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Lecture 70

BIOL 1010 Lecture 70: Lecture 70

4 Pages

Biological Sciences
Course Code
BIOL 1010
Emily A.Mc Kinnon

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o78P Bio 1010 In addition to sugars, Phloem sap may also contain inorganic ions, amino acids and hormones in transit from one part of the plant to another, but its main solute is usually the disaccharide sugar sucrose. Xylem sap flows upward from the roots, Phloem sap moves throughout the plant in various directions. Sieve tubes always carry sugars from a sugar source to a sugar sink. Sugar source is a plant organ that is a net producer of sugar by photosynthesis or breakdown starch. Leaves are the primary sugar sources in mature plants. Sugar sink is an organ that is a net consumer or storer of sugar. Growing roots, buds, stems, and fruits are sugar sinks. The pressure flow mechanism explains why phloem sap always flows from a sugar source to a sugar sink, regardless of their locations in the plant. o Contrast the forces that move Phloem sap with the forces that move Xylem sap Pressure is generated at the source end of a sieve tube by the loading of sugar and the resulting osmotic flow of water into the phloem. This pressure pushes phloem sap from the end to the sink end of the tube. In contrast, transpiration generates a pulling force that drives the ascent of xylem sap. 23.6 : Plan Nutrition in
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