BIOL 206 Lecture Notes - Lecture 20: Apoplast, Symplast, Phloem
Document Summary
Apoplast, symplast, xylem and phloem: bulk flow of phloem sap. Phloem sap moves due to a positive pressure gradient, created by osmosis. Ex: source to sink: xylem sap moves root to shoot, phloem moves source to sink, long distance movement of syntate. How do companion cells arise and why are they necessary: apoplastic- sucrose enters cell wall of sieve tube member, to get into apoplast must cross the cell wall to get into cell membrane. All of folds are lined with plasma membrane. = more membrane in small cell=more transporters: moving through symplastic pathway is passive/ free. Atp transport energetically expensive: this is secondary active transport, pressure flow model. Loading of solutes from source cell to sieve cell. Loading of phloem sap, resulting in osmotic water transport from xylem to phloem. Influx of water into phloem increase pressure potential, and water is pushed down the hydrostatic pressure gradient. At the sink, solutes unloaded which increases water potential\]
