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Chapter 7 Photosynthesis: Using Light to Make Food PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko Figure 7.0_1 Chapter 7: Big Ideas An Overview of Photosynthesis The Calvin Cycle: Reducing CO2 to Sugar The Light Reactions: Converting Solar Energy to Chemical Energy Photosynthesis Reviewed and Extended AN OVERVIEW OF PHOTOSYNTHESIS © 2012 Pearson Education, Inc. 7.1 Autotrophs are the producers of the biosphere Autotrophs – make their own food through the process of photosynthesis, – sustain themselves, and – do not usually consume organic molecules derived from other organisms. © 2012 Pearson Education, Inc. 7.1 Autotrophs are the producers of the biosphere Photoautotrophs use the energy of light to produce organic molecules. Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source. Heterotrophs are consumers that feed on – plants or – animals, or – decompose organic material. © 2012 Pearson Education, Inc. Figure 7.1A-D 7.2 Photosynthesis occurs in chloroplasts in plant cells Chloroplasts are the major sites of photosynthesis in green plants. Chlorophyll – is an important light-absorbing pigment in chloroplasts, – is responsible for the green color of plants, and – plays a central role in converting solar energy to chemical energy. © 2012 Pearson Education, Inc. 7.2 Photosynthesis occurs in chloroplasts in plant cells Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf. Stomata are tiny pores in the leaf that allow – carbon dioxide to enter and – oxygen to exit. Veins in the leaf deliver water absorbed by roots. © 2012 Pearson Education, Inc. Figure 7.2 Leaf Leaf Cross Section Mesophyll Vein CO2 O2 Stoma Mesophyll Cell Chloroplast Inner and outer membranes Granum Thylakoid Thylakoid space Stroma Figure 7.2_1 Leaf Cross Section Leaf Mesophyll Vein Mesophyll Cell CO2 O2 Stoma Chloroplast 7.2 Photosynthesis occurs in chloroplasts in plant cells Chloroplasts consist of an envelope of two membranes, which – enclose an inner compartment filled with a thick fluid called stroma and – contain a system of interconnected membranous sacs called thylakoids. © 2012 Pearson Education, Inc. 7.2 Photosynthesis occurs in chloroplasts in plant cells Thylakoids – are often concentrated in stacks called grana and – have an internal compartment called the thylakoid space, which has functions analogous to the intermembrane space of a mitochondrion. – Thylakoid membranes also house much of the machinery that converts light energy to chemical energy. Chlorophyll molecules – are built into the thylakoid membrane and – capture light energy. © 2012 Pearson Education, Inc. Figure 7.2_2 Chloroplast Inner and outer membranes Granum Thylakoid Thylakoid space Stroma 7.4 Photosynthesis is a redox process, as is cellular respiration Photosynthesis, like respiration, is a redox (oxidation-reduction) process. – CO2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it. – Water molecules are oxidized when they lose electrons along with hydrogen ions. Becomes reduced Becomes oxidized © 2012 Pearson Education, Inc. 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH Photosynthesis occurs in two metabolic stages. 1. The light reactions occur in the thylakoid membranes. In these reactions – water is split, providing a source of electrons and giving off oxygen as a by-product, – ATP is generated from ADP and a phosphate group, and – light energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and H+ from water to the electron acceptor NADP+ reducing it to NADPH. – NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle. © 2012 Pearson Education, Inc. 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH 2. The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast. – The Calvin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-rich products of the light reactions. – During the Calvin cycle, CO2 is incorporated into organic compounds in a process called carbon fixation. – After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds. – The Calvin cycle is often called the dark reactions or lightindependent reactions, because none of the steps requires light directly. © 2012 Pearson Education, Inc. Figure 7.5_s3 H2O CO2 Light NADP+ ADP P Calvin Cycle (in stroma) Light Reactions (in thylakoids) ATP NADPH Chloroplast O2 Sugar THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY © 2012 Pearson Education, Inc. 7.6 Visible radiation absorbed by pigments drives the light reactions Sunlight contains energy called electromagnetic energy or electromagnetic radiation. – Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths. – Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves. © 2012 Pearson Education, Inc. Figure 7.6A Increasing energy 105 nm 103 nm Gamma rays X-rays 103 nm 1 nm UV 106 nm Infrared 103 m 1m Microwaves Radio waves Visible light 380 400 500 600 Wavelength (nm) 700 650 nm 750 7.6 Visible radiation absorbed by pigments drives the light reactions Pigments – absorb light and – are built into the thylakoid membrane. Plant pigments – absorb some wavelengths of light and – reflect or transmit other wavelengths. We see the color of the wavelengths that are transmitted. For example, chlorophyll transmits green wavelengths. Animation: Light and Pigments © 2012 Pearson Education, Inc. 7.7 Photosystems capture solar energy Pigments in chloroplasts absorb photons (capturing solar power), which – increases the potential energy of the pigment’s electrons and – sends the electrons into an unstable state. – These unstable electrons – drop back down to their “ground state,” and as they do, – release their excess energy as heat. © 2012 Pearson Education, Inc. Figure 7.7A_2 Excited state Photon of light Heat Photon (fluorescence) Ground state Chlorophyll molecule 7.7 Photosystems capture solar energy Within a thylakoid membrane, chlorophyll and other pigment molecules – absorb photons and – transfer the energy to other pigment molecules. In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and proteins into photosystems. © 2012 Pearson Education, Inc. Figure 7.7B Photosystem Light Light-harvesting Reaction-center complexes complex Thylakoid membrane Primary electron acceptor Transfer of energy Pair of chlorophyll a molecules Pigment molecules 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH In the light reactions, light energy is transformed into the chemical energy of ATP and NADPH. To accomplish this, electrons are – removed from water, – passed from photosystem II to photosystem I, and – accepted by NADP+, reducing it to NADPH. Between the two photosystems, the electrons – move down an electron transport chain and – provide energy for the synthesis of ATP. © 2012 Pearson Education, Inc. Figure 7.8A Light Photosystem II Stroma Electron transport chain Provides energy for synthesis of ATP by chemiosmosis NADP H Light Photosystem I 1 Primary acceptor Thylakoid membrane Primary acceptor 2 4 P700 P680 Thylakoid space 3 H2O 1 2 5 O2 2 H 6 NADPH Figure 7.8B ATP NADPH Mill makes ATP Photosystem II Photosystem I 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH The products of the light reactions are – NADPH, – ATP, and – oxygen. © 2012 Pearson Education, Inc. 7.9 Chemiosmosis powers ATP synthesis in the light reactions Interestingly, chemiosmosis is the mechanism that – is involved in oxidative phosphorylation in mitochondria and – generates ATP in chloroplasts. ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane. © 2012 Pearson Education, Inc. 7.9 Chemiosmosis powers ATP synthesis in the light reactions In photophosphorylation, using the initial energy input from light, – the electron transport chain pumps H+ into the thylakoid space, and – the resulting concentration gradient drives H+ back through ATP synthase, producing ATP. © 2012 Pearson Education, Inc. Figure 7.9 Chloroplast To Calvin Cycle Light Light Stroma (low H+ concentration) H+ ADP H+ NADP+ P NADPH H+ H+ H+ Thylakoid membrane H2O Thylakoid space (high H+ concentration) 1 O + 2 + H 2 2 Photosystem II H+ + H H+ Electron transport chain H+ H+ H+ H+ H+ H+ Photosystem I H+ H+ H+ H+ ATP synthase ATP THE CALVIN CYCLE: REDUCING CO2TO SUGAR © 2012 Pearson Education, Inc. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle makes sugar within a chloroplast. To produce sugar, the necessary ingredients are – atmospheric CO2 and – ATP and NADPH generated by the light reactions. The Calvin cycle uses these three ingredients to produce an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate (G3P). A plant cell may then use G3P to make glucose and other organic molecules. © 2012 Pearson Education, Inc. Figure 7.10A Input CO2 ATP NADPH Calvin Cycle Output: G3P 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The steps of the Calvin cycle include – carbon fixation, – reduction, – release of G3P, and – regeneration of the starting molecule ribulose bisphosphate (RuBP). © 2012 Pearson Education, Inc. Figure 7.10B_s4 Step 1 Carbon fixation Input: 3 CO2 Rubisco 1 3 P 6 P RuBP Step 2 Reduction P 3-PGA 6 ATP 3 ADP 6 ADP Step 3 Release of one molecule of G3P Calvin Cycle 4 3 ATP 4 Regeneration of RuBP 6 NADPH 6 NADP 5 P 6 P G3P Step 2 G3P 3 Output: 1 P G3P Glucose and other compounds P 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates Most plants use CO2 directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO2 to RuBP. Such plants are called C3 plants because the first product of carbon fixation is a three-carbon compound, 3-PGA. © 2012 Pearson Education, Inc. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates C4 plants have evolved a means of – carbon fixation that saves water during photosynthesis while – optimizing the Calvin cycle. C4 plants are so named because they first fix CO2 into a four-carbon compound. When the weather is hot and dry, C4 plants keep their stomata mostly closed, thus conserving water. © 2012 Pearson Education, Inc. 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates Another adaptation to hot and dry environments has evolved in the CAM plants, such as pineapples and cacti. CAM plants conserve water by opening their stomata and admitting CO2 only at night. CO2 is fixed into a four-carbon compound, – which banks CO2 at night and – releases it to the Calvin cycle during the day. © 2012 Pearson Education, Inc. Figure 7.11 Mesophyll cell Bundlesheath cell CO2 4-C compound 4-C compound CO2 CO2 Calvin Cycle Calvin Cycle 3-C sugar C4 plant Sugarcane CO2 Night 3-C sugar Day CAM plant Pineapple 7.12 Review: Photosynthesis uses light energy, carbon dioxide, and water to make organic molecules About half of the carbohydrates made by photosynthesis are consumed as fuel for cellular respiration in the mitochondria of plant cells. Sugars also serve as the starting material for making other organic molecules, such as proteins, lipids, and cellulose. Excess food made by plants is stockpiled as starch in roots, tubers, seeds, and fruits. © 2012 Pearson Education, Inc. Figure 7.12 H2O CO2 Chloroplast Light NADP Light Reactions ADP P RuBP Calvin Cycle 3-PGA (in stroma) Photosystem II Electron transport chain Thylakoids Photosystem I ATP NADPH O2 Stroma G3P Sugars Cellular respiration Cellulose Starch Other organic compounds Figure 7.UN04 Photosynthesis converts includes both (a) (c) (b) to in which chemical energy light-excited electrons of chlorophyll H2O is split in which CO2 is fixed to RuBP and then and are passed down (d) reduce NADP to (h) using (f) to produce (e) producing (g) by chemiosmosis sugar (G3P)