Chapter 6. Capturing Solar Energy: Photosynthesis

Transcription

1 Chapter 6 Capturing Solar Energy: Photosynthesis Including some materials from lectures by Gregory Ahearn University of North Florida Ammended by John Crocker Copyright 2009 Pearson Education, Inc.

2 6.1 What Is Photosynthesis? Life on earth depends on photosynthesis. Photosynthesis is the capturing and conversion of sunlight into chemical energy. Virtually all life depends directly or indirectly on the energy captured by plants and stored as sugars. Before photosynthesis, there was little oxygen on Earth, and therefore, no organisms that used oxygen. All present-day organisms that use oxygen as their respiratory gas depend upon photosynthesis to generate new oxygen.

3 6.1 What Is Photosynthesis? Photosynthesis converts carbon dioxide and water to glucose. The chemical reaction for photosynthesis: 6 CO H light energy C 6 H 12 O O 2 Plants, seaweeds, and single-celled organisms all show the basic aspects of photosynthesis.

4 6.1 What Is Photosynthesis? Plant photosynthesis takes place in leaves. Leaves are the main location of photosynthesis. Plants have thin leaves so sunlight can penetrate. Plant leaves have a large surface area to expose them to the sun. Plant leaves have pores to admit CO 2 and expel O 2, called stomata (singular, stoma).

5 6.1 What Is Photosynthesis? Leaf cells contain chloroplasts. Photosynthesis occurs in chloroplasts, in layers of cells called the mesophyll. Chloroplasts contain a semifluid medium called stroma, which contains sacs called thylakoids within which photosynthesis occurs.

6 6.1 What Is Photosynthesis? An overview of photosynthetic structures mesophyll cells (a) Leaves outer membrane inner membrane thylakoid stroma (b) vein Internal leaf structure chloroplasts stoma (c) Chloroplast in mesophyll cell Fig. 6-1

7 6.1 What Is Photosynthesis? Photosynthesis consists of light-dependent and light-independent reactions. These reactions occur at different locations in the chloroplast. The two types of reactions are linked by the energy-carrier molecules adenosine triphosphatase (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).

8 6.1 What Is Photosynthesis? Light-dependent reactions Occur in the membranes of the thylakoids Light is captured here and stored in ATP and NADPH. H 2 O is consumed and O 2 is given off.

9 6.1 What Is Photosynthesis? Light-independent reactions Enzymes in the stroma use ATP and NADPH produced by light-dependent reactions to make glucose and other molecules. CO 2 is consumed in the process. ATP and NADPH are converted to low-energy ADP and NADP +. These low-energy molecules are re-charged to ATP and NADPH when recycled in lightdependent reactions.

10 6.1 What Is Photosynthesis? An overview of photosynthesis: lightdependent and light-independent reactions H 2 O LIGHT-DEPENDENT REACTIONS O 2 (thylakoids) depleted carriers (ADP, NADP + ) energized carriers (ATP, NADPH) CO 2 LIGHT-INDEPENDENT REACTIONS (stroma) glucose Fig. 6-2

11 The Energy in Visible Light The sun radiates electromagnetic energy Visible light is radiation falling between nanometers of wavelength

12 Light Captured by Pigments Packets of energy called photons have different energy levels depending on wavelength Short-wavelength photons (indigo, UV, x-rays) are very energetic Longer-wavelength photons have lower energies (red, infrared)

13 Light Captured by Pigments Action of light-capturing pigments Absorption of certain wavelengths (light is trapped ) Reflection of certain wavelengths (light bounces back) Transmission of certain wavelengths (light passes through)

14 Light Captured by Pigments Absorbed light drives biological processes when it is converted to chemical energy Common pigments found in chloroplasts include: Chlorophyll a and b Accessory pigments such as carotenoids

15 Light Captured by Pigments Pigment absorbs visible light Chlorophyll a and b absorb violet, blue, and red light but reflect green light (hence they appear green) Carotenoids absorb blue and green light but reflect yellow, orange, or red (hence they appear yellow-orange)

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17 Why Autumn Leaves Turn Color Both chlorophylls and carotenoids are present in leaves Chlorophyll breaks down before carotenoids in dying autumn leaves revealing yellow colors Red (anthocyanin) pigments are synthesized by some autumn leaves, producing red colors

18 6.2 How Is Light Energy Converted To Chemical Energy? Light is first captured by pigments in chloroplasts. Membranes of choroplast thylakoids contain several types of pigments (light-absorbing molecules). Chlorophyll Other accessory pigments including carotenoids

19 6.2 How Is Light Energy Converted To Chemical Energy? Captured sunlight energy is stored as chemical energy in two carrier molecules Adenosine triphosphate (ATP) Nicotinamide adenine dinucleotide phosphate (NADPH)

20 6.2 How Is Light Energy Converted To Chemical Energy? The light-dependent reactions generate energy-carrier molecules. Light-dependent reactions take place in photosystems found in the thylakoid membranes. Each photosystem consists of an assemblage of proteins, chlorophyll, accessory pigment molecules, and electron-carrier molecules.

21 6.2 How Is Light Energy Converted To Chemical Energy? In thylakoids, there are thousands of photosystems of two types. Photosystem I Photosystem II Each Photosystem is associated with a chain of electron carriers

22 6.2 How Is Light Energy Converted To Chemical Energy? Each photosystem consists of two major subsystems. A light-harvesting complex collects light energy and passes it on to a specific chlorophyll molecule called the reaction center. An electron transport system (ETS) transports energized electrons from one molecule to another.

23 6.2 How Is Light Energy Converted To Chemical Energy? Structures associated with the lightdependent reactions thylakoids chloroplast within thylakoid membrane PS II ETC reaction centers PS I ETC Fig. 6-4

24 6.2 How Is Light Energy Converted To Chemical Energy? Photosystem II generates ATP. Step 1: The light-harvesting complex passes light to the reaction center. Step 2: Electrons of the reaction center become energized. Step 3: The energized electrons jump to the ETS and jump from molecule to molecule, releasing energy at each step. Step 4: The released energy powers reactions that synthesize ATP.

25 6.2 How Is Light Energy Converted To Chemical Energy? Photosystem I generates NADPH. Step 5: The light-harvesting complex passes light to the reaction center. Step 6: Activated electrons from the reaction center are passed to the ETS and are replaced by electrons coming from the ETS of photosystem II. Step 7: Electrons jump from one molecule of the ETS to another, until they reach NADP +. Step 8: Each molecule of NADP + picks up two electrons, forming NADPH.

26 6.2 How Is Light Energy Converted To Chemical Energy? Step 9: The breakdown of H 2 O provides the replacement electrons to keep the process continuing, through the reaction: H 2 O ½ O 2 + 2H + + 2e The two electrons are donated to photosystem II. The hydrogen ions are used to convert NADP + to NADPH. Oxygen atoms combine to form a molecule of oxygen gas (O 2 ), which is given off to the atmosphere.

27 sunlight 7 e NADPH 8 electron transport chain energy level of electrons e 4 reaction center energy to drive ATP synthesis 6 e 5 photosystem I NADP + + H + within thylakoid membrane e photosystem II H 2 O 9 2 H + 1/2 O 2 Fig. 6-5

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29 6.2 How Is Light Energy Converted To Chemical Energy? Splitting water maintains the flow of electrons through the photosystems. Electrons from the reaction center of photosystem II flow through the ETS of photosystem II to the reaction center of photosystem I, forming NADPH. Photosystem II s reaction center must be supplied with new electrons to keep the process continuing.

30 Maintaining Electron Flow Redux Electrons leaving PS II replaced when H 2 O is split: H 2 O ½O 2 + 2H + + 2e - Two electrons from water replace those lost when 2 photons boost 2 electrons out of PSII Two hydrogen ions used to form NADPH Oxygen atoms combine to form O 2

31 6.3 How Is Chemical Energy Stored in Glucose Molecules? The ATP and NADPH generated in lightdependent reactions are used in lightindependent reactions to make molecules for long-term storage. These reactions occur in the fluid stroma that surrounds the thalakoids, and do not require light. In the stroma, ATP and NADPH are used with CO 2 and H 2 O to synthesize the storage form of energy glucose.

32 6.3 How Is Chemical Energy Stored in Glucose Molecules? The C 3 cycle captures carbon dioxide. Step 1: CO 2 from air combines with a five-carbon sugar, ribulose biphosphate (RuBP), and H 2 O to form phosphoglyceric acid (PGA). Step 2: PGA receives energy input from ATP and NADPH to form glyceraldehyde-3-phosphate (G 3 P). Step 3: Two G 3 P molecules (three carbons each) combine to form one molecule of glucose (six carbons). Step 4: 10 G 3 P molecules powered by ATP are used to regenerate six molecules of RuBP to restart the cycle.

33 6.3 How Is Chemical Energy Stored in Glucose Molecules? The C 3 cycle of carbon fixation 1 Carbon fixation combines CO 2 with RuBP 6 CO 2 C 6 4 RuBP synthesis uses energy and 10 G3Ps C C C C C RuBP C 3 cycle 12 C C C PGA 2 G3P synthesis uses energy ATP ADP 6 6 ADP ATP 12 C C C G3P 12 NADPH 12 NADP G3Ps available for synthesis of glucose C C C C C C glucose Fig. 6-6

34 6.4 What Is The Relationship Between Light-Dependent And Light-Independent Reactions? Photosynthesis includes two separate sets of reactions (light-dependent and lightindependent) that are closely linked.

35 6.4 What Is The Relationship Between Light-Dependent And Light-Independent Reactions? Light-dependent reactions capture solar energy; light-independent reactions use captured energy to make glucose. Energy-carrier molecules provide the link between these two sets of reactions. Light-dependent reactions of thylakoids use light to charge ADP and NADP + to make ATP and NADPH. ATP and NADPH move to the stroma where they provide energy to synthesize glucose.

36 6.4 What Is The Relationship Between Light-Dependent And Light-Independent Reactions? Two sets of reactions are connected in photosynthesis. energy from sunlight O 2 CO 2 ATP NADPH Light-dependent reactions occur in thylakoids ADP Lightindependent reactions (C 3 cycle) occur in stroma H 2 O NADP + chloroplast glucose Fig. 6-7

37 6.5 How Does the Need To Conserve Water Affect Photosynthesis? Photosynthesis requires carbon dioxide; porous leaves would allow the entry of CO 2, but would also result in the loss of H 2 O. Evolution of the stomata resulted in pores that could open, letting in CO 2, but also to close, to restrict H 2 O losses. Closing stomata to prevent H 2 O loss also restricts the release of O 2, produced by photosynthesis, to the atmosphere.

38 6.5 How Does the Need To Conserve Water Affect Photosynthesis? Stomata Fig. 6-8

39 6.5 How Does the Need To Conserve Water Affect Photosynthesis? When stomata are closed to conserve water, wasteful photorespiration occurs. In hot, dry conditions, plant stomata are closed much of the time, reducing internal CO 2 concentrations and increasing O 2 concentrations. Increased O 2 reacts with RuBP (instead of CO 2 ) in a process called photorespiration. Photorespiration does not produce useful cellular energy, and prevents the C 3 synthesis of glucose.

40 6.5 How Does the Need To Conserve Water Affect Photosynthesis? Alternative pathways reduce photorespiration. Some plants have evolved metabolic pathways that reduce photorespiration. These plants can produce glucose even under hot and dry conditions. The two most important alternative pathways are: The C 4 pathway Crassulacean acid metabolism (CAM)

41 6.5 How Does the Need To Conserve Water Affect Photosynthesis? Plants capture carbon and synthesize glucose in different places. Typical plants (C 3 plants) fix carbon and synthesize glucose as a result of the C 3 cycle in mesophyll cells.

42 6.5 How Does the Need To Conserve Water Affect Photosynthesis? C 3 plant In a C 3 plant, carbon capture and glucose synthesis are in mesophyll cells bundlesheath cell mesophyll cell (a) C 3 plant Fig. 6-9a

43 6.5 How Does the Need To Conserve Water Affect Photosynthesis The C 4 pathway includes two stages that take place in different parts of the leaf. In the first stage, CO 2 is captured in mesophyll cells in the presence of high O 2, producing a four-carbon molecule. The four-carbon molecule is transferred from mesophyll cells to the bundle-sheath cells where the four-carbon molecule is broken down to CO 2.

44 6.5 How Does the Need to Conserve Water Affect Photosynthesis C 4 plant (b) C 4 plant In a C 4 plant, carbon capture is in mesophyll cells, but glucose is synthesized in bundle-sheath cells bundlesheath cell mesophyll cell Fig. 6-9b

45 6.5 How Does the Need to Conserve Water Affect Photosynthesis C 4 plants capture carbon and synthesize glucose in different places. In the sheath-bundle cells, the released CO 2 proceeds to the second stage of the pathway the regular C 3 cycle without excess O 2 interfering with the process. Many C 4 plant species are grasses, and are agriculturally important species such as sugar cane, corn, and sorghum.

46 6.5 How Does the Need to Conserve Water Affect Photosynthesis CAM plants capture carbon and synthesize glucose at different times. In CAM plants, photorespiration is reduced by fixing carbon in two stages that take place in the same cells but at different times of the day. At night, with open stoma, reactions in mesophyll cells incorporate CO 2 into the organic acid molecules that are stored in vacuoles. During the day, with stoma closed, the organic acids release their CO 2 and the regular C 3 cycle proceeds.

47 6.5 How Does the Need to Conserve Water Affect Photosynthesis Two ways to reduce photorespiration in different places and times mesophyll cell bundle-sheath cell CO 2 CO 2 night C C C C 1 CO 2 is incorporated into four-carbon molecules C C C C CO 2 2 Four-carbon CO 2 day molecules release CO 2 to C 3 C the C 3 cycle 3 cycle cycle mesophyll cell C 4 CAM (a) Steps in separate places (b) Steps at separate times Fig. 6-10