AP Biology 2006-2007

Transport in Plants

AP Biology

Review: Transport proteins Facilitate diffusion via carrier or selective

channel formation Carrier proteins

Selective to solute molecule Produces conformational change of protein Releases molecule to opposite side

Selective channel Passageways for certain solutes May be gated - open/close

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Transport in plants H2O & minerals

transport in xylem transpiration

evaporation, adhesion & cohesion negative pressure

Sugars transport in phloem bulk flow

Calvin cycle in leaves loads sucrose into phloem positive pressure

Gas exchange photosynthesis

CO2 in; O2 out stomates

respiration O2 in; CO2 out roots exchange gases within air spaces in soil

Why doesover-wateringkill a plant?

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Ascent of xylem fluid

Transpiration pull generated by leaf

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Water & mineral absorption Water absorption from soil

osmosis aquaporins

Mineral absorption active transport proton pumps

active transport of H+

H2O

root hair

aquaporin

proton pumps

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Mineral absorption Proton pumps

active transport of H+ ions out of cell chemiosmosis H+ gradient

creates membranepotential difference in charge drives cation uptake

creates gradient cotransport of other

solutes against theirgradient

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Water flow through root Porous cell wall

water can flow through cell wall route & not enter cells

plant needs to force water into cellsCasparian strip

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Controlling the route of water in root Endodermis

cell layer surrounding vascular cylinder of root lined with impermeable Casparian strip forces fluid through selective cell membrane

filtered & forced into xylem cells

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Mycorrhizae increase absorption Symbiotic relationship between fungi & plant

symbiotic fungi greatly increases surface area for absorption of water & minerals

increases volume of soil reached by plant increases transport to host plant

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Mycorrhizae

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Chloroplasts

Epidermal cell

NucleusGuard cell

Thickened innercell wall (rigid)

Stoma open Stoma closed

H2O

water moves into guard cells

H2O H2O H2O

H2O H2O

H2O

H2O

H2O H2O H2O H2O

Control of Stomates

K+

K+

K+

K+

K+ K+

K+ K+

K+ K+K+K+

water moves out of guard cells

Uptake of K+ ions by guard cells

proton pumps water enters by

osmosis guard cells

become turgid Loss of K+ ions

by guard cells water leaves by

osmosis guard cells

become flaccid

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Water potential and osmosis Osmosis

Net absorption or loss of water Water potential ()

Combined effects of solute concentration and pressure

Determines direction of water movement

Water moves from areas of high water potential to areas of low water potential

0 Mpa = pure water open at sea level & room temp

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Water potential () Solute potential (s)

Proportional to number of dissolved solutes Osmotic potential s = 0 in pure water s = -0.23 in 0.1M solution of sugar Addition of solutes lowers the

Pressure Potential (P) Physical pressure on a solution Can be + or – Water in xylem is often negative Water in cell is often positive Additional pressure raises the

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= p + s

p - pressure potential

s - solute potential = osmotic potential

Flaccid cell (P = 0) placed in hypertonic solution will lose water via osmosis

Plasmolysis - protoplast pulls away from cell walls

Same cell placed within a hypotonic solution will gain water via osmosis = turgid

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Transport of sugars in phloem Loading of sucrose into phloem

flow through cells via plasmodesmata proton pumps

cotransport of sucrose into cells down proton gradient

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can flow 1m/hr

Pressure flow in phloem Mass flow hypothesis

“source to sink” flow direction of transport in phloem is

dependent on plant’s needs phloem loading

active transport of sucrose into phloem

increased sucrose concentration decreases H2O potential

water flows in from xylem cells increase in pressure due to

increase in H2O causes flow

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Experimentation Testing pressure flow

hypothesis using aphids to measure sap

flow & sugar concentration along plant stem