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Transport in plants occurs across a network of vessels and over long distances

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Lecture 6 Outline (Ch. 36 & 37)

I. Plant Transport Overview

II. Driving Forces

A. Water potential

B. Transpiration & Bulk Flow in Xylem

C. Stomata Control

D. Positive Pressure & Bulk Flow in Phloem

III. Mineral Acquisition

IV. Essential Nutrients

V. Relationships with other organisms

VI. Preparation for next lecture

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Physical forces drive the transport of materials in plants

over a range of distances

Transport occurs on three scales

1. Within a cell – cellular level2. Short-distance cell to cell –

tissue level3. Long-distance in xylem &

phloem - whole plant level

Transport in Plants

Transport occurs by 3 mechanisms:

A.Osmosis & Diffusion

B.Active Transport

C.Bulk Flow

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Transport in Plants – Water Potential

Roots xylem stomata

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To survive– Plants must balance water uptake and loss

•What is Osmosis? What is diffusion?

•Water potential : predicts water movement due to solute concentration & pressure

– designated as psi (ψ)

Water Potential

Water molecules are attracted to:•  Each other (cohesion)•  Solid surfaces (adhesion)

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• Free water flows from regions of high water potential to regions of low water potential

Water Potential

• Adding solutes

• Adding pressure

Water potential = Potential energy of water = Energy per volume of water in megapascals (MPa)

ψTotal = ψsolute + ψpressure

Ψ changes with:

0.1 Msolution

H2O

Purewater

P = 0

S = 0.23

= 0.23 MPa = 0 MPa

(a)

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• Solutes added

decreases ψ(water less likely to cross

membrane)

Water Potential

(in an open area, no pressure, so ψp = 0)

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• Application of physical pressure increases ψ

(water more likely to cross membrane)

H2OP

= 0.23S

= 0.23

= 0 MPa = 0 MPa

(b)

H2O

P = 0.30

S = 0.23

= 0.07 MPa = 0 MPa

(c)

Water Potential

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Water Potential

ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa

ψ = ψs + ψp

ψsolution = –0.3 MPa (solution has no pressure potential)

Water Potential

Which direction will water move?

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• Water potential– Affects uptake and loss of water by plant cells

• If a flaccid cell is placed in an environment with a higher solute concentration– The cell will lose water and become plasmolyzed

0.4 M sucrose solution:

Initial flaccid cell:

Plasmolyzed cellat osmotic equilibriumwith its surroundings

P = 0

S = 0.7

P = 0

S = 0.9

P = 0

S = 0.9

= 0.9 MPa

= 0.7 MPa

= 0.9 MPa

Water Potential

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Uses of turgor pressure:

• Inexpensive cell growth

• Hydrostatic skeleton

• Phloem transport

Water Potential

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Most plant tissues- cell walls and cytosol are continuous cell to cell (via?)

- cytoplasmic continuum called the symplast

apoplast = continuum of cell walls plus extracellular spaces

Water Route

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Symporters (cotransporters) contribute to the gradient that determines the directional flow of water.

SoilH2O

Mineralions

Symporter

Water

Soil

Cytosol

H+

Water Route

Water enters plants via the roots.

How do water and minerals get from the soil to vascular tissue?

Here, pumps in H+ and mineral ions

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Minerals & ions pumped into root cells, then moved past endodermis

What happens to ψ between soil and endodermis?

Where is osmosis occurring?

Water Potential

Once water & minerals cross the endodermis, they are transported through the xylem to upper parts of the plant.

Water Potential

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Water exits plantthrough stomata.

Smoothsurface

Rippledsurface

Water film that coats mesophyll cell walls evaporates.

Water moves up plant through xylem.

Adhesion to xylem cells

Cohesion between watermolecules

H2O

Xylem

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Bulk Flow = movement of fluid due to pressure gradient

• Transpiration drives bulk flow of xylem sap.

• Water is PULLED up a plant.

• Ring/spiral wall thickening protects against vessel collapse

Transpiration = loss of water from the shoot system to the surrounding environment.

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Xylem Ascent by Bulk Flow

• The movement of xylem sap is against gravity– maintained by the transpiration-cohesion-tension

• Stomata help regulate the rate of transpiration• Leaves generally have broad surface areas

• These characteristics– Increase photosynthesis– Increase water loss through stomata

20 µm

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What happens if rate of transpiration nears zero?

• Guttation

Xylem

i.e. – at night, water pressure builds up in the roots

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Stomata ControlH+ pumped out

K+ flow in

H2O flow in

stomata open

Why?

Why?

K+ channels, aquaporins and radially oriented cellulose fibers play important roles.

Cues for opening stomata:

Light

Depleted CO2

Internal cell “clocks”

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Phloem tissue

• Direction is source to sink• Near source to near sink• Phloem under positive

pressure

Phloem

Are tubers and bulbs sources or sinks?

Phloem sap composition:

• Sugar (mainly sucrose)• amino acids• hormones• minerals• enzymes

Aphid

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Vessel(xylem)

H2O

H2O

Sieve tube(phloem)

Source cell(leaf)

Sucrose

H2O

Sink cell(storageroot)

1

Sucrose

2

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1

2

3

4

Tra

nsp

irat

ion

str

eam

Pre

ssu

re f

low

Phloem

Pressure Flow Hypothesis

Where are sugars made?

Sugars actively transported into companion cells plasmodesmata to sieve tube elements

Via H+/sucrose cotransporters

Water potential increased, turgor pressure increased, sap PUSHED through phloem

Sugars removed (actively) at sink water potential decreased, water leaves phloem

Water follows (WHY?!)

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Overview: A Nutritional Network• Every organism

– Continually exchanges energy and materials with its environment

• The branching root and shoot system provides high SA:V to collect resources

– Plants’ resources are diffuse (scattered, at low concentration)

What are these diffuse resources?

What’s in dirt?!

Mineral Acquisition

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• After heavy rainfall, water drains away from the larger spaces in soil– But smaller spaces retain water

– attraction to surfaces, clay and other particles

• The film of loosely bound water available to plants

Soil particle surrounded byfilm of water

Root hairWater available to plant

Air space

Mineral Acquisition

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H2O

Root hair

K+

Cu2+ Ca2+Mg2+

K+

K+

H+

H+

Soil particle–

– – – – – – ––

Mineral Acquisition

CO2

Steps:1.  Roots acidify soil solution via respired CO2 and H+/ATPase pumps

2.  H+ attracted to soil particle (-) which “releases” cations3.  Roots absorb cations

Cation Exchange

•  Makes cations available for uptake.

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27Essential Nutrients and Deficiencies

• Plants require certain chemicals to thrive

• Plants derive most organic mass from the CO2 of air

– Also depend on soil nutrients like water and minerals

Essential elements:Required for a plant to complete its life cycle

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• Photosynthesis = major source of plant nutrition• Overall need

– Macronutrients – used in larger amounts• Nine = C, O, H, N, K, Ca, Mg, P, and S

– Micronutrients – used in minute amounts• Seven = Cl, Fe, Mn, Zn, B, Cu, and Mo

Essential Nutrients and Deficiencies

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

Deficiency of any one can have severe effects on plant growth

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• Mycorrhizae• Root nodulation• Parasitic plants• Carnivorous plants

Relationship with other organisms

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• Symbiotic associations with mycorrhizal fungi are found in about 90% of vascular plants – Substantially expand the surface area available for

nutrient uptake– Enhance uptake of phosphorus and micronutrients

Relationship with other organisms

The fungus gets: sugars from plant

Agriculturally, farmers and foresters …Often inoculate seeds with spores of mycorrhizae to promote mycorrhizal relationships.

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Nitrogen, Soil Bacteria and Nitrogen Availability• Plants need ammonia (NH3) or nitrate (NO3

–) for: Proteins, nucleic acids, chlorophyll…

• Nitrogen-fixing soil bacteria convert atmospheric N2 to nitrogenous minerals that plants can absorb

N2

Soil

N2 N2

Nitrogen-fixingbacteria

Organicmaterial (humus)

NH3

(ammonia)

NH4+

(ammonium)

H+

(From soil)

NO3–

(nitrate)Nitrifyingbacteria

Denitrifyingbacteria

Root

NH4+

Soil

Atmosphere

Nitrate and nitrogenous

organiccompoundsexported in

xylem toshoot system

Ammonifyingbacteria

Symbiotic relationships form between nitrogen-fixing bacteria and certain plants - Mainly legume family (e.g. peas, beans)

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• Nodules: Swellings of plant cells “infected” by Rhizobium bacteria

(a) Pea plant root

Nodules

Roots

• Inside the nodule– Rhizobium bacteria assume a

form called bacteroids, which are contained within vesicles formed by the root cell

(b) Bacteroids in a soybean root nodule. In this TEM, a cell froma root nodule of soybean is filledwith bacteroids in vesicles. The cells on the left are uninfected.

5 m

Bacteroidswithinvesicle

Epiphytes, Parasitic, and Carnivorous PlantsStaghorn fern,

an epiphyteEPIPHYTESAnchored on another plant, self-nourished

PARASITIC PLANTSAbsorb sugar/minerals

from host plant

Mistletoe, a photosynthetic parasite

Pitcher plantscavity filled with digestive fluid

Venus flytrap

To gain extra nitrogen

Things To Do After Lecture 6…Reading and Preparation:

1. Re-read today’s lecture, highlight all vocabulary you do not understand, and look up terms.

2. Ch. 36 Self-Quiz: #2, 3, 4, 6, 7, 8, 9 (correct answers in back of book)

Ch. 37 Self-Quiz: #1, 2, 8, 9, 10 (correct answers in back of book)

3. Read chapters 36 & 37, focus on material covered in lecture (terms, concepts, and figures!)

4. Skim next lecture.

“HOMEWORK” (NOT COLLECTED – but things to think about for studying):

1. Explain the two components of water potential – which of these is due to osmosis?

2. Diagram the movement of water in a plant via xylem versus sugar movement through phloem. List similarities and differences.

3. Discuss how mycorrhizae and Rhizobium are different and the benefits each provide to plants.

4. Think about what types of environments might be more likely to have carnivorous plants – what do plants gain by digesting insects?