plant nutrition and transport - accountax school of …accountax.us/secondary education biology...

© Cengage Learning 2015

Biology Concepts and Applications | 9e

Starr | Evers | Starr


Chapter 26

Plant Nutrition and Transport


26.1 Where Do Plants Get the Nutrients

They Require?

• A plant needs sixteen elements to survive

and grow

– Macronutrients: carbon, oxygen, hydrogen,

nitrogen, phosphorous, sulfur, potassium,

calcium and magnesium

– Micronutrients: chlorine, iron, boron,

manganese, zinc, copper, and molybednum


Properties of Soil

• Carbon, oxygen, and hydrogen atoms are

abundantly available in carbon dioxide and

water

• Plants get the other elements they need

when their roots take up minerals

dissolved in soil water


Properties of Soil (cont’d.)

• Soil consists mainly of mineral particles:

– Clay particles: attracts positively charged

mineral ions in soil water

– Sand and silt: intervene between tiny particles

of clay; allows roots to access oxygen



• Soils with the best oxygen and water

penetration are loams

– Have roughly equal proportions of sand, silt,

and clay



• Most plants do best in loams that contain

between 10 and 20 percent humus

– Decomposing organic material (e.g., fallen

leaves, feces) that releases nutrients and

traps minerals


How Soils Change

• Soils develop over thousands of years

– Most form in layers, or horizons, that are

distinct in color and other properties

– Topsoil: uppermost soil layer; most organic

matter and nutrients for plant growth

• Grasslands typically have a deep layer of topsoil;

tropical forests do not


How Soils Change (cont’d.)

O Horizon

A Horizon

B Horizon

C Horizon

Bedrock



• Minerals, salts, and other molecules

dissolve in water as it filters through soil

– Leaching: process by which water removes

soil nutrients and carries them away

– Soil erosion: loss of soil under the force of

wind and water


26.2 How Do Plant Roots Absorb Water

and Nutrients?

• Water moves from soil, through a root’s

epidermis and cortex, to the vascular

cylinder

– Osmosis drives this movement; fluid in the

plant typically contains more solutes than soil

water

• Xylem distributes water and mineral ions

to the rest of the plant


ANIMATION: Root organization

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videos_animations/root_structure.html


How Do Plant Roots Absorb Water and

Nutrients? (cont’d.)

• Soil water enters a root and moves

through cell walls

– Soil water diffuses from epidermis, through

the cortex, reaching the vascular cylinder

– A Casparian strip (waterproof band of plasma

membranes) forces water to enter a vascular

cylinder by passing through endodermal cells

– Water diffuses from cell to cell through

plasmodesmata until it enters xylem


ANIMATION: Root functioning




videos_animations/root_functioning.html


How Do Plant Roots Absorb Water and

Nutrients? (cont’d.)

• Ions cannot cross the plasma membrane

– Mineral ions dissolved in soil water enter a

cell’s cytoplasm through transport proteins

• Once actively transported into cells,

mineral ions diffuse from cell to cell

through plasmodesmata until they enter

xylem


26.3 What Mutualisms Affect Root

Function?

• Mycorrhiza: mutually beneficial interaction

between a root and a fungus

– Filaments of the fungus (hyphae) form a

velvety cloak around roots or penetrate their

cells

– Root cells get some scarce minerals that the

fungus is better able to absorb


What Mutualisms Affect Root Function?

(cont’d.)

• Mutualism with nitrogen-fixing Rhizobium

– Roots release certain compounds into the soil

that are recognized by compatible Rhizobium

– Roots encapsulate Rhizobium bacteria inside

swellings called root nodules

– Rhizobium in root nodules fix nitrogen gas to

ammonia for plant use

– Plant provides oxygen-free environment and

sugars to the anaerobic Rhizobium bacteria


What Mutualisms Affect Root Function?

(cont’d.)

a root nodule


26.4 How Does Water Move Through

Xylem?

• Water that enters a root travels to the rest

of the plant inside tubes of xylem

– Xylem cells deposit secondary wall material

– Just before the cells die, they digest away

most of their primary wall

– Holes/pits remain in secondary walls where

plasmodesmata once connected living cells

– In mature xylem tubes, water flows laterally

through the pits, between adjacent cells


How Does Water Move Through Xylem?

(cont’d.)

xylem

tube

pit

water



(cont’d.)

• Vessel elements:

– Cells that form in stacks in xylem and die

when mature

– Their pitted walls remain to form water-

conducting tubes

– Each tube consists of a stack of vessel

elements that meet end to end at perforation

plate



(cont’d.)

vessel element

perforation plate



(cont’d.)

• Tracheids:

– Tapered cells of xylem that die when mature

– Their interconnected, pitted walls remain and

form water-conducting tubes



(cont’d.)

tracheid


Cohesion–Tension Theory

• Tracheids and vessel elements that

compose xylem tubes are dead and

cannot pump water upward against gravity

• The movement of water in vascular plants

is driven by two features of water:

– Evaporation and cohesion


Cohesion–Tension Theory (cont’d.)

• Cohesion–tension theory: water in xylem

is pulled upward by air’s drying power,

which creates a continuous negative

pressure called tension



• Transpiration: evaporation of water from

aboveground plant parts

– Creates tension that pulls a cohesive column

of water upward through xylem


ANIMATION: Transpiration




videos_animations/transpiration_m.html



• Pits in the sides of vessels and tracheids

are often bordered by pectin-containing

secondary walls

– Pectins shrink when dry, and swell when wet



• When mineral-rich water flows through a

pectin-bordered pit, the border swells and

eventually plugs the hole

– Water tends to flow toward the thirstiest

regions of the plant, where pectin-bordered

pits are dry and open


26.5 How Do Land Plants Conserve

Water?

• In land plants, at least 90 percent of water

taken up by roots is lost by evaporation

• A waterproof cuticle helps a land plant

conserve water

– The cuticle also restricts gas exchanges with

air


How Do Land Plants Conserve Water?

(cont’d.)

• Stoma: an opening between a pair of

guard cells (specialized cells of epidermis)

• When guard cells swell with water, they

bend slightly forming a gap (the stoma)

• Open stomata allow gases and water

vapor to cross the epidermis

• When guard cells lose water, they collapse

against one another, closing the stoma


How Do Land Plants Conserve Water? (cont’d.)

guard cells

closed stoma open stoma



(cont’d.)

cuticle

epidermis stoma

guard cells



(cont’d.)

• Stomata open or close based on

environmental cues

– Example: light from the sun causes guard

cells to begin pumping potassium ions

• Water follows the ions by osmosis, plumping the

guard cells, opening the stomata

• Carbon dioxide diffuses through the open stomata

into the plant’s tissues, and photosynthesis begins


26.6 How Do Sugars Move Through

Phloem?

• Sieve tubes: sugar-conducting tube of

phloem; consists of stacked sieve

elements

– Sieve elements are living cells that meet end

to end at sieve plates

– Companion cells: parenchyma cells that

provide metabolic support to its partnered

sieve element


How Do Sugars Move Through Phloem?

(cont’d.)

phloem


Pressure Flow Theory

• Sugars are transported via sieve tubes

– Movement of sugars through phloem is called

translocation


Pressure Flow Theory (cont’d.)

• Inside sieve tubes, fluid rich in sugars flow

from a source to a sink because of a

pressure gradient

– Source: region where sugars are produced or

released

– Sink: region where sugars are being broken

down for energy



• Pressure flow theory:

– A difference in turgor between sieve elements

in source and sink regions pushes sugar-rich

fluid through a sieve tube


26.7 Application: Leafy Cleanup

• J-Field, Aberdeen Proving Ground:

– From World War I until the 1970s, the United

States Army tested and disposed of weapons

at this site

– Chemicals, chemical weapons, explosives,

plastics, and solvents were burned in pits

– Lead, arsenic, mercury, and TCE

contaminated the soil and groundwater


Application: Leafy Cleanup (cont’d.)

• To clean up the soil and protect nearby

Chesapeake Bay, the Army and the

Environmental Protection Agency turned to

phytoremediation

– The use of plants to take up and concentrate

or degrade environmental contaminants



• Ford Motor Company’s Rouge Center:

decades of steelmaking left soil

contaminated with highly carcinogenic

compounds

– Researchers developed a phytoremediation

system based on native plants

plant nutrition and transport - accountax school of …accountax.us/secondary education biology...

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