Photosynthesis Photosynthesis = Capturing Solar Energy Interconnection of photosynthesis & cellular respiration Copyright © 2005 Pearson Prentice Hall, Inc.

Photosynthesis Occurs in Chloroplasts of Some Leaf Cells cuticle upper Leaves Photosynthesis 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 Occurs in Chloroplasts of Some Leaf Cells granum (stack of thylakoids) channel interconnecting thylakoids

Photosynthesis? Photosynthesis Consists of: Light-Dependent Reactions Light-Independent Reactions Copyright © 2005 Pearson Prentice Hall, Inc.

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

Light Captured by Chloroplast Pigments in Thylakoid Membrane Light-Dependent Reactions Convert Light Energy to Chemical Energy I. Light Capture Light Captured by Chloroplast Pigments in Thylakoid Membrane Thylakoid Membrane, Thylakoid Space Light, chloroplast pigments, and photosynthesis (F7.3 p. 118) Copyright © 2005 Pearson Prentice Hall, Inc.

within thylakoid membrane thylakoids chloroplast within thylakoid membrane Figure: 7-UN2 Title: Electron Transport Chain ETC PS II ETC PS I r eaction centers

chlorophyll b carotenoids chlorophyll Visible light ("rainbow colors") Micro- waves Radio waves Gamma rays X-rays UV Infrared Visible light 400 450 500 550 600 650 700 750 Wavelength (nanometers) Absorbance of photosynthetic pigments 100 chlorophyll b 80 Figure :7-3 Title: Light, chloroplast pigments, and photosynthesis Caption: (a) Visible light, a small part of the electromagnetic spectrum (top line), consists of wavelengths that correspond to the colors of the rainbow. (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? 60 carotenoids light absorption (percent) 40 chlorophyll a 20 400 500 600 700 wavelength (nanometers)

Light-Dependent Reactions Convert Light Energy to Chemical Energy II Light-Dependent Reactions Convert Light Energy to Chemical Energy II. Production of ATP & NADPH Photosystem II Generates ATP Photosystem I Generates NADPH 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–

Light-Dependent Reactions Convert Light Energy to Chemical Energy III Light-Dependent Reactions Convert Light Energy to Chemical Energy III. Electrons, Protons & Oxygen Production Splitting Water: Source of Electrons for the Photosystems Chemiosmosis: produces a proton gradient (FE7.1 p121) Chemiosmosis: ATP Synthesis (F E7.2 p. 121) Oxygen: a by-product of photosynthesis (F7.5 p. 122) Copyright © 2005 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.

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)

7.3 Light-Independent Reactions: How Is Chemical Energy Stored in Glucose Molecules? 7.3.2 Copyright © 2005 Pearson Prentice Hall, Inc.

Light-Independent Reactions: Store Chemical Energy in Glucose C3 Cycle Captures Carbon Dioxide (F7.6 p122) Carbon Captured in C3 Cycle Used to Synthesize Glucose Copyright © 2005 Pearson Prentice Hall, Inc.

Interconnection Between Light-Dependent & Light-Independent Reactions Photosynthesis Summary Diagram (F7.7 p. 123) Copyright © 2005 Pearson Prentice Hall, Inc.

Interconnection Between Light-Dependent & Light-Independent Reactions 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 Photosynthesis Summary Diagram (F7.7 p. 123) chloroplast glucose