Energy Acquiring Pathways

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Energy - Acquiring Pathways

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life, 9e
• Chapter 7

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Photosynthesis - An Overview

• Reactions convert energy from the sun to chemical
  bond energy of ATP
• ATP energy drives reactions that produce glucose
  and other energy-rich compounds
• Autotrophs and Heterotrophs
• Survival of nearly all organisms depends on
  photosynthesis

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Key Concepts

• Organic compounds are the structural materials
  and energy stores of life
• Photoautotrophs produce organic compounds by
  photosynthesis
• Sunlight energy is trapped and converted to
  chemical energy and ATP

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Key Concepts

• Photosynthesis is the pathway by which carbon and
  energy enter the web of life
• In plants, photosynthesis takes place in
  chloroplasts
• Photosynthesis is summarized this way
• 12H20 6CO2 ---gt 602 C6H12O6 6H2O
• ATP supplies energy for reactions which
  synthesize glucose from CO2 and water

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Photosynthesis
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Where Reactions Take Place

• Chloroplasts
• Two outermost membranes surround interior stroma
• Inner thylakoid membrane system
• Light-dependent reactions occur at the thylakoid
  membrane system
• Light-independent reactions occur in the stroma

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Energy Materials for the Reactions

• Light-Dependent
• Sunlight drives ATP formation from ADP and Pi
• Water is split
• NADP picks up electrons and hydrogen
• Light - Independent
• ATP donates energy
• NADPH donates hydrogen
• CO2 donates carbon and oxygen
• Glucose (C6H12O6) is assembled

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Sunlight as an Energy Source

• Different forms of energy
• Wavelengths

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Electromagnetic Spectrum

• All wavelengths of radiant energy
• Visible light is 380 - 750 nm
• Photoautotrophs use visible light energy

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Englemanns Observational Test

• Oxygen-requiring bacteria congregated where
  oxygen was being produced by algae

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Plants capture energy from the sun, which drives
photosynthesis.
1. Sunlight energy is converted to chemical bond
energy in ATP. Typically, NADPH also forms.
Carbon dioxide, water are required
Oxygen is released
2. ATP and NADPH are used in reactions that form
glucose, other enery-rich organic compounds.
1. Usable energy is released when cells break
down glucose and other organic compounds.
Carbon dioxide, water are released
Oxygen is required
2. Released energy is coupled to electron
transfers that bring about the formation of many
ATP molecules.
ATP is available to drive cellular tasks
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12H2O 6CO2
6O2 C2H12O6 6H2O
WATER
CARBON DIOXIDE
OXYGEN
GLUCOSE
WATER
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energy input from sun
PHOTOAUTOTROPHS (plants, other producers)
nutrient cycling
HETEROTROPHS (consumers, decomposers)
energy output (mainly heat)
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Absorption Spectra

• Chlorophyll a and b are main pigments
• Accessory pigments
• Carotenoids
• Phycobilins
• Anthocyanins
• Extend the range of wavelengths that drive
  photosynthesis

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The Chemical Basis of Color

• Pigment strucure
• Hydrocarbon backbones dissolve readily in lipid
  bilayer of photosythetic cell membranes

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Photosystems

• Pigments are organized in photosystems

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What Happens to the Absorbed Energy?

• Energy flows randomly among pigments of a
  photosystem until trapped by reaction center

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Light - Dependent Reactions

• Occurs on thylakoid membrane
• Pigments absorb photon energy
• Transfer of electrons and hydrogen through
  electron transport system
• Replacement electrons flow to pigments
• Electron flow may be cyclic or noncyclic

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ATP
LIGHT-DEPENDENT REACTION
ADP Pi
LIGHT-INDEPENDENT REACTION
NADPH
NAD
glucose
P
photolysis in the thylakoid compartment
H2O
e
acceptor
ATP SYNTHASE
H shunted across membrane by some components of
the first electron transport system
ATP
ADP Pi
PHOTOSYSTEM II
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Light-Independent Reactions

• Synthesis
• ATP delivers energy
• NADPH delivers hydrogen and electrons
• CO2 provides carbon and oxygen

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Pathways of ATP Formation

• Cyclic Pathway
• ATP forms
• Electrons cycle from and back to a Type I
  photosystem
• Noncyclic Pathway
• ATP and NADH form
• Electrons flow from water, through Type I and
  Type II photosystems, then to NADP
• Oxygen is a by-product

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ATP Formation in Chloroplasts

• Chemiosmotic Theory
• H released by photolysis of water
• More H accumulates as electron transport
  systems operate
• H concentration and electric gradients form
  across the thylakoid membrane
• Flow of ions from thylakoid compartment into the
  stroma drives ATP formation

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Calvin-Benson Cycle

• Carbon Fixation
• Occurs in the stroma
• Carbon atom of CO2 is attached to RuBP (ribulose
  biphosphate) by the enzyme rubisco
• Unstable six-carbon intermediate forms
• Intermediate splits to form two three-carbon
  molecules of phosphoglycerate (PGA)

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Calvin-Benson Cycle

• Building Glucose
• Each PGA accepts a phosphate group from ATP and
  electrons from NADPH
• Two PGAL are formed
• To build one six-carbon sugar phosphate, twelve
  PGAL must form
• 10 PGAL rearrange to regenerate RuBP
• 2 PGAL combine to form phosphorylated glucose

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CO2 or O2
CO2 H2O
Rubisco affixes O2 to RuBP.
one glycolate only one PGA (not two) decreased
CO2 uptake, fewer sugars can form
CALVIN- BENSON CYCLE
CO2
carbon fixation in mesophyll cells
oxaloacetate
C3 PLANTS. With low CO2 / high O2,
photorespiration predominates.
that carbon fixed again in bundle-sheath
cells CO2 level in leaf enhanced no
photorespiration
CALVIN- BENSON CYCLE
(See next slide.)
CO2
stomata open at night CO2 uptake but no water
loss
C4 PLANTS. With low CO2 / high O2, Calvin-Benson
cycle predominates.
CALVIN- BENSON CYCLE
stomata close during day CO2 in leaf used
CAM PLANTS. With low CO2 / high O2, Calvin-Benson
cycle predominates.
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upper leaf surface
vein
mesophyll cell
lower leaf surface
bundle-sheath cell
CO2 moves through stoma, into air spaces in leaf
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Variation in Carbon Fixation

• C3 Plants
• 3-Carbon PGA is first intermediate
• Evergreen trees and shrubs, temperate nonwoody
  plants
• C4 Plants
• 4-Carbon oxaloacetate is first intermediate
• Carbon is fixed twice two different locations
• Grasses and plants that evolved in tropics
• CAM Plants
• Fix carbon in same cells at different times
• Stomata open at night
• Adaptation to desert conditions

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In Conclusion

• Photoautotrophs use wavelengths of visible light
  to build organic compounds
• Photosynthesis is the main biosynthetic pathway
  by which carbon and energy enter the web of life
• In plants, light-dependent reactions take place
  in the thylakoid membrane system
• The light-independent reactions take place in the
  stroma

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In Conclusion

• The sun radiates energy for photosynthesis
• The shorter the wavelength, the more energetic
  the photons
• All but one group of photoautotrophs have
  chlorophyll a and accessory pigments
• Chlorophylls absorb all wavelengths of visible
  light except green and yellow-green ones, which
  they transmit
• Each pigment absorbs photons

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In Conclusion

• Photosystems are clusters of pigments
• Thylakoid membranes have photosystems I II
• Photosystem I operates in a cyclic pathway of ATP
  formation
• Photosystems I II operate together during the
  noncyclic pathway of ATP formation
• Light-dependent pathways entail photolysis of
  water to yield oxygen, H ions and electrons

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In Conclusion

• The light-independent reactions in the stroma
  complete the Calvin-Benson cycle
• Carbon from CO2 is fixed
• Photorespiration predominates in C3 plants
• C4 plants fix carbon in two different cells and
  CAM plants fix carbon at night
• developed by M. Roig