Botany 130, Lecture 14

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Photosynthesis Botany 130 Lecture 14
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Acquisition of carbon dioxide by plants

• Photosynthesis requires water and carbon dioxide
• Water is taken up by the roots
• The water used in photosynthesis metabolism is
  small relative to the total water use by the
  plant
• Carbon dioxide is taken up from the atmosphere
• Sugars are made out of thin air
• Most plants, and all early plants, relied on
  diffusion of CO2 from the air through stomata

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CO2 diffuses through stomatal pores on leaves
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Acquisition of carbon dioxide by plants

• After eons of photosynthesis, plants have altered
  the Earth and made two problems for themselves
• The atmosphere is now about 20 O2
• The atmosphere now has 0.04 CO2
• Both of these problems affect photosynthesis
• A major part of the problem is that the enzyme
  that uses CO2 also uses O2
• The low ratio of CO2 to O2 in the atmosphere is a
  problem

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Acquisition of carbon dioxide by plants

• Some plants, especially corn, use energy to pump
  up CO2, improving the CO2/O2 ratio

CO2 ?? PEP ? Oxaloacetate ? Malate
Malate Pyruvate Pyruvate
AMPPP
CO2
ATP P
C4 plants use a biochemical pump to concentrate
CO2 using ATP as an energy source.
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Acquisition of carbon dioxide by plants

• Using energy in ATP, this metabolism pumps CO2
  from one place to another

CO2 ?? PEP ? Oxaloacetate ? Malate
Malate Pyruvate Pyruvate
AMPPP
CO2 X10
ATP P
Mesophyll Cell Reactions
Bundle Sheath Cells
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Acquisition of carbon dioxide by plants

• Because the first product of carbon fixation is a
  four-carbon compound (can also be aspartate),
  this is called C4 metabolism

CO2 ?? PEP ? Oxaloacetate ? Malate
Malate (or Aspartate) Pyruvate
Pyruvate
AMPPP
CO2
ATP P
Mesophyll Cell Reactions
Bundle Sheath Cells
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Acquisition of carbon dioxide by plants

• C4 metabolism requires the interaction of two
  locations

Mesophyll Cells Bundle Sheath Cells
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Acquisition of carbon dioxide by plants

• C4 metabolism is especially useful at high
  temperature
• Most tropical grasses are C4, most Wisconsin
  native grasses are C3
• Summer active grasses (crabgrass) are C4
• C4 metabolism more efficient at high temperature
• At low temperature, C4 is less efficient because
  the cost of the pump does not pay for itself

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Acquisition of carbon dioxide by plants

• Cacti and other desert succulents do a different
  version of C4 (called Crassulacean Acid
  Metabolism, CAM)
• They make the C4 compound (only malate) during
  the night, then release the CO2 inside their
  leaves during the day
• Some algae use energy to actively accumulate
  inorganic carbon from water
• All of these systems are mechanisms for acquiring
  carbon dioxide for photosynthesis

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Start here Monday
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In photosynthetic carbon metabolism (the dark
reactions)
Net 3 CO2 6 NADPH 9 ATP ? triose phosphate
6 NADP 9 ADP 8 Pi

• These reactions occur in the chloroplast stroma
• Compare the Krebs Cycle in the matrix of the
  mitochondrion
• Both reducing power (NADPH) and ATP is used
• All CO2 metabolism occurs here
• No O2 metabolism occurs here

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How carbon dioxide is converted to sugars

• Evolutionarily, appears to be a collection of
  pathways originally used for other things
• Some glycolytic reactions
• Some reactions plants and animals use to make
  ribose (called pentose phosphate pathway)
• Plus three reactions unique to photosynthesis
• Consistent with the idea that photosynthesis and
  autotrophism were late additions to life on Earth

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How carbon dioxide is converted to sugars

• The unique reactions
• A five-carbon sugar gets a second phosphate
• Carbon dioxide is added and the six carbons break
  apart into two 3-carbon molecules
• Carboxylation is done by an enzyme that now has
  the common name Rubisco

Ru5P ATP ? RuBP ADP
Rubisco
RuBP CO2 ? 2 (3-phosphoglyceric acid)
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How carbon dioxide is converted to sugars
Unique reactions Carboxylation
CO2
Glycolytic reactions
PGA (x2)
RuBP
ADP
ATP NADPH
ATP
Ru5P
ADP NADP Pi
Triose phosphates
Sucrose and starch
Regeneration
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How carbon dioxide is converted to sugars

• The unique reactions
• A five carbon sugar gets a second phosphate
• Carbon dioxide is added and the six carbons break
  apart into two 3-carbon molecules
• Carboxylation is done by an enzyme that now has
  the common name rubisco

Ru5P ATP ? RuBP ADP
Rubisco
RuBP CO2 ? 2 (3-phosphoglyceric acid)
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How carbon dioxide is converted to sugars
Ru5P ATP ? RuBP ADP
RuBP CO2 ? 2 PGA

• Paper chromatography and radioactive CO2 were two
  new technologies used by Melvin Calvin (plus
  Andrew Benson and Al Bassham) to establish that
  PGA was the first product of photosynthesis
• Calvin figured out that the precursor was not a
  two-carbon molecule, but a five-carbon molecule

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How carbon dioxide is converted to sugars
Ru5P ATP ? RuBP ADP
RuBP CO2 ? 2 (Phosphoglyceric acid)

• Because PGA has three carbons, regular
  photosynthesis is now called C3 photosynthesis,
  to distinguish it from what happens in corn
• BUT, corn does all of C3 photosynthesis plus the
  CO2 pump

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How carbon dioxide is converted to sugars
RuBP CO2 ? 2 (Phosphoglyceric acid)
PGA ? glycolytic reactions

• PGA is one of the intermediates in glycolysis
• The next steps in photosynthesis are the reverse
  of glycolytic reactions
• In glycolysis, triose phosphates are oxidized to
  an acid
• In the Calvin Cycle, those reactions are reversed
• Requires reducing power

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Fig. 7.18
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Fig. 7.19
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How carbon dioxide is converted to sugars
PGA NADPH ATP ? Triose phosphates

• These reactions are the same as found in
  glycolysis
• These reactions require a source of ATP and NADPH
• NADPH is almost identical to NADH, but is used in
  anabolic reactions while NADH is used in
  catabolic reactions

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How carbon dioxide is converted to sugars
Triose phosphates ? Regeneration (5/6) Starch
and sucrose (1/6)

• Triose phosphates are used for two things
• Regeneration of the acceptor molecule (5/6)
• Synthesis of sugars, especially starch and
  sucrose (1/6)

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Fig. 7.20
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How carbon dioxide is converted to sugars
Unique reactions Carboxylation
CO2
Glycolytic reactions
PGA (x2)
RuBP
ADP
ATP NADPH
ATP
Ru5P
ADP NADP Pi
Triose phosphates
Sucrose and starch
Regeneration
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These reactions do not require light and are
often called the Dark Reactions of
photosynthesis. Photosynthetic carbon metabolism
is a better term.
CO2
PGA (x2)
RuBP
ADP
ATP NADPH
ATP
Ru5P
ADP NADP Pi
Triose phosphates
Sucrose and starch
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Rubisco will also use O2 which is the start of a
wasteful process called photorespiration
O2
PGA phosphoglycolate (toxic)
RuBP
ADP
ATP NADPH
ATP
Ru5P
ADP NADP Pi
Triose phosphates
Sucrose and starch
Carbon is lost during the metabolism of
P-glycolate. In extreme cases starch and
sucrose are consumed to run photorespiration.
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Photorespiration -when O2 is used instead of
CO2 - wastes energy - releases CO2 - requires
three organelles - is reduced as atmospheric CO2
goes up - recovers 3/4 of the carbon lost to
glycolate
Fig. 7.22
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C4 metabolism decreases photorespiration by
improving the CO2 to O2 ratio
10x CO2 O2
PGA phosphglycolate (toxic)
RuBP
ADP
ATP NADPH
ATP
Ru5P
ADP NADP Pi
Triose phosphates
C4 plants have 1/10 the rate of
photo- respiration of C3 plants.
Sucrose and starch
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Keeping track of the numbers StoichiometryHow
much ATP and reducing power is needed to make one
triose phosphate?
3 CO2
6 PGA (x2)
3 RuBP
ADP
6 ATP 6 NADPH
3 ATP
3 Ru5P
ADP NADP Pi
6 Triose phosphates
1 triose phosphate
5 Triose phosphates
Net 3 CO2 6 NADPH 9 ATP ? one triose phosphate
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Sucrose is synthesized from triose phosphates
Glycolysis ? Pyruvate ?
Sucrose synthesis ? Triose phosphate ?
Krebs Cycle ? CO2
Calvin Cycle ? CO2
Carbon oxidation
Carbon reduction
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Sucrose is synthesized from triose phosphates
CO2 ? Calvin Cycle ? Triose phosphate ?
Sucrose synthesis
Pi
Phosphate is released during sucrose synthesis.
Triose phosphate is exchanged for phosphate on
an antiporter in the inner chloroplast membrane.
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Sucrose synthesis

• Making F1,6BP, the reverse of that seen in
  respiration

P
P
?
?
P
O
O
Glyceraldehyde
3P and Dihydroxyacetone P
Fructose 1,6-bisP
Triose phosphates
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Sucrose synthesis

• FBP to F6P is a highly regulated step

P
P
P
O
O
?
Pi
Fructose 1,6-bisP
Fructose 6P
The reverse of this step requires ATP. If the
above reaction and the reverse reaction seen
in respiration operate together, ATP can be
burned with no net gain. This is a futile cycle.
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Sucrose synthesis

• Hexose monophosphates can be interconverted
• Sucrose is glucose plus fructose

P
P
O
O
O
H
H
O
?
?
O
Sucrose
Glucose 6P
Fructose 6P
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Products of photosynthesis

• Typically, about half of the triose phosphates
  not used for regeneration are used to make
  sucrose
• The other half are used to make starch inside the
  chloroplast
• Some plants make sugar alcohols in place of some
  of the sucrose
• Examples, apples and celery
• Some plants make tri-, tetra-, and
  pentasaccharides
• Possibly related to phloem transport

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The oxidation states of carbon
Anabolic metabolism
Catabolic metabolism
Reduced
Fats Sugars Acids CO2 Carbon dioxide
Oxidized
Respiration
Photosynthesis
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Respiration and Photosynthesis
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Respiration and Photosynthesis