PHOTOSYNTHESIS

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BIOLOGY 171 CHAPTER 10
PHOTOSYNTHESIS
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PHOTOSYNTHESIS

• Autotrophs
• Heterotrophs
• Autotrophic Nutrition
• Photoautotrophs
• Chemoautotrophs
• Heterotrophic Nutrition
• Consumers
• Decomposers

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Photosynthesis
Photosynthesis The production of organic
compounds (sugars) from simple inorganic
compounds (CO2 and H2O) using the energy of
light in the endergonic process. Oxygen is a
byproduct of the process. Light is converted
into chemical energy.
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OVERVIEW

• The Splitting of Water
• C. B. van Niel
• O2 from H2O Rather than CO2
• 18O
• A Redox ProcessLight Reactions
• Photophosphorylation
• Calvin Cycle
• Carbon Fixation

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PHOTOSYNTHESIS

• Summary equation
• 6 CO2 12 H20 C6H1206 6O2 6H2O
• Location - Chloroplast
• Essentials of Photosynthesis
• Chlorophyll a and Accessory Pigments
• Photosystems
• Carbon Dioxide
• Water, Light

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THE LEAF AND PHOTOSYNTHESIS

• The leaf is the organ of photosynthesis
• in most plants.
• The blade is the flattened portion of the leaf
  that collects light energy for photosynthesis.
• Most photosynthesis occurs in the mesophyll of
  the leaf blade.

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THE LEAF
Blade
Petiole
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INTERNAL LEAF ANATOMY DICOT
Upper Epidermis
Xylem
Palisade Mesophyll
Spongy Mesophyll
Lower Epidermis
Stomata and Guard Cells
Bundle Sheath
Phloem
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LIGHT REACTIONS

• Nature of Sunlight
• Electromagnetic Energy
• Travels in Waves - Wavelength
• Visible Light 380 to 750 nm
• Photons
• Photosynthetic Pigments
• Chlorophyll a Blue-green
• Chlorophyll b Yellow-green
• Carotenoids Yellow to Orange

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LIGHT AND PHOTOSYNTHESIS

• Absorption Spectrum of Chlorophyll a
• Violet/Blue and red
• Action Spectrum of Photosynthesis
• Violet, Blue, Red
• Accessory Pigments
• Photosystems
• Reactions Center
• Primary Electron Acceptor
• Photoexcitation of Chlorophyll
• Ground State to Excited State

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Figure 10.5 The electromagnetic spectrum
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Figure 10.8 Evidence that chloroplast pigments
participate in photosynthesis absorption and
action spectra for photosynthesis in an alga
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Figure 10.10 Excitation of isolated chlorophyll
by light
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THE CHLOROPLAST
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PHOTOSYSTEMS

• Light Harvesting Complex
• Reaction Center Complex
• Photosystem I
• P700 Chlorophyll
• Photosystem II
• P680 Chlorophyll

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Fig. 10-5-1
H2O
Light
NADP
ADP
P

i
Light Reactions
Chloroplast
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Fig. 10-5-2
H2O
Light
NADP
ADP
P

i
Light Reactions
ATP
NADPH
Chloroplast
O2
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Fig. 10-5-3
H2O
CO2
Light
NADP
ADP
P

i
Calvin Cycle
Light Reactions
ATP
NADPH
Chloroplast
O2
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Fig. 10-5-4
H2O
CO2
Light
NADP
ADP
P

i
Calvin Cycle
Light Reactions
ATP
NADPH
Chloroplast
CH2O (sugar)
O2
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PHOTOPHOSPHORYLATION

• Cyclic Electron Flow or
• Cyclic Photophosphorylation
• Produces ATP Only
• Involves Photosystem I
• Noncyclic Electron Flow or Linear
• Noncyclic Photophosphorylation
• Produces ATP, NADPH and O2
• Involves Photosystem I and II

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Figure 10.12 How noncyclic electron flow during
the light reactions generates ATP and NADPH
(Layer 3)

• Produces ATP
• only.
• Electrons of
• chlorophyll
• excited by light.
• Electrons return to same chlorophyll.
• ATP produced by chemiosmosis.

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Figure 10.12 How noncyclic electron flow during
the light reactions generates ATP and NADPH
(Layer 5)
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Fig. 10-17
STROMA (low H concentration)
Cytochrome complex
Photosystem I
Photosystem II
Light
4 H
NADP reductase
Light
3
Fd
NADP H
NADPH
Pq
Pc
e
2
e
H2O
O2
1/2
1
THYLAKOID SPACE (high H concentration)
4 H
2 H
To Calvin Cycle
Thylakoid membrane
ATP synthase
STROMA (low H concentration)
ADP
ATP
P
i
H
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NONCYCLIC PHOTOPHOSPHORYLATION

• Electrons do not return to the chlorophyll from
  which they originated.
• ATP, NADPH and O2 produced.
• Electrons from photosystem I reduce NADP to
  NADPH.
• Electrons of photosystem I replaced by electrons
  from
• photosystem II.
• Energy from the electrons of photosystem II
  result in the production of ATP through
  chemiosmosis. As water is split, electrons
  from water replace those lost from photosystem
  II. This also results in the production of O
  and 2H.

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LIGHT INDEPENDENT REACTIONS
Occurs in the stroma of the chloroplast Calvin
cycle CO2 is reduced by NADPH ATP provides
energy Carbohydrate and water are produced
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Fig. 10-18-1
(Entering one at a time)
Input
3
CO2
Phase 1 Carbon fixation
Rubisco
3
P
P
Short-lived intermediate
6
P
3
P
P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
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Fig. 10-18-2
(Entering one at a time)
Input
3
CO2
Phase 1 Carbon fixation
Rubisco
3
P
P
Short-lived intermediate
6
P
3
P
P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
ATP
6
6 ADP
Calvin Cycle
P
P
6
1,3-Bisphosphoglycerate
6
NADPH
6 NADP
P
6
i
P
6
Glyceraldehyde-3-phosphate (G3P)
Phase 2 Reduction
1
P
Glucose and other organic compounds
Output
G3P (a sugar)
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Fig. 10-18-3
(Entering one at a time)
Input
3
CO2
Phase 1 Carbon fixation
Rubisco
3
P
P
Short-lived intermediate
6
P
3
P
P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
ATP
6
6 ADP
3 ADP
Calvin Cycle
P
6
P
3
ATP
1,3-Bisphosphoglycerate
6
NADPH
Phase 3 Regeneration of the CO2 acceptor (RuBP)
6 NADP
P
6
i
P
5
G3P
P
6
Glyceraldehyde-3-phosphate (G3P)
Phase 2 Reduction
1
P
Glucose and other organic compounds
Output
G3P (a sugar)
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PROBLEMS

• Photorespiration
• Bright light and warm temperatures
• O2 competes with CO2 for the binding site on
  ribulose bisphosphate carboxylase
• (Rubisco)
• Uses O2 and Releases CO2
• RuBP is oxidized thus reducing photosynthetic
  output (up to 50)

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SOLUTION TO PHOTORESPIRATION

• C4 photosynthesis is the solution to the
  problem.
• In C4 photosynthesis CO2 is pumped form the
  mesophyll to the bundle sheath cells as an
  organic acid, malate.
• Malate releases CO2 in the bundle sheath cells
  where C3 photosynthesis is completed.

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C4 PHOTOSYNTHESIS

• CO2 bonds to PEP
• forming oxaloacetate,
• which becomes malic
• acid.
• Malic acid diffuses to
• the bundle sheath cells,
• releasing CO2.
• Malic acid becomes
• Pyruvic acid, which
• diffuses to the mesophyll,
• picking up a phosphate
• from ATP to become PEP.

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Fig. 10-19
The C4 pathway
C4 leaf anatomy
Mesophyll cell
Mesophyll cell
CO2
PEP carboxylase
Photosynthetic cells of C4 plant leaf
Bundle- sheath cell
PEP (3C)
Oxaloacetate (4C)
Vein (vascular tissue)
ADP
Malate (4C)
ATP
Pyruvate (3C)
Bundle- sheath cell
CO2
Stoma
Calvin Cycle
Sugar
Vascular tissue
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C4 LEAF CROSS SECTION
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SOLUTION TO DRY CONDITIONS

• CAM Metabolism - Crassulacean
• Acid Metabolism
• Allows plants to conserve water.
• Stomates are closed during the day, open at
  night.
• CO2 is absorbed and converted to organic acids
  at night.
• Organic acids release CO2 during the day.

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Fig. 10-20
Sugarcane
Pineapple
C4
CAM
CO2
CO2
Mesophyll cell
Night
CO2 incorporated into four-carbon organic
acids (carbon fixation)
1
Organic acid
Organic acid
Bundle- sheath cell
Day
CO2
CO2
Organic acids release CO2 to Calvin cycle
2
Calvin Cycle
Calvin Cycle
Sugar
Sugar
(a) Spatial separation of steps
(b) Temporal separation of steps