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Lecture 15

Lecture 15 - Water and Fluid Transport - Moving Fluids in Plants II

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Biology 2601A/B
Graeme Taylor

LECTURE 15: WATER AND FLUID TRANSPORT – MOVING FLUIDS IN PLANTS II Phloem  Living cells (sieve elements) connected by sieve plates  Transport the products of photosynthesis o Also hormonal and RNA signals  Number of sieve elements make a sieve tube – pores on sieve plate and pores on the sides of the cell wall that connects the cells  Live cells  Usually connected to companion cell – some of the needs of the cell are supplied by this companion cell Sieve Element Cell  No nucleus, vacuole, filaments, microtubules, or ribosomes  Have mitochondria, plastids and smooth ER  Under high turgor pressure (lots of solute) – large drive for water to enter cells due to high sugar content  Sieve plates o Holes in cell wall between sieve elements o Covered by smooth ER Companion Cells  Load and unload sugars  ATP production and protein synthesis Phloem Transport Mechanisms  Phloem loaded with sucrose by companion cells at source tissues o May be symplastic (via plasmodesmata) or apopplastic (active transport)  Osmotic gradient in phloem creates pressure differential and moves both water and solutes  Source cell (such as mesophyll cell) – generation of sucrose which is transported into the companion cell across the cell wall (active transport)  Concentration of sucrose in companion cells increases, highly connected to sieve element with plasmodesmata – active loading into companion cells which diffuses into the phloem itself and into the sieve element cells  As you load up those sugars, the solute concentration of the cell increases – the more solutes that are pumped in, the more negative the solute potential becomes  This lowers the water potential of the cell which drives water movement from adjacent cells that have a less negative water potential  Turgor pressure of the cell is increased – this pressure drives the movement of the fluids in the cell o Forces water down the sieve tube, down the phloem, towards areas that are under low pressure  Osmotic potential of phloem becomes more negative (as solutes accumulate)  Water potential declines, leading to water flow from xylem  This generates a turgor pressure in the phloem  The phloem sap then flows by bulk flow  Pressure gradient driven movement of water and solute  Much faster than diffusion  Can be against the water potential gradient  Phloem unloaded at sink tissue  Concurrent water flow in and out of xylem driven by water potential gradie
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