Chapter 6 Where It Starts – Photosynthesis

Electromagnetic Spectrum of Radiant Energy range of heat escaping from Earth’s surface shortest wavelengths (highest energy) range of most radiation reaching Earth’s surface longest wavelengths (lowest energy) visible light gamma rays ultraviolet radiation near-infrared radiation radiation infrared x-rays microwaves radio waves Figure 6.2 Properties of light. 400 nm 500 nm 600 nm 700 nm

Absorption Spectra chlorophyll b phycoerythrobilin phycocyanobilin β-carotene chlorophyll a Light absorption Figure 6.4 Animated Discovery that photosynthesis is driven by particular wavelengths of light. A Light micrograph of cells in a strand of Chladophora. Theodor Engelmann used this green alga in an early experiment to show that certain colors of light are best for photosynthesis. B Engelmann directed light through a prism so that bands of colors crossed a water droplet on a microscope slide. The water held a strand of Chladophora and oxygen-requiring bacteria. The bacteria clustered around the algal cells that were releasing the most oxygen—the ones that were most actively engaged in photosynthesis. Those cells were under red and violet light. These results constituted one of the first absorption spectra. C Absorption spectra of chlorophylls a and b , β-carotene, and two phycobilins reveal the efficiency with which these pigments absorb different wavelengths of visible light. Line color indicates the characteristic color of each pigment. 400 nm 500 nm 600 nm 700 nm Wavelength

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

two outer membranes of chloroplast stroma part of thylakoid membrane system: Figure 6.5 Animated The chloroplast: site of photosynthesis in the cells of typical leafy plants. The micrograph shows chloroplast-stuffed cells in a leaf of Plagiomnium ellipticum, a type of moss. thylakoid compartment, cutaway view Figure 6-5b p105

Figure 6.7 Summary of the inputs and outputs of photosynthesis. Figure 6-7 p106

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

Noncyclic Pathway of Photosynthesis to second stage of reactions light energy electron transfer chain light energy ADP + Pi ATP synthase photosystem II photosystem I thylakoid compartment stroma

Photophosphorylation 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 Figure 6.8 Animated Noncyclic pathway of photosynthesis. Electrons that travel through two different electron transfer chains end up in NADPH, which delivers them to sugar-building reactions in the stroma. The cyclic pathway (not shown) uses a third type of electron transfer chain. 1 Light energy ejects electrons from a photosystem II. 2 The photosystem pulls replacement electrons from water molecules, which break apart into oxygen and hydro-gen ions. The oxygen leaves the cell as O2. 3 The electrons enter an electron transfer chain in the thylakoid membrane. 4 Energy lost by the electrons as they move through the electron transfer chain is used to pump hydrogen ions from the stroma into the thylakoid compartment. A hydrogen ion gradient forms across the thylakoid membrane. 5 Light energy ejects electrons from a photosystem I. Replacement elec-trons come from an electron transfer chain in the thylakoid membrane. 6 The electrons move through a second electron transfer chain, then combine with NADP+ and H+, so NADPH forms. 7 Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane. 8 Hydrogen ion flow causes ATP synthases to attach phosphate to ADP, so ATP forms in the stroma.

Energy flow in the noncyclic reactions of photosynthesis light energy Excited P680 energy P680 (photosystem II) light energy Excited P700 P700 (photosystem I) Figure 6.9 Energy flow in the light-dependent reactions of photosynthesis. The P700 in photosystem I absorbs photons of a 700-nanometer wavelength. The P680 of photosystem II absorbs photons at 680 nanometers. Energy inputs boost P700 and P680 to an excited state in which they lose electrons. A The noncyclic pathway involves a one-way flow of electrons from water, to photosystem II, to photosystem I, to NADPH. As long as electrons continue to flow through the two electron transfer chains, H+ continues to be carried across the thylakoid membrane, and ATP and NADPH keep forming. Light provides the energy boosts that keep the pathway going. Energy flow in the noncyclic reactions of photosynthesis Stepped Art Figure 6-9a p108

P700 (photosystem I) Excited P700 light energy Excited P700 energy Figure 6.9 Energy flow in the light-dependent reactions of photosynthesis. The P700 in photosystem I absorbs photons of a 700-nanometer wavelength. The P680 of photosystem II absorbs photons at 680 nanometers. Energy inputs boost P700 and P680 to an excited state in which they lose electrons. B In the cyclic pathway, electrons ejected from photosystem I are returned to it. As long as electrons continue to pass through its electron transfer chain, H+ continues to be carried across the thylakoid membrane, and ATP continues to form. Light provides the energy boost that keeps the cycle going. Energy flow in the cyclic reactions of photosynthesis Stepped Art Figure 6-9b p108

Take-Home Message: What happens during the light-dependent reactions of photosynthesis? 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

Take-Home Message: How does energy flow during the reactions of photosynthesis? 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

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)

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

Calvin– Benson Cycle other molecules glucose 1 4 2 4 Calvin– Benson Cycle Figure 6.10 Animated Light-independent reactions of photosynthesis. The sketch shows a cross-section of a chloroplast with the light-independent reactions cycling in the stroma. The steps shown are a summary of six cycles of the Calvin–Benson reactions. Black balls signify carbon atoms. Appendix V details the reaction steps. 1 Six CO2 diffuse into a photosynthetic cell, and then into a chloroplast. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve molecules of PGA form. 2 Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve PGAL form. 3 Two PGAL combine to form one glucose molecule. 4 The remaining ten PGAL receive phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP. glucose 3 other molecules Stepped Art Figure 6-10 p109

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 light- dependent reactions Collectively called the Calvin–Benson cycle, these carbon-fixing reaction use hydrogen (from NADPH), and carbon and oxygen (from CO2) to build sugars

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

C3 Plants palisade mesophyll cell spongy mesophyll cell C3 plants Plants that use only the Calvin–Benson cycle to fix carbon Forms 3-carbon PGA in mesophyll cells 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 spongy mesophyll cell

mesophyll cell CO2 O2 glycolate RuBP Calvin– Benson PGA Cycle ATP NADPH Figure 6.11 Animated Light-independent reactions in C3 plants. 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 24

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.

C4 Cycle Calvin– Benson Cycle mesophyll cell CO2 from inside plant B C4 plants. Oxygen also builds up inside leaves when stomata close during photosynthesis. mesophyll cell oxaloacetate C4 Cycle bundle-sheath cell CO2 An additional pathway in these plants keeps the CO2 concentration high enough in bundle-sheath cells to prevent photorespiration. RuBP Calvin– Benson Cycle Figure 6.12 Animated Light-independent reactions in C4 plants. PGA bundle-sheath cell sugars Figure 6-12b p110

A CAM Plant: Jade Plant CAM Plants 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 CO2 from outside plant oxaloacetate C4 Cycle night day RuBP Calvin– Benson Cycle Figure 6.13 Animated Light-independent reactions in CAM plants. A A CAM plant: Crassula argentea, or jade plant. B CAM plants open stomata and fix carbon using a C4 pathway at night. When the plant’s stomata are closed during the day, the organic compounds made during the night are converted to CO2 that enters the Calvin–Benson cycle. PGA sugars Figure 6-13a p111

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