Chapter 29 Phloem

You Must Know How bulk flow affects movement of solutes in plants.

Animation: Phloem Translocation Spring Right click slide / Select play

Animation: Phloem Translocation Summer Right click slide / Select play

Concept 29.7: Sugars are transported from sources to sinks via the phloem The products of photosynthesis are transported through phloem by the process of translocation. The translocation of phloem sap through sieve tubes by bulk flow is driven by positive pressure. © 2014 Pearson Education, Inc. Phloem

Bulk flow differs from diffusion – It is driven by differences in pressure potential, not solute potential. – It moves the entire solution, not just water or solutes. – It is much faster. © 2014 Pearson Education, Inc.

Mesophyll cells have lots of chloroplasts Apoplast Symplast Mesophyll cell Sieve-tube Element (part of the phloem) Mesophyll cell Bundle- sheath cell Phloem parenchyma cell (b) A chemiosmotic mechanism is responsible for the active transport of sucrose. High H  concentration Low H  concentration Proton pump Cotransporter Sucrose S S HH HH HH Loading of sucrose into phloem

Figure Vessel (xylem) Bulk flow by negative pressure Loading of sugar H2OH2O Sucrose Source cell (leaf) 1 Sieve tube (phloem) Bulk flow by positive pressure 1 2 Uptake of water 2 Unloading of sugar Sink cell (storage root) Sucrose 3 3 Recycling of water H2OH2O Sucrose 4 3 4