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Where It Starts: Photosynthesis. Chapter 6. Biology Concepts and Applications , Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011. 6.1 Green Energy. Before photosynthesis evolved, Earth’s atmosphere had little free oxygen
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Where It Starts:Photosynthesis Chapter 6 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.
6.1 Green Energy • Before photosynthesis evolved, Earth’s atmosphere had little free oxygen • Oxygen released during photosynthesis changed the atmosphere • Favored evolution of new metabolic pathways, including aerobic respiration
Introduction • Autotroph organism that makes its own food using carbon from inorganic molecules (CO2) and energy • Heterotroph organism that obtains energy and carbon form organic compounds • Photosynthesis metabolic pathway by which most autotrophs capture light energy and use it to make sugars from CO2 and water
6.2 Sunlight as an Energy Source • Electromagnetic energy • Travels in waves • Is organized as photons • Wavelength • Distance between the crests of two successive waves of light • Measured in nanometers (nm): 25 million nm = 1 inch • Visible light A small part of a spectrum of electromagnetic energy radiating from the sun • Between 380 and 750 nm
Photosynthetic Pigments • Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules • Pigment is an organic molecule that absorbs only light of particular wavelengths • Photons not captured are reflected as color
Major Photosynthetic Pigments • Chlorophyll a • Main photosynthetic pigment • Absorbs violet and red light (appears green) • Chlorophyll b, carotenoids, phycobilins • Absorb additional wavelengths • Collectively, photosynthetic pigments absorb almost all of wavelengths of visible light • Figure 6.3 in Text
Conclusion: Violet and red light are the best for driving photosynthesis Prism Bacteria (oxygen requiring) alga a Outcome of T. Engelmann’s experiment. Fig. 6.4a, p.96
80 60 Light absorption (%) 40 20 0 400 500 600 700 Wavelength (nanometers) b Absorption spectra for chlorophyll a (solid graph line) and chlorophyll b (dashed line). Compare these graphs with the clustering of bacteria shown in (a). Fig. 6.4b, p.96
80 60 Light absorption (%) 40 20 0 400 500 600 700 Wavelength (nanometers) c Absorption spectra for beta-carotene (solid line) and one of the phycobilins (dashed line). Fig. 6.4c, p.96
Key Concepts: THE RAINBOW CATCHERS • A great one-way flow of energy through the world of life starts after chlorophylls and other pigments absorb the energy of visible light from the sun’s rays • In plants, some bacteria, and many protists, that energy ultimately drives the synthesis of glucose and other carbohydrates
6.4 Overview of Photosynthesis • In the Chloroplast!! • Photosynthesis proceeds in two stages • Light-dependent reactions • Light-independent reactions Summary equation: 6H2O + 6CO2 6O2 + C6H12O6
sunlight Light- Dependent Reactions H2O O2 NADPH ADP + Pi NADP+ ATP Light- Independent Reactions Calvin-Benson cycle H2O CO2 phosphorylated glucose end products (e.g., sucrose, starch, cellulose) Fig. 6.13, p.104
Sites of Photosynthesis: Chloroplasts • Light-dependent reactions occur at a much-folded thylakoid membrane • Forms a single, continuous compartment inside the stroma • First stage of photosynthesis • light energy + H2O chemical energy (ATP & NADPH) • Light-independent reactions occur in the stroma (chloroplast’s semifluid interior) • Second stage of photosynthesis • Use ATP & NADPH to assemble sugars from H2O and CO2
Products of Light-Dependent Reactions • Typically, sunlight energy drives the formation of ATP and NADPH • Oxygen is released from the chloroplast (and the cell)
Key Concepts:OVERVIEW OF PHOTOSYNTHESIS • Photosynthesis proceeds through two stages in chloroplasts of plants and many types of protists • First, pigments in a membrane inside the chloroplast capture light energy, which is converted to chemical energy • Second, chemical energy drives synthesis of carbohydrates
6.5 Light-Dependent Reactions • In the thylakoid membrane • Light-harvesting complexes • Absorb light energy and pass it to photosystems which then release electrons • Photosystem a cluster of pigments and proteins that converts light energy to chemical energy in photosynthesis • Electrons enter light-dependent reactions
1. Noncyclic Photophosphorylation • Electrons released from photosystem II flow through an electron transfer chain (ETC) • Electron transfer phosphorylation occurs • Electrons that flow through the ETC set up a hydrogen ion gradient that drives ATP formation • At end of chain, they enter photosystem I • Photon energy causes photosystem I to release electrons, which end up in NADPH • Photosystem II replaces lost electrons by pulling them from water (photolysis) • Photolysis process by which light energy breaks down a molecule
electron transfer chain light energy electron transfer chain light energy NADPH Photosystem II Photosystem I THYLAKOID COMPARTMENT THYLAKOID MEMBRANE oxygen (diffuses away) STROMA Fig. 6.8b, p.99
2. Cyclic Photophosphorylation • Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I • NADPH does not form, oxygen is not released
ATP Formation • In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment • A hydrogen ion gradient builds up across the thylakoid membrane • H+ flows back across the membrane through ATP synthases • Results in formation of ATP in the stroma
Key Concepts:MAKING ATP AND NADPH • In the first stage of photosynthesis, sunlight energy is converted to the chemical bond energy of ATP • The coenzyme NADPH forms in a pathway that also releases oxygen
6.7 Light Independent Reactions:The Sugar Factory • Light-independent reactions proceed in the stroma • Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP to start the Calvin–Benson cycle • Calvin Benson cycle light-independent reactions of photosynthesis • Carbon fixation process by which carbon from an inorganic source gets incorporated into an organic molecule. • Rubisco carbon fixing enzyme
Calvin–Benson Cycle • Cyclic pathway makes phosphorylated glucose • Uses energy from ATP, carbon and oxygen from CO2, and hydrogen and electrons from NADPH • Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose) • Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2
a CO2 in air spaces inside a leaf diffuses into a photosynthetic cell. Six times, rubisco attaches a carbon atom from CO2 to the RuBP that is the starting compound for the Calvin–Benson cycle. f It takes six turns of the Calvin–Benson cycle (six carbon atoms) to produce one glucose molecule and regenerate six RuBP. 6CO2 b Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. The resulting intermediate is called PGAL. e Ten of the PGAL get phosphate groups from ATP. In terms of energy, this primes them for an uphill run—for the endergonic synthesis reactions that regenerate RuBP. 12 ATP 12 PGA 6 RuBP 6 ADP 12 ADP + 12 Pi Calvin-Benson cycle 6 ATP 12 NADPH 4 Pi 12 NADP+ d The phosphorylated glucose enters reactions that form carbohydrate products—mainly sucrose, starch, and cellulose. c Two of the twelve PGAL molecules combine to form a molecule of glucose with an attached phosphate group. 10 PGAL 12 PGAL 1 Pi phosphorylated glucose Fig. 6.10, p.101
6.8 Adaptations: Different Carbon-Fixing Pathways • Environments differ • Plants have different details of sugar production in light-independent reactions • On dry days, plants conserve water by closing their stomata gaps that open on plant surfaces that allow water vapor and gases to diffuse across the epidermis • O2 from photosynthesis cannot escape
Plant Adaptations to Environment • C3 plants • High O2 level; Rubisco attaches to O2 instead of CO2 to RuBP; Photorespiration reduces efficiency of sugar production
Plant Adaptations to Environment • C3 plants • Photorespiration • Reaction in which rubisco attaches oxygen instead of CO2 to ribulose bisphosphate • Plant loses carbon instead of fixing it. • Extra energy is need to make sugars on dry days
Plant Adaptations to Environment • C4 plants • Carbon fixation occurs twice, in two different cells to minimize photorespiration • First reactions release CO2 near rubisco, limit photorespiration when stomata are closed Examples: Corn, bamboo
CO2 from inside plant C4 cycle oxaloacetate CO2 RuBP Calvin-Benson cycle PGA sugar b C4 plants. Oxygen also builds up in the air spaces inside the leaves when stomata close. An additional pathway in these plants keeps the CO2 concentration high enough to prevent rubisco from using oxygen. Fig. 6.11b2, p.102
Plant Adaptations to Environment • CAM plants • Type of C4 plant that conserves water by fixing carbon twice, at different times of the day in the same cell • Day time C4 reactions • Night time Calvin-Benson cycle • Open stomata and fix carbon at night
CO2 from outside plant oxaloacetate C4 cycle night day CO2 RuBP Calvin-Benson cycle PGA sugar c CAM plants open stomata and fix carbon with a C4 pathway at night. When stomata are closed during the day, organic compounds made during the night are converted to CO2 that enters the Calvin–Benson cycle. Fig. 6.11c2, p.102
Key Concepts:MAKING SUGARS • Second stage is the “synthesis” part of photosynthesis • Enzymes speed assembly of sugars from carbon and oxygen atoms, both from carbon dioxide • Reactions use ATP and NADPH that form in the first stage of photosynthesis • ATP delivers energy, and NADPH delivers electrons and hydrogens to the reaction sites • Details of the reactions vary among organisms
