Photosynthesis

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Photosynthesis

• Chapter 4

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Greenhouse Effect

• Burning of fuels, respiration, etc increases CO2
  in atmosphere
• CO2 absorbs infrared radiation (1000 nm) and
  converts it to heat
• Heat increases and the CO2 increase leads to an
  increased rate of photosynthesis

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Chloroplast

• Thought to be a single cell organism originally
• Invaded a cell and has remained and evolved over
  time
• Chloroplast has its own genetic code and
  information

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How does CO2 from the gaseous form change to a
soluble form where the enzymes can fix it?
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Mechanism of CO2 Absorption and Transport
CO2
CO2 H2O H2CO3 (carbonic acid)
H2CO3 Na NaHCO3 (K also works)
NaHCO3 Na HCO3-
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Gases are more soluble in cold water than in warm
water. Additionally, CO2 is more sensitive to
this relationship, so when the temperature rises
the ratio of dissolved O2 to CO2 increases. This
can affect photorespiration as is discussed later.
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Summary Photosynthesis Reaction
6 CO2 6 H2O C6H12O6 6 O2
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Where Do the C, O, and H Come From and In What
Products Are They Found?
6CO2 6 H2O C6H12O6 6O2
Is this balanced?
Yes, but ...
6CO2 12 H2O -----gt C6H12O6 6O2 6H2O
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Light

• Bright sunlight 2000 ?mol m-2 s-1
  (approximately 10,000 fc)
• C3 plants can use up to
  700 ?mol m-2 s-1
• C4 plants can use up to
  2000 ?mol m-2 s-1

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Light Absorption By Pigments
400
500
600
700
Wavelength (nm)
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Photosynthesis Rate vs. Wavelength
Wavelength (nm)
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Rate of Photosynthesis vs. Respiration
Relative Light Intensity
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The Two Phases of Photosynthesis

• Light Reactions (aka Z Scheme)
• Photolysis (H2O 2H 1/2O2)
• Photophosphorylation (ADP P ATP)
• NADP Reduction (NADP H NADPH)
• Dark Reactions (aka C3 or Calvin-Benson Cycle)
• Carbon Dioxide Fixation
• (CO2 C6H12O6)

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Step 1 of PhotosynthesisConversion of Radiant
Energy to Electrical Energy(Z Scheme)
Light
Chlorophyll Chlorophyll e-
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Electrical Energy
ADP
ATP
PS I
PS II
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ADP
Electrical Energy
e-
ADP
ATP
ATP
Red Cyclic Phosphorylation
PS I
PS II
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Summary of Light Reactions

• Photolysis
• (H2O 2H 1/2O2)
• Photophosphorylation
• (ADP P ATP)
• NADP Reduction
• (NADP H NADPH)

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The Two Phases of Photosynthesis

• Light Reactions (aka Z Scheme)
• Photolysis (H2O 2H 1/2O2)
• Photophosphorylation (ADP P ATP)
• NADP Reduction (NADP H NADPH)
• Dark Reactions (aka Calvin-Benson Cycle)
• Carbon Fixation (CO2 C6H12O6)

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From Whitmarsh, USDA-ARS
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C3 Photosynthesis not as Efficient as C4

• No CO2 pump (PEP carboxylase more efficient than
  Rubisco)
• Photorespiration
• Up to 50 of fixed CO2 (RuBP) entering Rubisco
  can be converted back to PGA and phosphoglycolate
  (which goes to CO2)
• Occurs only in light
• RuBP Carboxylase acts as an Oxygenase

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Photorespiratory Glycolate Pathway
Glycine
RuBP O2
2 Glycine
-NH2
Glyoxylate
NH3
CO2
Glycolate
PGA P-Glycolate
Serine
NADH
Serine
Glycerate
Glycerate
peroxisome
mitochondria
chloroplast
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Photorespiration

• Purpose is unknown
• The following conditions reduce photorespiration
• High CO2
• Low O2
• Low light

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C4 Photosynthesis

• Actually has two carbon fixation cycles
• C4 and C4

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Often called Kranz anatomy
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C4 Plant
Vascular Bundle surrounded by Bundle Sheath Cells
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Maize
Soybean
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When is it advantageous to have a C4 or a C3
plant? Below 80ºF (26.7ºC) C3 has the advantage
and above 90ºF (32.2ºC), C4 has the advantage.
Under low O2, C3 would have the advantage.
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Crassulacean Acid Metabolism - CAM
At night, stomates are open and PEP carboxylase
fixes CO2 producing oxaloacetate (OAA). OAA is
then reduced by NADH to malate. Malate is then
stored in the vacuole.
During the day, stomates close but malate is
decarboxylated. The CO2 is converted sugar and
starch via normal C3 metabolism. Light reactions
proceed to generate the needed ATP and NADPH.
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C4 vs CAM

• C4
• C4 Photosynthesis in mesophyll cells
• C3 Photosyntheis in bundle sheath cells
• Tight cells, no air space
• Low O2
• Negligible Photorespiration
• CAM
• C4 at night when stomates are open
• C3 during daylight when stomates are closed