Photosynthesis Chapter 10. Photosynthesis Light energy stored as chemical energy for future use Original source of energy for other organisms Except for

Photosynthesis

Chapter 10

Photosynthesis

• Light energy stored as chemical energy for future use

• Original source of energy for other organisms

• Except for a few species of bacteria, all life depends on the energy-storing reactions of photosynthesis

Discoveries Leading to the Understanding of Photosynthesis

Until 17th century, scholars believed that plants derived the bulk of their substance from soil humus.


• Joannes van Helmont– Disproved idea that plants get bulk of

substance from soil humus• Planted 5 lb. willow in 200 lbs. of dried soil• Over 5 year time span, only watered plant with

rainwater• At end of 5 years

– Plant grew from 5 lbs. to 169 lbs.– Soil only lost 2 oz. during the 5 years

• Reasoned plant substance must have come from water


• Joseph Priestly– 1772– Reported sprig of mint could restore air that

had been made impure by a burning candle– Plant changed air so mouse could live in it– Experiment not always successful

• Sometimes didn’t provide adequate light for plant


• Jean Senebier– 1780– pointed out that “fixed air,” carbon dioxide was

required for photosynthesis

• Antoine Lavoisier– Stated that green plants use carbon dioxide

and produce oxygen


• Jan Ingen-Housz– 1796– Found that carbon went into the nutrition of

the plant

• Nicolas de Saussure– 1804– Observed that water was involved in the

photosynthetic process


• Julius von Sachs– Between 1862 and 1864 observed

• Starch grains are present in chloroplasts of higher plants

• If leaves containing starch are kept in darkness for some time, starch disappears

• If same leaves are exposed to light, starch reappears in chloroplasts

• First person to connect appearance of starch (carbohydrate) with both fixation of carbon in the chloroplasts and the presence of light


• Cornelis van Niel– 1930s– Compared photosynthesis in different groups of

photosynthetic bacteria• Green and sulfur bacteria use H2S instead of H2O to reduce

CO2

• Found that sulfur was liberated instead of O2

• Since sulfur could only come from H2S, van Niel reasoned that O2 liberated by higher plants comes from H2O not CO2


• Cornelis van Niel– His general equation for photosynthesis

6CO2 + 12H2A C6H12O6 + 6H2O + 12Alight

carbohydrateHydrogen donor

Carbon dioxide

water A

H2A could be H2O, H2S, H2 or any molecule capable of donating an electron. Reaction requires energy input. When H2A gives up electrons, it is oxidized to A.

Specific Photosynthetic Reactions

• T.W. Engelmann– Between 1883 and 1885– Demonstrated which colors of light are used in

photosynthesis– Found that red and blue light were trapped by

algal photosynthetic organelles


• J. Reinke– Studied effect of changing the intensity of light on

photosynthesis– Observed rate of photosynthesis increased

proportionally to increase in light intensity at low-to-moderate light intensities

– At greater light intensities, rate of photosynthesis was not affected by changing light intensities

– Indicated reaction was already proceeding at maximum rate


• F.F. Blackman– 1905– Reasoned photosynthesis could be divided

into two general parts• Photochemical reactions (light reactions)• Temperature-sensitive reactions (previously called

dark reactions)


• Photochemical reactions– Light reactions– Insensitive to temperature changes

• Temperature-sensitive reactions– Previously called dark reactions– Enzymatic reactions– Do not depend directly on light– Chloroplast proteins, thioredoxins, regulate

activities of some dark reactions

Chloroplast Research

• Robin Hill– 1932– Demonstrated chloroplasts isolated from cell

could still trap light energy and liberate oxygen

• Daniel Amon– 1954– Proved isolated chloroplasts could convert

light energy to chemical energy and use this energy to reduce CO2

Chloroplast Structure

• Double-membrane envelope

• Two types of internal membranes– Grana (singular, granum)– Stroma lamella – interconnect grana

• Stroma– Made up of grana and stroma lamella

Division of Labor in Chloroplasts

• Research has shown that– Intact chloroplasts carry out complete process

of photosynthesis– Broken plastids

• Carry out only part of photosynthetic reactions• Will liberate oxygen

Division of Labor in Chloroplasts

• Division of labor– Green thylakoids

• Capture light• Liberate O2 from H2O • Form ATP from ADP and phosphate• Reduce NADP+ to NADPH

– Colorless stroma• Contain water-soluble enzymes• Captures CO2

• Uses energy from ATP and NADPH in sugar synthesis

Characteristics of Light

• Two models describing nature of light– Interpret light as electromagnetic waves– Light acts as if it were composed of discrete

packets of energy called photons


• Light is small portion of electromagnetic energy spectrum that comes from sun

• Longest waves– Cannot see– Infrared and radio waves– Longer than visible red wavelength

• Shortest waves– Cannot see– Ultraviolet waves, X-rays, gamma rays– Shorter than violet


• White light (visible light)– Separate into component colors to form

visible spectrum– Visible wavelengths range from

• Red (640 – 740 nm)• Violet (400 – 425 nm)

Photons

• Packet of energy making up light

• Contains amount of energy inversely proportional to wavelength of light characteristic for that photon– Blue light has more energy per photon than

does red light

Photons

• Only one photon is absorbed by one pigment molecule at a time

• Energy of photon is absorbed by an electron of pigment molecule– Gives electron more energy

Absorption of Light Energy by Plant Pigments

• Spectrophotometer– Instrument used to measure amount of specific

wavelength of light absorbed by a pigment– Absorption spectrum

• Graph of data obtained

• Chlorophyll– Reflects green light– Absorbs blue and red wavelengths

• Wavelengths used in photosynthesis

Absorption of Light Energy by Plant Pigments

• Chlorophyll– Two major types of chlorophyll in vascular

plants• Chlorophylls a and b• In solution absorb much of red, blue, indigo, and

violet light• In thin green leaf

– Absorption spectrum similar to but not identical to that of chlorophyll in solution

Absorption of Light Energy by Chlorophyll

• Chlorophyll molecule absorbs or traps photon

• Energy of photon causes electron from one of chlorophyll’s atoms to move to higher energy state

• Unstable condition

• Electron moves back to original energy level

Absorption of Light Energy by Chlorophyll

• Absorbed energy transferred to adjacent pigment molecule– Process called resonance

• Energy eventually transferred to chlorophyll a reception center– Series of steps drives electrons from water to reduce NADP+

– Formation of NADPH represents conversion of light energy to chemical energy

– NADPH reduces CO2 in enzymatic reactions leading to sugar formation

Two Photosystems

• Robert Emerson– 1950s– Made observations that led to realization that

there are two light reactions and two pigment systems

• Photosystem I• Photosystem II

Two PhotosystemsPigments

Reaction Center

Description

Photosystem I Chlorophyll a and b P700

Greater proportion of chlorophyll a than b in *light-harvesting complex, sensitive to longer wavelength light

Photosystem IIChlorophyll a and b, carotene

P680

Equal amounts of chlorophyll a and b, *light-harvesting complex sensitive to shorter wavelength light

* light-harvesting complex – functional pigment units that act as light traps

Adenosine Triphosphate Synthesis

• Photophosporylation– Light-driven production of ATP in chloroplasts

• Two types– Cyclic photophosphorylation– Noncyclic photophosphorylation


• Cyclic Photophosphorylation– Electrons flow from light-excited chlorophyll

molecules to electron acceptors and cyclically back to chlorophyll

– No O2 liberated

– No NADP+ is reduced– Produces H+ gradient that leads to energy

conservation in ATP production– Only photosystem I involved


• Noncyclic photophosphorylation– Electrons from excited chlorophyll molecules

are trapped in NADP+ to form NADPH– Electrons do not cycle back to chlorophyll– Photosystems I and II are involved– ATP and NADPH are formed

• Energy drives CO2 reduction reactions of photosynthesis

Enzymes of Light-Independent Reactions

• All enzymes participating directly in photosynthesis occur in chloroplasts– Many are water-soluble– Many found in stroma

• Ribulose biphosphate carboxylase/oxygenase (rubisco)– Catalyzes first step in carbon cycle of

photosynthesis

Enzymes of Light-Independent Reactions

rubisco

Carbon dioxide + ribulose biphosphate 2 phosphoglyceric acid

*(RuBP)

•RuBP 5-C sugar present in plastid stroma, spontaneous reaction

Photosynthetic Carbon Reduction Cycle

• Methods used to isolate carbon compounds formed during enzymatic reactions– Used radioactive carbon (14C) in CO2 to trace

each intermediate product– Two-dimensional paper chromatography

Photosynthetic Carbon Reduction Cycle

• Melvin Calvin– 1950s

– Used radioactive C (14C) in CO2 to trace intermediate products of carbon reduction cycle

– Nobel Prize

C3 Pathway

• First product PGA contains 3 Cs• Calvin cycle (in honor of discoverer, Melvin

Calvin)• Key points

– CO2 enters cycle and combines with RuBP produced in stroma

• 2 molecules of PGA are produced

– Energy stored in NADPH and ATP transferred into stored energy in phosphoglyceraldehyde (PGAL)

C3 Pathway

– PGAL may be enzymatically converted to 3-C sugar phosphate, dihydroxyacetone phosphate

– Two molecules of dihydroxyacetone phosphate combine to form a sugar phosphate, fructose 1,6 - biphosphate

C3 Pathway

– Some fructose 1,6 – biphosphate transformed into other carbohydrates, including starch (reactions not part of C3 cycle)

– RuBP is regenerated • Free to accept more CO2

Photorespiration

• Differs from aerobic respiration– Yields no energized energy carriers– Does not occur in the dark

• Involves interaction with chloroplasts, peroxisomes, mitochondria

Photorespiration

C3 Plants

High rates of photorespiration (particularly on hot, bright days)

Produce less sugar during hot, bright days of summer, under milder conditions are more efficient because they expend less energy to capture CO2

C4 Plants

Show little or no photorespiration

Produce 2 or 3 times more sugar than C3 plants during hot, bright days of summer

Environmental Stress and Photorespiration

• Succulents– Developed methods of storing and conserving

water• Highly developed parenchyma tissue• Large vacuoles• Reduced intercellular spaces• Absorb and store water when moisture is available

Environmental Stress and Photorespiration

• Succulents– Stoma closed during the day and open at

night• Advantage

– Reduces water loss during day

• Disadvantage– Reduces CO2 uptake in daylight when photosynthesis

can occur

– Exhibit type of carbon metabolism called crassulacean acid metabolism (CAM)

Major Features of CAM

– Stomata open at night

– Leaves rapidly absorb CO2

– Enzyme phosphoenolpyruvate (PEP) carboxylase initiates fixation of CO2

– Malate, 4-C compound is usually produced– Total amount of organic acids rapidly increases in

leaf-cell vacuoles at night– Leaf acidity rapidly decreases during following day

• Organic acids are decarboxylated and CO2 released into leaf mesophyll

Major Features of CAM

– Stomata closed during the day• Prevents or greatly reduces CO2 absorption and

water loss

• C3 cycle of photosynthesis usually takes place and converts the internally released CO2 into carbohydrate

C4 Pathway

• Discovered in 1965– H.P. Kortschak, C.E. Hartt, G.O. Burr

• Extensively studied by M.D. Hatch and C.R. Slack

• Pathway also known as Hatch-Slack cycle

• Differs from C3 or Calvin cycle

– Ensures an efficient absorption of CO2 and results in low CO2 compensation point

C4 Pathway

• Compensation point– Concentration of CO2 remaining in closed

chamber at the point when CO2 produced by respiration balances or compensates for CO2 absorbed during photosynthesis

– Varies among different plants

C4 Pathway

– Example of compensation point• Place bean plant and corn plant in chamber in light• Bean plant will die before corn plant

• Corn plant has very low CO2 compensation point

• Both plants eventually die of starvation

Factors Affecting Productivity

• Only about 0.3% to 0.5% of light energy that strikes leaf is stored in photosynthesis

• Yield could be increased by factor of 10 under ideal conditions


• Breed productivity into plants– Norman Borlaug– Nobel Prize 1970– Developed high-yielding wheat strains

• Disadvantages– Strains require high levels of fertilizer

» Expensive» Create pollution

– Potential for genetic problems


• Breeding programs or use of recombinant DNA technology may lead to new C4 and C3 plants less prone to photorespiration

Environmental Fluctuations Alter Photosynthesis Rate

• To some extent, environmental factors under control of plant grower

• Water and mineral content control of soil most easily controlled

• Control of temperature, light (intensity, quality, duration), and CO2 require special equipment

Environmental Factors

• Temperature– Most plants function best between

temperatures of 10C and 25C– Above 25C

• Continuous decrease in photosynthesis rate as temperature increases

– Under low light intensity, increase in temperature beyond certain minimum does not produce increase in photosynthesis


• Light– Light intensity and wavelength affect

photosynthesis rate• Intensity to which chloroplasts are exposed affects

photosynthesis more than intensity of light falling on leaf surface

• Structural adaptations that diminish light intensity that reaches chloroplasts

– Surface hairs, thick cuticle, thick epidermis


• Light– Sunflecks

• Brief exposure to light received by plants on forest floor when breezes move upper canopy

• Contribute to majority of light used by understory vines, shrubs, and herbs

– Plants adapt to quality of light to survive• Plants growing in deep water have developed

accessory pigments to absorb blue-green wavelengths and use it in photosynthesis


• Carbon dioxide– Not possible to deplete atmospheric carbon dioxide– Continual increase in carbon dioxide contributes to

threat of global warming– Atmospheric carbon dioxide around leaves limits rate

of photosynthesis in C3 plants

– Experimentally determined an artificial increase in carbon dioxide (up to 0.6%) may increase rate of photosynthesis for limited period

• Level injurious to some plants after 10 to 15 days of exposure


• Water– Rate of photosynthesis may be changed by

small differences in water content of chlorophyll-bearing cells

– Drought reduces rate of photosynthesis in some plants


• Mineral nutrients– Poor soils can result in plants with poorly

developed photosynthetic capacities• Can increase yields by effective fertilizer programs

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


• 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


• 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


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

• Stiffens cells and tissues


• 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


• 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


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

water present• Dry soil – stronger tension


• 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


• 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


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

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

water flow


• 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


• 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


• 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


• 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

Documents

Photosynthesis Chapter 10. Photosynthesis Light energy stored as chemical energy for future use Original source of energy for other organisms Except for