BIOL 1010 Lecture Notes - Lecture 70: Phloem, Xylem, Transpiration
Document Summary
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. 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.