Copyright © 2009 Pearson Education, Inc. Lectures by Gregory Ahearn University of North Florida Chapter 6 Capturing Solar Energy: Photosynthesis

Copyright © 2009 Pearson Education Inc. 6.1 What Is Photosynthesis?  Life on earth depends on photosynthesis. Photosynthesis is the capturing of sunlight energy and the conversion of it into chemical energy. All present-day organisms that use oxygen as their respiratory gas depend upon photosynthesis. Equally important is the stored sugars from photosynthesis, since virtually all life depends on this energy, either directly or indirectly.

Copyright © 2009 Pearson Education Inc. 6.1 What Is Photosynthesis?  Photosynthesis removes ________ ________ from the atmosphere and adds ________ to it. 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. Energy captured is stored in the bonds of glucose.

Copyright © 2009 Pearson Education Inc. 6.1 What Is Photosynthesis? Leaves are the main location of photosynthesis. Plants have _____ leaves so sunlight can penetrate. Plant leaves have a large _________ _______ to expose them to the sun. Plant leaves have pores to admit CO 2, called _________ (singular, stoma) that can open and close.

Copyright © 2009 Pearson Education Inc. 6.1 What Is Photosynthesis?  Inside the leaf are a few layers of cells called the _____________.  A system of veins supplies water and minerals to the mesophyll and brings the sugars they make to other parts of the plant.  Photosynthesis occurs in ____________ inside the mesophyll.  Chloroplasts contain a semifluid medium called stroma, which contains sacs called _____________ within which photosynthesis occurs.

Copyright © 2009 Pearson Education Inc. Fig What Is Photosynthesis?  An overview of photosynthetic structures mesophyll cells chloroplasts veinstoma outer membrane inner membrane thylakoid stroma Internal leaf structure Chloroplast in mesophyll cell Leaves (b) (c) (a)

Copyright © 2009 Pearson Education Inc. 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).

Copyright © 2009 Pearson Education Inc. Fig What Is Photosynthesis?  An overview of photosynthesis: light- dependent and light-independent reactions LIGHT-DEPENDENT REACTIONS (thylakoids) LIGHT-INDEPENDENT REACTIONS (stroma) depleted carriers (ADP, NADP + ) H2OH2O O2O2 CO 2 glucose energized carriers (ATP, NADPH)

Copyright © 2009 Pearson Education Inc. 6.1 What Is Photosynthesis?  Light-dependent reactions Occur in the membranes of the ___________. Light is captured here and stored in ATP and NADPH. _________is consumed and _________ is given off.

Copyright © 2009 Pearson Education Inc. 6.1 What Is Photosynthesis?  Light-independent reactions Occurs in the __________. ATP and NADPH produced by light-dependent reactions are used to make __________ and other molecules. ____________ ___________ is consumed in the process. ATP and NADPH are converted to low-energy ADP and NADP +. They return to the light-dependent reactions to be recharged into ATP and NADPH

Copyright © 2009 Pearson Education Inc. LIGHT- DEPENDENT REACTIONS

Copyright © 2009 Pearson Education Inc. 6.2 How Is Light Energy Converted To Chemical Energy?  Light is first captured by pigments in chloroplasts. Membranes of thylakoids contain several types of pigments (light-absorbing molecules). _______________ is one light-absorbing molecule that absorbs violet, blue, and red light, but reflects green. Other accessory pigments include ____________, which absorb blue and green light, but reflect yellow and orange.

Copyright © 2009 Pearson Education Inc. Fig How Is Light Energy Converted To Chemical Energy?  Light, chloroplast pigments, and photosynthesis carotenoids chlorophyll Absorbance of photosynthetic pigments Visible light Gamma raysX-raysUVInfrared Micro- waves Radio waves Wavelength (nanometers) light absorption (percent)

Copyright © 2009 Pearson Education Inc. 6.2 How Is Light Energy Converted To Chemical Energy?  Light-dependent reactions take place in photosystems found in the thylakoid membranes. In thylakoids, there are thousands of photosystems of two types. Photosystem I Photosystem II  Each photosystem consists of an assemblage of proteins, chlorophyll, accessory pigment molecules, and electron-carrier molecules.  The light-dependent reactions generate energy- carrier molecules.

Copyright © 2009 Pearson Education Inc. 6.2 How Is Light Energy Converted To Chemical Energy?  Each photosystem consists of two major parts: 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.

Copyright © 2009 Pearson Education Inc. Fig How Is Light Energy Converted To Chemical Energy?  Structures associated with the light- dependent reactions thylakoids PS II chloroplast within thylakoid membrane PS I ETC reaction centers

Copyright © 2009 Pearson Education Inc. 6.2 How Is Light Energy Converted To Chemical Energy?  Photosystem II generates ________. 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.

Copyright © 2009 Pearson Education Inc. 6.2 How Is Light Energy Converted To Chemical Energy?  Photosystem I generates _________. 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.

Copyright © 2009 Pearson Education Inc. reaction center photosystem II photosystem I synthesis energy to drive sunlight NADPH 2 H + H2OH2O e–e– e–e– H+H+ NADP + ATP e–e– e–e– + 1/2 O 2 energy level of electrons within thylakoid membrane electron transport chain Fig. 6-5

Copyright © 2009 Pearson Education Inc. 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. Where do the new electrons come from?

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. LIGHT-INDEPENDENT REACTIONS

Copyright © 2009 Pearson Education Inc. 6.3 How Is Chemical Energy Stored in Glucose Molecules?  The ATP and NADPH generated in light-dependent reactions are used in light-independent reactions to make molecules for long-term storage. These reactions occur in the fluid stroma that surrounds the thylakoids, 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.

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. Fig How Is Chemical Energy Stored in Glucose Molecules?  The C 3 cycle of carbon fixation 12 G3P 12 PGA G3P synthesis uses energy RuBP CO 2 6 RuBP synthesis uses energy and 10 G3Ps glucose Carbon fixation combines CO 2 with RuBP 2 G3Ps available for synthesis of glucose C 3 cycle ADP ATP NADPH NADP + ADP C CC C C C CC C C C C C C C C CC

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. O2O2 H2OH2O ATP ADP glucose NADP + CO 2 Light-dependent reactions occur in thylakoids energy from sunlight Light- independent reactions (C 3 cycle) occur in stroma chloroplast NADPH Fig What Is The Relationship Between Light- Dependent And Light-Independent Reactions? Photosynthesis includes two separate sets of reactions (light- dependent and light- independent) that are closely linked.

Copyright © 2009 Pearson Education Inc. 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 _____, but also to close, to restrict _____ losses.  Closing stomata to prevent H 2 O loss also restricts the release of _____, produced by photosynthesis, to the atmosphere.

Copyright © 2009 Pearson Education Inc. 6.5 How Does the Need To Conserve Water Affect Photosynthesis?  When stomata are closed to conserve water, wasteful __________________ 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.

Copyright © 2009 Pearson Education Inc. 6.5 How Does the Need To Conserve Water Affect Photosynthesis?  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)

Copyright © 2009 Pearson Education Inc.  Typical plants (C 3 plants) fix carbon and synthesize glucose as a result of the C 3 cycle in mesophyll cells.  C4 plants are plants that use a supplementary method of CO2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle.  These plants are able to handle higher temperatures and higher light intensity than C3 plants. C4 plants do require more ATP than C3 plants but also make more glucose per given leaf area and grow more quickly.

Copyright © 2009 Pearson Education Inc. bundle- sheath cell mesophyll cell C 3 plant In a C 3 plant, carbon capture and glucose synthesis are in mesophyll cells (a) Fig. 6-9a 6.5 How Does the Need To Conserve Water Affect Photosynthesis?  C 3 plant

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. bundle- sheath cell mesophyll cell C 4 plant In a C 4 plant, carbon capture is in mesophyll cells, but glucose is synthesized in bundle-sheath cells (b) Fig. 6-9b 6.5 How Does the Need to Conserve Water Affect Photosynthesis  C 4 plant

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 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. Many desert species are CAM plants

Copyright © 2009 Pearson Education Inc. Fig How Does the Need to Conserve Water Affect Photosynthesis C4 CAM Steps in separate placesSteps at separate times CO 2 is incorporated into four-carbon molecules Four-carbon molecules release CO 2 to the C 3 cycle C 3 cycle CO 2 C4C4 CAM mesophyll cell bundle-sheath cell night day mesophyll cell C 3 cycle (a)(b) 1 2 C C C C C C C C