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Chapter 3

BIO204 CH37/38 - Lecture 5

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Sanja Hinic- Frlog

5(CH37/38) October-09-13 8:31 PM Key Concepts – Solute transport • Transport of sugars in phloem Source tissue -> Sink tissues release sugars for use elsewhere -> tissues using or storing sugars • Loading of sucrose by source cells into nearby phloem cells ○ requires energy (ATP) ○ creates pressure gradient (water follows sugars) -> movement of water to sinks through osmosis Solute transport = Transport of sugars = Translocation How does translocation work? Pressure-Flow Hypothesis • High turgor pressure near sources causes phloem sap to flow from sources to sinks • Involves energy dependent ○ Phloem loading - carbs into the system ○ Phloem unloading - carbs out of the system Phloem loading  Active transport of sucrose from source leaf cells through companion cells to sieve-tube members  Phloem sap near source: -> High sucrose concentrations -> Low water potential  Water moves from the nearby xylem into the sieve-tube members Depletion of carbohydrates Phloem unloading  Active removal of sucrose from sieve-tube members by companion cells into sink cells (e.g. roots)  Phloem sap near sink: -> low sucrose concentrations -> high water potential  Water moves from nearby sieve-tube members into the xylem Sugar beets - they accumulate large amount of sucrose in vacuole Summary of phloem loading & unloading ○ High turgor pressure near source ○ Low turgor pressure near sink ○ Water in phloem sap translocated via a steep pressure potential gradient:  source -> sink tissue ○ Sugar molecules are carried along by bulk flow:  mass movement of molecules along a pressure gradient ○ Water returns to the source via xylem Active transport for concentrating sugars in sieve-tube members at sources • ΔpH between interior of companion cells and exterior of cells • Model for Phloem loading ○ Proton pump (or H+-ATPase) ○ Cotransporter (protein acting as a conduit for protons and sucrose to enter the companion cell together) • H -ATPase hydrolyzes ATP, pumps H+ to exterior of phloem cell ○ Produces a proton gradient ○ Proton gradient required for sucrose transport into companion cell against concentration gradient ○ Translocation of sucrose from companion cell into sieve-tube members via direct cytoplasmic connection A Model for Cotransport of Protons and Sucrose A Model for Cotransport of Protons and Sucrose Phloem Unloading • Phloem unloading also requires ○ ATP ○ A membrane transport mechanisms • Different membrane proteins and mechanism of movement • Vary dependent on type of sink within same plant as well as among different species • Occurs when Sucrose Is Taken Up by Sink Cells Key Concepts – Plant nutrition • Plants need ○ carbon dioxide and water ○ essential nutrients - required for normal growth and reproduction, and a specific structure or metabolic function  Ions dissolved in soil water  Uptake by roots • Nutrient absorption ○ via specialized proteins in plasma membranes of root cells ○ Nitrogen or phosphorus often obtained from fungi associated with their roots ○ Exclusion of toxins from xylem by endodermal cells or ○ Active transport into cell vacuoles for storage • Specialized methods of obtaining nutrients ○ Association of plants with nitrogen-fixing bacteria ○ Parasitism ○ Carnivory Macronutrient - building blocks of nucleic acids, proteins, carbohydrates, phospholipids, and other key molecules Micronutrient - required in small quantities, but is still important What Happens When Key Nutrients Are in Short Supply? • Law of the Minimum (Justus von Liebig, 1803-1873 ): “Plant yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be” • Supply of the deficient nutrient can improve yield until some other nutrient is limiting • Liebig developed first mineral fertilizers in agriculture to improve crop yield N-P-K: Nitrogen-phosphorus-potassium - needs to be in various proportion to maximize plant growth Hydroponic cultures are used for studying nutrient deficiencies Hydroponic growth - takes place in liquid cultures, without soil - availability of nutrients can be precisely controlled Distinctive symptoms of nutrient deficiencies When nitrogen is in short supply, which processes are affected? • Photosynthesis ○ Chlorophyll ○ Rubisco • Amino acid synthesis ○ Structural proteins ○ Enzymes • ATP synthesis ○ Cellular respiration ○ Metabolic energy requirements • Nucleic acid synthesis (DNA, RNA) ○ Growth, development, reproduction, regulation All above except photosynthesis applies to animals as well! Soil building - Weathering - forces applied by rain, running water, and wind which breaks pieces of large rocks ○ Soil texture is categorized by particle size  Gravel: > 2 mm  Sand: 0.02 - 2 mm  Silt : 0.002 - 0.02 mm  Silt : 0.002 - 0.02 mm  Clay: < 0.002 mm - Humus - decaying organic matter - Texture - proportion of gravel, sand, silt, clay ○ Importance:  Affects ability of roots to penetrate soil  Affects soil's ability to hold water
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