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

Light(-dependent) reactions

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

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

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

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

Compare to respiration

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

Dark reactions or Light-independent reactions

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.

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.

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

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

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. 10-10, p. 158

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. 10-12, p. 159

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. 10-11, p. 159

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.

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.

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.