biology in focus - chapter 29

CAMPBELL BIOLOGY IN FOCUS

© 2014 Pearson Education, Inc.

Urry • Cain • Wasserman • Minorsky • Jackson • Reece

Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge

29Resource Acquisition, Nutrition, and Transport in Vascular Plants

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



Figure 29.1

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


Concept 29.1: Adaptations for acquiring resources were key steps in the evolution of vascular plants

The evolution of adaptations enabling plants to acquire resources from both above and below ground sources allowed for the successful colonization of land by vascular plants


The algal ancestors of land plants absorbed water, minerals, and CO2 directly from surrounding 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


The evolution of xylem and phloem in land plants made possible the development of extensive root and shoot systems that carry out long-distance transport

Xylem transports water and minerals from roots to shoots

Phloem transports photosynthetic products from sources to sinks



Figure 29.2-1

H2Oandminerals

H2O


Figure 29.2-2

H2Oandminerals

H2O

O2

CO2

O2CO2


Figure 29.2-3

Light

H2Oandminerals

H2O Sugar

O2

CO2

O2CO2

Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss


Shoot Architecture and Light Capture

Stems serve as conduits for water and nutrients and as supporting structures for leaves

Shoot height and branching pattern affect light capture

There is a trade-off between growing tall and branching


Phyllotaxy, the arrangement of leaves on a stem, is specific to each species

Most angiosperms have alternate phyllotaxy with leaves arranged in a spiral

The angle between leaves is 137.5 and likely minimizes shading of lower leaves



Figure 29.3

Shootapicalmeristem

Buds

34 21

42 2916 11 19

27

3224 40

28

15

10

2331

18

1326

22

1217

2520

1

2

3

4

5

6

7

8

9

14

1 mm


Figure 29.3a


Figure 29.3b

Shootapicalmeristem

34 21

42 2916 11 19

27

3224 40

28

15

10

2331

18

1326

22

1217

2520

1

2

3

4

5

6

7

8

9

14

1 mm

The productivity of each plant is affected by the depth of the canopy, the leafy portion of all the plants in the community

Shedding of lower shaded leaves when they respire more than photosynthesize, self-pruning, occurs when the canopy is too thick


Leaf orientation affects light absorption In low-light conditions, horizontal leaves capture

more sunlight In sunny conditions, vertical leaves are less damaged

by sun and allow light to reach lower leaves


Root Architecture and Acquisition of Water and Minerals

Soil is a resource mined by the root system Root growth can adjust to local conditions

For example, roots branch more in a pocket of high nitrate than in a pocket of low nitrate

Roots are less competitive with other roots from the same plant than with roots from different plants


Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae

Mutualisms with fungi helped plants colonize land Mycorrhizal fungi increase the surface area for

absorbing water and minerals


Concept 29.2: Different mechanisms transport substances over short or long distances

There are two major transport pathways through plants The apoplast The symplast


The Apoplast and Symplast: Transport Continuums

The apoplast consists of everything external to the plasma membrane

It includes 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


Three transport routes for water and solutes are The apoplastic route, through cell walls and

extracellular spaces The symplastic route, through the cytosol The transmembrane route, across cell walls



Figure 29.4

Apoplastic route

Cell wall

Symplastic route

Transmembrane route

Cytosol

KeyPlasmodesma

Plasma membrane Apoplast

Symplast

Short-Distance Transport of Solutes Across Plasma Membranes

Plasma membrane permeability controls short-distance movement of substances

Both active and passive transport occur in plants In plants, membrane potential is established through

pumping H by proton pumps In animals, membrane potential is established

through pumping Na by sodium-potassium pumps



Figure 29.5

CYTOPLASM EXTRACELLULARFLUID

Protonpump

Hydrogen ion

Sucrose(neutral solute)

Potassium ion

Ion channel

Nitrate

(d) Ion channels(c) H and cotransport of ions

(b) H and cotransport of neutral solutes(a) H and membrane potential

H/sucrosecotransporter

H/NO3−

cotransporter

S

NO3−

K

K

K

K

K K

K

H H

HH

H

H

H

H

H

H

H

H

S

S

S

S

S

H

H

H

H

H

H

H

H

H

H

H

H

HH

H

HH

H

NO3

−

NO 3−

NO3−

NO3−

NO 3−


Figure 29.5a

CYTOPLASM EXTRACELLULARFLUID

Protonpump

Hydrogen ion

(a) H and membrane potential

H

H

H

H

H

H

H

H


Figure 29.5b

Sucrose(neutral solute)

(b) H and cotransport of neutral solutes

H/sucrosecotransporter

S H H

HH

H

H

H

H

H

H

H

S

S

S

S

S


Figure 29.5c

Nitrate

(c) H and cotransport of ions

H/NO3−

cotransporter

NO3−

H

H

H

H

H

HH

H

HH

H

NO3

−

NO3−

NO3−

NO3−

NO 3−


Figure 29.5d

Potassium ion

Ion channel

(d) Ion channels

K

K

K

K

K K

K

Plant cells use the energy of H gradients to cotransport other solutes by active transport


Plant cell membranes have ion channels that allow only certain ions to pass


Short-Distance Transport of Water Across Plasma Membranes

To survive, plants must balance water uptake and loss

Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure


Water potential is a measurement that combines the effects of solute concentration and pressure

Water potential determines the direction of movement of water

Water flows from regions of higher water potential to regions of lower water potential

Potential refers to water’s capacity to perform work


Water potential is abbreviated as and measured in a unit of pressure called the megapascal (MPa)

0 MPa for pure water at sea level and at room temperature


How Solutes and Pressure Affect Water Potential

Both pressure and solute concentration affect water potential

This is expressed by the water potential equation: S P

The solute potential (S) of a solution is directly proportional to its molarity

Solute potential is also called osmotic potential


Pressure potential (P) is the physical pressure on a solution

Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast

The protoplast is the living part of the cell, which also includes the plasma membrane


Water potential affects uptake and loss of water by plant cells

If a flaccid (limp) cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis

Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall

Water Movement Across Plant Cell Membranes



Figure 29.6

Plasmolyzedcell at osmoticequilibriumwith itssurroundings

Turgid cellat osmoticequilibriumwith itssurroundings

0.4 M sucrosesolution:

(a) Initial conditions:cellular environmental

−0.9 MPa 0 MPa

Initial flaccid cell:

0 MPa

00

P

S

Pure water:

(b) Initial conditions:cellular environmental

−0.7 MPa

−0.7

P

S

−0.9

P

S

0

−0.9 MPa

−0.9

P

S

0

0

0.7

P

S−0.7


−0.9

Figure 29.6a

Plasmolyzedcell at osmoticequilibriumwith itssurroundings

0.4 M sucrosesolution:

(a) Initial conditions:cellular environmental

−0.9 MPa

−0.9

P

S


−0.9 MPa

P

S

−0.7 MPa

−0.7

P

S

0

0

0


−0.7

Figure 29.6b


−0.7 MPa

P

S

Turgid cellat osmoticequilibriumwith itssurroundings

Pure water:

(b) Initial conditions:cellular environmental

0 MPa

00

P

S

0 MPa

0.7

P

S

0

−0.7

If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid (firm)

Turgor loss in plants causes wilting, which can be reversed when the plant is watered


Video: Turgid Elodea


Figure 29.7

Turgid

Wilted


Figure 29.7a

Wilted


Figure 29.7b

Turgid

Aquaporins: Facilitating Diffusion of Water

Aquaporins are transport proteins in the cell membrane that allow the passage of water

These affect the rate of water movement across the membrane


Long-Distance Transport: The Role of Bulk Flow

Efficient long-distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure

Water and solutes move together through tracheids and vessel elements of xylem and sieve-tube elements of phloem

Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm


Concept 29.3: Plants roots absorb essential elements from the soil

Water, air, and soil minerals contribute to plant growth 80–90% of a plant’s fresh mass is water 96% of plant’s dry mass consists of carbohydrates

from the CO2 assimilated during photosynthesis

4% of a plant’s dry mass is inorganic substances from soil


Macronutrients and Micronutrients

More than 50 chemical elements have been identified among the inorganic substances in plants, but not all of these are essential to plants

There are 17 essential elements, chemical elements required for a plant to complete its life cycle

Researchers use hydroponic culture, the growth of plants in mineral solutions, to determine which chemical elements are essential



Figure 29.8

Technique

Control: Solutioncontaining all minerals

Experimental: Solutionwithout potassium


Table 29.1

Nine of the essential elements are called macronutrients because plants require them in relatively large amounts

The macronutrients are carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium


The remaining eight are called micronutrients because plants need them in very small amounts

The micronutrients are chlorine, iron, manganese, boron, zinc, copper, nickel, and molybdenum

Plants with C4 and CAM photosynthetic pathways also need sodium

Micronutrients function as cofactors, nonprotein helpers in enzymatic reactions


Symptoms of Mineral Deficiency

Symptoms of mineral deficiency depend on the nutrient’s function and mobility within the plant

Deficiency of a mobile nutrient usually affects older organs more than young ones

Deficiency of a less mobile nutrient usually affects younger organs more than older ones

The most common deficiencies are those of nitrogen, potassium, and phosphorus



Figure 29.9

Nitrogen-deficient

Potassium-deficient

Phosphate-deficient

Healthy

Soil Management

Ancient farmers recognized that crop yields would decrease on a particular plot over the years

Soil management, by fertilization and other practices, allowed for agriculture and cities


Fertilization

In natural ecosystems, nutrients are recycled through decomposition of feces and humus, dead organic material

Soils can become depleted of nutrients as plants and the nutrients they contain are harvested

Fertilization replaces mineral nutrients that have been lost from the soil


Commercial fertilizers are enriched in nitrogen (N), phosphorus (P), and potassium (K)

Excess minerals are often leached from the soil and can cause algal blooms in lakes

Organic fertilizers are composed of manure, fishmeal, or compost

They release N, P, and K as they decompose


Adjusting Soil pH

Soil pH affects cation exchange and the chemical form of minerals

Cations are more available in slightly acidic soil, as H+ ions displace mineral cations from clay particles

The availability of different minerals varies with pH For example, at pH 8 plants can absorb calcium but

not iron


At present, 30% of the world’s farmland has reduced productivity because of soil mismanagement


The Living, Complex Ecosystem of Soil

Plants obtain most of their mineral nutrients from the topsoil

The basic physical properties of soil are Texture Composition


Soil Texture

Soil particles are classified by size; from largest to smallest they are called sand, silt, and clay

Topsoil is formed when mineral particles released from weathered rock mix with living organisms and humus


Soil solution consists of water and dissolved minerals in the pores between soil particles

After a heavy rainfall, water drains from the larger spaces in the soil, but smaller spaces retain water because of its attraction to clay and other particles

Loams are the most fertile topsoils and contain equal amounts of sand, silt, and clay


Topsoil Composition

A soil’s composition refers to its inorganic (mineral) and organic chemical components


Inorganic components of the soil include positively charged ions (cations) and negatively charged ions (anions)

Most soil particles are negatively charged

Anions (for example, NO3–, H2PO4

–, SO42–) do not

bind with negatively charged soil particles and can be lost from the soil by leaching


Cations (for example, K, Ca2, Mg2) adhere to negatively charged soil particles; this prevents them from leaching out of the soil through percolating groundwater

During cation exchange, cations are displaced from soil particles by other cations

Displaced cations enter the soil solution and can be taken up by plant roots


Animation: Minerals from Soil


Figure 29.10

Soil particle K

Root hair

Cell wall

Mg2K

H

H

K

Ca2 Ca2

H2O CO2 H2CO3 HCO3−

Organic components of the soil include decomposed leaves, feces, dead organisms, and other organic matter, which are collectively named humus

Humus forms a crumbly soil that retains water but is still porous

It also increases the soil’s capacity to exchange cations and serves as a reservoir of mineral nutrients


Living components of topsoil include bacteria, fungi, algae and other protists, insects, earthworms, nematodes, and plant roots

These organisms help to decompose organic material and mix the soil


Concept 29.4: Plant nutrition often involves relationships with other organisms

Plants and soil microbes have a mutualistic relationship Dead plants provide energy needed by soil-dwelling

microorganisms Secretions from living roots support a wide variety of

microbes in the near-root environment


Soil Bacteria and Plant Nutrition

Soil bacteria exchange chemicals with plant roots, enhance decomposition, and increase nutrient availability


Rhizobacteria

The soil layer surrounding the plant’s roots is the rhizosphere

Rhizobacteria thrive in the rhizosphere, and some can enter roots

The rhizosphere has high microbial activity because of sugars, amino acids, and organic acids secreted by roots


Rhizobacteria known as plant-growth-promoting rhizobacteria can play several roles Produce hormones that stimulate plant growth Produce antibiotics that protect roots from disease Absorb toxic metals or make nutrients more available

to roots


Bacteria in the Nitrogen Cycle

Nitrogen can be an important limiting nutrient for plant growth

The nitrogen cycle transforms atmospheric nitrogen and nitrogen-containing compounds

Plants can only absorb nitrogen as either NO3– or

NH4

Most usable soil nitrogen comes from actions of soil bacteria



Figure 29.11ATMOSPHERE

ATMOSPHERE

SOIL

SOIL

Nitrogen-fixingbacteria

N2

N2 N2

NH3(ammonia)

H

(from soil)

Ammonifyingbacteria

Amino acids

NH4

(ammonium) NO2

−

(nitrite)Nitrifying bacteria

NO3−

(nitrate)Nitrifying bacteria

Denitrifyingbacteria

Microbialdecomposition

Nitrate andnitrogenous

organiccompoundsexported in

xylem to shoot system

NH4

Root

Proteins from humus(dead organic material)


Figure 29.11a

ATMOSPHERE

SOIL


N2 N2

NH3(ammonia)

H

(from soil)

Ammonifyingbacteria

Amino acids

NH4

(ammonium) NO2

−


NO3−







NH4

Root



Figure 29.11b

ATMOSPHERE

SOIL


N2

NH3(ammonia)

H

(from soil)

Ammonifyingbacteria

Amino acids

NH4

(ammonium) NO2

−





Figure 29.11c

N2

NO2−

(nitrite) NO3

−






NH4

Root

Conversion to NH4

Ammonifying bacteria break down organic compounds and release ammonium (NH4

)

Nitrogen-fixing bacteria convert N2 gas into NH3

NH3 is converted to NH4

Conversion to NO3–

Nitrifying bacteria oxidize NH4 to nitrite (NO2

–) then nitrite to nitrate (NO3

–)

Different nitrifying bacteria mediate each step© 2014 Pearson Education, Inc.

Nitrogen is lost to the atmosphere when denitrifying bacteria convert NO3

– to N2


Nitrogen-Fixing Bacteria: A Closer Look

Nitrogen is abundant in the atmosphere but unavailable to plants due to the triple bond between atoms in N2

Nitrogen fixation is the conversion of nitrogen from N2 to NH3:

Some nitrogen-fixing bacteria are free-living; others form intimate associations with plant roots


N2 8 e– 8 H 16 ATP 2 NH3 H2 16 ADP 16

Symbiotic relationships with nitrogen-fixing Rhizobium bacteria provide some legumes with a source of fixed nitrogen

Along a legume’s roots are swellings called nodules, composed of plant cells “infected” by nitrogen-fixing Rhizobium bacteria



Figure 29.12

Roots

Nodules

Inside the root nodule, Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell

The plant obtains fixed nitrogen from Rhizobium, and Rhizobium obtains sugar and an anaerobic environment

Each legume species is associated with a particular strain of Rhizobium


Fungi and Plant Nutrition

Mycorrhizae are mutualistic associations of fungi and roots

The fungus benefits from a steady supply of sugar from the host plant

The host plant benefits because the fungus increases the surface area for water uptake and mineral absorption

Mycorrhizal fungi also secrete growth factors that stimulate root growth and branching


The Two Main Types of Mycorrhizae

Mycorrhizal associations consist of two major types Ectomycorrhizae Arbuscular mycorrhizae



Figure 29.13

Epidermis Cortex

Epidermalcell

Endodermis

Mantle (fungal sheath)


Fungalhyphaebetweencorticalcells (LM) 50 m

1.5 mm(C

olor

ized

SEM

)

Epidermis Cortex

Fungalvesicle

Endodermis

Cortical cell

Casparianstrip

Plasmamembrane

Arbuscules

(LM)

10

m

Roothair

Fungalhyphae

(a) Ectomycorrhizae

(b) Arbuscular mycorrhizae (endomycorrhizae)


Figure 29.13a

Epidermis Cortex

Epidermalcell

Endodermis



Fungalhyphaebetweencorticalcells (LM) 50 m

1.5 mm

(Col

oriz

ed S

EM)

(a) Ectomycorrhizae


Figure 29.13aa


1.5 mm

(Col

oriz

ed S

EM)


Figure 29.13ab

Epidermalcell

(LM) 50 m

Fungalhyphaebetweencorticalcells


Figure 29.13b

Epidermis Cortex

Fungalvesicle

Endodermis

Cortical cell

Casparianstrip

Plasmamembrane

Arbuscules

(LM)

10

m

Roothair

Fungalhyphae

(b) Arbuscular mycorrhizae (endomycorrhizae)


Figure 29.13ba

Cortical cell

Arbuscules

(LM)

10

m

In ectomycorrhizae, the mycelium of the fungus forms a dense sheath over the surface of the root

These hyphae form a network in the apoplast but do not penetrate the root cells

Ectomycorrhizae occur in about 10% of plant families, most of which are woody (for example, pine, birch, and eucalyptus)


In arbuscular mycorrhizae, microscopic fungal hyphae extend into the root

These mycorrhizae penetrate the cell wall but not the plasma membrane to form branched arbuscules within root cells

The arbuscules are important sites of nutrient transfer

Arbuscular mycorrhizae occur in about 85% of plant species, including most crops


Agricultural and Ecological Importance of Mycorrhizae

Seeds can be inoculated with fungal spores to promote formation of mycorrhizae

Some invasive exotic plants disrupt interactions between native plants and their mycorrhizal fungi For example, garlic mustard slows growth of other

plants by preventing the growth of mycorrhizal fungi



Figure 29.14

Experiment Results

Invaded Uninvaded Sterilizedinvaded

Sterilizeduninvaded

Soil type

Invaded UninvadedSoil type White ash

Sugar mapleSeedlings

Red maple

Myc

orrh

izal

colo

niza

tion

(%)

Incr

ease

inpl

ant b

iom

ass

(%) 300

200

100

0

3020100

40


Figure 29.14a

Experiment


Figure 29.14b

Results

Invaded Uninvaded Sterilizedinvaded

Sterilizeduninvaded

Soil type

Invaded UninvadedSoil type White ash

Sugar mapleSeedlings

Red maple

Myc

orrh

izal

colo

niza

tion

(%)

Incr

ease

inpl

ant b

iom

ass

(%) 300

200

100

0

3020100

40

Epiphytes, Parasitic Plants, and Carnivorous Plants

Some plants have nutritional adaptations that use other organisms in nonmutualistic ways

Three unusual adaptations are Epiphytes Parasitic plants Carnivorous plants


Video: Sundew Traps Prey

Epiphytes grow on other plants and obtain water and minerals from rain, rather than tapping their hosts for sustenance



Figure 29.15a

Staghorn fern, an epiphyte

Parasitic plants absorb water, sugars, and minerals from their living host plant

Some species also photosynthesize, but others rely entirely on the host plant for sustenance

Some species parasitize the mycorrhizal hyphae of other plants



Figure 29.15b

Parasitic plants

Mistletoe, a photosyntheticparasite

Dodder, a nonphoto-synthetic parasite(orange)

Indian pipe, a nonphoto-synthetic parasite ofmycorrhizae


Figure 29.15ba

Mistletoe, a photosyntheticparasite


Figure 29.15bb

Dodder, a nonphoto-synthetic parasite(orange)


Figure 29.15bc

Indian pipe, a nonphoto-synthetic parasite ofmycorrhizae

Carnivorous plants are photosynthetic but obtain nitrogen by killing and digesting mostly insects



Figure 29.15c

Carnivorous plants

Pitcher plants

Venus flytraps

Sundew


Figure 29.15ca

Pitcher plants


Figure 29.15cb

Pitcher plants


Figure 29.15cc

Sundew


Figure 29.15cd

Venus flytrap


Figure 29.15ce

Venus flytrap

Concept 29.5: Transpiration drives the transport of water and minerals from roots to shoots via the xylem

Plants can move a large volume of water from their roots to shoots


Absorption of Water and Minerals by Root Cells

Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water

Root hairs account for much of the absorption of water by roots

After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals


The concentration of essential minerals is greater in the roots than in the soil because of active transport


The endodermis is the innermost layer of cells in the root cortex

It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue

Water can cross the cortex via the symplast or apoplast

Transport of Water and Minerals into the Xylem


Animation: Transport in Roots


Figure 29.16

Apoplasticroute

Symplasticroute

Transmembraneroute

The endodermis: controlled entryto the vascular cylinder (stele)

Apoplasticroute

Symplasticroute

Roothair

Plasmamembrane

Epidermis

Casparian strip

Vascularcylinder(stele)

Vessels(xylem)

Cortex

Endodermis

Pathway alongapoplast

Transport in the xylem

Casparian strip

Pathway throughsymplast

Endodermalcell

1

12

2

4

5

3

34

5

54


Figure 29.16a

Apoplasticroute

Symplasticroute

Transmembraneroute


Apoplasticroute

Symplasticroute

Roothair

Plasmamembrane

Epidermis

Casparian strip

Vascularcylinder(stele)

Vessels(xylem)

Cortex

Endodermis


1

12

2

3

4

34

5

5


Figure 29.16b


Pathway alongapoplast


Casparian strip

Pathway throughsymplast

Endodermalcell

54

5

4

Water and minerals can travel to the vascular cylinder through the cortex via The apoplastic route, along cell walls and

extracellular spaces The symplastic route, in the cytoplasm, moving

between cells through plasmodesmata The transmembrane route, moving from cell to cell by

crossing cell membranes and cell walls


The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder

Water and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder


The endodermis regulates and transports needed minerals from the soil into the xylem

Water and minerals move from the protoplasts of endodermal cells into their cell walls

Diffusion and active transport are involved in this movement from symplast to apoplast

Water and minerals now enter the tracheids and vessel elements


Bulk Flow Transport via the Xylem

Xylem sap, water and dissolved minerals, is transported from roots to leaves by bulk flow

The transport of xylem sap involves transpiration, the loss of water vapor from a plant’s surface

Transpired water is replaced as water travels up from the roots


Pulling Xylem Sap: The Cohesion-Tension Hypothesis

According to the cohesion-tension hypothesis, transpiration and water cohesion pull water from shoots to roots

Xylem sap is normally under negative pressure, or tension


Transpirational pull is generated when water vapor in the air spaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata

As water evaporates, the air-water interface retreats farther into the mesophyll cell walls and becomes more curved

Due to the high surface tension of water, the curvature of the interface creates a negative pressure potential


This negative pressure pulls water in the xylem into the leaf

The pulling effect results from the cohesive binding between water molecules

The transpirational pull on xylem sap is transmitted from leaves to roots


Animation: Transpiration

Animation: Water Transport in Plants

Animation: Water Transport


Figure 29.17

Xylem

Microfibrils incell wall of

mesophyll cell

Microfibril (cross section) Air-water

interfaceWaterfilm

Cuticle

Mesophyll

Cuticle Stoma

Air space

Upperepidermis

Lowerepidermis


Figure 29.17a

Xylem


mesophyll cell

Cuticle

Mesophyll

Cuticle Stoma

Air space

Upperepidermis

Lowerepidermis


Figure 29.17b


mesophyll cell

Microfibril (cross section) Air-water

interfaceWaterfilm


Figure 29.18

Cohesionby hydrogenbonding

Water molecule

Cohesionand adhesionin the xylem

Water uptake from soilWaterSoil particle

Root hair

Cell wall

Transpiration

Xylemcells

Water molecule

Adhesion by hydrogenbonding

Xylem sap

Mesophyll cells

Stoma

Atmosphere

Outside air

Wat

er p

oten

tial g

radi

ent

Leaf (air spaces)

Trunk xylem

Leaf (cell walls)

Trunk xylem

Soil

−7.0 MPa

−100.0 MPa

−1.0 MPa

−0.8 MPa

−0.6 MPa

−0.3 MPa


Figure 29.18a

Water molecule

Water uptake from soilWaterSoil particle

Root hair


Figure 29.18b

Cohesionby hydrogenbonding

Cohesionand adhesionin the xylem

Cell wall

Xylemcells

Adhesion by hydrogenbonding


Figure 29.18c

Transpiration

Water molecule

Xylem sap

Mesophyll cells

Stoma

Atmosphere

Cohesion and adhesion in the ascent of xylem sap: Water molecules are attracted to each other through cohesion

Cohesion makes it possible to pull a column of xylem sap

Water molecules are attracted to hydrophilic walls of xylem cell walls through adhesion

Adhesion of water molecules to xylem cell walls helps offset the force of gravity


Thick secondary walls prevent vessel elements and tracheids from collapsing under negative pressure

Drought stress or freezing can cause cavitation, the formation of a water vapor pocket by a break in the chain of water molecules


Xylem Sap Ascent by Bulk Flow: A Review

Bulk flow is driven by a water potential difference at opposite ends of xylem tissue

Bulk flow is driven by evaporation and does not require energy from the plant; like photosynthesis, it is solar powered


Bulk flow differs from diffusion It is driven by differences in pressure potential, not

solute potential It occurs in hollow dead cells, not across the

membranes of living cells It moves the entire solution, not just water or solutes It is much faster


Concept 29.6: The rate of transpiration is regulated by stomata

Leaves generally have broad surface areas and high surface-to-volume ratios

These characteristics increase photosynthesis and increase water loss through stomata

Guard cells help balance water conservation with gas exchange for photosynthesis


Stomata: Major Pathways for Water Loss

About 95% of the water a plant loses escapes through stomata

Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape

Stomatal density is under genetic and environmental control


Mechanisms of Stomatal Opening and Closing

Changes in turgor pressure open and close stomata When turgid, guard cells bow outward and the pore

between them opens When flaccid, guard cells become less bowed and

the pore closes



Figure 29.19 Guard cells turgid/Stoma open

K

Guard cells flaccid/Stoma closed

H2O

Radially orientedcellulose microfibrils

Cellwall

Vacuole

(a) Changes in guard cell shape and stomatal opening andclosing (surface view)

Guard cell

(b) Role of potassium ions (K) in stomatal opening and closing

H2O H2O

H2O

H2OH2O

H2O

H2O

H2OH2O


Figure 29.19a

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

Radially orientedcellulose microfibrils

Cellwall

Vacuole

(a) Changes in guard cell shape and stomatal opening andclosing (surface view)

Guard cell


Figure 29.19b

K

H2O

(b) Role of potassium ions (K) in stomatal opening and closing

H2O H2O

H2O

H2OH2O

H2O

H2O

H2OH2O

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

Changes in turgor pressure result primarily from the reversible uptake and loss of potassium ions (K) by the guard cells


Stimuli for Stomatal Opening and Closing

Generally, stomata open during the day and close at night to minimize water loss

Stomatal opening at dawn is triggered by Light

CO2 depletion

An internal “clock” in guard cells

All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles


Drought stress can cause stomata to close during the daytime

The hormone abscisic acid (ABA) is produced in response to water deficiency and causes the closure of stomata


Effects of Transpiration on Wilting and Leaf Temperature

Plants lose a large amount of water by transpiration If the lost water is not replaced by sufficient transport

of water, the plant will lose water and wilt Transpiration also results in evaporative cooling,

which can lower the temperature of a leaf and prevent protein denaturation


Adaptations That Reduce Evaporative Water Loss

Xerophytes are plants adapted to arid climates



Figure 29.20

Old man cactus(Cephalocereussenilis)

Ocotillo(Fouquieriasplendens)

Oleander (Nerium oleander)Thick cuticle Upper epidermal tissue

Stoma Lower epidermal tissue

CryptTrichomes(“hairs”)

100

m


Figure 29.20a

Ocotillo (leafless)


Figure 29.20b

Ocotillo after heavy rain


Figure 29.20c

Ocotillo leaves


Figure 29.20d

Oleander leaf cross section

Thick cuticle Upper epidermal tissue

Stoma Lower epidermal tissue

CryptTrichomes(“hairs”)

100

m


Figure 29.20e

Oleander flowers


Figure 29.20f

Old man cactus

Some desert plants complete their life cycle during the rainy season

Others have leaf modifications that reduce the rate of transpiration

Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night


Concept 29.7: Sugars are transported from sources to sinks via the phloem

The products of photosynthesis are transported through phloem by the process of translocation


Movement from Sugar Sources to Sugar Sinks

In angiosperms, sieve-tube elements are the conduits for translocation

Phloem sap is an aqueous solution that is high in sucrose

It travels from a sugar source to a sugar sink A sugar source is an organ that is a net producer of

sugar, such as mature leaves A sugar sink is an organ that is a net consumer or

storer of sugar, such as a tuber or bulb


A storage organ can be both a sugar sink in summer and sugar source in winter

Sugar must be loaded into sieve-tube elements before being exported to sinks

Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways

Companion cells enhance solute movement between the apoplast and symplast



Figure 29.21

ApoplastSymplast

Cell walls (apoplast)

Plasmodesmata

Mesophyll cell

Plasmamembrane

Companion(transfer) cell

Sieve-tubeelement

Mesophyllcell

Bundle-sheath cell

Phloemparenchyma cell

(a) Sucrose manufactured in mesophyll cellscan travel via the symplast (blue arrows)to sieve-tube elements.

(b) A chemiosmotic mechanism isresponsible for the active transport ofsucrose.

High H concentration

Low H concentration

Protonpump

Cotransporter

SucroseS

SH

H H


Figure 29.21a

Apoplast

Symplast

Cell walls (apoplast)

Plasmodesmata

Mesophyll cell

Plasmamembrane

Companion(transfer) cell

Sieve-tubeelement

Mesophyllcell

Bundle-sheath cell

Phloemparenchyma cell

(a) Sucrose manufactured in mesophyll cellscan travel via the symplast (blue arrows)to sieve-tube elements.


Figure 29.21b

(b) A chemiosmotic mechanism isresponsible for the active transport ofsucrose.

High H concentration

Low H concentration

Protonpump

Cotransporter

SucroseS

SH

H H

In most plants, phloem loading requires active transport

Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose

At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water


Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms

Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow


Animation: Phloem Translocation Spring

Animation: Phloem Translocation Summer


Figure 29.22

Loading of sugar

H2O

Vessel(xylem)

Source cell(leaf)

Sievetube

(phloem)

H2OSucrose

Uptake of water

Unloading of sugar

Recycling of water

Sink cell(storageroot)

H2OSucrose

Bul

k flo

w b

y ne

gativ

e pr

essu

re

Bul

k flo

w b

y po

sitiv

e pr

essu

re

1

1

2

2

3

4 3 4

Sometimes there are more sinks than can be supported by sources

Self-thinning is the dropping of sugar sinks such as flowers, seeds, or fruits



Figure 29.UN01a


Figure 29.UN01b


Figure 29.UN02

Minerals H2O

O2

CO2

H2O

O2

CO2


Figure 29.UN03

biology in focus - chapter 29

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