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Chapter 6 Where It Starts – Photosynthesis Electromagnetic Spectrum of Radiant Energy shortest wavelengths (highest energy) gamma rays x-rays range of heat range of most radiation escaping from reaching Earth’s surface Earth’s surface visible light near-infrared ultraviolet radiation radiation 400 nm 500 nm longest wavelengths (lowest energy) radiation microwaves radio waves infrared 600 nm 700 nm Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin Light absorption β-carotene 400 nm chlorophyll a 500 nm 600 nm Wavelength 700 nm Take-Home Message: Why do cells use more than one photosynthetic pigment? A combination of pigments allows a photosynthetic organism to most efficiently capture the particular range of light wavelengths that reaches the habitat in which it evolved 5 two outer membranes of chloroplast stroma part of thylakoid membrane system: thylakoid compartment, cutaway view Figure 6-5b p105 6 Figure 6-7 p106 7 The Noncyclic Pathway Photosystems (type II and type I) contain “special pairs” of chlorophyll a molecules that eject electrons Electrons lost from photosystem II are replaced by photolysis of water molecules – the process by which light energy breaks down a water molecule into hydrogen and oxygen Electrons lost from a photosystem enter an electron transfer chain (ETC) in the thylakoid membrane In the ETC, electron energy is used to build up a H+ gradient across the membrane H+ flows through ATP synthase, which attaches a phosphate group to ADP ATP is formed in the stroma by chemiosmosis, or electron transfer phosphorylation Electrons from the first electron transfer chain (from photosystem II) are accepted by photosystem I Electrons ejected from photosystem I enter a different electron transfer chain in which the coenzyme NADP+ accepts the electrons and H+, forming NADPH ATP and NADPH are the energy products of light-dependent reactions in the noncyclic pathway 8 Noncyclic Pathway of Photosynthesis H+ light energy electron transfer chain light energy to second stage of reactions ADP + Pi ATP synthase photosystem II photosystem I thylakoid compartment stroma 9 The Cyclic Pathway When NADPH accumulates in the stroma, the noncyclic pathway stalls A cyclic pathway runs in type I photosystems to make ATP; electrons are cycled back to photosystem I and NADPH does not form Photophosphorylation • Photophosphorylation is a light-driven reaction that attaches a phosphate group to a molecule • In noncyclic photophosphorylation, electrons move from water to photosystem II, to photosystem I, to NADPH • In cyclic photophosphorylation, electrons cycle within photosystem I 10 Excited P700 energy Excited P680 P700 (photosystem I) P680 (photosystem II) light energy light energy Energy flow in the noncyclic reactions of photosynthesis Stepped Art Figure 6-9a p108 11 energy Excited P700 P700 (photosystem I) light energy Energy flow in the cyclic reactions of photosynthesis Stepped Art Figure 6-9b p108 12 What happens during the light-dependent reactions of photosynthesis? Take-Home Message: In light-dependent reactions, chlorophylls and other pigments in thylakoid membrane transfer light energy to photosystems Photosystems eject electrons that enter electron transfer chains in the membrane; electron flow through ETCs sets up hydrogen ion gradients that drive ATP formation In the noncyclic pathway, oxygen is released and electrons end up in NADPH A cyclic pathway involving only photosystem I allows the cell to continue making ATP when the noncyclic pathway is not running; NADPH does not form; O2 is not released 13 How does energy flow during the reactions of photosynthesis? Take-Home Message: Light provides energy inputs that keep electrons flowing through electron transfer chains Energy lost by electrons as they flow through the chains sets up a hydrogen ion gradient that drives the synthesis of ATP alone, or ATP and NADPH 14 Light-Independent Reactions The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis Calvin-Benson cycle Enzyme-mediated reactions that build sugars in the stroma of chloroplasts Carbon fixation Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule Builds glucose from CO2 Uses bond energy of molecules formed in light-dependent reactions (ATP, NADPH) 15 The Calvin-Benson Cycle The enzyme rubisco attaches CO2 to RuBP Forms two 3-carbon PGA molecules PGAL is formed PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH Two PGAL combine to form a 6-carbon sugar Rubisco is regenerated 16 1 4 Calvin– Benson Cycle 2 other molecules 3 glucose Stepped Art Figure 6-10 p109 18 19 Take-Home Message: What happens in light- independent reactions of photosynthesis? Light-independent reactions of photosynthesis run on the bond energy of ATP and energy of electrons donated by NADPH; both formed in the lightdependent reactions Collectively called the Calvin–Benson cycle, these carbon-fixing reaction use hydrogen (from NADPH), and carbon and oxygen (from CO2) to build sugars 20 21 22 Adaptations: Different Carbon-Fixing Pathways Environments differ, and so do details of photosynthesis: C3 plants C4 plants CAM plants Stomata Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems Allow CO2 in and O2 out Close on dry days to minimize water loss 23 C3 Plants palisade mesophyll cell C3 plants Plants that use only the Calvin–Benson cycle to fix carbon Forms 3-carbon PGA in mesophyll cells spongy mesophyll cell Used by most plants, but inefficient in dry weather when stomata are closed Example: barley When stomata are closed, CO2 needed for light-independent reactions can’t enter, O2 produced by light-dependent reactions can’t leave Photorespiration At high O2 levels, rubisco attaches to oxygen instead of carbon CO2 is produced rather than fixed 24 mesophyll cell CO2 O2 glycolate RuBP Calvin– Benson PGA Cycle ATP NADPH B On dry days, stomata close and oxygen accumulates inside leaves. The excess causes rubisco to attach oxygen instead of carbon to RuBP. This is photorespiration, and it makes sugar production inefficient in C3 plants. sugars Figure 6-11b p110 25 C4 Plants C4 plants Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar Examples: Corn, switchgrass, bamboo C4 plants. Oxygen also builds up inside leaves when stomata close during photosynthesis. An additional pathway in these plants keeps the CO2 concentration high enough in bundle-sheath cells to prevent photorespiration. mesophyll cell 26 mesophyll cell B C4 plants. Oxygen also builds up inside leaves oxaloacetate C4 when stomata Cycle close during photosynthesis. bundle-sheath cell bundle-sheath cell CO2 from inside plant An additional pathway in these plants keeps the CO2 concentration high enough in bundle-sheath cells to prevent photorespiration. CO2 PGA RuBP Calvin– Benson Cycle sugars Figure 6-12b p110 27 CAM Plants A CAM Plant: Jade Plant CAM plants (Crassulacean Acid Metabolism) Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for sugar production Example: succulents, cactuses mesophyll cell 28 CO2 from outside plant oxaloacetate C4 Cycle night day CO2 PGA RuBP Calvin– Benson Cycle sugars Figure 6-13a p111 29 Take-Home Message: How do carbon-fixing reactions vary? When stomata are closed, oxygen builds up inside leaves of C3 plants; rubisco then can attach oxygen (instead of carbon dioxide) to RuBP; photorespiration reduces the efficiency of sugar production, so it can limit the plant’s growth Plants adapted to dry conditions limit photorespiration by fixing carbon twice: C4 plants separate the two sets of reactions in space; CAM plants separate them in time
