CHAPTER 36 TRANSPORT IN PLANTS

I. AN OVERVIEW OF TRANSPORT MECHANISMS IN PLANTS TRANSPORT AT THE CELLULAR LEVEL DEPENDS ON THE SELECTIVE PERMEABILITY OF MEMBRANES SPECIFIC TRANSPORT PROTEINS ENABLE PLANT CELLS TO MAINTAIN AN INTERNAL ENVIRONMENT DIFFERENT FROM THEIR SURROUNDINGS.

B. DIFFERENCES IN WATER POTENTIAL DRIVE WATER TRANSPORT SOLUTES DECREASE WATER POTENTIAL, WHILE PRESSURE INCREASES WATER POTENTIAL. WATER FLOWS BY OSMOSIS FROM A COMPARTMENT WITH A HIGHER WATER POTENTIAL TO ONE WITH A LOWER WATER POTENTIAL.

C. VACUOLATED CELLS HAVE THREE MAJOR COMPARTMENTS THE PLASMA MEMBRANE REGULATES TRANSPORT BETWEEN THE CYTOSOL AND THE WALL SOLUTION THE TONOPLAST REGULATES TRANSPORT BETWEEN THE CYTOSOL AND THE VACUOLE.

THE SYMPLAST IS THE CONTINUUM OF CYTOSOL LINKED BY PLASMODESMATA. D.    THE SYMPLAST AND APOPLAST BOTH FUNCTION IN TRANSPORT WITHIN TISSUES AND ORGANS THE SYMPLAST IS THE CONTINUUM OF CYTOSOL LINKED BY PLASMODESMATA. THE APOPLAST IS THE CONTINUUM OF CELL WALLS.

E. BULK FLOW FUNCTIONS IN LONG-DISTANCE TRANSPORT TRANSPORT OF XYLEM SAP AND PHLOEM SAP IS DUE TO PRESSURE DIFFERENCES AT OPPOSITE ENDS OF CONDUITS, THE XYLEM VESSELS AND SIEVE TUBES.

II. ABSORPTION OF WATER AND MINERAL BY ROOTS A. ROOT HAIRS, MYCORRHIZAE, AND A LARGE SURFACE AREA OF CORTICAL CELLS ENHANCE WATER AND MINERAL ABSORPTION ROOT HAIRS ARE THE MOST IMPORTANT AVENUES OF ABSORPTION NEAR ROOT TIPS, BUT MYCORRHIZAE (SYMBIOTIC ASSOCIATIONS OF FUNGI AND ROOTS) ARE RESPONSIBLE FOR MOST ABSORPTION BY THE WHOLE ROOT SYSTEM. ONCE SOIL SOLUTION ENTERS THE ROOT, THE EXTENSIVE SURFACE AREA OF CORTICAL CELL MEMBRANES ENHANCES UPTAKE OF WATER AND SELECTED MINERALS INTO CELLS.

B. THE ENDODERMIS FUNCTIONS AS A SELECTIVE ENTRY BETWEEN THE ROOT CORTEX AND VASCULAR TISSUE WATER CAN CROSS THE CORTEX VIA THE SYMPLAST OR APOPLAST, BUT MINERALS THAT REACH THE ENDODERMIS VIA THE APOPLAST MUST FINALLY CROSS THE SELECTIVE MEMBRANES OF ENDODERMAL CELLS. THE WAXY CASPARIAN STRIP OF THE ENDODERMAL WALL BLOCKS APOPLASTIC TRANSFER OF MINERALS FROM THE CORTEX TO THE STELE.

III. TRANSPORT OF XYLEM SAP THE ASCENT OF XYLEM SAP DEPENDS MAINLY ON TRANSPIRATION AND THE PHYSICAL PROPERTIES OF WATER LOSS OF WATER VAPOR (TRANSPIRATION) LOWERS WATER POTENTIAL IN THE LEAF BY PRODUCING A NEGATIVE PRESSURE (TENSION) THIS LOW WATER POTENTIAL DRAWS WATER FROM THE XYLEM. COHESION AND ADHESION OF THE WATER TRANSMITS THE PULLING FORCE ALL THE WAY DOWN THE ROOTS.

B. REVIEW: XYLEM SAP ASCENDS BY SOLAR-POWERED BULK FLOW THE MOVEMENT OF XYLEM SAP AGAINST GRAVITY IS MAINTAINED BY TRANSPIRATION.

IV. THE CONTROL OF TRANSPIRATION GUARD CELLS MEDIATE THE PHOTOSYNTHESIS TRANSPIRATION COMPROMISE STOMATA SUPPORT PHOTOSYNTHESIS BY ALLOWING C02 AND 02 EXCHANGE BETWEEN THE LEAF AND ATMOSPHERE, BUT THESE PORES ARE ALSO THE MAIN AVENUES FOR TRANSPIRATIONAL LOSS OF WATER FROM THE PLANT TURGOR CHANGES IN GUARD CELLS, WHICH DEPEND ON K+ AND WATER TRANSPORT INTO AND OUT OF THE CELLS, REGULATE THE SIZE OF THE STOMATAL OPENINGS

V. TRANSLOCATION OF PHLOEM SAP PHLOEM TRANSLOCATES ITS SAP FROM SUGAR SOURCES TO SUGAR SINKS MATURE LEAVES ARE THE MAIN SOURCES, THOUGH STORAGE ORGANS SUCH AS BULBS CAN BE SOURCES DURING CERTAIN SEASONS. DEVELOPING ROOTS AND SHOOT TIPS ARE EXAMPLES OF SUGAR SINKS. PHLOEM LOADING AND UNLOADING DEPEND ON ACTIVE TRANSPORT OF SUCROSE. THE SUCROSE IS COTRANSPORTED ALONG WITH H+, WHICH IS DIFFUSING DOWN A GRADIENT GENERATED BY PROTON PUMPS.

B. PRESSURE FLOW IS THE MECHANISM OF TRANSLOCATION IN ANGIOSPERMS LOADING OF SUGAR AT THE SOURCE END OF A SIEVE TUBE AND UNLOADING AT THE SINK END MAINTAIN A PRESSURE DIFFERENCE THAT KEEPS THE SAP FLOWING THROUGH THE TUBE.