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Chapter 10 Photosynthesis
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Plants and other autotrophs
Photosynthesis Is the process that converts solar energy into chemical energy Occurs in plants, algae, certain protists, some prokaryote Plants and other autotrophs Are the producers of the biosphere Use energy of sunlight to make organic molecules from water and carbon dioxide.
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Heterotrophs Obtain their organic material from other organisms
Are the consumers of the biosphere
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Chloroplasts: The Sites of Photosynthesis in Plants
The leaves of plants Are the major sites of photosynthesis Vein Leaf cross section Figure 10.3 Mesophyll CO2 O2 Stomata
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Chloroplasts Are the organelles in which photosynthesis occurs
Contain thylakoids and grana Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner Thylakoid Granum Stroma 1 µm
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Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis is summarized as 6 CO H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
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Chloroplasts split water into
The Splitting of Water Chloroplasts split water into Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules 6 CO2 12 H2O Reactants: Products: C6H12O6 6 H2O 6 O2 Figure 10.4
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Photosynthesis as a Redox Process
Photosynthesis is a redox process Water is oxidized, carbon dioxide is reduced
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The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of two processes The light reactions The Calvin cycle
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The light reactions Occur in the grana
Split water, release oxygen, produce ATP, and form NADPH
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The Calvin cycle Occurs in the stroma
Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power
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An overview of photosynthesis
CO2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH2O] (sugar) NADPH NADP ADP + P O2 Figure 10.5 ATP
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Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
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The Nature of Sunlight Light
Is a form of electromagnetic energy, which travels in waves
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Wavelength Is the distance between the crests of waves
Determines the type of electromagnetic energy
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The electromagnetic spectrum
Is the entire range of electromagnetic energy, or radiation 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 10.6
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The visible light spectrum
Includes the colors of light we can see Includes the wavelengths that drive photosynthesis
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Photosynthetic Pigments: The Light Receptors
Are substances that absorb visible light
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Reflect light, which include the colors we see
Reflected Chloroplast Absorbed light Granum Transmitted Figure 10.7
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The spectrophotometer
Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength
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An absorption spectrum
Is a graph plotting light absorption versus wavelength Figure 10.8 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 chlorophyll absorbs most blue light. Blue 1 2 3 4 100
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The absorption spectra of chloroplast pigments
Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis
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Wavelength of light (nm)
The absorption spectra of three types of pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Absorption of light by chloroplast pigments Chlorophyll a (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Wavelength of light (nm) Chlorophyll b Carotenoids Figure 10.9
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The action spectrum of a pigment
Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis (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)
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The action spectrum for photosynthesis
Was first demonstrated by Theodor W. Engelmann 400 500 600 700 Aerobic bacteria 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
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Chlorophyll a Chlorophyll b Is the main photosynthetic pigment
Is an accessory pigment C CH CH2 N 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 10.10
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Other accessory pigments
Absorb different wavelengths of light and pass the energy to chlorophyll a
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Excitation of Chlorophyll by Light
When a pigment absorbs light It goes from a ground state to an excited state, which is unstable Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground e– Figure A
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If an isolated solution of chlorophyll is illuminated
It will fluoresce, giving off light and heat Figure B
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A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
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(INTERIOR OF THYLAKOID)
A photosystem Is composed of a reaction center surrounded by a number of light-harvesting complexes 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 10.12 e–
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The light-harvesting complexes
Consist of pigment molecules bound to particular proteins Funnel the energy of photons of light to the reaction center
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When a reaction-center chlorophyll molecule absorbs energy
One of its electrons gets bumped up to a primary electron acceptor
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The thylakoid membrane
Is populated by two types of photosystems, I and II
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Noncyclic Electron Flow
Is the primary pathway of energy transformation in the light reactions
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Electron transport chain
Produces NADPH, ATP, and oxygen Figure 10.13 Photosystem II (PS II) Photosystem-I (PS I) ATP NADPH NADP+ ADP CALVIN CYCLE CO2 H2O O2 [CH2O] (sugar) LIGHT REACTIONS Light Primary acceptor Pq Cytochrome complex PC e P680 e– + 2 H+ Fd reductase Electron Transport chain Electron transport chain P700 + 2 H+ + H+ 7 4 2 8 3 5 1 6
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A mechanical analogy for the light reactions
Mill makes ATP e– Photon Photosystem II Photosystem I NADPH Figure 10.14
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Under certain conditions
Cyclic Electron Flow Under certain conditions Photoexcited electrons take an alternative path
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In cyclic electron flow
Only photosystem I is used Only ATP is produced Primary acceptor Pq Fd Cytochrome complex Pc NADP+ reductase NADPH ATP Figure 10.15 Photosystem II Photosystem I
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A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Generate ATP by the same basic mechanism: chemiosmosis But use different sources of energy to accomplish this
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The spatial organization of chemiosmosis
Differs in chloroplasts and mitochondria 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 10.16
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In both organelles ATP synthase
Redox reactions of electron transport chains generate a H+ gradient across a membrane ATP synthase Uses this proton-motive force to make ATP
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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 10.17
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Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
Is similar to the citric acid cycle Occurs in the stroma
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The Calvin cycle has three phases
Carbon fixation Reduction Regeneration of the CO2 acceptor
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The Calvin cycle Figure 10.18 Input Light 3 CO2 CALVIN CYCLE
(G3P) Input (Entering one at a time) CO2 3 Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ Glyceraldehyde-3-phosphate 6 ATP ATP 3 ADP CALVIN CYCLE 5 1 G3P (a sugar) Output Light H2O LIGHT REACTION ATP NADPH NADP+ ADP [CH2O] (sugar) CALVIN CYCLE Figure 10.18 O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction
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Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates
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On hot, dry days, plants close their stomata
Conserving water but limiting access to CO2 Causing oxygen to build up
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Photorespiration: An Evolutionary Relic?
In photorespiration O2 substitutes for CO2 in the active site of the enzyme rubisco The photosynthetic rate is reduced
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C4 plants minimize the cost of photorespiration
By incorporating CO2 into four carbon compounds in mesophyll cells
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These four carbon compounds
Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle
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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 10.19
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CAM Plants CAM plants Open their stomata at night, incorporating CO2 into organic acids
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During the day, the stomata close
And the CO2 is released from the organic acids for use in the Calvin cycle
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The CAM pathway is similar to the C4 pathway
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 Organic acids release CO2 to Calvin cycle Figure 10.20
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The Importance of Photosynthesis: A Review
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 reaction 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 10.21
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Organic compounds produced by photosynthesis
Provide the energy and building material for ecosystems
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