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Chapter 10. Photosynthesis. Overview: The Process That Feeds the Biosphere Photosynthesis Is the process that converts solar energy into chemical energy. Plants and other autotrophs Are the producers of the biosphere. Figure 10.1. Plants are photoautotrophs
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Chapter 10 Photosynthesis
Overview: The Process That Feeds the Biosphere • Photosynthesis • Is the process that converts solar energy into chemical energy
Plants and other autotrophs • Are the producers of the biosphere
Figure 10.1 • Plants are photoautotrophs • They use the energy of sunlight to make organic molecules from water and carbon dioxide
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 (c) Unicellular protist 10 m (e) Pruple sulfur bacteria 1.5 m Figure 10.2 (d) Cyanobacteria (b) Multicellular algae 40 m • Photosynthesis • Occurs in plants, algae, certain other protists, and some prokaryotes
Heterotrophs • Obtain their organic material from other organisms • Are the consumers of the biosphere
Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
Leaf cross section Vein Mesophyll CO2 O2 Stomata Figure 10.3 Chloroplasts: The Sites of Photosynthesis in Plants • The leaves of plants • Are the major sites of photosynthesis
Mesophyll Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Granum Stroma Inner membrane 1 µm • Chloroplasts • Are the organelles in which photosynthesis occurs • Contain thylakoids and grana
Tracking Atoms Through Photosynthesis: Scientific Inquiry • Photosynthesis is summarized as 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
Reactants: 12 H2O 6 CO2 6 H2O 6 O2 C6H12O6 Products: Figure 10.4 The Splitting of Water • Chloroplasts split water into • Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
Photosynthesis as a Redox Process • Photosynthesis is a redox process • Water is oxidized, carbon dioxide is reduced
The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of two processes • The light reactions • The Calvin cycle
The light reactions • Occur in the grana • Split water, release oxygen, produce ATP, & form NADPH, which are used in the Calvin cycle, this is the primary function of the light reactions
The Calvin cycle • Occurs in the stroma of the chloroplast • Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power
H2O CO2 Light NADP ADP + P LIGHT REACTIONS CALVIN CYCLE ATP NADPH Chloroplast [CH2O] (sugar) O2 Figure 10.5 • An overview of photosynthesis
Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
The Nature of Sunlight • Light • Is a form of electromagnetic energy, which travels in waves
Wavelength • Is the distance between the crests of waves • Determines the type of electromagnetic energy
1 m 106 nm 10–5 nm 106 nm 1 nm 10–3 nm 103 nm 103 m Micro- waves Radio waves Gamma rays X-rays UV Infrared Visible light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Lower energy Higher energy Figure 10.6 • The electromagnetic spectrum • Is the entire range of electromagnetic energy, or radiation
The visible light spectrum • Includes the colors of light we can see • Includes the wavelengths that drive photosynthesis
Photosynthetic Pigments: The Light Receptors • Pigments • Are substances that absorb visible light • Ex. Red & yellow leaves absorb other colors, but reflect red & yellow so we see these colors.
Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7 • Reflect light, which include the colors we see
The spectrophotometer • Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength
Refracting prism Chlorophyll solution Photoelectric tube White light Galvanometer 2 3 1 0 100 4 Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light Figure 10.8 • An absorption spectrum • Is a graph plotting light absorption versus wavelength
The absorption spectra of chloroplast pigments • Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis • Visible light excites electrons during 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 Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Figure 10.9 • The absorption spectra of three types of pigments in chloroplasts
Rate of photosynthesis (measured by O2 release) (b) 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. Most effective wavelength for photosynthesis: 420nm • The action spectrum of a pigment • Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis
Aerobic bacteria Filament of alga 500 600 700 400 (c) 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. Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. In other words, to see the relationship between wavelenths of light & the O2 released CONCLUSION • The action spectrum for photosynthesis • Was first demonstrated by Theodor W. Engelmann
CH3 in chlorophyll a in chlorophyll b CHO CH2 CH3 CH H C C C Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center C C CH3 C C H3C CH2 C N C N H C C Mg H N C C N H3C C C CH3 C C C C C H H CH2 H C C O CH2 O O C O O CH3 CH2 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10 • Chlorophyll a • Is the main photosynthetic pigment • Chlorophyll b • Is an accessory pigment
AND there are others… • Other accessory pigments • Absorb different wavelengths of light and pass the energy to chlorophyll a
Excited state e– Heat Energy of election Photon (fluorescence) Ground state Chlorophyll molecule Photon Figure 10.11 A Excitation of Chlorophyll by Light • When a pigment absorbs light • It goes from a ground state to an excited state, which is unstable
Figure 10.11 B • If an isolated solution of chlorophyll is illuminated • It will fluoresce, giving off light and heat
Thylakoid Photosystem Photon STROMA Light-harvesting complexes Reaction center Primary election acceptor e– Thylakoid membrane Special chlorophyll a molecules Transfer of energy Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 • A photosystem • Is composed of a reaction center surrounded by a number of light-harvesting complexes
The light-harvesting complexes • Consist of pigment molecules bound to particular proteins • Funnel the energy of photons of light to the reaction center
When a reaction-center chlorophyll molecule absorbs energy • One of its electrons gets bumped up to a primary electron acceptor
The thylakoid membrane • Is populated by two types of photosystems, I and II, these are the electron transport chains in plants
Noncyclic Electron Flow • Noncyclic electron flow • Is the primary pathway of energy transformation in the light reactions • Cooperation of both photosystems is required for the reduction of NADP+
H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH Electron Transport chain O2 [CH2O] (sugar) Primary acceptor 7 Primary acceptor 4 Fd Electron transport chain Pq 2 e 8 e– e H2O NADP+ + 2 H+ Cytochrome complex 2 H+ NADP+ reductase + 3 NADPH O2 PC e– + H+ P700 e– 5 Light P680 Light 1 6 ATP Photosystem-I (PS I) Photosystem II (PS II) Figure 10.13 NONCYCLIC electron flow: Produces NADPH, ATP, and oxygen
e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem I Photosystem II Figure 10.14 • A mechanical analogy for the light reactions
Cyclic Electron Flow • Under certain conditions • Photoexcited electrons take an alternative path
Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II Figure 10.15 • In cyclic electron flow • Only photosystem I is used • Only ATP is produced
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria • Chloroplasts and mitochondria • Generate ATP by the same basic mechanism: chemiosmosis, thus both have ATP synthase complexes • But use different sources of energy to accomplish this
Key Higher [H+] Lower [H+] Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H+ Diffusion Thylakoid space Intermembrance space Electron transport chain Membrance ATP Synthase Stroma Matrix ADP+ P ATP H+ Figure 10.16 • The spatial organization of chemiosmosis • Differs in chloroplasts and mitochondria
In both organelles, by chemiosmosis • Redox reactions of electron transport chains generate a H+ gradient across a membrane • ATP synthase • Uses this proton-motive force to make ATP
H2O CO2 LIGHT NADP+ ADP CALVIN CYCLE LIGHT REACTOR ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Photosystem I NADP+ reductase Light 2 H+ 3 NADP+ + 2H+ Fd NADPH + H+ Pq Pc 2 H2O 1⁄2 O2 THYLAKOID SPACE (High H+ concentration) 1 2 H+ +2 H+ To Calvin cycle ATP synthase Thylakoid membrane STROMA (Low H+ concentration) ADP ATP P H+ Figure 10.17 Notice: NADP+ is reduced to NADPH • The light reactions and chemiosmosis: the organization of the thylakoid membrane
Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar, which is then used by respiration to release energy. • The Calvin cycle • Is similar to the citric acid cycle • Occurs in the stroma
The Calvin cycle has three phases • Carbon fixation: addition of carbon dioxide to RuBP • Reduction (NADP+ to NADPH) • Regeneration of the CO2 acceptor
H2O Input CO2 Light 3 (Entering one at a time) NADP+ CO2 ADP CALVINCYCLE LIGHTREACTION ATP NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-livedintermediate P 6 3 P P Ribulose bisphosphate(RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 6 P P 3 ATP 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Glucose andother organiccompounds G3P(a sugar)Output Figure 10.18 • The Calvin cycle Phase 1: Carbon fixation Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction
Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates