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39.1 Plants require a variety of nutrients in addition to the direct products of photosynthesis.
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• Plants require a few macronutrients in large amounts and several micronutrients in trace amounts. Most of these are obtained from the soil through the roots.
• Plant growth is significantly influenced by the nature of the soil. Soils vary in terms of nutrient composition and water-holding capacity.
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1. What is the difference between a macronutrient and a micronutrient? Explain how a plant would utilize each of the macronutrients.
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39.2 Some plants have novel strategies for obtaining nutrients.
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• Some plants entice bacteria to produce organic nitrogen for them. These bacteria may be free-living or form a symbiotic relationship with a host plant.
• About 90% of all vascular plants rely on fungal associations
to gather essential nutrients.
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2. The atmosphere is full of nitrogen yet it is inaccessible to most plants. Why is that? What solution has evolved in legumes?
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39.3 Water and minerals move upward through the xylem.
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• Water and minerals enter the plant through the roots. Energy is required for active transport.
• The bulk movement of water and minerals is the result of movement between cells, across cell membranes, and through tubes of xylem. Aquaporins are water channels that enhance osmosis.
• A combination of the properties of water, structure of xylem, and transpiration of water through the leaves results in the passive movement of water to incredible heights. The ultimate energy source for pulling water through xylem vessels and tracheids is the sun.
• Water leaves the plant through openings in the leaves called stomata. Stomata open when their guard cells are turgid and bulge, causing the thickened inner walls of these cells to bow away from the opening.
• Plants can tolerate long submersion in water, if they
can deliver oxygen to their submerged tissues.
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3. What is pressure potential? How does it differ from solute potential? How do these pressures cause water to rise in a plant?
4. What proportion of water that enters a plant leaves it via transpiration?
5. Why are root hairs usually turgid? Does the accumulation of minerals within a plant root require the expenditure of energy? Why or why not?
6. Under what environmental condition is water transport through the xylem reduced to near zero? How much transpiration occurs under these circumstances?
7. Does stomatal control require energy? Explain.
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39.4 Dissolved sugars and hormones are transported in the phloem.
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• Sucrose and hormones can move up and down in the phloem between sources and sinks.
• The movement of water containing dissolved sucrose
and other substances in the phloem requires energy. Sucrose is loaded
into the phloem near sites of synthesis, or sources, using energy supplied
by the companion cells or other nearby parenchyma cells.
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8. What is translocation? What is the driving force behind translocation?
9. Describe the movement of carbohydrates through a plant, beginning with the source and ending with the sink. Is this process active or passive?
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