1. 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
Figure 10.5: Absorption spectra of chlorophylls a and b at different wavelengths of light. Graph shows the fraction of received light that is absorbed when the pigment is exposed to various wavelengths of light. The relation between wavelength and color is also shown. 20
400
500
600
700
Wavelength (nm)
Fig. 10-5, p. 152

4. NONCYCLIC ELECTRON TRANSPORT
Fig. 10-7, p. 154
NONCYCLIC ELECTRON TRANSPORT
P700* e−
-0.6
sunlight
energy
Electron Transport System
NADPH
P680* e− e−
potential to transfer electrons (measured in volts)
H+ + NADP+
sunlight
energy
ADP + Pi
electron transport system 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
Figure 10.7: The pathway of noncyclic electron transport from water to reduced nicotinamide adenine dinucleotide phosphate (NADPH), with the associated adenosine triphosphate (ATP) synthesis. Pi, Inorganic phosphate.
photosystem II e−
H2O
photolysis
P680: reaction center of photosystem II
P700: reaction center of photosystem I

5. CYCLIC ELECTRON TRANSPORT
Fig. 10-7, p. 154
potential to transfer electrons (measured in volts)
+0.8
+0.4
-0.6
CYCLIC ELECTRON TRANSPORT
P700* e−
sunlight
energy
Electron Transport System
NADPH
P680* e− e−
H+ + NADP+
sunlight
energy
ADP + Pi
electron transport system e− e−
P700
photosystem I
released energy used to form ATP
from ADP and phosphate
photosystem II
Figure 10.7: The pathway of noncyclic electron transport from water to reduced nicotinamide adenine dinucleotide phosphate (NADPH), with the associated adenosine triphosphate (ATP) synthesis. Pi, Inorganic phosphate. e−
H2O
photolysis

6. Light-independent reactions
oxygen released
sunlight energy
photosystem II e− H+
electron transport system
H2O is split
Light-dependent reactions H+
lumen
(H+ reservoir) H+
photosystem I e−
electron transport system
NADP+
carbon dioxide used
Figure 10.3: Diagram of a section of chloroplast granum showing where reactions take place. ADP, Adenosine diphosphate.
ADP + Pi H+ H+
Light-independent reactions
sugar phosphate
Stroma
carbohydrate end product (e.g. sucrose, starch, cellulose)
Fig. 10-3, p. 151

7. Compare to respiration

8. TCA cycle INTERMEMBRANE space MATRIX
pyruvate from cytoplasm
inner membrane H+
electron transport system
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
Figure 9.8c: Detail of membranes, showing location of electron transport system and adenosine triphosphate (ATP) synthesis.
Oxygen accepts electrons, joins with 2H+, forms water
ATP
synthesized 2
ATP Pi
ADP
oxygen H+
INTERMEMBRANE
space H+
MATRIX
H+ flows in H+ H+
Fig. 9-8c, p. 142

9. Dark reactions or Light-independent reactions

10. cyclic production of intermediate sugar phosphates
Fig. 10-9, p. 157
(CO2 from the air)
stroma
Carbon dioxide
fixation
(intermediates)
(PGA)
(RuBP)
rubisco
H2O
ADP Pi
NADP+
(PGAL)
cyclic production of intermediate sugar phosphates
Calvin cycle
sugar phosphate synthesis
typically used at once to form carbohydrates (mainly
sucrose, starch, cellulose)
sugar phosphate
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 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!
Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.

11. Figure 10. 9: Some major steps in the C3 pathway of photosynthesis
Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.

12. cyclic production of intermediate sugar phosphates
Fig. 10-9, p. 157
(CO2 from the air)
stroma
Carbon dioxide
fixation
(intermediates)
(PGA)
(RuBP)
rubisco
H2O
ADP Pi
NADP+
(PGAL)
cyclic production of intermediate sugar phosphates
Calvin cycle
sugar phosphate synthesis
typically used at once to form carbohydrates (mainly
sucrose, starch, cellulose)
sugar phosphate
Using ATP and NADPH to generate high energy containing covalent bonds
PGA: phosphoglyceric acid
PGAL: phosphoglyceraldehyde H H P C C OH
PGA H C O
Low energy electrons
O H
ATP + NADPH
Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate. H H P C C OH
PGAL H C O
High energy electrons H

13. 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.

14. 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

15. 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.
Figure 10.10: Corn (Zea mays), a C4 plant (right), with its low CO2 compensation point is able to survive at a lower CO2 concentration than bean (Phaseolus vulgaris), a C3 plant (left), when they are grown together in a closed chamber in light for 10 days.
Fig , p. 158

16. The C4 pathway concentrates CO2
Interaction between the C4 cycle and the C3 cycle
C4 cycle
AMP
mesophyll cells
Figure 10.12: Interaction between the C4 cycle in mesophyll cells and the C3 cycle in bundle sheath cells.
C3 cycle
bundle sheath cells
Fig , p. 159

17. The C4 pathway concentrates CO2
air
space
bundle
sheath cell
upper
epidermis
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.
Figure 10.11: Photosynthesis in corn (Zea mays). A section through a leaf shows the concentric arrangement of bundle sheath and mesophyll cells. Compare this diagram with Figure 6.10a.
CO2 movement
mesophyll cells
guard
cell
vascular bundle
lower
epidermis
Fig , p. 159

18. 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.

19. 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…).
Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.

20. 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?
Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.