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PSU BIOL 240W - Bulk Flow in Plant Vascular Tissue

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BIOL 240W 1st Edition Lecture 9 Outline of Last Lecture I. Short Distance Water MovementII. Bulk FlowIII. TranspirationIV. Water and Mineral Uptake by RootsV. TransportOutline of Current Lecture I. Pull of Water and Nutrients Due to TranspirationII. Bulk Flow vs. DiffusionIII. StomataIV. Transport of SugarsV. Phloem as an “Information Superhighway”Current LectureI. Pull of Water and Nutrients Due to Transpirationa. Outside of leaf is often drier than the inside. Water vapor diffuses down its waterpotential gradient and exits via the stomatab. Loss of water by diffusion and evaporation provides pulling force for upward movementc. Cell walls act like a thin capillary network. Their surface tension with a negative pressure potential drives the water movement from the inside of the leaf to outside of the leafd. Water is drawn out of the xylem so it can be used for photosynthesise. Cohesion of water molecules is pulling the water up the xylemi. Drought stress or freezing can cause failure of cohesionii. Water vapor pockets or air bubbles can be formed, but water molecules can move around these bubbles to adapt to these circumstancesf. Adhesion to cell walls of tracheids and vessel elements helps the water to fight gravityg. **The negative water potential created by transpiration creates the bulk flow of water upward in the xylem (ΨP)**II. Bulk Flow vs. DiffusionThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. Bulk flowi. Driven by differences in pressure potentialii. Occurs in hollow dead cellsiii. Moves the entire solutioniv. Fast processb. Diffusioni. Driven by solute potentialii. Occurs across membranes of living cellsiii. Moves only water or solute; does not move both at onceiv. Slow processIII. Stomataa. Conflict occurs in the stomata because they must be open to capture carbon dioxide molecules, but 95% of the water a plant loses escapes through the stomatab. Guard cells regulate water movement from inside to outside of the cellc. Turgor pressure changes guard cells, which also changes stoma sizei. Turgid guard cells are inflated and have an increased stoma size. This allows movement of moisture through stomaii. Flaccid guard cells close stomaiii. Potassium is an osmoregulator. More potassium inside the cell causes water to move into the cell. When water moves in, the guard cells become turgid and the stomata are open.d. Opening and Closing Stomatai. Stomata are normally open during the day1. Triggered by light, the need for carbon dioxide molecules in the plant, and the internal clock of guard cells 2. Drought, high temperature, and wind can cause stomata to close during the daytime. In addition, the hormone abscisic acid is produced in response to water deficiency and closes the stomataii. Stomata are closed at night to minimize water loss (little sunlight at night to drive photosynthesis anyway)iii. Xerophytes are plants adapted to arid climates (xero=dry)1. Some complete their life cycle during the rainy season2. Others have fleshy stems to store water3. May have leaf modification that reduce rates of transpiration4. CAM plants can complete stomatal gas exchange during the nightIV. Transport of Sugarsa. Products of photosynthesis are transported through phloem by the process of translocationb. Phloem sap is an aqueous solution that moves by bulk flow through the sieve tube elementsc. A source is an organ that is a net producer of a product (ex: sugar). Mature leaveswould be a source of sugar in the plantd. A sink is the target organ for the sugar. Sinks are the net consumer or storage sites of sugar. Tubers and bulbs can be sinks in plantse. Movement is always source  sink, but the role of a certain organ can change based on the plants’ needs or time of the season. For example, a storage organ can be both a sugar sink in the summer and a sugar source in the winterf. Transporti. Sugars are loaded into sieve tube elements by symplastic diffusion and active loading from the apoplastii. Active transport of sugar depends on energy from proton pump1. As discussed before, sucrose must be co-transported with a proton because it cannot pass through a membrane on its own2. ATP is the primary source of energy. Then, the hydrogen ion gradient facilitates the co-transport of sugarsg. Pressure flow hypothesis for phloem movement (4 parts)i. Phloem near source has a low solute potential (solute is present). Sugar must travel against concentration gradient to enter sieve tube in the phloem.ii. When water rushes into the phloem as well, the pressure potential increases (positive pressure)iii. Sugars are withdrawn in sink cells (ex: storage roots) as the solutes travel down the phloem. Increase of solute potential is created.iv. Water moves out of phloem is recycled in the xylem. V. Phloem as an “Information Superhighway”a. Phloem is not just responsible for sugar and water movementb. Plant viruses are able to spread through phloemc. mRNA can be transported long distance through the phloemd. Signals can also travel in


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