Questions for Biology IIB (SS 2006) Wilhelm Gruissem

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1 Questions for Biology IIB (SS 2006) Plant biology Wilhelm Gruissem The questions for my part of Biology IIB, Plant Biology, are provided for self-study and as material for the exam. Please note that the text of the questions may be different in the exam, but the scope of the question will be the same. Also, questions may be given in the form of multiple choice. The questions below are provided in English for the reason shown in the following Gary Larson cartoon. The questions in the exam will be provided in English and German. Photosynthesis and carbohydrate metabolism 1. Photosystem II has a special chlorophyll pair (P680) that delivers electrons to pheophytin, the primary electron acceptor. The electron transfer between P680 and pheophytin is called charge separation and is very rapid (1 to 3 x sec), while subsequent transfer reactions take longer. Explain why a rapid charge separation between P680 and pheophytin is necessary. 2. Photosynthetic bacteria have a single photosystem, while algae and higher plants have two photosystems (PSI and PSII). What is the advantage of two photosystems?

2 Questions for Biology IIB (SS 2006), cont. Plant Biology 3. You are investigating the mechanisms of photosynthesis using isolated chloroplasts that are suspended in a suitable buffer. Using sensitive microelectrodes you find that in the dark, the ph in the thylakoid lumen and the stroma is very similar. After illumination, you measure a significant difference in ph between the two compartments, with ph 5 in the thylakoid lumen and ph 8 in the stroma. How do you explain this 1000-fold ph difference in the light between the two compartments? 4. Ribulose-1,5-bisphosphate carboxylase is an abundant plant enzyme required for the fixation of CO 2. a. Under ambient conditions (O 2, light, temperature), which are the two most important parameters that favor the forward reaction in CO 2 -fixation? b. Raising the concentration of O 2 from an ambient concentration to 40% reduces the rate of photosynthesis to 50%, while reducing the concentration of O 2 from an ambient concentration to 2% increases the rate of photosynthesis two-fold. Explain this observation. 5. What is one aspect common between the Calvin-Benson cycle (photosynthesis) and the citric acid cycle (respiration)? 6. Why has maize become an important commercial crop in the mid-west of the United States, while wheat is more prevalent in Canada and northern Europe? 7. Explain the major differences in morphology and CO 2 uptake between plants that use C4 photosynthesis and plants that use the crassulacean acid metabolism. 8. Imagine you have to design a plant and make decisions on how the sugars glucose, fructose and sucrose should be transported in the phloem from the leaf to the root. What is your decision and why did you make it? 9. After sunrise photosynthesis is activated and the triose-phosphate pool fills up rapidly. Although triose-phosphates from the pool are used by the plant cell for metabolic processes (e.g., amino acid biosynthesis, respiration), these reactions use only a small fraction of the trioses produced by photosynthetic CO2 fixation. How does the plant assure that photosynthesis continues during the day? 10. Which five intermediates of glycolysis and the citric acid cycle are substrates for the synthesis of all 20 amino acids in plants? 11. What are the reaction products of ribulose-1,5-bisphosphate carboxylase with a. ribulose-1,5-bisphosphate and CO 2? b. ribulose-1,5-bisphosphate and O 2? 12. What is the primary fixation product of CO 2 in C4 plants? 2

3 Questions for Biology IIB (SS 2006), cont. Plant Biology 13. Metabolite pools in plant cells have important functions during photosynthesis and for the regulation of biochemical pathways. a. What are the primary functions of the hexose pools in the cytoplasm and in the chloroplast? b. Name three biosynthesis pathways for which the triose-phosphate/pentosephosphate pool provides the substrates. 14. Explain why C4 plans can fix CO 2 more efficiently at higher temperatures. 15. During illumination the thylakoid lumen has a ph value of 5 (ph5), and the stroma side of the thylakoid membrane has a ph value of 8 (ph8). Without illumination, the ph difference disappears. a. Which enzyme complex is responsible for the ph gradient in the chloroplast? b. Explain the disappearance of the ph gradient in the dark. 3

4 Questions for Biology IIB (SS 2006) Plant biology Wilhelm Gruissem Respiration and lipid metabolism 1. The alternative oxidase is a unique plant mitochondrial enzyme that transfers electrons to oxygen, bypassing complex III and IV in the electron transport chain. a. What happens to the energy that is not used for proton pumping? b. Why would the enzyme be of advantage to plants but not animals? 2. Many plants store pre-dominantly oils and triacylglycerols in their seeds rather than starch. What would be the advantage? 3. A biological membrane is composed of lipids and proteins. a. How would two membranes differ in their fluidity at 20ºC if they would contain either only saturated fatty acids or only unsaturated fatty acids? b. What would you predict for the movement of proteins in these two types of membranes? c. What is the mechanism that can explain why cold-sensitive plants wilt after they have been exposed to low but above-freezing temperatures? 4. Animal cells produce only eukaryote-type lipids, while plants produce both eukaryote- and prokaryote-type lipids. How do you explain the difference? 5. Explain the principal steps in fatty acid biosynthesis. 6. Glycolysis and mitochondrial respiration are mainly responsible for energy production in non-photosynthetic plant organs. What are the primary respiration substrates in: germinating seed cells (e.g., maize or canola)? leaf cells (e.g., mesophyll cells)? root cells? 7. Explain the three aspects in which mitochondrial electron transport differs between plant and animals. 8. Plants use both NADH and NADPH as redox molecules. a. Which biochemical processes use NADH or NADPH? b. What is the difference between NADH and NADPH

5 Questions for Biology IIB (SS 2006), cont. Plant Biology 9. Which problems do you expect to occur in plant mutants that cannot synthesize the following lipids? a. Phosphatidylcholine b. Galactolipids c. Triacylglycerine 2

6 Questions for Biology IIB (SS 2006) Plant biology Wilhelm Gruissem Hormones 1. Which classes of molecules provide the substrates for the synthesis of all known non-peptide plant hormones? 2. Agrobacterium tumefasciens transforms plant cells with genes that encode enzymes for hormone biosynthesis and synthesis of complex compounds that can be metabolized only by the bacterial cell. a. Which hormomes do you predict will be synthesized by the enzymes to explain the production of tumors (crown gall tumors)? b. In what ratio do you predict will the transformed cells produce the hormones to develop the non-differentiated cells in the tumor? c. How can you test your prediction? 3. You insert into the genome of tobacco plants the ipt gene, which encodes the enzyme isopentenyl transferase under the control of a senescence-specific promoter (i.e., a promoter that is activated only during senescence). As a result, you find that senescence is strongly delayed in the transformed plant but not in your control plant. How do you explain this observation and what hormones do you predict will be antagonized by the ipt gene? 4. During seed germination, GA is produced by the embryo and activates genes in aleurone cells that encode enzymes required for starch mobilization. In order to test if it is indeed GA that is required, you take several germinating seeds and treat one of each with one of the other plant hormones. a. Which other hormone do you predict will antagonize the GA-induced gene activity in the germinating seed? b. Why do you think this hormone has the predicted effect? 5. In the late 19 th century, farmers stored apples for the winter in large warehouses. To keep the apples from freezing (which would destroy their texture), they used kerosene stoves to keep the temperature above 0ºC. But they found that the apples ripened quickly. Why? Using genetic engineering technology, which enzymes would you target to develop apples in which the ripening process in delayed? 6. Auxin induces the expansion of cells by softening the cell wall through (among other processes) reduction of the ph. a. Which enzyme is required for the ph change in the cell wall? b. Explain two hypotheses by which the regulation of this enzyme could be explained.

7 Questions for Biology IIB (SS 2006), cont. Plant Biology 7. Gibberellins (GA), abscisic acid (ABA), cytokinins and ethylene (C 2 H 4 ) have different functions during plant development. a. For each hormone, please describe two processes (briefly) that are controlled by the hormone. b. Which of the hormones show antagonistic interactions? 8. Assign the plant hormones abscisic acid (ABA), auxin (IAA), brassinosteroid (BR), cytokinin (CK), ethylene (C 2 H 4 ), gibberellic acid (GA), jasmonic acid (JA) and salicylic acid (SA) to the following processes in which these hormones have a primary function: a. Development b. Cell elongation c. Fruit ripening d. Pathogen defense and resistance e. Seed dehydration f. Stomata opening and closing 9. Auxin (indole-3-acetic acid, IAA) is synthesized in the shoot apical meristem. The polar transport of IAA from cell to cell (chemiosmotic model) establishes an auxin gradient in the plant. Describe a simple experiment in which you can demonstrate the polar transport of labeled IAA. 2

8 Questions for Biology IIB (SS 2006) Plant biology Wilhelm Gruissem Development 1. Are animal embryonic stem cells and plant meristem cells comparable? If yes, why? If not, why not? 2. The shoot apical meristem consists of three layers (L1, L2, L3). In what planes must cell division occur in these layers to allow growth of the meristem? 3. Explain the two principal mechanisms that contribute to plant growth. In which zones of the shoot and the root does each mechanism contribute pre-dominantly to the growth of the plant? 4. The development of the flower can be explained by the action of different homeotic genes, referred to as A, B and C functions. A function controls the development of sepals and petals, B function the development of petals and stamens, and C function the development of stamens and carpels. a. What is the phenotype (=appearance) of flowers in which the A function is missing? b. What is the phenotype of flowers in which the C function is missing? c. What would be the phenotype of flowers in which all three functions are missing, and what inference does this have for the evolution of flower organs? 5. Describe the development and organization of the plant female gametophyte. 6. Explain using a simple schematic drawing how the combinatorial action of the three genetically identified functions of the Arabidopsis floral meristem (A-, B- and C- function) can regulate the development of the flower into sepals, petals, anthers and the gynoecium. 7. The apical shoot meristem contains the L1, L2 and L2 cell layers. a. Which tissues develop from the three layers? b. What is the orientation of cell division in the three layers relative to the meristem surface? c. In which layer do you find the quiescent center (= stem cells)?