change in pH loosens the cross links within the cell wall - Water comes in- Cell elongates- When the signal is removed, we stop it. Stop the proton pump and everything goes back to normal - You have a mutant that doesn’t respond with phototropism. 1. May respond to auxin wrongly 2. Enzymes in the cell wall don’t respond to the pH change- The tip of the plant (chemical signal) IV. Design an experiment: The distribution of a chemical signal is what causes phototropism Independent: distribution of chemical signal Dependent: phototropism Controls: a plant that hasn’t been manipulated; cut off the tip and put it back
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