Chapter 7 hotosynthesis: Using to Make Food Introduction lants, algae, and certain prokaryotes convert light energy to chemical energy and store the chemical energy in sugar, made from carbon dioxide and water. oweroint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey Lecture by Edward J. Zalisko Introduction Figure 7.0_ Chapter 7: Big Ideas Algae farms can be used to produce oils for biodiesel or carbohydrates to generate ethanol. An Overview of hotosynthesis The Reactions: Converting Solar Energy to Chemical Energy The : Reducing CO to Sugar hotosynthesis Reviewed and Extended Figure 7.0_ AN OVERVIEW OF HOTOSYNTHESIS
7. Autotrophs are the producers of the biosphere 7. 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. hotoautotrophs 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. 7. Autotrophs are the producers of the biosphere Figure 7.A-D hotosynthesis in plants takes place in chloroplasts, converts carbon dioxide and water into organic molecules, and releases oxygen. Figure 7.A Figure 7.B
O Figure 7.C Figure 7.D 7. hotosynthesis occurs in chloroplasts in plant cells s 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. 7. hotosynthesis occurs in chloroplasts in plant cells s 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. Figure 7. Figure 7._ Leaf Cross Section Leaf Mesophyll Vein Leaf Cross Section Leaf CO Stoma Mesophyll Cell Mesophyll Vein Mesophyll Cell Inner and outer membranes Granum Thylakoid Thylakoid space Stroma CO O Stoma 3
7. hotosynthesis occurs in chloroplasts in plant cells s 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. 7. hotosynthesis 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. Figure 7._ Figure 7._3 Mesophyll Cell Thylakoid Thylakoid space Stroma Inner and outer membranes Granum Figure 7._4 7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes Stroma Granum Scientists have known since the 800s that plants produce O. But does this oxygen come from carbon dioxide or water? For many years, it was assumed that oxygen was extracted from CO taken into the plant. However, later research using a heavy isotope of oxygen, 8 O, confirmed that oxygen produced by photosynthesis comes from H O. 4
Figure 7.3A 7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes Experiment : CO H O C H O H O O Experiment : CO H O C H O H O O Figure 7.3B 7.4 hotosynthesis is a redox process, as is cellular respiration Reactants: roducts: hotosynthesis, like respiration, is a redox (oxidation-reduction) process. CO 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. Figure 7.4A 7.4 hotosynthesis is a redox process, as is cellular respiration Cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule. Becomes reduced Becomes oxidized This is accomplished by oxidizing the sugar and reducing O to H O. The electrons lose potential as they travel down the electron transport chain to O. In contrast, the food-producing redox reactions of photosynthesis require energy. 5
7.4 hotosynthesis is a redox process, as is cellular respiration Figure 7.4B In photosynthesis, light energy is captured by chlorophyll molecules to boost the energy of electrons, light energy is converted to chemical energy, and chemical energy is stored in the chemical bonds of sugars. Becomes oxidized Becomes reduced 7.5 Overview: The two stages of photosynthesis are linked by AT and NADH hotosynthesis occurs in two metabolic stages.. 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, AT is generated from AD and a phosphate group, and light energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and from water to the electron acceptor NAD + reducing it to NADH. NADH produced by the light reactions provides the electrons for reducing carbon in the cycle. 7.5 Overview: The two stages of photosynthesis are linked by AT and NADH. The second stage is the cycle, which occurs in the stroma of the chloroplast. The cycle is a cyclic series of reactions that assembles sugar molecules using CO and the energy-rich products of the light reactions. During the cycle, CO 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 cycle is often called the dark reactions or lightindependent reactions, because none of the steps requires light directly. Figure 7.5_s H O Figure 7.5_s H O NAD + AD NAD + AD Reactions (in thylakoids) Reactions (in thylakoids) AT NADH O
Figure 7.5_s3 H O CO Reactions (in thylakoids) NAD + AD AT (in stroma) THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY NADH O Sugar 7. 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. 7. Visible radiation absorbed by pigments drives the light reactions behaves as discrete packets of energy called photons. A photon is a fixed quantity of light energy. The shorter the wavelength, the greater the energy. Figure 7.A Increasing energy 0 5 nm 0 3 nm nm 0 3 nm 0 nm m 0 3 m Gamma rays X-rays UV Infrared Visible light Microwaves 380 400 500 00 700 750 Wavelength (nm) 50 nm Radio waves 7. Visible radiation absorbed by pigments drives the light reactions igments absorb light and are built into the thylakoid membrane. lant 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: and igments 7
Figure 7.B Figure 7.B_ Reflected light Thylakoid Absorbed light Transmitted light 7. Visible radiation absorbed by pigments drives the light reactions s contain several different pigments, which absorb light of different wavelengths. Chlorophyll a absorbs blue-violet and red light and reflects green. Chlorophyll b absorbs blue and orange and reflects yellow-green. Carotenoids broaden the spectrum of colors that can drive photosynthesis and provide photoprotection, absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules. 7.7 hotosystems capture solar energy igments 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. Figure 7.7A Figure 7.7A_ Excited state hoton of light Heat hoton (fluorescence) Ground state Chlorophyll molecule 8
Figure 7.7A_ Excited state 7.7 hotosystems capture solar energy hoton of light Heat hoton (fluorescence) Ground state 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. Chlorophyll molecule 7.7 hotosystems capture solar energy Figure 7.7B hotosystem A photosystem consists of a number of lightharvesting complexes surrounding a reactioncenter complex. A light-harvesting complex contains various pigment molecules bound to proteins. Collectively, the light-harvesting complexes function as a light-gathering antenna. Thylakoid membrane -harvesting complexes Reaction-center complex rimary electron acceptor Transfer of energy air of chlorophyll a molecules igment molecules 7.7 hotosystems capture solar energy 7.7 hotosystems capture solar energy The light energy is passed from molecule to molecule within the photosystem. Finally it reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced. This solar-powered transfer of an electron from the reaction-center pigment to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light reactions. Two types of photosystems (photosystem I and photosystem II) cooperate in the light reactions. Each type of photosystem has a characteristic reaction center. hotosystem II, which functions first, is called 80 because its pigment absorbs light with a wavelength of 80 nm. hotosystem I, which functions second, is called 700 because it absorbs light with a wavelength of 700 nm. 9
7.8 Two photosystems connected by an electron transport chain generate AT and NADH Figure 7.8A In the light reactions, light energy is transformed into the chemical energy of AT and NADH. To accomplish this, electrons are removed from water, passed from photosystem II to photosystem I, and accepted by NAD +, reducing it to NADH. Between the two photosystems, the electrons move down an electron transport chain and Thylakoid membrane Stroma Thylakoid space hotosystem II rimary acceptor 80 3 H O O H Electron transport chain rovides energy for synthesis of AT by chemiosmosis 4 NAD H hotosystem I rimary acceptor 5 700 NADH provide energy for the synthesis of AT. Figure 7.8A_ Figure 7.8A_ Stroma hotosystem II Electron transport chain rovides energy for synthesis of AT by chemiosmosis Electron transport chain rovides energy for synthesis of AT by chemiosmosis NAD hotosystem I NADH Thylakoid membrane rimary acceptor 80 4 4 rimary acceptor 5 700 Thylakoid space H O 3 O H Figure 7.8B AT 7.8 Two photosystems connected by an electron transport chain generate AT and NADH The products of the light reactions are NADH NADH, AT, and Mill makes AT oxygen. hotosystem II hotosystem I 0
7.9 Chemiosmosis powers AT synthesis in the light reactions Interestingly, chemiosmosis is the mechanism that is involved in oxidative phosphorylation in mitochondria and generates AT in chloroplasts. AT is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane. 7.9 Chemiosmosis powers AT synthesis in the light reactions In photophosphorylation, using the initial energy input from light, the electron transport chain pumps into the thylakoid space, and the resulting concentration gradient drives back through AT synthase, producing AT. Figure 7.9 Figure 7.9_ To AD AT Stroma (low concentration) NAD + NADH To AD AT NAD + NADH Thylakoid membrane Thylakoid space (high concentration) H O O + hotosystem II H+ H+ Electron transport chain hotosystem I H+ AT synthase H O O Electron hotosystem II transport chain hotosystem I + H AT synthase 7.9 Chemiosmosis powers AT synthesis in the light reactions How does photophosphorylation compare with oxidative phosphorylation? Mitochondria use oxidative phosphorylation to transfer chemical energy from food into the chemical energy of AT. s use photophosphorylation to transfer light energy into the chemical energy of AT. THE CALVIN CYCLE: REDUCING CO TO SUGAR
7.0 AT and NADH power sugar synthesis in the cycle The cycle makes sugar within a chloroplast. To produce sugar, the necessary ingredients are Figure 7.0A Input CO AT NADH atmospheric CO and AT and NADH generated by the light reactions. The cycle uses these three ingredients to produce an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate (G3). A plant cell may then use G3 to make glucose and other organic molecules. Output: G3 7.0 AT and NADH power sugar synthesis in the cycle Figure 7.0B_s Step Carbon fixation Input: 3 CO The steps of the cycle include carbon fixation, 3 RuB Rubisco 3-GA reduction, release of G3, and regeneration of the starting molecule ribulose bisphosphate (RuB). Figure 7.0B_s Step Carbon fixation Input: 3 Figure 7.0B_s3 Step Carbon fixation Input: 3 CO CO Rubisco Rubisco Step Reduction 3 RuB 3-GA AT Step Reduction 3 RuB 3-GA AT AD NADH AD NADH G3 NAD Step 3 Release of one molecule of G3 5 G3 3 G3 NAD Output: G3 Glucose and other compounds
Figure 7.0B_s4 Step Carbon fixation Input: 3 CO Rubisco 7. EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates 3 Step Reduction 3 AD 3 AT Step 3 Release of one molecule of G3 5 RuB 4 G3 3 3-GA G3 AT AD NADH NAD Most plants use CO directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO to RuB. Such plants are called C 3 plants because the first product of carbon fixation is a three-carbon compound, 3-GA. Step 4 Regeneration of RuB Output: G3 Glucose and other compounds 7. EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates In hot and dry weather, C 3 plants close their stomata to reduce water loss but prevent CO from entering the leaf and O from leaving. As O builds up in a leaf, rubisco adds O instead of CO to RuB, and a two-carbon product of this reaction is then broken down in the cell. This process is called photorespiration because it occurs in the light, consumes O, and releases CO. But unlike cellular respiration, it uses AT instead of producing it. 7. EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates C 4 plants have evolved a means of carbon fixation that saves water during photosynthesis while optimizing the cycle. C 4 plants are so named because they first fix CO into a four-carbon compound. When the weather is hot and dry, C 4 plants keep their stomata mostly closed, thus conserving water. 7. EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates Figure 7. Mesophyll cell CO Night CO Another adaptation to hot and dry environments has evolved in the CAM plants, such as pineapples and cacti. Bundlesheath cell 4-C compound 4-C compound CO CO CAM plants conserve water by opening their stomata and admitting CO only at night. CO is fixed into a four-carbon compound, which banks CO at night and 3-C sugar C 4 plant Day 3-C sugar CAM plant releases it to the cycle during the day. Sugarcane ineapple 3
Figure 7._ Figure 7._ Mesophyll cell CO Night CO 4-C compound 4-C compound Bundlesheath cell CO CO Sugarcane 3-C sugar C 4 plant Day 3-C sugar CAM plant Figure 7._3 HOTOSYNTHESIS REVIEWED AND EXTENDED ineapple 7. Review: hotosynthesis uses light energy, carbon dioxide, and water to make organic molecules Most of the living world depends on the foodmaking machinery of photosynthesis. The chloroplast integrates the two stages of photosynthesis and makes sugar from CO. 7. Review: hotosynthesis 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. 4
Figure 7. Thylakoids H O CO Reactions hotosystem II Electron transport chain hotosystem I O NAD AD AT NADH RuB 3-GA (in stroma) G3 Sugars Stroma Cellular respiration Cellulose Starch Other organic compounds 7.3 CONNECTION: hotosynthesis may moderate global climate change The greenhouse effect operates on a global scale. Solar radiation includes visible light that penetrates the Earth s atmosphere and warms the planet s surface. Heat radiating from the warmed planet is absorbed by gases in the atmosphere, which then reflects some of the heat back to Earth. Without the warming of the greenhouse effect, the Earth would be much colder and most life as we know it could not exist. Figure 7.3A Figure 7.3B Sunlight Some heat energy escapes into space Atmosphere Radiant heat trapped by CO and other gases 7.3 CONNECTION: hotosynthesis may moderate global climate change The gases in the atmosphere that absorb heat radiation are called greenhouse gases. These include water vapor, carbon dioxide, and methane. 7.3 CONNECTION: hotosynthesis may moderate global climate change Increasing concentrations of greenhouse gases have been linked to global climate change (also called global warming), a slow but steady rise in Earth s surface temperature. Since 850, the atmospheric concentration of CO has increased by about 40%, mostly due to the combustion of fossil fuels including coal, oil, and gasoline. 5
7.3 CONNECTION: hotosynthesis may moderate global climate change The predicted consequences of continued warming include melting of polar ice, rising sea levels, extreme weather patterns, droughts, increased extinction rates, and the spread of tropical diseases. 7.3 CONNECTION: hotosynthesis may moderate global climate change Widespread deforestation has aggravated the global warming problem by reducing an effective CO sink. Global warming caused by increasing CO levels may be reduced by limiting deforestation, reducing fossil fuel consumption, and growing biofuel crops that remove CO from the atmosphere. 7.4 SCIENTIFIC DISCOVERY: Scientific study of Earth s ozone layer has global significance Solar radiation converts O high in the atmosphere to ozone (O 3 ), which shields organisms from damaging UV radiation. Industrial chemicals called CFCs have caused dangerous thinning of the ozone layer, but international restrictions on CFC use are allowing a slow recovery. Figure 7.4A Southern tip of South America Antarctica Figure 7.4B You should now be able to. Define autotrophs, heterotrophs, producers, and photoautotrophs.. Describe the structure of chloroplasts and their location in a leaf. 3. Explain how plants produce oxygen. 4. Describe the role of redox reactions in photosynthesis and cellular respiration. 5. Compare the reactants and products of the light reactions and the cycle.
You should now be able to You should now be able to. Describe the properties and functions of the different photosynthetic pigments. 7. Explain how photosystems capture solar energy. 8. Explain how the electron transport chain and chemiosmosis generate AT, NADH, and oxygen in the light reactions. 9. Compare photophosphorylation and oxidative phosphorylation. 0. Describe the reactants and products of the cycle.. Compare the mechanisms that C 3, C 4, and CAM plants use to obtain and use carbon dioxide.. Review the overall process of the light reactions and the cycle, noting the products, reactants, and locations of every major step. 3. Describe the greenhouse effect. 4. Explain how the ozone layer forms, how human activities have damaged it, and the consequences of the destruction of the ozone layer. Figure 7.UN0 Figure 7.UN0 H O CO energy CO H O C H O O Carbon dioxide Water Oxygen gas hotosynthesis Glucose Thylakoids Reactions NAD AD AT Stroma NADH O Sugar Figure 7.UN03 Figure 7.UN04 hotosynthesis converts includes both (a) (b) (c) to Mitochondrion Intermembrane space H c. chemical energy H O is split and in which light-excited electrons of chlorophyll in which CO is fixed to RuB and then Membrane (d) are passed down reduce NAD to using (h) Matrix a. b. d. e. (e) chemiosmosis (f) producing (g) by to produce sugar (G3) 7