Chapter 6 Where It Starts – Photosynthesis (Sections 6.5 - 6.8)

6.5 Light-Dependent Reactions The light-dependent reactions of the first stage of photosynthesis convert the energy of light to the energy of chemical bonds There are two different sets of light-dependent reactions : a noncyclic pathway and a cyclic pathway

Capturing Light for Photosynthesis Light-harvesting complexes in the thylakoid membrane absorb photons and pass the energy to photosystems, which then release electrons photosystem A cluster of hundreds of chlorophylls, accessory pigments, and other molecules that converts light energy to chemical energy in photosynthesis

The Thylakoid Membrane Some components of the thylakoid membrane as seen from the stroma

The Thylakoid Membrane Figure 6.6 Artist’s view of some of the components of the thylakoid membrane as seen from the stroma. Molecules of electron transfer chains and ATP synthases are also present, but not shown for clarity. light-harvesting complex photosystem Fig. 6.6, p. 98

The Noncyclic Pathway Electrons released from photosystem II flow through an electron transfer chain, then to photosystem I Photon energy causes photosystem I to release electrons, which end up in NADPH

Replacing Lost Electrons Photosystem II replaces lost electrons by pulling them from water, which then dissociates into H+ and O2 (photolysis) photolysis Process by which light energy breaks down a molecule

Harvesting Electron Energy The process by which the flow of electrons through electron transfer chains drives ATP formation is called electron transfer phosphorylation electron transfer phosphorylation Electron flow through electron transfer chains sets up a hydrogen ion gradient that drives ATP formation

Steps in Noncyclic Reactions 1. Light energy ejects electrons from photosystem II 2. Photosystem II pulls replacement electrons from water molecules, which break apart into oxygen and hydrogen ions; the oxygen leaves the cell as O2 3. Electrons enter transfer chains in the thylakoid membrane 4. Energy from electrons in the transfer chain pumps hydrogen ions from the stroma into the thylakoid compartment; a hydrogen ion gradient forms across the thylakoid membrane

Steps in Noncyclic Reactions (cont.) 5. Light energy ejects electrons from photosystem I; replacement electrons come from an electron transfer chain 6. The electrons move through a second electron transfer chain, then combine with NADP+ and H+ to form NADPH 7. Hydrogen ions in the thylakoid compartment diffuse through the interior of ATP synthases and across the thylakoid membrane; hydrogen ion flow causes ATP synthases to attach phosphate to ADP, forming ATP in the stroma

Noncyclic Light-Dependent Reactions

to light-independent reactions Noncyclic Light-Dependent Reactions to light-independent reactions light energy light energy 1 4 5 7 3 6 2 Figure 6.7 Light-dependent reactions of photosynthesis. This example shows the noncyclic reactions in a thylakoid membrane. 1 Light energy ejects electrons from a photosystem II. 2 The photosystem pulls replacement electrons from water molecules, which break apart into oxygen and hydrogen 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 transfer chain causes hydrogen ions to be pumped 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 electrons come from an electron transfer chain. 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 synth ases by their gradient across the thylakoid membrane. Hydrogen ion flow causes ATP synth ases to attach phosphate to ADP, so ATP forms in the stroma. The Light-Dependent Reactions of Photosynthesis Fig. 6.7, p. 99

ANIMATION: Sites of photosynthesis To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

The Cyclic Pathway Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I NADPH does not form – ATP forms by electron transfer phosphorylation Electrons flowing through electron transfer chains cause H+ to accumulate in the thylakoid compartment H+ follows its gradient back across the membrane through ATP synthases, driving ATP synthesis

Animation: Noncyclic pathway of electron flow To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

ANIMATION: Photosynthesis - Light system To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

6.6 Energy Flow in Photosynthesis Energy flow in light-dependent reactions is an example of how organisms use energy harvested from the environment to drive cellular processes The simpler cyclic pathway evolved first, and operates in nearly all photosynthesizers Some organisms became modified to add photosystem II, beginning a sequence of reactions that removes electrons from water molecules, releasing hydrogen ions and oxygen

Making ATP and NADPH Having alternate pathways is efficient because cells can produce NADPH and ATP, or produce ATP alone NADPH accumulates when it is not being used, which backs up the noncyclic pathway, so the cyclic pathway predominates – the cell makes ATP, but not NADPH When sugar production is high, NADPH is used quickly, and does not accumulate – the noncyclic pathway predominates

The Cyclic Pathway

The Cyclic Pathway Excited P700 energy 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. Figure 6.8 Energy flow in the light-dependent reactions of photosynthesis. An energy input of an appropriate wavelength ejects electrons from a photosystem’s special pair of chlorophylls. The special pair in photosystem I absorbs photons of a 680-nanometer wavelength, so it is called P680. The special pair in photosystem II absorbs photons of a 700-nanometer wavelength, so it is called P700. P700 (photosystem I) light energy Energy flow in the cyclic reactions of photosynthesis Fig. 6.8b, p. 100

The Cyclic Pathway Excited P700 energy P700 (photosystem I) light energy Excited P700 energy The Cyclic Pathway Figure 6.8 Energy flow in the light-dependent reactions of photosynthesis. An energy input of an appropriate wavelength ejects electrons from a photosystem’s special pair of chlorophylls. The special pair in photosystem I absorbs photons of a 680-nanometer wavelength, so it is called P680. The special pair in photosystem II absorbs photons of a 700-nanometer wavelength, so it is called P700. Energy flow in the cyclic reactions of photosynthesis Stepped Art Fig. 6.8b, p. 100

The Noncyclic Pathway

The Noncyclic Pathway Excited P700 Excited P680 energy light energy P700 (photosystem I) light energy P680 (photosystem II) light energy Figure 6.8 Energy flow in the light-dependent reactions of photosynthesis. An energy input of an appropriate wavelength ejects electrons from a photosystem’s special pair of chlorophylls. The special pair in photosystem I absorbs photons of a 680-nanometer wavelength, so it is called P680. The special pair in photosystem II absorbs photons of a 700-nanometer wavelength, so it is called P700. Energy flow in the noncyclic reactions of photosynthesis A The noncyclic pathway is 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. Fig. 6.8a, p. 100

The Noncyclic Pathway Excited P700 Excited P680 energy light energy (photosystem II) The Noncyclic Pathway light energy Excited P700 P700 (photosystem I) Figure 6.8 Energy flow in the light-dependent reactions of photosynthesis. An energy input of an appropriate wavelength ejects electrons from a photosystem’s special pair of chlorophylls. The special pair in photosystem I absorbs photons of a 680-nanometer wavelength, so it is called P680. The special pair in photosystem II absorbs photons of a 700-nanometer wavelength, so it is called P700. Energy flow in the noncyclic reactions of photosynthesis Stepped Art Fig. 6.8a, p. 100

Key Concepts Making ATP and NADPH ATP forms in the first stage of photosynthesis, which is light-dependent because the reactions run on the energy of light The coenzyme NADPH forms in a noncyclic pathway that also releases oxygen ATP also forms in a cyclic pathway that does not release oxygen

ANIMATION: Energy Changes in Photosynthesis To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

6.7 Light-Independent Reactions: The Sugar Factory The cyclic, light-independent reactions of the Calvin–Benson cycle are the “synthesis” part of photosynthesis Carbon fixation occurs, and sugars are synthesized Inside the stroma, the enzyme rubisco attaches a carbon from CO2 to RuBP to start the Calvin–Benson cycle

Key Terms Calvin–Benson cycle Light-independent reactions of photosynthesis; cyclic carbon-fixing pathway that forms sugars from CO2 carbon fixation Process by which carbon from an inorganic source such as CO2 gets incorporated into an organic molecule rubisco (ribulose bisphosphate carboxylase) Carbon-fixing enzyme of the Calvin–Benson cycle

Energy for Sugar Synthesis Photo: ATP and NADPH are produced by the light-dependent reactions using light energy Synthesis: Light-independent reactions use energy from ATP, and hydrogen and electrons from NADPH, to synthesize sugars from CO2

Steps of the Calvin–Benson Cycle 6 CO2 enter a chloroplast; rubisco attaches each to a RuBP molecule – resulting intermediates split –12 PGA form 2. Each PGA gets a phosphate group from ATP, plus hydrogen and electrons from NADPH – 12 PGAL form 3. 2 PGAL combine to form 1 glucose molecule 4. Remaining 10 PGAL receive phosphate groups from ATP –endergonic reactions regenerate 6 RuBP

Steps of the Calvin–Benson Cycle

Steps of the Calvin–Benson Cycle 1 4 Calvin– Benson Cycle Figure 6.9 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 VI 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. 2 other molecules 3 glucose Fig. 6.9, p. 101

Steps of the Calvin–Benson Cycle 1 2 4 Calvin– Benson Cycle Figure 6.9 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 VI 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 Fig. 6.9, p. 101

ANIMATION: Calvin-Benson cycle To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Key Concepts Making Sugars The second stage is the “synthesis” part of photosynthesis Sugars are assembled with carbon and oxygen atoms from CO2 The reactions run on the chemical bond energy of ATP, and electrons donated by NADPH—molecules that formed in the first stage of photosynthesis

6.8 Adaptations: Carbon-Fixing Pathways When environments differ, so do details of light-independent reactions Three pathways of sugar synthesis: C3 plants C4 plants CAM plants

Key Terms C3 plant Type of plant that uses only the Calvin–Benson cycle to fix carbon C4 plant Type of plant that minimizes photorespiration by fixing carbon twice, in two cell types CAM plant Type of C4 plant that conserves water by fixing carbon twice, at different times of day

Photorespiration On dry days, plants conserve water by closing their stomata When stomata are closed, O2 from photosynthesis can’t escape, and CO2 for photosynthesis can’t enter In C3 plants, high O2 levels cause rubisco to attach O2 to RuBP instead of CO2 This pathway (photorespiration) reduces the efficiency of sugar production on dry days

Key Terms stomata Openings through plant surfaces Allow water vapor and gases to diffuse across the epidermis (through the cuticle) photorespiration Reaction in which rubisco attaches oxygen instead of carbon dioxide to ribulose bisphosphate

Photorespiration

CO2 O2 glycolate RuBP Calvin– Benson Cycle PGA ATP NADPH sugars Figure 6.10 Carbon-fixing adaptations. Most plants, including basswood (Tilia americana A), are C3 plants. B Photorespiration in C3 plants makes sugar production inefficient on dry days. Additional reactions minimize photorespiration in C4 plants such as corn (Zea mays C), and CAM plants such as jade plants (Crassula argentea D). ATP NADPH sugars Fig. 6.10b, p. 102

ANIMATION: C3-C4 comparison To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

C3 Plants C3 plants use only the Calvin–Benson cycle Most plants, including basswood (Tilia americana), are C3 plants

C4 Plants In C4 plants, carbon fixation occurs twice The first reactions release CO2 near rubisco, which limits photorespiration when stomata are closed Example: corn (Zea mays)

CAM Plants CAM plants minimize photorespiration by opening stomata and fixing carbon at night Example: Jade plants (Crassula argentea)

Key Concepts Alternate Pathways Details of light-independent reactions that vary among organisms are evolutionary adaptations to different environmental conditions

Green Energy (revisited) Photosynthesis removes carbon dioxide from the atmosphere, and locks its carbon atoms inside organic compounds When aerobic organisms break down the organic compounds for energy, carbon atoms are released in the form of CO2 Since photosynthesis evolved, these two processes have constituted a balanced cycle of the biosphere Burning fossil fuels for energy has put Earth’s atmospheric cycle of carbon dioxide out of balance

Fossil Fuel Emissions The sky over New York City on a sunny day

ANIMATION: Photosynthesis - Carbon Fixing To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE