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Plants. Transport and Tissue. Transport in plants. H 2 O & minerals transport in xylem transpiration Sugars transport in phloem bulk flow Gas exchange photosynthesis CO 2 in; O 2 out stomates respiration O 2 in; CO 2 out roots exchange gases within air spaces in soil. - PowerPoint PPT Presentation
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Plants
Transport and Tissue
2005-2006
Transport in plants
• H2O & minerals– transport in xylem – transpiration
• Sugars– transport in phloem– bulk flow
• Gas exchange– photosynthesis
• CO2 in; O2 out• stomates
– respiration• O2 in; CO2 out• roots exchange gases
within air spaces in soil
Why does over-watering kill a plant?
2005-2006
Transport in plants• Physical forces drive transport at different scales– cellular
• from environment into plant cells• transport of H2O & solutes
into root hairs– short-distance transport
• from cell to cell• loading of sugar from
photosynthetic leaves into phloem sieve tubes
– long-distance transport• transport in xylem & phloem
throughout whole plant
2005-2006
Cellular transport
• Active transport– solutes are moved into
plant cells via active transport
– central role of proton pumps• chemiosmosis
proton pumps
2005-2006
Short distance (cell-to-cell) transport
• Compartmentalized plant cells– cell wall– cell membrane
• cytosol– vacuole
• Movement from cell to cell– move through cytosol
• plasmodesmata junctions connect cytosol of neighboring cells
• symplast– move through cell wall
• continuum of cell wall connecting cell to cell
• apoplastsymplast
apoplast
2005-2006
Routes from cell to cell • Moving water & solutes between cells
– transmembrane route• repeated crossing of plasma membranes• slowest route but offers more control
– symplast route• move from cell to cell within cytosol
– apoplast route• move through connected cell wall without crossing cell membrane• fastest route but never enter cell
2005-2006
Long distance transport• Bulk flow– movement of fluid driven by pressure• flow in xylem tracheids & vessels
– negative pressure– transpiration creates negative pressure pulling xylem sap
upwards from roots • flow in phloem sieve tubes
– positive pressure– loading of sugar from photosynthetic leaf cells generates
high positive pressure pushing phloem sap through tube
2005-2006
Movement of water in plants
cells are turgid
cells are flaccidplant is wilting
• Water relations in plant cells is based on water potential– osmosis through
aquaporins• transport proteins
– water flows from high potential to low potential
2005-2006
Water & mineral uptake by roots• Mineral uptake by root hairs– dilute solution in soil– active transport pumps
• this concentrates solutes (~100x) in root cells
• Water uptake by root hairs– flow from high H2O potential to low H2O potential– creates root pressure
2005-2006
Controlling the route of water in root
• Endodermis– cell layer surrounding vascular cylinder of root– lined with impervious Casparian strip– forces fluid through
selective cell membrane & into symplast• filtered &
forced into xylem vessels
Plant TISSUES
• Dermal– epidermis (“skin” of plant)– single layer of tightly packed
cells that covers & protects plant
• Ground– bulk of plant tissue – photosynthetic mesophyll,
storage • Vascular– transport system in
shoots & roots – xylem & phloem
Plant CELL types in plant tissues• Parenchyma– “typical” plant cells = least specialized– photosynthetic cells, storage cells– tissue of leaves, stem, fruit, storage roots
• Collenchyma– unevenly thickened primary walls– support
• Sclerenchyma – very thick, “woody” secondary walls– support– rigid cells that can’t elongate– dead at functional maturity
Parenchyma Parenchyma cells are unspecialized, thin, flexible & carry out many metabolic
functions all other cell types in plants develop from parenchyma
Collenchyma Collenchyma cells have thicker primary walls & provide support
help support without restraining growth remain alive in maturity
the strings in celery stalksare collenchyma
Sclerenchyma• Thick, rigid cell wall– lignin (wood)– cannot elongate– mostly dead at maturity
• Cells for support– xylem vessels– xylem tracheids– fibers
• rope fibers– sclereids
• nutshells• seed coats • grittiness in pears
tracheids
vessel elements Vascular tissue
Aaaah…Structure–Functionagain!
vessel element
dead cells
Xylem move water & minerals up from roots dead cells at functional maturity
only cell walls remain need empty pipes to efficiently move H2O transpirational pull
Phloem: food-conducting cells carry sugars & nutrients throughout plant
sieve tube
companion cell
living cells
plasmodesmata sieve plate
Phloem• Living cells at functional maturity– cell membrane, cytoplasm
• control of diffusion– lose their nucleus, ribosomes & vacuole
• more room for specialized transport of liquid food (sucrose)
• Cells – sieve tubes
• sieve plates — end walls — have pores to facilitate flow of fluid between cells
– companion cells• nucleated cells connected to the sieve-tube • help sieve tubes