Division of Labor in Chloroplasts

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Division of Labor in Chloroplasts

• Green thylakoids
• Capture light
• Liberate O2 from H2O
• Form ATP from ADP and phosphate
• Reduce NADP to NADPH

Light reactions

• Colorless stroma
• Contains water-soluble enzymes
• Captures CO2
• Uses energy from ATP and NADPH for sugar
  synthesis

Dark reactions
2
Light(-dependent) reactions
3
Absorption spectra of Chlorophyll a and b
100
80
chlorophyll b
Percent of light absorbed
60
40
chlorophyll a
20
0
400
500
600
700
Wavelength (nm)
Fig. 10-5, p. 152
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NONCYCLIC ELECTRON TRANSPORT
e-
P700
-0.6
sunlight energy
Electron Transport System
NADPH
e-
P680
e-
potential to transfer electrons (measured in
volts)
0
H NADP
ADP Pi
electron transport system
sunlight energy
e-
e-
P700
0.4
Pigments from the light harvesting complex
photosystem I
released energy used to form ATP from ADP and
phosphate
0.8
photosystem II
e-
H2O
photolysis
P680 reaction center of photosystem II
P700 reaction center of photosystem I
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CYCLIC ELECTRON TRANSPORT
e-
P700
sunlight energy
Electron Transport System
NADPH
e-
P680
e-
H NADP
electron transport system
ADP Pi
sunlight energy
e-
e-
P700
photosystem I
released energy used to form ATP from ADP and
phosphate
photosystem II
e-
H2O
photolysis
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oxygen released
sunlight energy
photosystem II
e-
H
electron transport system
Light-dependent reactions
H2O is split
H
lumen (H reservoir)
H
photosystem I
e-
electron transport system
NADP
carbon dioxide used
ADP Pi
H
H
Light-independent reactions
sugar phosphate
Stroma
Fig. 10-3, p. 151
carbohydrate end product (e.g. sucrose, starch,
cellulose)
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Compare to respiration
8
inner membrane
pyruvate from cytoplasm
electron transport system
H
Coenzymes give up electrons, hydrogen (H) to
transport system
e-
NADH
acetyl-CoA
e-
NADH
H
TCA cycle
H
FADH2
As electrons pass through system, H is pumped
out from matrix
e-
carbon dioxide
Oxygen accepts electrons, joins with 2H, forms
water
ATP synthesized
ATP
2
Pi
ADP
oxygen
H
INTERMEMBRANE space
H
MATRIX
H
H flows in
H
Fig. 9-8c, p. 142
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Dark reactionsor Light-independent reactions
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The Calvin cycle (C3 pathway of
photosynthesis) PGA phosphoglyceric acid PGAL
phosphoglyceraldehyde RuBP ribulose
bisphosphate Rubisco ribulose bisphosphate
carboxylase The energy carriers ATP and NADPH
(formed by photosystems I and II) are used to
form high energy containing C-C and C-H bonds
starting from H2O and CO2. Through the Calvin
cycle, plants capture CO2 and H2O and transform
low energy containing CO and H-O bonds into the
high energy containing C-C and C-H bonds of
sugar. Rubisco is the worlds most abundant
protein!
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(No Transcript)
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Using ATP and NADPH to generate high energy
containing covalent bonds PGA phosphoglyceric
acid PGAL phosphoglyceraldehyde
H
H
C
C
OH
P
PGA
H
O
C
Low energy electrons
O H
ATP NADPH
H
H
C
C
OH
P
PGAL
H
O
C
High energy electrons
H
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Photorespiration

• When Rubisco uses O2, this will result in one
  molecule of PGA and one molecule of
  phosphoglycolate (a two-carbon molecule), instead
  of two PGA molecules (see the Calvin Cycle).
• Phosphoglycolate cannot be used in the calvin
  cycle and thus represents a loss of efficiency in
  photosynthesis.
• Photorespiration can cause up to a 25 reduction
  in photosynthesis in C3 plants.

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Photorespiration
C3 Plants High rates of photorespiration (particularly on hot, bright days) Produce less sugar during hot, bright days of summer
C4 Plants Show little or no photorespiration Produce 2 or 3 times more sugar than C3 plants during hot, bright days of summer
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Corn, a C4 plant (right), is able to survive at
a lower CO2 concentration than bean, a C3 plant
(left), when they are grown together in a closed
chamber in light for 10 days.
Fig. 10-10, p. 158
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The C4 pathway concentrates CO2
Interaction between the C4 cycle and the C3
cycle
C4 cycle
AMP
mesophyll cells
C3 cycle
bundle sheath cells
Fig. 10-12, p. 159
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The C4 pathway concentrates CO2
In C4 plants, CO2 is first captured by PEP
carboxylase in mesophyll cells to make
oxaloacetate which is subsequently turned into
malate. This malate then diffuses into the
chloroplasts of bundle sheath cells where it
releases CO2. Thus, bundle sheath chloroplasts
contain higher CO2 concentrations compared to
chloroplasts in mesophyll cells and therefore
have higher photosynthesis and lower
photorespiration rates.
upper epidermis
bundle sheath cell
air space
CO2 movement
mesophyll cells
guard cell
lower epidermis
vascular bundle
Fig. 10-11, p. 159
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However!

• The C4 pathway requires additional ATP for CO2
  fixation.
• Thus, C4 plants only grow better than C3 plants
  under hot and dry environmental conditions.

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SUMMARY Transforming Light Energy into Chemical
Energy
Transforming CO2 and H2O into food Light energy
is captured to make ATP and NADPH via the action
of photosystems I and II. This ATP and NADPH
is used via the Calvin cycle to transform the low
energy containing C-O and H-O bonds of CO2 and
H2O into the high energy containing C-C and C-H
bonds of sugar. In other words Light energy
from the sun is used by plants to increase the
potential energy of electrons in the bonding
orbitals of covalent bonds. This is done by
replacing oxygen in C-O and H-O bonds by carbon
or hydrogen, leading to the production of O2 and
carbohydrates (sugars, starch, etc).
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• Consumption of photosynthesis products
• Agriculture
• Annual accumulation of light energy as C-H and
  C-C bonds (FOOD).
• 2. Fossil fuels
• Accumulation of light energy as C-C and C-H bonds
  over millions of years (accumulation of
  photosynthesis products over millions of years).
• 3. Energy intensive agriculture
• use of fossil fuels to increase agricultural
  yields (fertilizer and pesticide production,
  irrigation, harvest, storage, transportation,
  etc). Use of photosynthesis products of the past
  to increase FOOD yields (present photosynthesis
  productivity).
• How do we maintain present levels of food
  production when fossil fuel sources become
  depleted?