Chapter 10 Photosynthesis

Figure 10.1 Sunlight consists of a spectrum of colors, visible here in a rainbow

Figure 10.2 Photoautotrophs These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (a) Plants (b) Multicellular algae (c) Unicellular protist 10 m 40 m (c) Cyanobacteria 1.5 m (d) Pruple sulfur bacteria

Figure 10.3 Focusing in on the location of photosynthesis in a plant Mesophyll cell Mesophyll Vein Stomata CO2 O2 Chloroplast 5 µm 1 µm Leaf cross section Outer membrane Granum Storma Thylakoid Thylakoid Space Intermembrane space Inner membrane

Figure 10.4 Tracking atoms through photosynthesis 6 CO2 12 H2O Reactants: Products: C6H12O6 6 H2O 6 O2

Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle Chloroplast

Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle ATP NADPH Chloroplast O2

Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle [CH2O] (sugar) NADP ADP + P i H2O Light LIGHT REACTIONS ATP NADPH Chloroplast

Figure 10.6 The electromagnetic spectrum Gamma rays X-rays UV Infrared Micro- waves Radio 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m 380 450 500 550 600 650 700 750 nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy

Figure 10.7 Why leaves are green: interaction of light with chloroplasts Reflected Chloroplast Absorbed light Granum Transmitted

Figure 10.8 Research Method Determining an Absorption Spectrum APPLICATION An absorption spectrum is a visual representation of how well a particular pigment absorbs different wavelengths of visible light. Absorption spectra of various chloroplast pigments help scientists decipher each pigment’s role in a plant. TECNIQUE A spectrophotometer measures the relative amounts of light of different wavelengths absorbed and transmitted by a pigment solution. 2 The transmitted light strikes a photoelectric tube, which converts the light energy to electricity. 3 4 The electrical current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed. White light is separated into colors (wavelengths) by a prism. 1 One by one, the different colors of light are passed through the sample (chlorophyll in this example). Green light and blue light are shown here.

White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer Slit moves to pass light of selected wavelength Green The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. Blue 1 2 3 4 100 See Figure 10.9a for absorption spectra of three types of chloroplast pigments. Result

Wavelength of light (nm) Figure 10.9 Inquiry Which wavelengths of light are most effective in driving photosynthesis? Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Chlorophyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids 400 500 600 700 Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

(measured by O2 release) Rate of photosynthesis Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. (b)

Aerobic bacteria 400 500 600 700 Filament of alga Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. (c) Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CONCLUSION

Figure 10.10 Structure of chlorophyll molecules in chloroplasts of plants H3C Mg H CH3 O CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown

Figure 10.11 Excitation of isolated chlorophyll by light Excited state Energy of election e– Heat Photon (fluorescence) Chlorophyll molecule Ground (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

Figure 10.21 A review of photosynthesis Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions O2 CO2 H2O Light Light reactions Calvin cycle NADP+ ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast

Figure 10.12 How a photosystem harvests light Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Figure 10.13 How noncyclic electron flow during the light reactions generates ATP and NADPH CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor 2 e Energy of electrons Light P680 1 Photosystem II (PS II)

Figure 10.13 How noncyclic electron flow during the light reactions generates ATP and NADPH CO2 Light LIGHT REACTIONS CALVIN CYCLE O2 NADP+ NADPH [CH2O] (sugar) Photosystem II (PS II) e Primary acceptor ADP ATP 2 H+ + 1⁄2 1 3 2 Energy of electrons P680

Electron transport chain Figure 10.13 How noncyclic electron flow during the light reactions generates ATP and NADPH O2 + H2O CO2 Light LIGHT REACTIONS CALVIN CYCLE NADP+ NADPH [CH2O] (sugar) Photosystem II (PS II) e Primary acceptor ATP 2 H+ 1⁄2 2 Energy of electrons ADP Pq Cytochrome complex Pc Electron transport chain 4 3 e 5 e P680 1

Electron transport chain Figure 10.13 How noncyclic electron flow during the light reactions generates ATP and NADPH O2 H2O CO2 Light LIGHT REACTIONS CALVIN CYCLE NADPH [CH2O] (sugar) Photosystem II (PS II) e Primary acceptor 2 H+ 1⁄2 2 Energy of electrons ADP Pq Cytochrome complex Pc ATP Electron transport chain 5 NADP+ Photosystem I (PS I) 6 1 3 4 + e e P700 P680

Electron transport chain Figure 10.13 How noncyclic electron flow during the light reactions generates ATP and NADPH P700 + CO2 Photosystem II (PS II) H2O Light LIGHT REACTIONS CALVIN CYCLE O2 NADPH [CH2O] (sugar) e Primary acceptor 2 H+ 1⁄2 1 Energy of electrons Pq Cytochrome complex Pc ATP Electron transport chain NADP+ Photosystem I (PS I) 6 2 ADP 5 Fd Electron Transport chain 7 reductase + 2 H+ 8 + H+ 3 4 P680

Figure 10.14 A mechanical analogy for the light reactions Mill makes ATP e– Photon Photosystem II Photosystem I NADPH

Figure 10.15 Cyclic electron flow Primary acceptor Pq Fd Cytochrome complex Pc NADP+ reductase NADPH ATP NADP+ Photosystem II (PS II) Photosystem I (PS I)

Figure 10.16 Comparison of chemiosmosis in mitochondria and chloroplasts Key Higher [H+] Lower [H+] Mitochondrion Chloroplast MITOCHONDRION STRUCTURE Intermembrance space Membrance Matrix Electron transport chain H+ Diffusion Thylakoid Stroma ATP P ADP+ Synthase CHLOROPLAST

Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+

Figure 10.18 The Calvin cycle Light H2O CO2 LIGHT REACTIONS ATP NADPH NADP+ [CH2O] (sugar) CALVIN CYCLE ADP (Entering one at a time) CO2 3 Phase 1: Carbon fixation Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP Input O2 6 CALVIN CYCLE

Figure 10.18 The Calvin cycle (Entering one at a time) CO2 3 Phase 1: Carbon fixation Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 P i 6 Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 6 ATP CALVIN CYCLE 1 G3P (a sugar) Output Glucose and other organic compounds 6 ADP Input Light H2O LIGHT REACTIONS ATP NADP+ [CH2O] (sugar) CALVIN CYCLE NADPH ADP O2 6

Figure 10.18 The Calvin cycle (Entering one at a time) CO2 3 Phase 1: Carbon fixation Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 P i 6 Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 6 ATP 3 ATP 3 ADP CALVIN CYCLE 5 Phase 3: Regeneration of the CO2 acceptor (RuBP) 1 G3P (a sugar) Output Glucose and other organic compounds G3P 6 ADP Light H2O LIGHT REACTIONS NADPH NADP+ [CH2O] (sugar) CALVIN CYCLE Input ATP ADP O2 6

Figure 10.19 C4 leaf anatomy and the C4 pathway CO2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C4 plant leaf Stoma Mesophyll C4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Sheath Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue

Figure 10.20 C4 and CAM photosynthesis compared Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO2 C4 CAM CO2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 CO2 Organic acids release CO2 to Calvin cycle Organic acids release CO2 to Calvin cycle

Figure 10.21 A review of photosynthesis Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions O2 CO2 H2O Light Light reactions Calvin cycle NADP+ ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast