BISC 101 Lecture Notes - Lecture 8: Sieve Tube Element, Water Potential, Phloem
25 May 2016
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Document Summary
Xylem = tracheids and vessel elements (like sclerenchyma) Phloem = sieve tube elements and companion cells (like parenchyma) Transpiration: loss of water that creates a force that drives the passive transport of water from roots to shoots. Transport in plants involve water potential/osmosis: 2 factors solute concentration and pressure gradients. Water potential = osmotic (solute) potential + pressure potential. Water transport can occur over short distances from soil to xylem via the apoplastic pathway (outside plasma membrane/between cells), transmembrane pathway (through water channels), or symplastic pathway (plasmodesmata through cytoplasm) Transport can occur over long distances via bulk flow (roots to shoots + sources to sinks) Negative pressure results from water loss at the stomata (diffusion + evaporation) which sucks water up the plant. Rate of transpiration is regulated by stomata opening and closing (blue light photoreceptors vs. aba hormone) Xylem transports water and nutrients from roots to shoots; phloem transports sugar/phloem sap from sources to sinks.
