Bot 305; Test #3 Name:
Fall 1997
Provide succinct answers in the space provided after each question. Unless otherwise noted each question is worth 4 points.
1. (a) What activity of a fungus could signal to a plant that it was being attacked by that fungus?
Ans: The breakdown products of the fungus' digestion of the wall can serve as signal of attack.
(b) What response in the infected leaf of a resistant plant prevents the spread of a viral infection?
Ans.: Hypersensitive response--controlled cell death in localized necrotic lesion.
(c) What does the "A" stand for in SAR, and explain how "A" happens.
Ans: Acquired; acquisition happens when it is induced by hormone signal from site of infection.
2. (a) Plants produce hydrogen peroxide in response to pathogen attack. In what two ways does hydrogen peroxide provide protection to plants?
Ans.: Kills pathogens; cross-links cell wall (making it "tougher").
(b) Plants produce salicylic acid (SA) in response to pathogen attack. Is SA itself a poison? If so how does it poison pathogens? If not, explain what SA does to promote defense against pathogens.
Ans.: No, rather it serves as a signal to induce the production of PR proteins.
3. In Fig. 1: (a) How was plant N modified so that it did not accumulate salicylic acid?
Ans.: It was genetically transformed by the addition of a gene that makes an enzyme that breaks down SA to catechol.
(b) Why was the X/X graft combination done?
Ans.: Test to see if hormone signal can cross graft border.
(c) If the leaf on the rootstock N plant is inoculated with a virus, does the X leaf grafted onto this rootstock plant show resistance to subsequent viral infection? What hypothesis does this result disprove?
Ans.: Yes; this shows that SA is not the systemic signal, because the signal can be exported from a leaf that does not accumulate SA.
4. (a) Diagram the two major portions of a membrane lipid, and for each indicate its affinity for water.
Ans.: head is hydrophilic
tail is hydrophobic
(b) Phospholipids typically do not flip flop from one layer of a membrane to the other. Why?
Ans.: Because the hydrophilic head group is excluded from the hydrophobic middle region of the membrane, so it cannot cross this region.
(c) What is the difference in shape between a saturated and an unsaturated hydrocarbon tail of a lipid, and how does this shape difference in membrane lipids affect the phase transition temperature of a membrane?
Ans.: Unsaturated has kink or bend, so it makes membrane more disordered, and thus lowers the phase transition temperature.
5. Name 5 different general functions for membrane proteins, and for each function give a specific example of a membrane protein which has that function, citing a protein we have studied in this course.
Ans.: receptor (hormone receptor); cell-cell junction (plasmodesmatal protein); enzyme (PLC); cytoskeleton-ECM connector (integrin); transport (calcium channel or pump).
6. (a) Frye & Edinin labeled membrane proteins on mouse cells with a red dye and the membrane proteins on human cells with a green dye, then they fused these cells to form mouse-human hybrid cells. What question about membrane proteins were they testing with this experiment and what answer did they get?
Ans.: Can membrane protein move around in membrane? Answer they got is Yes.
(b) Why do certain membrane proteins move around in mosaic clusters, keeping a fixed positional relationship to neighboring proteins?
Ans.: Because they are connected or anchored to surrounding proteins by proteins on the cytoplasmic face of the membrane.
7. (a) What is a key advantage of having several steps (instead of only 1) in a signal transduction chain?
Ans.: More steps = more amplification.
(b) Describe 3 things that the subunit of a G-protein does after it is "turned on" by an activated receptor.
Ans.: release beta & gamma; activate enzymes (e.g., PLC); convert bound GTP to GDP.
(c) What are the two major products of PLC activity and how does each one promote signal transduction?
Ans.: IP3, which opens calcium channels & DAG which activates protein kinase.
8. (a) How is a G-protein returned to its inactive state?
Ans.: It converts its GTP to GDP.
(b) After IP3 opens calcium channels, how is the [Ca2+] of the cytoplasm returned to its normal low level?
Ans.: By calcium pumps activated by calcium activated calmodulin.
(c) What activity reverses the action of a protein kinase?
Ans.: protein that removes phosphate groups added by the kinase (=phosphatase)
9. (a) What chemical mimics an activated receptor in activating a G-protein?
(b) How could cholera toxin substitute for Pfr in initiating a signal transduction chain?
Ans.: Keeps G-protein turned on by blocking its ability to convert GTP to GDP.
(c) How does lithium interfere with a signal transduction chain?
Ans.: blocks recyling of IP3 so that PIP2 is not replenished.