Post on 14-Jan-2016
Division of Labor in Chloroplasts
Green thylakoids• Capture light
• Liberate O2 from H2O
• Form ATP from ADP and phosphate
• Reduce NADP+ to NADPH
Colorless stroma• Contains water-soluble
enzymes
• Captures CO2
• Uses energy from ATP and NADPH for sugar synthesis
Light reactions
Dark reactions
Light(-dependent) reactions
Fig. 10-5, p. 152Wavelength (nm)400 500 600 700
0
20
40
60
80
100
chlorophyll b
chlorophyll a
Perc
ent
of
ligh
t abso
rbed
Absorption spectra of Chlorophyll a and b
Fig. 10-7, p. 154
pote
nti
al to
tra
nsfe
r ele
ctr
on
s (
measu
red
in
volt
s)
+0.8
+0.4
0
-0.6
ADP + Pi e
−
e−
e−
e−
e−
e−
NADPH
H+ + NADP+
P700
*
P70
0photosystem
I
photosystem IIreleased energy used to form ATPfrom ADP and phosphate
sunlightenergy
H2
O
sunlightenergy
P680
*
NONCYCLIC ELECTRON TRANSPORT
photolysis
P680: reaction center of photosystem II P700: reaction center of photosystem I
Pigments from the light harvesting complex
Fig. 10-7, p. 154
pote
nti
al to
tra
nsfe
r ele
ctr
on
s (
measu
red
in
volt
s)
+0.8
+0.4
0
-0.6
ADP + Pi e
−
e−
e−
e−
e−
e−
NADPH
H+ + NADP+
P700
*
P70
0
photosystem I
photosystem IIreleased energy used to form ATPfrom ADP and phosphate
sunlightenergy
H2
O
sunlightenergy
P680
*
CYCLIC ELECTRON TRANSPORT
photolysis
Fig. 10-3, p. 151
sunlight energy
oxygen released
H2O is split
H+
e−
H+
H+
NADP+
e−
H+
H+
carbohydrate end product (e.g. sucrose, starch, cellulose)
carbon dioxide used
Light-independe
nt reactions
sugar phosphate
Light-dependent reactionslumen
(H+ reservoir)
ADP + Pi
Stroma
electron transport system
photosystem II
photosystem I
electron transport system
Compare to respiration
Fig. 9-8c, p. 142
inner membranepyruvate from cytoplasm
Coenzymes give up electrons, hydrogen (H+) to transport system
NADH
NADH
FADH2
acetyl-CoA
TCA cycle
ATP
carbon dioxide
2
As electrons pass through system, H+ is pumped out from matrix
Oxygen accepts electrons, joins with 2H+, forms water
oxygen
INTERMEMBRANEspace
ATPsynthesized
ADPPi
H+
H+ H+ flows inH+
H+
H+
H+
electron transport system
H+
MATRIX
e−
e−
e−
Dark reactions
or
Light-independent reactions
Fig. 10-9, p. 157
(CO2 from the air)
stroma
Carbon dioxidefixation
(intermediates)
(PGA)
(RuBP)
rubiscoH2O
ADPPi
NADP+
(PGAL)
cyclic production of intermediate sugar phosphates
Calvin cycle
ADP
(PGAL)Pi sugar phosphate synthesis
typically used at once to form carbohydrates (mainlysucrose, starch, cellulose)sugar
phosphate
The Calvin cycle (C3 pathway of photosynthesis)
PGA: phosphoglyceric acidPGAL: phosphoglyceraldehydeRuBP: ribulose bisphosphateRubisco: 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 C=O and H-O bonds into the high energy containing C-C and C-H bonds of sugar.
Rubisco is the worlds most abundant protein!
Fig. 10-9, p. 157
(CO2 from the air)
stroma
Carbon dioxidefixation
(intermediates)
(PGA)
(RuBP)
rubiscoH2O
ADPPi
NADP+
(PGAL)
cyclic production of intermediate sugar phosphates
Calvin cycle
ADP
(PGAL)Pi sugar phosphate synthesis
typically used at once to form carbohydrates (mainlysucrose, starch, cellulose)sugar
phosphate
Using ATP and NADPH to generate high energy containing covalent bondsPGA: phosphoglyceric acidPGAL: phosphoglyceraldehyde
C C OH
H C O
O H
HH
P
C C OH
H C O
H
HH
P
PGA
PGAL
ATP + NADPH
Low energy electrons
High energy electrons
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.
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
Fig. 10-10, p. 158
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. 10-12, p. 159
C4 cycle
AMP
mesophyll cells
C3 cycle
bundle sheath cells
Interaction between the C4 cycle and the C3 cycle
The C4 pathway concentrates CO2
Fig. 10-11, p. 159
airspace
mesophyll cells
vascular bundle
CO2 movementguardcell lower
epidermis
upperepidermis
bundlesheath cell
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.
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.
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…).
SUMMARY: Transforming Light Energy into Chemical Energy
Consumption of photosynthesis products
1. AgricultureAnnual accumulation of light energy as C-H and C-C bonds (FOOD).
2. Fossil fuelsAccumulation 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?