Chapter 7 Capturing Solar Energy: Photosynthesis 7.1 What is photosynthesis? 7.2 Light-dependent reactions: How is light energy converted to chemical energy? 7.3 Light-independent reactions: How is chemical energy stored in glucose molecules? 7.4 What is the relationship between light-dependent and light-independent reactions? 7.5 Water, CO2, and the C4 pathway

7.1 What Is Photosynthesis? ~ 2 billion years ago, some cells acquired the ability to do photosynthesis (through chance genetic mutation) Oxygen levels in the atmosphere increased greatly Free oxygen (O2) is corrosive But as time went on (more mutation), some organisms used oxygen to break down glucose in cellular respiration

(chloroplast) photosynthesis H2O CO2 ATP sugar O2 cellular respiration Figure :7-1 Title: Interconnections between photosynthesis and cellular respiration Caption: Chloroplasts in green plants use the energy of sunlight to synthesize high-energy carbon compounds such as glucose from low-energy molecules of water and carbon dioxide. Plants themselves, and other organisms that eat plants or one another, extract energy from these organic molecules by cellular respiration, yielding water and carbon dioxide once again. This energy in turn drives all the reactions of life. cellular respiration (mitochondrion)

Relationships Photosynthesis: Cellular Respiration: 6 CO2 + 6 H20 + light energy -> C6H12O6 + 6 O2 Cellular Respiration: C6H12O6 + 6 O2 -> 6 CO2 + 6 H20 + chemical and heat energy

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis A leaf is just a few cells thick so the sun can penetrate Stomata (stoma is singular) adjust to let more or less CO2 in

channel interconnecting thylakoids Leaves Internal leaf structure cuticle upper epidermis mesophyll cells stoma lower epidermis chloroplasts Figure :7-2 Title: An overview of photosynthetic structures Caption: (a) Photosynthesis occurs primarily in the leaves of land plants. (b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf's upper epidermis. (c) A single chloroplast, showing the stroma and thylakoids where photosynthesis occurs. Chloroplast in mesophyll cell bundle sheath outer membrane vascular bundle (vein) inner membrane thylakoid stroma granum (stack of thylakoids) channel interconnecting thylakoids

Internal leaf structure cuticle upper epidermis mesophyll cells Figure :7-2 part b Title: An overview of photosynthetic structures part b Internal leaf structure Caption: (b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf's upper epidermis. stoma lower epidermis chloroplasts bundle sheath vascular bundle (vein)

FIGURE 7-3 Stoma in leaf of pea plant Figure 7-3 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Mesophyll cell containing chloroplasts FIGURE 7-2c An overview of photosynthetic structures (c) A mesophyll cell packed with green chloroplasts. Figure 7-2c Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Chloroplast in mesophyll cell outer membrane inner membrane thylakoid stroma Figure :7-2 part c Title: An overview of photosynthetic structures part c Chloroplast in mesophyll cell Caption: (c) A single chloroplast, showing the stroma and thylakoids where photosynthesis occurs. channel interconnecting thylakoids granum (stack of thylakoids

7.1 What Is Photosynthesis? Light-dependent reactions Chlorophyll captures sunlight energy and transfer to energy carrier molecules (ATP & NADPH) Uses H20 and releases O2 Light-independent reactions Enzymes use ATP & NADPH to drive synthesis of glucose Uses CO2 and H20 and releases glucose

LIGHT-DEPENDENT REACTIONS (thylakoids) H2O O2 depleted carriers (ADP, NADP+) energized carriers (ATP, NADPH) Figure: 7-UN1 Title: Overview of Photosynthesis Caption: In light-dependent reactions, chlorophyll and other molecules embedded in the membranes of the thylakoids capture sunlight energy and convert some of it into the chemical energy stored in energy-carrier molecules (ATP and NADPH). Oxygen gas is released as a by-product. In light-independent reactions, enzymes in the stroma use the chemical energy of the carrier molecules to drive the synthesis of glucose or other organic molecules. LIGHT-INDEPENDENT REACTIONS (stroma) CO2 + H2O glucose

How Is Light Energy Converted to Chemical Energy? Sun emits energy in a broad spectrum of electromagnetic radiation A packet of energy is called a photon, and the level of energy corresponds to a wavelength Light can be absorbed, reflected or transmitted

Sun emits energy in a broad spectrum of electromagnetic radiation Visible light ("rainbow colors") Micro- waves Radio waves Gamma rays X-rays UV Infrared Visible light Figure :7-3 part a Title: Light, chloroplast pigments, and photosynthesis part a Visible light ("rainbow colors") Caption: (a) Visible light, a small part of the electromagnetic spectrum (top line), consists of wavelengths that correspond to the colors of the rainbow. 400 450 500 550 600 650 700 750 Wavelength (nanometers)

How Is Light Energy Converted to Chemical Energy? During photosynthesis, light is first captured by pigments in chloroplasts Chloroplasts contain different pigments that can absorb certain wavelengths of photons

light absorption (percent) Absorbance of photosynthetic pigments 100 chlorophyll b 80 60 carotenoids light absorption (percent) 40 chlorophyll a Figure :7-3 part b Title: Light, chloroplast pigments, and photosynthesis part b Absorbance of photosynthetic pigments Caption: (b) Chlorophyll (blue and green curves) strongly absorbs violet, blue, and red light. Carotenoids (orange curve) absorb blue and green wavelengths. Question Based on the information in this graph, what color are carotenoids? What color is phycocyanin? 20 400 500 600 700 wavelength (nanometers)

Chloroplast Pigments Chlorophyll a and b strongly absorb violet, blue and red light Carotenoids absorb blue and red light So what colors do you see? Carotenoids are also vitamin A and forms the visual pigment in our eyes (captures light energy so we can see!)

FIGURE 7-6 Loss of chlorophyll reveals yellow carotenoids Figure 7-6 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

The Light-Dependent Reactions Occur Within the Thylakoid Membranes Thylakoid membranes contain photosystems I and II These are highly organized assemblies of proteins, chlorophyll and accessory pigment molecules The pigment molecules absorb light energy (photon)

A mechanical analogy for the light reactions Mill makes ATP e– Photon Photosystem II Photosystem I NADPH Figure 10.14  The light reactions use the solar power of photons absorbed by PS II and PS I to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH to the carbohydrate-synthesizing reactions of the Calvin cycle. Copyright © 2005 Pearson Prentice Hall, Inc.

energy level of electrons 2e– NADPH sunlight energy level of electrons 2e– NADPH within thylakoid membrane + H+ NADP+ electron transport chain 2e– synthesis energy to drive ATP photosystem I 2e– Figure :7-4 Title: The light-dependent reactions of photosynthesis Caption: 1 Light is absorbed by photosystem II, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 2 Energized electrons leave the reaction center. 3 The electrons move into the adjacent electron transport chain. 4 The chain passes the electrons along, and some of their energy is used to drive ATP synthesis by chemiosmosis. Energy-depleted electrons replace those lost by photosystem I. 5 Light strikes photosystem I, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 6 Energized electrons leave the reaction center. 7 The electrons move into the electron transport chain. 8 The energetic electrons from photosystem I are captured in molecules of NADPH. 9 The electrons lost from the reaction center of photosystem II are replaced by electrons obtained from splitting water, a reaction that also releases oxygen, and H+ used to form NADPH. Question If these reactions produce ATP and NADPH, then why do plant cells need mitochondria? photosystem II reaction center 1 /2 O2 + 2 H+ H2O 2e–

electron transport chain sunlight 2e– NADPH NADP+ + H+ 2e– electron transport chain energy level of electrons within thylakoid membrane 2e– Figure :7-4 Title: The light-dependent reactions of photosynthesis Caption: 1 Light is absorbed by photosystem II, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 2 Energized electrons leave the reaction center. 3 The electrons move into the adjacent electron transport chain. 4 The chain passes the electrons along, and some of their energy is used to drive ATP synthesis by chemiosmosis. Energy-depleted electrons replace those lost by photosystem I. 5 Light strikes photosystem I, and the energy is passed to electrons in the reaction-center chlorophyll molecules. 6 Energized electrons leave the reaction center. 7 The electrons move into the electron transport chain. 8 The energetic electrons from photosystem I are captured in molecules of NADPH. 9 The electrons lost from the reaction center of photosystem II are replaced by electrons obtained from splitting water, a reaction that also releases oxygen, and H+ used to form NADPH. Question If these reactions produce ATP and NADPH, then why do plant cells need mitochondria? photosystem I energy to drive reaction center ATP synthesis 2e– photosystem II H2O 1/2 O2 + 2 H+

How Is Light Energy Converted to Chemical Energy? Photosystem II generates ATP Photosystem I generates NADPH Splitting water maintains the flow of electrons through the photosystems Chemiosmosis: creating the hydrogen ion gradient Chemiosmosis: ATP synthesis Oxygen is a by-product of photosynthesis

thylakoid chloroplast FIGURE 7-8 (part 1) Events of the light-dependent reactions occur in and near the thylakoid membranes chloroplast Figure 7-8 (part 1) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

electrons powers active transport of H+ by ETC. Energy-carrier Energy from energized electrons powers active transport of H+ by ETC. Energy-carrier molecules power the C3 cycle. ETC PSII PSI stroma ETC C3 cycle FIGURE 7-8 (part 2) Events of the light-dependent reactions occur in and near the thylakoid membranes Energy from energized electrons powers NADPH synthesis. thylakoid space High H+ concentration generated by active transport. H+ channel coupled to ATP-synthesizing enzyme. Flow of H+ down concentration gradient powers ATP synthesis. Figure 7-8 (part 2) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

1 Energy is released as water flows downhill. 2 Energy is harnessed to rotate turbine. FIGURE E7-1 Energy stored in a water "gradient" can be used to generate electricity 3 Energy of rotating turbine is used to generate electricity. Figure E7-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

photosystem II thylakoid membrane H+ H+ 2e– H+ H+ (thylakoid interior) Figure: E7-1 Title: Chemiosmosis: creating the hydrogen ion gradient Caption: The energy released from the exergonic reaction of these electron transfers in Photosystem II is used to power active transport of hydrogen ions across the thylakoid membrane from the stroma into the thylakoid interior. (stroma)

(high H+ concentration in thylakoid) H+ H+ (low H+ concentration in stroma) H+ H+ H+ H+ (high H+ concentration in thylakoid) H+ H+ Figure: E7-2 Title: Chemiosmosis: ATP Synthesis Caption: The thylakoid membrane does not allow hydrogen ions to leak out, except at specific protein channels that are coupled to ATP-synthesizing enzymes. When hydrogen ions flow through these channels, down their gradients of charge and concentration, the energy released drives the synthesis of ATP. H+ ADP ATP P

Figure :7-5 Title: Oxygen is a by-product of photosynthesis Caption: The bubbles released by the leaves of this aquatic plant (Elodea) are composed of oxygen, a by-product of photosynthesis.

Light-Independent Reactions: How Is Chemical Energy Stored in Glucose Molecules? The C3 cycle captures carbon dioxide The C3 (3 carbons) cycle of carbon fixation It is also called the Calvin-Benson cycle Carbon is fixed during the C3 cycle is used to synthesize glucose You do not need to know the details of this cycle, nor Figure 7-10 (ed. 7 Figure 7-6)

6 CO2 6 H2O 6 12 RuBP PGA C3 cycle 12 6 12 G3P glucose Figure :7-6 Title: The C3 cycle of carbon fixation Caption: 1 Six molecules of RuBP react with 6 molecules of CO2 and 6 molecules of H2O to form 12 molecules of PGA. This reaction is carbon fixation, the capture of carbon from CO2 into organic molecules. 2 The energy of 12 ATPs and the electrons and hydrogens of 12 NADPHs are used to convert the 12 PGA molecules to 12 G3Ps. 3 Two G3P molecules are available to synthesize glucose or other organic molecules. This occurs outside the chloroplast and is not part of the C3 cycle. 4 Energy from 6 ATPs is used to rearrange 10 G3Ps into 6 RuBPs, completing one turn of the C3 cycle. G3P 12 glucose (or other organic compounds)

6 6 6 CO2 C 6 H2O 6 C C C C C 12 C C C RuBP PGA C3 cycle 12 ATP 12 ADP Figure :7-6 Title: The C3 cycle of carbon fixation Caption: 1 Six molecules of RuBP react with 6 molecules of CO2 and 6 molecules of H2O to form 12 molecules of PGA. This reaction is carbon fixation, the capture of carbon from CO2 into organic molecules. 2 The energy of 12 ATPs and the electrons and hydrogens of 12 NADPHs are used to convert the 12 PGA molecules to 12 G3Ps. 3 Two G3P molecules are available to synthesize glucose or other organic molecules. This occurs outside the chloroplast and is not part of the C3 cycle. 4 Energy from 6 ATPs is used to rearrange 10 G3Ps into 6 RuBPs, completing one turn of the C3 cycle. 6 ADP 12 NADPH 12 C C C 6 ATP 12 NADP+ G3P C C C C C C glucose (or other organic compounds)

energy from sunlight O2 CO2 H2O ATP NADPH Light-dependent reactions occur in thylakoids. Light- independent reactions (C3 cycle) occur in stroma. Figure :7-7 Title: A summary diagram of photosynthesis Caption: The light-dependent reactions in the thylakoids convert the energy of sunlight into the chemical energy of ATP and NADPH. Part of the sunlight energy is also used to split H2O, forming O2. In the stroma, the light-independent reactions use the energy of ATP and NADPH to convert CO2 and H2O to glucose. The depleted carriers, ADP and NADP+, return to the thylakoids to be recharged by the light-dependent reactions. Question Could a plant survive in an oxygen-free atmosphere? ADP NADP+ H20 chloroplast glucose

Water, CO2, C3 and C4 Pathways cuticle upper epidermis mesophyll cells Figure :7-2 part b Title: An overview of photosynthetic structures part b Internal leaf structure Caption: (b) A section of a leaf, showing mesophyll cells where chloroplasts are concentrated and the waterproof cuticle that coats the leaf's upper epidermis. stoma lower epidermis chloroplasts bundle sheath vascular bundle (vein)

7.5 Water, CO2, and the C4 Pathway When stomata are closed to conserve water, CO2 cannot enter, and O2 cannot leave A wasteful process called photorespiration occurs when it is too hot for C3 plants Glucose is not made, and seedlings may die. Some plants have a C4 pathway Compare pathways of C3 and C4 plants

C3 plants use the C3 pathway within chloroplast in mesophyll cell CO2 O2 PGA CO2 C3 CYCLE RuBP G3P glucose bundle- sheath cells C4 plants use the C4 pathway within chloroplast in mesophyll cell CO2 PEP 4-carbon molecule AMP C4 Pathway ATP Figure :7-8 Title: Comparison of C3 and C4 plants Caption: (a) In C3 plants, only the mesophyll cells carry out photosynthesis. All carbon fixation occurs by the C3 pathway. With low CO2 and high O2 levels, photorespiration dominates in C3 plants, because the enzyme that should catalyze the RuBP plus CO2 reaction catalyzes the RuBP plus O2 reaction instead. (b) In C4 plants, both the mesophyll cells and bundle sheath cells contain chloroplasts and participate in photosynthesis. CO2 is combined with PEP by a more selective enzyme, and the carbon is shuttled into bundle sheath cells by a four-carbon molecule, which releases CO2 into the bundle sheath cells. Higher CO2 levels allow efficient carbon fixation (with little photorespiration) in the C3 pathway of the bundle sheath cells. Notice that the regeneration of PEP requires energy from ATP. Question Why do C3 plants have an advantage over C4 plants under conditions that are not hot and dry? pyruvate CO2 O2 bundle- sheath cells PGA CO2 C3 CYCLE RuBP G3P glucose within chloroplast in bundle-sheath cell

7.5 Water, CO2, and the C4 Pathway C4 plants reduce photorespiration by means of a two-stage carbon-fixation process C3 and C4 plants are each adapted to different environmental conditions