Absorption and Transport

Chapter 11

Transport and Life

• Plants have same general needs as animals for transporting substances from one organ to another

• Plants need supply of water – Maintain structures– Photosynthesis– Growth– Die if dehydrated

Transport and Life

• Replacement water comes from soil through roots

• Need transport system to get water from soil into roots and up to leaves

• Growth requires mineral nutrients– Must have system to transport minerals to

meristematic regions

Transport and Life

• Carbohydrates produced in photosynthesis provide energy and C skeleton for synthesis of other organic molecules– Energy needed in all plant parts but especially

in meristematic regions of stems and roots and in flowers, seeds, and fruits

• Must have system for transporting carbohydrates from photosynthetic organs to living cells in plant

Water

• Most abundant compound in living cell

• Solvent

• Moves solutes from place to place

• Substrate or reactant for many biochemical reactions

• Provides strength and structure to herbaceous organs

Factors Affecting Flow of Water in Air, Cells, and Soil

• Five major forces– Diffusion– Osmosis– Capillary forces– Hydrostatic pressure– Gravity

Factors Affecting Flow of Water in Air, Cells, and Soil

• Diffusion– Flow of molecules from regions of higher to

lower concentrations– Major force for directing flow of water in gas

phase– Liquid water and solute molecules also diffuse

• Example: place drop of dye in glass of water

Factors Affecting Flow of Water in Air, Cells, and Soil

• Osmosis– Diffusion of water across selectively

permeable membrane from a dilute solution (less solute, more water) to a more concentrated solution (more solute, less water)

– Osmotic pump• Device that uses osmosis to power the flow of

water out of a chamber• Works by pressure generated through osmosis

Factors Affecting Flow of Water in Air, Cells, and Soil

• Hydrostatic pressure– In cells, called turgor pressure– Opposes flow of water into cells– Importance of turgor

• Stiffens cells and tissues

Factors Affecting Flow of Water in Air, Cells, and Soil

• Capillary forces– Water molecules are cohesive

• Stick to each other

– Water molecules are adhesive• Stick to hydrophilic molecules• Example: carbohydrates

– Cohesion and adhesion can generate tension that pulls water into small spaces

Factors Affecting Flow of Water in Air, Cells, and Soil

• Capillary forces– Forces pulling water into tube– Produce a tension in water like a stretched

rubber band– Maximum tension that can develop in capillary

tube depends on cross-sectional area of bore• Smallest bores produce greatest tensions

Factors Affecting Flow of Water in Air, Cells, and Soil

• Water pulled into soil and held there by capillary forces– Strength of forces depends on amount of

water present• Dry soil – stronger tension

Factors Affecting Flow of Water in Air, Cells, and Soil

• Gravity– Takes force to move water upward– Significant factor in tall trees

Water Potential

• Takes into account all the forces that move water

• Combines them to determine when and where water will move through a plant

• Water always tends to flow from a region of high water potential to a region of low water potential– If water potential of soil around root is less than water

potential of root cells, water will flow out of root into the soil

Water Potential

• Can calculate water potential from physical measurements– Useful to agriculturists who estimate water

needs

Transpiration

• Flow of water through plant is usually powered by loss of water from leaves

• Transpiration pulls water up the plant– Major event is diffusion of water vapor from

humid air inside leaf to drier air outside the leaf

– Loss of water from leaf generates force that pulls water into leaf from vascular system, from roots, and from soil into roots

Diffusion of Water Vapor Through Stomata

• Intercellular air spaces in leaves close to equilibrium with solution in cellulose fibrils of cell walls

• Bulk of air outside leaves generally dry

• Strong tendency for diffusion of water vapor out of leaf

• Water vapor diffuses out of stomata– Route by which most water is lost from plant

Diffusion of Water Vapor Through Stomata

• Anatomical leaf features that slow diffusion rate– Dense layer of trichomes on leaf surface– Stomatal crypts (sunken stomata)

• Depressions in leaf surface into which stomata open

• Warm air holds more water than cool air– Plants lose water faster when temperature is

high

Flow of Water Into Leaves

• Water vapor evaporates from surrounding cell walls when water vapor is lost from intercellular spaces of leaf– Partially dries cell walls– Produces capillary forces that attract water

from adjacent area in leaf• Some replacement water comes from inside leaf

cells across plasma membrane– Too much water lost, plant wilts

Flow of Water Into Leaves

• In well-watered plant, water from cell walls and from inside cell replaced by water from xylem

Flow of Water Through Xylem

• Removal of one water molecule out of central space of tracheid

• Results in hydrostatic tension on rest of water in tracheids and vessels

• If water continues to flow from leaf tracheid into leaf cell walls– Constant stream of water flowing from xylem– Powered by tension gradient

Flow of Water Through Xylem

• Tracheids– Fairly high resistance to water flow– Require fairly steep tension gradient to

maintain adequate flow– Air bubble in one tracheid has no effect on

overall flow

Flow of Water Through Xylem

• Vessels– Lower resistance to water flow– More easily inactivated by air bubbles

• Few vessels• Bubble in vessel may block substantial amount of

water flow

Flow of Water Through Xylem

• Conifers– Only tracheids, no vessels– Advantage in dry, cold climates

• Conditions most likely to produce air bubbles in xylem

Symplastic and Apoplastic Flow Through Roots

• Pathway– Loss of water through xylem decreases water

potential in xylem of growing primary root– Pulls water from apoplast of stele of root– Water from apoplast of stele is replaced by

water flowing into stele from root cortex– Water from soil moves into root cortex

Symplastic and Apoplastic Flow Through Roots

• Because no cuticle over epidermis of primary root– Water can flow between cells of epidermis

directly into apoplast of cortex and to endodermis

• Water cannot cross endodermis because of Casparian strip

Symplastic and Apoplastic Flow Through Roots

• To go further into root– Water must enter symplast by crossing

plasma membrane of endodermal cell– Can also cross plasma membrane of cells at

root hairs or in cortex– Can flow from cell to cell through symplast via

plasmodesmata• Cross endodermis in symplast• Enters apoplast• Flows into xylem

Symplastic and Apoplastic Flow Through Roots

• Water must pass through at least two plasma membranes to reach root xylem from soil

Flow Through Soil

• Can be considerable resistance to flow of water through soil– Capillary spaces are small– Distances may be long

• Limits rate at which water can reach leaves

Flow Through Soil

• Temporary wilt– Occurs when water does not move quickly

enough to replace water lost from leaves– Plant recovers if water loss is stopped

• Permanent wilt– Occurs when osmotic forces pulling water into

cells are not as great as the attractive forces holding water to soil particles

– Plant does not recover

Control of Water Flow

• Transpiration– Slow at night– Increases after sun comes up– Peaks middle of day– Decreases to night level over afternoon

• Rate of transpiration directly related to intensity of light on leaves

Control of Water Flow

• Other environmental factors affecting rate– Temperature– Relative humidity of bulk air– Wind speed

Stomata

• Primary sensing organs are guard cells– Illumination

• Concentration of solutes in vacuoles of guard cells increases

• Starch in chloroplasts of guard cells converted to malic acid

Stomata

• Proton pump in guard cell plasma membrane activated

– Moves H+ across plasma membrane– K+ and Cl- ions flow through different channels into cells

• Accumulation of malate, K+, Cl- increase osmotic effect drawing water into guard cells

• Extra water volume in guard cells expands walls increasing turgor pressure

Stomata

• Guard cells bend away from each other opening stoma between them

– Specialized cell walls of guard cells» Cellulose microfibrils wrapped around long axis of

cells (radial micellation)» Heavier, less extensible wall adjacent to stoma

• Darkness reverses process

Mineral Uptake and Transport

• Plants synthesize organic growth compounds– Do not need to take them in

• Need to take in elements that are substrates or catalysts for synthetic reactions

Mineral Uptake and Transport

• Plant cells take up mineral elements only when elements are in solution– Dissolution of crystals in rock and soil

particles– Decomposition of organic matter in soil

Roles of Mineral Elements in PlantsElement Primary Roles

Potassium (K) Osmotic solute, activation of some enzymes

Nitrogen (N) Structure of amino acids and nucleic acid bases

Phosphorus (P) Structure of phospholipids, nucleic acids, adenosine triphosphate

Sulfur (S) Structure of some amino acids

Calcium (Ca) Structure of cell walls, transmission of developmental signals

Magnesium (Mg) Structure of chlorophyll, activation of some enzymes

Iron (Fe) Structure of heme in respiratory, photosynthetic enzymes

Manganese (Mn) Activation of photosynthetic enzyme

Chloride (Cl) Activation of photosynthetic enzyme, osmotic solute

Boron (B), cobalt (Co), copper (Cu), zinc (Zn)

Activation of some enzymes

C. HOPKiNS CaFe – Mighty good (mnemonic for remembering elements)

Soil Types

• Soil– Part of Earth’s crust that has been changed

by contact with biotic and abiotic parts of environment

– 1-3 m in thickness– Made up of

• Physically and chemically modified mineral matter• Organic matter in various stages of decomposition

Soil Types

• Soils differ in– Depth– Texture– Chemistry– Sequence of layers

Soil Types

• Soil type– Basic soil classification unit

• Soil types grouped into– Soil series– Families– Orders

• 11 soil orders

• Distribution of specific types of plants often correlated with presence of particular soil types

Soil Formation

• Dissolving elements from rock– Begins with acidic rain– Rain dissolves crystals in rock– Rate of dissolving depends on crystal surface

area in contact with water– Freezing and thawing of water in cracks of

rocks• Breaks off pieces of rock• Forms new fissures

Soil Formation

• Starts soil formation process

• Water and wind erosion pulverize rock particles

• Lichens and small plants start to grow – Rhizoids and roots enlarge fissures in rocks

Soil Formation

• Best soils– Do not have greatest concentration of

minerals in soil solution– High ion concentration increases osmotic

effect of soil and limits movement of water into plant

– High concentration of some ions • Toxic to plants• Al3+, Na+

Soil Formation

• Best to have lower concentration of nutrients with source that releases ions into solution as they are taken up by plants

Nitrogen Fixation

• Nitrogen– Needed in large amounts by plants

– Plants cannot use atmospheric nitrogen (N2)

• Must be converted to NH4+ or NO3

- through process of nitrogen fixation

• Nitrogen fixation– Catalyzed by enzymes in bacteria

• Bacteria free living in soil• Bacteria in association with roots of plants (legumes)

– Rhizobium

Nitrogen Fixation

• NH4+ NO3

- – Nitrification– NO3

- very soluble and easily leached from soil

• NO3- NH4

+

– occurs in plants– Nitrate reduction

• NO3- N2

– Denitrification– Carried out by certain soil bacteria

Minerals Accumulated by Root Cells

• All plant cells require mineral source– Especially meristematic regions

• Minerals in solution– Passive transport in stream of water pulled

through plant by transpiration– Active processes contributing to uptake and

transport• Require input of energy from ATP or NADPH

Maintenance of Mineral Supply

• Three processes replenish mineral supply– Bulk flow of water in response to transpiration– Diffusion– Growth

• As root grows, comes in contact with new soil region and new supply of ions

Uptake of Minerals Into Root Cells

• Ion transported across plasma membrane into root cell

• Enter epidermis– Moves along symplast– Travels as far as endodermis through

apoplastic pathway

Uptake of Minerals Into Root Cells

– Reaches endodermis• Crosses plasma membrane

– Allows plant to exclude toxic ions– Concentrate needed nutrients in low concentration in soil

solution– Requires ATP energy

Mycorrhizae

• Association of filamentous fungi with roots of some plants

• Plants with mycorrhizae often grow better than plants with mycorrhizae

Mycorrhizae

• Mutualistic relationship– Mycorrhizal fungi have high-affinity system for

taking up phosphate– Fungus provides phosphate for uptake into

plant roots– Plant roots provide carbon and nutrients to

fungus

Ion Transport From Root to Shoot

• Ions secreted into apoplast– Enter xylem

• Takes ions to wherever stomata are open and transpiration is occurring

– Transported to shoot• Taken up into shoot cells• Greater concentration of ions accumulate and

solvent water evaporates

Ion Transport From Root to Shoot

• Example of ion accumulation– Dead tips of older leaves of slow-growing

house plants• Sign ions have accumulated to toxic level• Water this type of plant infrequently but thoroughly• Allow excess water to drain through pot• Fertilize infrequently

Root Pressure

• Root pressure is result of osmotic pump

• Accumulation of ions in stele has osmotic effect

• Soil saturated with water– Water tends to enter root and stele– Builds up root pressure in xylem– Forces xylem sap up into shoot

Root Pressure

• Hydathode– Specialized opening in leaves of some

grasses and small herbs

• Guttation– Water forced out of hydathodes by root

pressure

Phloem Transport

• Translocation – transport of carbohydrates in plant

• Carbohydrates– Product of photosynthesis– Source of carbon for synthesis of all other organic

compounds– Can be stored temporarily in chloroplast of mature

leaf cells– May be exported from leaf in form of sucrose or other

sugars

Phloem Transport

• Carbohydrate pathway through phloem traced using radioactive CO2

– Rate of transport is faster than diffusion or transport from individual cell to cell

– Not as fast as the rate at which water is pulled through xylem

• Phloem transport can change direction

Phloem Transport

• Current idea of transport– Sucrose flows through sieve tubes as one

component in bulk flow of solution– Flow directed by gradient of hydrostatic

pressure– Powered by osmotic pump

Phloem Transport

• Phloem– Dynamic osmotic pump– Source of solute at one end and sink at the

other– Sucrose is main osmotically active solute in

phloem– Sucrose pumped from photosynthetically

active parenchyma cells into sieve tubes of minor veins

• Exact pathway unknown

Phloem Transport

– Accumulation of sucrose in sieve tube pulls water into sieve tube from apoplast by osmosis

• Increases hydrostatic pressure inside sieve tube at source

• Pressure starts flow of solution that will travel to any attached sieve tube in which pressure is less

Phloem Transport

– Loss of concentration prevented by• Continual pumping of sucrose at source• Removal of sucrose at the sink