Plant Nutrition and Transport

Chapter 29

Impacts, Issues

Leafy Cleanup Crews

The EPA is using hybrid plants to remove some

dangerous toxins from highly contaminated sites

– a process known as phytoremediation

29.1 Plant Nutrients

and Availability in Soil

Nutrients

• Elements or molecules essential for an

organism’s growth and survival

Plants require sixteen elemental nutrients

available from soil, water, and air

• Nine macronutrients, required in large amounts

• Seven micronutrients, required in trace amounts

Plant Nutrients

and Deficiency Symptoms

Properties of Soil

Soil consists of mineral particles mixed with

decomposing organic material (humus)

• Water and air in spaces between particles

Mineral particles in soil differ in size (sand, silt,

and clay) which affects compaction

• Clay particles are negatively charged, and can

hold positively charged ions dissolved in water

Soils and Plant Growth

Different soil types affect growth of different plants

• Most plants grow best in soils containing 10 to 20

percent humus

• Soils with equal proportions of sand, silt, and

humus (loams) have the best oxygen and water

penetration

• Swamps and bogs have too much organic matter

How Soils Develop

Soils develop over thousands of years

Most form in layers (horizons) with distinct

properties (soil profiles)

Topsoil (the A horizon) contains the most

organic material

Soil Horizons

Fig. 29-2, p. 495

O HORIZON Fallen leaves and

other organic material littering

the surface of mineral soil

A HORIZON Topsoil, with decomposed

organic material; variably deep [only a few

centimeters in deserts, elsewhere extending

as far as 30 centimeters (1 foot) below the

soil surface]

B HORIZON Compared with A horizon, larger

soil particles, not much organic material,

more minerals; extends 30 to 60 centimeters

(1 to 2 feet) below soil surface

C HORIZON No organic material, but partially

weathered fragments and grains of rock from

which soil forms; extends to underlying

bedrock

BEDROCK

Leaching and Erosion

Leaching

• Process by which water removes soil nutrients

and carries them away

• Fastest in sandy soils

Soil erosion

• A loss of soil under the force of wind and water

• Increases with sparse vegetation and poor

farming practices

Erosion Due to Poor Farming Practices

29.1 Key Concepts

Plant Nutrients and Soil

Many plant structures are adaptations to limited

amounts of water and essential nutrients

The amount of water and nutrients available for

plants to take up depends on the composition of

soil

Soil is vulnerable to leaching and erosion

29.2 How Do Roots

Absorb Water and Nutrients?

Root specializations such as root hairs,

mycorrhizae, and nodules help the plant absorb

water and nutrients

Root Hairs

Root hairs

• Thin extensions of root epidermal cells that

enormously increase surface area available for

absorbing water and dissolved mineral ions

• New root hairs constantly form just behind the

root tip

Mycorrhizae

Mycorrhizae

• Forms of mutualism between root and fungi in

which both species benefit

• Fungal hyphae share minerals absorbed from soil

• Root cells provide fungus with food

Root Nodules

Root nodules

• Masses of root cells infected with bacteria that fix

atmospheric nitrogen into a form usable by plants

(nitrogen fixation)

• A mutualism between certain types of soil

bacteria and legumes

Root Specializations

How Roots Control Water Uptake

Osmosis drives water from soil into the walls of

parenchyma cells of the root cortex

Water enters cell cytoplasm by diffusion or

through aquaporins; active transporters pump

dissolved mineral ions into cells

Water and ions move from cell to cell through

plasmodesmata

The Casparian Strip

Endodermis between the cortex and vascular

cylinder secretes a waxy substance which forms

a waterproof band (Casparian strip) between

plasma membranes of endodermal cells

The Casparian strip forces water and ions to

enter the vascular cylinder through

plasmodesmata or through endodermal cell

membranes (controlled by transport proteins)

Exodermis

Exodermis

• A layer of cells just below the root surface that

can deposit a Casparian strip that functions like

the one next to the vascular cylinder

Control of Water and Ion Uptake

by Transport Proteins

Fig. 29-5a, p. 497

Fig. 29-5a, p. 497

vascular cylinder

epidermis

endodermis

primary

phloem

primary

xylem

cortex

A In roots, the

vascular cylinder’s

outer layer is a sheet

of endodermis, one

cell thick.

Fig. 29-5b, p. 497

Fig. 29-5b, p. 497

vascular cylinder

tracheids

and vessels

in xylem

sieve tubes

in phloem

B Parenchyma cells that

make up the layer secrete a

waxy substance into their

walls wherever they touch.

The secretions form a

Casparian strip, which

prevents water from

seeping around the cells

into the vascular cylinder.endodermal cell

Casparian strip

Fig. 29-5c, p. 497

Fig. 29-5c, p. 497

C Water and ions can

only enter the

vascular cylinder by

moving through cells

of the endodermis.

They enter the cells

via plasmodesmata or

via transport proteins

in the cells’ plasma

membranes.

Vascular

cylinder

Casparian

strip

water and nutrients

Cortex

Animation: Root functioning

29.3 How Does Water

Move Through Plants?

The upward movement of water through xylem,

from roots to leaves, is driven by two properties

of water: evaporation and cohesion

Tracheids and vessel members

• Water conducting tubes of xylem

• Cells are dead at maturity

• Lignin-impregnated walls remain

Tracheids and Vessel Members

Fig. 29-6a, p. 498

Fig. 29-6a, p. 498

perforation

in the side

wall of

tracheid

a Tracheids have

tapered, unperforated

end walls. Perforations

in the side walls of

adjoining tracheids

match up.

Fig. 29-6b, p. 498

Fig. 29-6b, p. 498

vessel member

b Three adjoining vessel

members. The thick, finely

perforated end walls of dead cells

connect to make long tubes that

conduct water through xylem.

Fig. 29-6c, p. 498

Fig. 29-6c, p. 498

perforation

plate

c Perforation plate at the end

wall of one type of vessel

member. The perforated ends

allow water to flow freely

through the tube.

Cohesion-Tension Theory

Continuous negative pressure (tension) created

by evaporation of water from leaves and stems

(transpiration) pulls water upward through xylem

Hydrogen bonds among water molecules

(cohesion) in continuous columns inside xylem

tubes keep water from breaking into droplets

Cohesion-Tension Theory

Fig. 29-7a, p. 499

Fig. 29-7a, p. 499

A The driving force of

transpiration

Evaporation of water

molecules from above

ground plant parts puts

water in xylem into a

state of tension that

extends from roots to

leaves. For clarity,

tissues inside the vein

are not shown.

mesophyll

(photosynthetic cells) upper epidermisvein

stoma

Fig. 29-7b, p. 499

Fig. 29-7b, p. 499

B Cohesion of water

inside xylem tubes

Even though long

columns of water that

fill narrow xylem tubes

are under continuous

tension, they resist

breaking apart. The

collective strength of

many hydrogen bonds

keeps individual water

molecules together.

phloemxylem

vascular

cambium

Fig. 29-7c, p. 499

Fig. 29-7c, p. 499

C Ongoing water

uptake at roots

Water molecules

lost from the plant

are being

continually

replaced by water

molecules taken

up from soil.

Tissues in the vein

not shown.

cortex

vascular

cylinder endodermis

water

molecule

root hair

cell

Animation: Transpiration

29.2-29.3 Key Concepts: Water Uptake

and Movement Through Plants

Certain specializations help roots of vascular

plants take up water and nutrients

Xylem distributes absorbed water and solutes

from roots to leaves

29.4 How Do Stems

and Leaves Conserve Water?

Water is an essential resource for all land plants

Water-conserving structures (cuticle and

stomata) and processes are key to the survival

of land plants

The Water-Conserving Cuticle

Cuticle

• A translucent, water-impermeable layer coating

the walls of all plant cells exposed to air

• Consists of epidermal cell secretions: waxes,

pectin, and cellulose fibers embedded in cutin

Controlling Water Loss at Stomata

Stomata

• Openings through the plant epidermis that

regulate water vapor loss and gas exchange

• Formed by two guard cells

Guard cells open or close the stoma depending

on the amount of water in their cytoplasm

• Swollen cells open stoma

• Collapsed cells close stoma

Controlling Water Loss at Stomata

Environmental cues open or close stomata

• Water availability (abscisic acid released by root

cells)

• Carbon dioxide levels in leaf (aerobic respiration)

• Light intensity (triggers potassium pumps)

• Air pollution (prevents photosynthesis)

Stomata and Industrial Smog

29.4 Key Concepts

Water Loss Versus Gas Exchange

A cuticle and stomata help plants conserve

water, a limited resource in most land habitats

Closed stomata stop water loss but also stop

gas exchange

Some plant adaptations are trade-offs between

water conservation and gas exchange

29.5 How Do Organic Compounds

Move Through Plants?

Phloem distributes the organic products of

photosynthesis through plants

Concentration and pressure gradients in the

sieve-tube system of phloem force organic

compounds to flow to different parts of the plant

Phloem:

Sieve-Tube Members and Sieve Plates

Fig. 29-10a, p. 502

one of a series of

living cells that abut,

end to end, and form

a sieve tube

companion cell (in

the background,

pressed tightly

against sieve tube)

perforated end plate

of sieve-tube cell, of

the sort shown in (b)

Organic Products of Photosynthesis

Plants store carbohydrates as starch, and

distribute them as sucrose and other small,

water-soluble molecules

Pressure-Flow Theory

Translocation

• Gradients set up by companion cells move

organic molecules into sieve tubes at sources,

and unload them at sinks

Pressure-flow theory

• Internal pressure (turgor) builds up in sieve tubes

at a source, pushing solute-rich fluid to a sink,

where sucrose is removed from the phloem

Translocation of Organic Compounds:

Sources and Sinks

Fig. 29-12a, p. 503

Fig. 29-12a, p. 503

Translocation

inter-

connected

sieve tubesSOURCE (e.g.,mature leaf cells)

A Solutes move into a sieve tube against their concentration gradients by active transport.

WATER

B As a result of increased solute concentration, the fluid in the sieve tube becomes hypertonic.

C The pressure difference pushes the fluid from the source to the sink. Water moves into and out of the sieve tube along the way.

flow

D Both pressure and solute concentrations gradually decrease as the fluid moves from source to sink.

E Solutes are unloaded into sink cells, which then become hypertonic with respect to the sieve tube. Water moves from the sieve tube into sink cells.

SINK (e.g., developing root cells)

Fig. 29-12b, p. 503

Fig. 29-12b, p. 503

upper leaf epidermis

photosynthetic cell

sieve tube in leaf

vein

companion cell next

to sieve tube

lower leaf epidermis

Typical source region

Photosynthetic tissue in a leaf

Fig. 29-12c, p. 503

Fig. 29-12c, p. 503

sieve

tube

Typical sink region

Actively growing cells in a young root

29.5 Key Concepts

Sugar Distribution Through Plants

Phloem distributes sucrose and other organic

compounds from photosynthetic cells in leaves

to living cells throughout the plant

Organic compounds are actively loaded into

conducting cells, then unloaded in growing

tissues or storage tissues

Summary:

Processes that Sustain Plant Growth

Fig. 29-13, p. 504

ATP formation

by roots

absorption of

minerals and

water by roots

transport of minerals

and water to leaves

respiration of

sucrose by rootstransport of

sucrose to rootsphotosynthesis

Animation: Cohesion-tension theory (or

Water transport)

Animation: Interdependent processes

Animation: Soil profile

Animation: Stomata

Animation: Translocation in phloem

Animation: Uptake of nutrients by plants

Animation: Water absorption

Video: Leafy clean-up crews

Video: Sequoias