BOT 331 Exam 1 Questions With Correct Answers

There are two types of nucleic acids: DNA and RNA. There is both DNA and RNA inside the nucleus, mitochondria and plastids. Is there both DNA and RNA in the cytosol? Explain your answer - Answer No, not both. DNA remains in the nucleus. RNA copies of genes are synthesized in the nucleus, but then moved out pores into the cytosol, where ribosomes (also containing RNA) bind RNA and use the RNA to direct synthesis of proteins. In the case of plastids and mitochondria, RNA copies of the organelle's genes remain in the organelle, and are translated into protein by ribosomes in the plastid stroma or mitochondrial matrix. The conducting cells of the xylem experience negative water pressures that could cause the cell's walls to collapse, but the walls typically resist collapsing. State two properties of a conducting cell's wall that resists collapse. - Answer The walls resist collapse because the walls are very thick. Most of the thickness is secondary wall that is dense with cellulose microfibrils and the phenolic polymer lignin, which is very rigid. The phenols of lignin also cross-link to microfibrils, strengthening the wall. the tension is what potentially could cause collapse and the thick, lignified walls resist the collapse. What is stress relaxation AND how does a cell cause stress relaxation? - Answer Stress relaxation is when microfibrils slide relative to each other, which relieves the tension that is otherwise pulling on microfibrils. Tension in the wall is due to the cell's turgor pressure. A cell causes relaxation by breaking the bonds between hemicellulose and cellulose, which allows the fibers to slide. A cell breaks the bonds by pumping protons into the wall (acidification), which activates a protein called Expansin. Expansin catalyzes breaking of the hemicelluose cross-links. Be sure to answer all of the following 3 questions about water potential before moving to the next question. For each, explain your answer. (a) If the water potential in the apoplast is -0.5 MPa, and the water potential inside a cell is -0.25 MPa, then will water flow into the cell or out of the cell. (b) Considering the scenario in (a), at what point does net flow of water stop? (c) If water flows into a plant cell with a rigid cell wall, will the osmotic potential or the pressure potential change the most? - Answer a) Water flows from a region of high water potential (less negative) to a region of low water potential (more negative). Because the apoplast has a lower water potential than the cell, water will flow out of the cell. b) Net flow stops when the water potential inside the cell is equal to the apoplast; when there is no longer a gradient of high to low. c)The pressure potential will change the most, because the volume of the cell will not change much. As soon as water flowing in causes the cell's volume to push on the wall, the rigid wall resists causing the pressure potential to increase (become more positive). Because the volume does not change much, the concentration of solutes does not change, so the osmotic potential (solute potential) does not change. True/false, if false explain why Guard cells are specialized for secreting waxes into their cell wall, so that the majority of the plant's shoot surface is covered in a cuticle. - Answer False, epidermal cells cover most of a plant's shoot surface with their secretions of cutin that make up the cuticle. Guard cells are specialized for converting changes in cell size to opening or closing stomatal pores. True/false, if false explain why A meristem is a region of a plant that has stopped growing and is fully mature. - Answer False, a meristem is a region where many cells are going through the cell cycle - growing and dividing. Many of the cells in a meristem have not yet fully differentiated, so are not mature; have not taken on their final organization for specialized functions. True/false, if false explain why The CSI1 protein catalyzes synthesis of cellulose microfibrils in a direction parallel to microtubules. - Answer False, the CSI1 protein does NOT catalyze synthesis of cellulose. Cellulose Synthase is the enzyme that synthesizes cellulose microfibrils. CSI1 is believed to be a protein that binds to both a Cellulose Synthase and a microtubule, so that the Synthase, when synthesizing cellulose, moves in the direction of the microtubule. True/false, if false explain why Cellulose microfibrils are synthesized in a direction that is perpendicular to microtubules in the cell cortex. - Answer False, cellulose microfibrils are synthesized parallel to the microtubule. The CSI1 protein is believed to couple the Cellulose Synthase rosette to a microtubule, so has the rosette moves, it moves along the microtubule resulting in a microfibril parallel to the microtubule.