Photosynthesis

1
Photosynthesis

• Chapter 10

2
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

3
Discoveries Leading to the Understanding of
Photosynthesis

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

4
Discoveries Leading to the Understanding of
Photosynthesis

• 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

5
Discoveries Leading to the Understanding of
Photosynthesis

• 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 didnt provide adequate light for plant

6
Discoveries Leading to the Understanding of
Photosynthesis

• 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

7
Discoveries Leading to the Understanding of
Photosynthesis

• 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

8
Discoveries Leading to the Understanding of
Photosynthesis

• 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

9
Discoveries Leading to the Understanding of
Photosynthesis

• 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

10
Discoveries Leading to the Understanding of
Photosynthesis

• Cornelis van Niel
• His general equation for photosynthesis
• 6CO2 12H2A ? C6H12O6 6H2O 12A

light
carbohydrate
Hydrogen 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.
11
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

12
Specific Photosynthetic Reactions

• 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

13
Specific Photosynthetic Reactions

• F.F. Blackman
• 1905
• Reasoned photosynthesis could be divided into two
  general parts
• Photochemical reactions (light reactions)
• Temperature-sensitive reactions (previously
  called dark reactions)

14
Specific Photosynthetic 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

15
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

16
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

17
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

18
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

19
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

20
Characteristics of Light

• 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

21
Characteristics of Light

• White light (visible light)
• Separate into component colors to form visible
  spectrum
• Visible wavelengths range from
• Red (640 740 nm)
• Violet (400 425 nm)

22
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

23
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

24
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

25
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

26
Absorption of Light Energy by Chlorophyll

• Chlorophyll molecule absorbs or traps photon
• Energy of photon causes electron from one of
  chlorophylls atoms to move to higher energy
  state
• Unstable condition
• Electron moves back to original energy level

27
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

28
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

29
Two Photosystems
Pigments 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 II Chlorophyll 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
30
Adenosine Triphosphate Synthesis

• Photophosporylation
• Light-driven production of ATP in chloroplasts
• Two types
• Cyclic photophosphorylation
• Noncyclic photophosphorylation

31
Adenosine Triphosphate Synthesis

• 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

32
Adenosine Triphosphate Synthesis

• 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

33
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

34
Enzymes of Light-Independent Reactions

rubisco Carbon dioxide ribulose
biphosphate ? 2 phosphoglyceric acid
(RuBP)

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

35
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

36
Photosynthetic Carbon Reduction Cycle

• Melvin Calvin
• 1950s
• Used radioactive C (14C) in CO2 to trace
  intermediate products of carbon reduction cycle
• Nobel Prize

37
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)

38
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

39
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

40
Photorespiration

• Differs from aerobic respiration
• Yields no energized energy carriers
• Does not occur in the dark
• Involves interaction with chloroplasts,
  peroxisomes, mitochondria

41
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
42
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

43
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)

44
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

45
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

46
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

47
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

48
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

49
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

50
Factors Affecting Productivity

• 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

51
Factors Affecting Productivity

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

52
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

53
Environmental Factors

• Temperature
• Most plants function best between temperatures of
  10?C and 25?C
• Above 25?C
• Continuous decrease in photosynthesis rate as
  temperature increases
• Under low light intensity, increase in
  temperature beyond certain minimum does not
  produce increase in photosynthesis

54
Environmental Factors

• 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

55
Environmental Factors

• 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

56
Environmental Factors

• 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

57
Environmental Factors

• 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

58
Environmental Factors

• Mineral nutrients
• Poor soils can result in plants with poorly
  developed photosynthetic capacities
• Can increase yields by effective fertilizer
  programs

59
Absorption and Transport

• Chapter 11

60
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

61
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

62
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

63
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

64
Factors Affecting Flow of Water in Air, Cells,
and Soil

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

65
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

66
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

67
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

68
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

69
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

70
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

71
Factors Affecting Flow of Water in Air, Cells,
and Soil

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

72
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

73
Water Potential

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

74
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

75
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

76
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

77
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

78
Flow of Water Into Leaves

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

79
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

80
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

81
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

82
Flow of Water Through Xylem

• Conifers
• Only tracheids, no vessels
• Advantage in dry, cold climates
• Conditions most likely to produce air bubbles in
  xylem

83
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

84
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

85
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

86
Symplastic and Apoplastic Flow Through Roots

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

87
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

88
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

89
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

90
Control of Water Flow

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

91
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

92
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

93
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

94
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

95
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

96
Roles of Mineral Elements in Plants
Element 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)
97
Soil Types

• Soil
• Part of Earths 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

98
Soil Types

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

99
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

100
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

101
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

102
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

103
Soil Formation

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

104
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

105
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

106
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

107
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

108
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

109
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

110
Mycorrhizae

• Association of filamentous fungi with roots of
  some plants
• Plants with mycorrhizae often grow better than
  plants with mycorrhizae

111
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

112
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

113
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

114
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

115
Root Pressure

• Hydathode
• Specialized opening in leaves of some grasses and
  small herbs
• Guttation
• Water forced out of hydathodes by root pressure

116
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

117
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

118
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

119
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

120
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

121
Phloem Transport

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