AP Biology 2011-2012 Chapter 36: Transport in Plants.

2011-2012 AP Biology

Chapter 36:Transport in

Plants

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Overview: Underground Plants

Stone plants (Lithops) are adapted to life in the desert

Two succulent leaf tips are exposed above ground; the rest of the plant lives below ground

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The success of plants depends on their ability to gather and conserve resources from their environment

The transport of materials is central to the integrated functioning of the whole plant

Overview: Underground Plants (cont.)

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Plant Evolution

Algal ancestors absorbed minerals and CO2 directly from water

Early nonvascular land plants lived in shallow water and had aerial shoots

Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport

evolution of xylem & phloem made long-distance transport of water, minerals, and products of photosynthesis

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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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Apoplast & Symplast

The apoplast consists of everything external to the plasma membrane cell walls, extracellular spaces, and the interior

of vessel elements and tracheids

The symplast consists of the cytosol of the living cells in a plant, as well as the plasmodesmata

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Figure 36.6

Cell wall

Cytosol

Plasmodesma

Plasma membrane

Apoplastic route

Symplastic route

Transmembrane route

Key

Apoplast

Symplast

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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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Figure 36.7CYTOPLASM EXTRACELLULAR FLUID

ATP

Proton pump

Hydrogen ion

H+

(a) H+ and membrane potential

H+

H+

H+

H+

H+

H+

H+

+

+

+

+

+

(b) H+ and cotransport of neutralsolutes

H+/sucrosecotransporter

Sucrose(neutral solute)

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

S

S

S S

S

S

+

+

+

+

+

+

(c) H+ and cotransport of ions

Nitrate

H+NO3

cotransporter

H+

H+H+

H+

H+

H+

H+

H+

H+

H+ H+

NO 3

NO 3

NO3

NO3

NO3

NO3

++

+

+

+

+

(d) Ion channelsPotassium ion

Ion channel

K+

K+

K+

K+

K+

K+

K+

+

+

+

+

+

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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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Endodermis & Casparian strip

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Root anatomy

dicot monocot

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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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Roots

Fungus

Figure 36.5

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Mycorrhizae

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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

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Maple sugaring

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Figure 36.8

Solutes have a negative effect on by bindingwater molecules.

Pure water at equilibrium

H2O

Adding solutes to theright arm makes lowerthere, resulting in netmovement of water tothe right arm:

H2O

Pure water

Membrane Solutes

Positive pressure has a positive effect on by pushing water.

Pure water at equilibrium

H2O

H2O

Positivepressure

Applying positivepressure to the right armmakes higher there,resulting in net movementof water to the left arm:

Solutes and positivepressure have opposingeffects on watermovement.

Pure water at equilibrium

H2O

H2O

Positivepressure

Solutes

In this example, the effectof adding solutes isoffset by positivepressure, resulting in nonet movement of water:

Negative pressure(tension) has a negativeeffect on by pullingwater.

Pure water at equilibrium

H2O

H2O

Negativepressure

Applying negativepressure to the right armmakes lower there,resulting in net movementof water to the right arm:

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Outside air 100.0 MPa

7.0 MPa

1.0 MPa

0.8 MPa

0.6 MPa

0.3 MPa

Leaf (air spaces)

Leaf (cell walls)

Trunk xylem

Trunk xylem

Soil

Wa

ter

po

ten

tia

l g

rad

ien

t

Xylem sap

Mesophyll cells

Stoma

Water moleculeAtmosphereTranspiration

Xylemcells

Adhesion byhydrogen bonding

Cell wall

Cohesion andadhesion inthe xylem

Cohesion byhydrogen bonding

Water molecule

Root hair

Soil particle

WaterWater uptakefrom soil

Figure 36.13

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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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Control of transpiration Balancing stomate function

always a compromise between photosynthesis & transpiration leaf may transpire more than its weight in

water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis