Capturing Solar Energy: Photosynthesis 7 Capturing Solar Energy: Photosynthesis 1

Chapter 7 At a Glance 7.1 What Is Photosynthesis? 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? 7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?

7.1 What Is Photosynthesis? For most organisms, energy is derived from sunlight, either directly or indirectly Those organisms that can directly trap sunlight do so by photosynthesis Photosynthesis is the process by which solar energy is trapped and stored as chemical energy in the bonds of a sugar

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis Photosynthesis in plants takes place in chlorophyll-containing organelles called chloroplasts, most of which are contained within leaf cells Chloroplasts contain chemical reactions that are able to convert energy in sunlight into stored energy in sugars Both the upper and lower surfaces of a leaf consist of a layer of transparent cells, the epidermis

Figure 7-1 An overview of photosynthetic structures cuticle upper epidermis mesophyll cells Leaves stoma lower epidermis stoma chloroplasts bundle sheath cells outer membrane vascular bundle (vein) inner membrane Internal leaf structure thylakoid stroma Chloroplast channel interconnecting thylakoids Mesophyll cell containing chloroplasts 5

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis (continued) The outer surface of both epidermal layers is covered by the cuticle, a transparent, waxy, waterproof covering that reduces the evaporation of water from the leaf Leaves obtain CO2 for photosynthesis from the air through pores in the epidermis called stomata (singular, stoma)

Figure 7-2 Stomata Stomata open Stomata closed 7

Zebrina Under Microscope Zebrina (Tradescantia zebrine)

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis (continued) Inside the leaf are layers of cells called the mesophyll, where the chloroplasts are located and where photosynthesis occurs Bundle sheath cells surround the vascular bundles, which lack chloroplasts in most plants Photosynthesis in plants takes place within chloroplasts, most of which are contained in mesophyll cells

Figure 7-1 An overview of photosynthetic structures cuticle upper epidermis mesophyll cells Leaves stoma lower epidermis stoma chloroplasts bundle sheath cells outer membrane vascular bundle (vein) inner membrane Internal leaf structure thylakoid stroma Chloroplast channel interconnecting thylakoids Mesophyll cell containing chloroplasts 10

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis (continued) Chloroplasts are organelles with a double membrane enclosing a fluid called the stroma Embedded in the stroma are disk-shaped membranous sacs called thylakoids The light-dependent reactions of photosynthesis occur in and adjacent to the membranes of the thylakoids

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis (continued) Reactions of the Calvin cycle that capture carbon dioxide and produce sugar occur in the stroma Starting with carbon dioxide (CO2) and water (H2O), photosynthesis converts sunlight energy into chemical energy stored in bonds of glucose and releases oxygen (O2) as a product by the following equation: 6 CO2  6 H2O  light energy  C6H12O6  6O2 carbon water sunlight glucose oxygen dioxide (sugar)

What Is Photosynthesis? An overview of photosynthesis: light-dependent and light-independent reactions LIGHT-DEPENDENT REACTIONS (thylakoids) H2O O2 depleted carriers (ADP, NADP+) energized carriers (ATP, NADPH) LIGHT-INDEPENDENT REACTIONS (stroma) CO2 glucose Fig. 6-2

Figure 7-3 An overview of the relationship between the light reactions and the Calvin cycle 6 H2O 6 CO2 energy from sunlight ATP Calvin cycle light reactions NADPH ADP NADP thylakoid sugar chloroplast (stroma) O2 C6H12O6 14

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis (continued) During the light reactions, chlorophyll and other molecules embedded in the chloroplast thylakoid membranes capture sunlight energy and convert some of it into chemical energy stored in the energy-carrier molecules ATP and NADPH

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis (continued) In the reactions of the Calvin cycle, enzymes in the stroma use CO2 from the air and chemical energy from the energy-carrier molecules to synthesize a three-carbon sugar that will be used to make glucose

7.1 What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis (continued) The “photo” part of photosynthesis refers to the capture of sunlight in the thylakoid membranes The “synthesis” part of photosynthesis refers to the Calvin cycle, which synthesizes sugar from the energy captured in ATP and NADPH in the light reactions

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? The light reactions capture the energy of the sunlight, storing it as chemical energy in ATP and NADPH These light reactions are made possible by molecules anchored within the membranes of the thylakoid

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? Light is captured by pigments in chloroplasts The sun emits energy within a broad spectrum of electromagnetic radiation This electromagnetic spectrum ranges from short-wavelength gamma rays, through ultraviolet, visible, and infrared light, to long-wavelength radio waves

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? Light is captured by pigments in chloroplasts (continued) Light is composed of individual packets of energy called photons Visible light has wavelengths with energies strong enough to alter biological pigment molecules such as chlorophyll a Chlorophyll a is a key light-capturing pigment molecule in chloroplasts, absorbing violet, blue, and red light Green light, however, is reflected, which is why leaves appear green

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? Light is captured by pigments in chloroplasts (continued) Chloroplasts also contain accessory pigments, which absorb additional wavelengths of light energy and transfer them to chlorophyll a Chlorophyll b absorbs blue and red-orange wavelengths of light missed by chlorophyll a Carotenoids are accessory pigments that absorb blue and green light, and appear yellow or orange to our eyes because they reflect these colors In autumn, more-abundant, green chlorophyll breaks down before the carotenoids do, revealing their yellow color, which in summer is masked

Figure 7-4 Light and chloroplast pigments 100 chlorophyll b 80 carotenoids 60 light absorption (percent) chlorophyll a 40 20 wavelength (nanometers) 400 450 500 550 600 650 700 750 visible light micro- waves radio waves gamma rays X-rays UV infrared higher energy lower energy 23

Figure 7-5 Loss of chlorophyll reveals carotenoid pigments 24

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? The light reactions occur in association with the thylakoid membranes The thylakoid membranes contain many photosystems, each consisting of a cluster of chlorophyll and accessory pigment molecules surrounded by various proteins Two photosystems, photosystem II (PS II) and photosystem I (PS I), work together during the light reactions

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? The light reactions occur in association with the thylakoid membranes (continued) Each type of photosystem has a unique electron transport chain located adjacent to it These electron transport chains (ETC) each consist of a series of electron-carrier molecules embedded in the thylakoid membrane Within the thylakoid membrane, the overall path of electrons is as follows: PS II  ETC II  PS I  ETC I  NADP

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? The light reactions occur in association with the thylakoid membranes (continued) The steps in the light reactions are as follows: Photons of light are absorbed by pigment molecules clustered in photosystem II The energized electron is ejected from the chlorophyll molecule The reaction center within each photosystem consists of a pair of specialized chlorophyll a molecules and a primary electron acceptor molecule embedded in a complex of proteins

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? Photosystem II uses light energy to create a hydrogen ion gradient and split water The steps in the light reactions are as follows: (continued) The electron acceptor passes the electron on to the first molecule of ETC II, and it is then passed from one electron-carrier molecule to the next, losing energy as it goes

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? Photosystem II uses light energy to create a hydrogen ion gradient and split water (continued) The steps in the light reactions are as follows: (continued) 5a. Some of the energy is harnessed to pump hydrogen ions (H) across the thylakoid membrane and into the thylakoid space, where they will be used to generate ATP 5b. The energy-depleted electron leaves ETC II and enters the reaction center of photosystem I, where it replaces an electron ejected when light strikes photosystem I

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? Photosystem II uses light energy to create a hydrogen ion gradient and split water (continued) The steps in the light reactions are as follows: (continued) Light energy striking PS I is captured by its pigment molecules and funneled to a chlorophyll a molecule in the reaction center This ejects an energized electron that is picked up by the primary electron acceptor of PS I

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? Photosystem II uses light energy to create a hydrogen ion gradient and split water (continued) The steps in the light reactions are as follows: (continued) From the primary electron acceptor of PS I, the energized electron is passed along ETC I until it reaches NADP The energy-carrier molecule NADPH is formed when each NADP molecule picks up two energetic electrons, along with one hydrogen ion

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? The hydrogen ion gradient generates ATP by chemiosmosis The energy of electron movement through the thylakoid membrane creates an H gradient that drives ATP synthesis in a process called chemiosmosis

7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy? The hydrogen ion gradient generates ATP by chemiosmosis (continued) Chemiosmosis occurs in three steps As the energized electron travels along ETC II, some of the energy it liberates is used to pump hydrogen ions into the thylakoid space This pumping creates a high concentration of H inside the space relative to the surrounding stroma H flows down its concentration gradient through a thylakoid channel protein called ATP synthase, generating ATP from ADP

Figure 7-7 Events of the light reactions occur in and near the thylakoid membrane chloroplast (stroma) CO2 light energy H is pumped into the thylakoid space H electron transport chain I Calvin cycle electron transport chain II NADP e H NADPH sugar e e e ATP synthase C6H12O6 e ADP e photosystem II photosystem I Pi H H H ATP 2 H H H2O ½ H O2 H H H The flow of H down its concentration gradient powers ATP synthesis A high H concentration is created in the thylakoid space thylakoid membrane (thylakoid space) 34

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules? The Calvin cycle captures carbon dioxide ATP and NADPH synthesized from light reactions are used to power the synthesis of a simple sugar (gyceraldehyde-3-phosphate, or G3P) This is accomplished through a series of reactions occurring in the stroma called the Calvin cycle

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules? The Calvin cycle captures carbon dioxide (continued) The Calvin cycle occurs in three steps Carbon fixation The synthesis of G3P The regeneration of ribulose bisphosphate (RuBP)

Figure 7-9 The Calvin cycle fixes carbon from CO2 and produces the simple sugar G3P H2O CO2 ATP Calvin cycle light reactions NADPH ADP NADP sugar O2 C6H12O6 3 Carbon fixation combines three CO2 with three RuBP using the enzyme rubisco CO2 3 6 RuBP PGA 6 ATP Calvin cycle 6 ADP 3 ADP 6 NADPH 3 ATP 6 NADP 5 6 G3P G3P Energy from ATP and NADPH is used to convert the six molecules of PGA to six molecules of G3P Using the energy from ATP, the five remaining molecules of G3P are converted to three molecules of RuBP 1 G3P One molecule of G3P leaves the cycle 1 1 1 G3P G3P glucose Two molecules of G3P combine to form glucose and other molecules 37

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules? The Calvin cycle captures carbon dioxide (continued) Carbon fixation During carbon fixation, carbon from CO2 is incorporated, or “fixed,” into a larger organic molecule The enzyme rubisco combines three CO2 molecules with three RuBP molecules, forming three unstable 6-carbon molecules that each quickly split in half Six molecules of a 3-carbon product, phosphoglyceric acid (PGA), result The generation of this 3-carbon PGA molecule is called the C3 pathway

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules? The Calvin cycle captures carbon dioxide (continued) The synthesis of G3P occurs in a series of reactions using energy received by ATP and NADPH (generated by light reactions) Energy donated by ATP and NADPH is used to convert six PGA molecules into six of the 3-carbon sugar molecule G3P

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules? The Calvin cycle captures carbon dioxide (continued) The regeneration of RuBP ATP from the light reactions is used with five of the six G3P molecules formed to regenerate the 5-carbon RuBP necessary to repeat the cycle The remaining G3P molecule, which is the end product of photosynthesis, exits the cycle

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules? The Calvin cycle captures carbon dioxide (continued) Why do some plants use alternate pathways for carbon fixation? When plant stomata are closed in hot environments to prevent water loss, oxygen builds up in the plant cells and RuBP combines with it, rather than CO2, in a wasteful process called photorespiration This process prevents the Calvin cycle from synthesizing sugar, and plants may die under these circumstances

7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules? The Calvin cycle captures carbon dioxide (continued) Flowering plants have evolved two different mechanisms to circumvent wasteful photorespiration The C4 pathway Crassulacean acid metabolism (CAM)

Figure E7-1 The C4 pathway and the CAM pathway CO2 night day mesophyll cell CO2 PEP (3C) PEP carboxylase mesophyll cell pyruvate (3C) oxaloacetate (4C) PEP (3C) pyruvate (3C) PEP carboxylase malate (4C) oxalo- acetate (4C) CO2 rubisco pyruvate (3C) malate (4C) Calvin cycle malate (4C) malate (4C) CO2 sugar rubisco Calvin cycle malic acid (4C) bundle sheath cell central vacuole sugar C4 plants CAM plants 43