Chapter 7 Photosynthesis Using Light to Make Food

Chapter 7 Photosynthesis: Using Light to Make Food Power. Point Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko

Introduction § Plants, algae, and certain prokaryotes – convert light energy to chemical energy – by the process of photosynthesis and – store the chemical energy in sugar, made from – carbon dioxide and – water. © 2012 Pearson Education, Inc.

Figure 7. 0_2 Unicellular algae

Introduction § Algae farms can be used to produce – oils for biodiesel or – carbohydrates to generate ethanol. – Unlike biofuel crops such as corn, algae don’t require large areas of fertile land, and they grow fast. © 2012 Pearson Education, Inc.

Figure 7. 0_1 Chapter 7: Big Ideas An Overview of Photosynthesis The Calvin Cycle: Reducing CO 2 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, and do not usually consume organic molecules derived from other organisms. – There are two kinds of autotrophs – Photoautotrophs- use the energy of light to produce organic molecules. Eg: Plants, algae, some protists & cyanobacteria – Chemoautotrophs- are prokaryotes that use energy in inorganic chemicals as their energy source. Eg: Sulfur bacteria that live in deep-ocean vents © 2012 Pearson Education, Inc.

Figure 7. 1 A-D Photoautotroph diversity Plants Cyanobacteria Photosynthetic Kelp, a multicellular alga protists

7. 1 Autotrophs are the producers of the biosphere § Heterotrophs – are consumers that feed on other organisms (such as plants, animals, or decomposing organic material) – All animals – Humans – Fungi – Protists (that are not autotrophs) – Bacteria (except cyanobacteria) © 2012 Pearson Education, Inc.

7. 1 Autotrophs are the producers of the biosphere § Photosynthesis – converts carbon dioxide and water – into organic molecules, and releases oxygen. Light energy 6 CO 2 © 2012 Pearson Education, Inc. 6 H 2 O C 6 H 12 O 6 6 O 2

Figure 7. 2_1 Cross section of a Leaf Cross Section Leaf Mesophyll Vein Mesophyll Cell CO 2 Stoma Chloroplast

7. 2 Photosynthesis occurs in chloroplasts in plant cells § Mesophyll is the green tissue in the interior of the leaf. Chloroplasts are concentrated in mesophyll cells § 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_2 Structure of Chloroplasts Chloroplast Inner and outer membranes Granum Thylakoid space Stroma

7. 2 Photosynthesis occurs in chloroplasts in plant cells § Chloroplasts – are the major sites of photosynthesis in green plants. – Chloroplasts consist of photosynthetic pigments, enzymes, and other molecules grouped together in membranes – There about half a million chloroplasts per square millimeter of leaf surface. © 2012 Pearson Education, Inc.

7. 2 Photosynthesis occurs in chloroplasts in plant cells § Chloroplasts – Consist of an envelope of two membranes, inner & outer membrane – Inner compartment is filled with a thick fluid called stroma – Contain a system of interconnected membranous sacs called thylakoids. – Stacks of thylakoides are called grana and © 2012 Pearson Education, Inc.

7. 2 Photosynthesis occurs in chloroplasts in plant cells § Chlorophyll – molecules are built into the thylakoid membrane and – is an important light-absorbing pigment in chloroplasts, – is responsible for the green color of plants, and – plays a pivotal role in converting solar energy to chemical energy. © 2012 Pearson Education, Inc.

Figure 7. 3 A Bubbles on the leaves of an aquatic plant Q: Which gas is in the bubbles?

7. 3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes § Scientists have known since the 1800 s that plants produce O 2. But does this oxygen come from carbon dioxide or water? – For many years, it was assumed that oxygen was extracted from CO 2 taken into the plant. – However, later research using a heavy isotope of oxygen, 18 O, confirmed that oxygen produced by photosynthesis comes from H 2 O. © 2012 Pearson Education, Inc.

Figure 7. UN 01 Photosynthesis Light energy 6 CO 2 Carbon dioxide 6 H 2 O Water C 6 H 12 O 6 Photosynthesis Glucose 6 O 2 Oxygen gas

7. 3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes Experiment 1: 6 CO 2 12 H 2 O → C 6 H 12 O 6 6 H 2 O 6 O 2 Experiment 2: 6 CO 2 12 H 2 O → C 6 H 12 O 6 6 H 2 O 6 O 2 © 2012 Pearson Education, Inc.

Figure 7. 3 B Reactants: Products: Q: Photosynthesis produces billions of tons of carbohydrates a year. Where does most of the mass of this huge amount of organic matter come from?

7. 4 Photosynthesis is a redox process, as is cellular respiration § Photosynthesis, – like respiration, is a redox (oxidation-reduction) process. – CO 2 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. © 2012 Pearson Education, Inc.

Figure 7. 4 A Photosynthesis is a redox (reduction & oxidation) process Becomes reduced Becomes oxidized

Figure 7. 4 A Photosynthesis (uses light energy) Becomes reduced Becomes oxidized Becomes reduced Cellular respiration (releases chemical energy)

7. 4 Photosynthesis is a redox process, as is cellular respiration § Cellular respiration – The electrons lose potential as they travel down the electron transport chain to O 2. – Harvests chemical energy from food molecules. § Photosynthesis – Light energy is captured by chlorophyll molecules to boost the energy of electrons, – The electrons gain energy as they travel climb uphill. – Light energy is converted to chemical energy, and – Chemical energy is stored in the chemical bonds of sugars. © 2012 Pearson Education, Inc.

7. 5 Overview: The two stages of photosynthesis are linked by ATP and NADPH § Photosynthesis occurs in two metabolic stages – Light reactions: Light energy is converted in the to chemical energy and O 2 in the thylakoid membranes – Calvin cycle: CO 2 is incorporated into organic compounds in the stroma – Calvin cycle is sometimes referred to as dark reactions because it does not need light directly Copyright © 2009 Pearson Education, Inc.

7. 5 Overview: The two stages of photosynthesis are linked by ATP and NADPH § 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 § Calvin cycle, – Occurs in the stroma of the chloroplasts – During the Calvin cycle, CO 2 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_s 1 H 2 O Light NADP+ ADP P Light Reactions (in thylakoids) Chloroplast

Figure 7. 5_s 2 H 2 O Light NADP+ ADP P Light Reactions (in thylakoids) ATP NADPH Chloroplast O 2

Figure 7. 5_s 3 H 2 O CO 2 Light NADP+ ADP P Calvin Cycle (in stroma) Light Reactions (in thylakoids) ATP NADPH Chloroplast O 2 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. – The shorter the wavelength, the greater the energy. © 2012 Pearson Education, Inc.

Figure 7. 6 A Increasing energy 10 5 nm 10 3 nm Gamma rays X-rays 103 nm 1 nm UV 106 nm Infrared 103 m 1 m Microwaves Radio waves Visible light 380 400 500 600 Wavelength (nm) The electromagnetic spectrum 700 650 nm 750

7. 6 Visible radiation absorbed by pigments drives the light reactions § Light – 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 in photons. © 2012 Pearson Education, Inc.

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. © 2012 Pearson Education, Inc.

Animation: Light and Pigments Right click on animation / Click play © 2012 Pearson Education, Inc.

Figure 7. 6 B Light Reflected light Chloroplast Thylakoid Absorbed light Transmitted light

7. 6 Visible radiation absorbed by pigments drives the light reactions § 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 – (accessory pigments) Their role is to capture sunlight and transfer to chlorophyll. – Eg: Carotene (Orange) ; Xanthophyll (Yellow) Yellow © 2012 Pearson Education, Inc.

7. 7 Photosystems capture solar energy § Pigments in chloroplasts absorb photons, 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 drop down, they release their energy as heat and light. © 2012 Pearson Education, Inc.

Figure 7. 7 A A solution of chlorophyll glowing red when illuminated Excited state Photon of light Heat Photon (fluorescence) Ground state Chlorophyll molecule

7. 7 Photosystems capture solar energy § Photosystems – In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and proteins into photosystems. – A photosystem consists of a number of light-harvesting complexes surrounding a reaction-center complex. – A light-harvesting complex contains various pigment molecules bound to proteins. – Collectively, the light-harvesting complexes function as a light-gathering antenna. © 2012 Pearson Education, Inc.

Figure 7. 7 B Photosystem Light-harvesting Reaction-center complexes complex Thylakoid membrane Primary electron acceptor Transfer of energy Pair of chlorophyll a molecules Pigment molecules

7. 7 Photosystems capture solar energy § Photosystems – 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. – There are two types of photosystems, I and II. © 2012 Pearson Education, Inc.

7. 7 Photosystems capture solar energy § Photosystem I – This photosystem functions second, – It is called P 700 because it absorbs light of 700 nm. § Photosystem II – This photosystem functions first, – It is called P 680 because its absorbs light of 680 nm. § The two photosystems are connected by an electron transport chain © 2012 Pearson Education, Inc.

Figure 7. 8 A Electron Flow in the Light Reactions 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 P 700 P 680 Thylakoid space 3 H 2 O 1 2 5 O 2 2 H 6 NADPH

7. 8 Two photosystems connected by an electron transport chain generate ATP and NADPH § Electron flow in the light reactions (from water to NADP) – Electrons removed from water – Gain energy passed from pigments and excited to a higher energy level – Photo excited high-energy electrons are captured by a primary electron acceptor – High energy electrons travel from photosystem II to photosystem I – Between the two photosystems, the electrons move down an electron transport chain. –. © 2012 Pearson Education, Inc.

Figure 7. 8 B A mechanical analogy of the light reactions ATP Photon Photosystem II Phot Mill makes ATP on NADPH Photosystem I

7. 9 Chemiosmosis powers ATP synthesis in the light reactions § Generation of ATP in the light reactions – The energy released by the electrons “falling” in the electron transport chain is used to pump H+ into the thylakoid space, and – This results in a concentration gradient of H+ across the thylakoid membrane – The energy in the concentration gradient drives H+ back to the stroma through ATP synthase, producing ATP. © 2012 Pearson Education, Inc.

Figure 7. 9 The production of ATP by Chemiosmosis Chloroplast To Calvin Cycle Light Stroma (low H+ concentration) ADP H+ NADP+ H+ P NADPH H+ H+ H+ Thylakoid membrane H 2 O Thylakoid space (high H+ concentration) 1 O + 2 + H 2 2 Photosystem II H+ H + H+ Electron transport chain H+ H+ Photosystem I H+ H+ H+ ATP synthase ATP

7. 9 Chemiosmosis powers ATP synthesis in the light reactions § Photophosphorylation, – Production of ATP using the initial energy input from light © 2012 Pearson Education, Inc.

7. 9 Chemiosmosis powers ATP 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 ATP. – Chloroplasts use photophosphorylation to transfer light energy into the chemical energy of ATP. © 2012 Pearson Education, Inc.

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.

THE CALVIN CYCLE: REDUCING CO 2 TO SUGAR © 2012 Pearson Education, Inc.

7. 10 ATP and NADPH power sugar synthesis in the Calvin cycle § Calvin cycle – Makes sugar within a chloroplast. – Using atmospheric CO 2 and – ATP and NADPH generated by the light reactions. – Produces an energy-rich, three-carbon sugar called glyceraldehyde-3 -phosphate (G 3 P). – A plant cell may then use G 3 P to make glucose and other organic molecules. © 2012 Pearson Education, Inc.

Figure 7. 10 A An overview of the Calvin cycle Input CO 2 ATP NADPH Calvin Cycle Output: G 3 P

7. 10 ATP and NADPH power sugar synthesis in the Calvin cycle § The steps of the Calvin cycle include 1. carbon fixation, 2. reduction, 3. release of G 3 P, and 4. regeneration of the starting molecule ribulose bisphosphate (Ru. BP). © 2012 Pearson Education, Inc.

Figure 7. 10 B_s 1 Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 3 P 6 P Ru. BP 3 -PGA Calvin Cycle P

Figure 7. 10 B_s 2 Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 3 P Step 2 Reduction 6 P Ru. BP 3 -PGA P 6 ATP 6 ADP Calvin Cycle 2 6 NADPH 6 P G 3 P 6 NADP P

Figure 7. 10 B_s 3 Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 3 P Step 2 6 P Ru. BP Reduction 3 -PGA P 6 ATP 6 ADP Calvin Cycle Step 3 Release of one molecule of G 3 P 5 G 3 P 2 6 NADPH 6 P P 6 NADP G 3 P 3 Output: 1 P G 3 P Glucose and other compounds P

Figure 7. 10 B_s 4 Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 3 P Step 2 6 P Ru. BP Reduction 3 -PGA P 3 ADP Calvin Cycle 4 5 G 3 P Step 4 Regeneration of Ru. BP ATP 6 ADP 3 ATP Step 3 Release of one molecule of G 3 P 6 2 6 NADPH 6 P P 6 NADP G 3 P 3 Output: 1 P G 3 P Glucose and other compounds P

7. 11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climates § Most plants use CO 2 directly from the air, and carbon fixation occurs when the enzyme Rubisco adds CO 2 to Ru. BP. § Such plants are called C 3 plants because the first product of carbon fixation is a three-carbon compound, 3 -PGA. § Skip C 4 and CAM plants © 2012 Pearson Education, Inc.

PHOTOSYNTHESIS REVIEWED AND EXTENDED © 2012 Pearson Education, Inc.

7. 12 Review: Photosynthesis 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 2. © 2012 Pearson Education, Inc.

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 H 2 O Light CO 2 Chloroplast NADP Light Reactions ADP P Ru. BP Calvin Cycle 3 -PGA (in stroma) Photosystem II Electron transport chain Thylakoids Photosystem I ATP NADPH O 2 Stroma G 3 P Sugars Cellular respiration Cellulose Starch Other organic compounds

7. 13 CONNECTION: Photosynthesis 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. © 2012 Pearson Education, Inc.

Figure 7. 13 A

Figure 7. 13 B CO 2 in the atmosphere and the greenhouse effect Some heat energy escapes into space Sunlight Atmosphere Radiant heat trapped by CO 2 and other gases

7. 13 CONNECTION: Photosynthesis 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. © 2012 Pearson Education, Inc.

7. 13 CONNECTION: Photosynthesis 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 1850, the atmospheric concentration of CO 2 has increased by about 40%, mostly due to the combustion of fossil fuels including – coal, – oil, and – gasoline. © 2012 Pearson Education, Inc.

7. 13 CONNECTION: Photosynthesis 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. © 2012 Pearson Education, Inc.

7. 13 CONNECTION: Photosynthesis may moderate global climate change § Widespread deforestation has aggravated the global warming problem by reducing an effective CO 2 sink. § Global warming caused by increasing CO 2 levels may be reduced by – limiting deforestation, – reducing fossil fuel consumption, and – growing biofuel crops that remove CO 2 from the atmosphere. © 2012 Pearson Education, Inc.

7. 14 SCIENTIFIC DISCOVERY: Scientific study of Earth’s ozone layer has global significance § Solar radiation converts O 2 high in the atmosphere to ozone (O 3), which shields organisms from damaging UV radiation. § Industrial chemicals called CFCs (Chloro-flurocarbons) have caused dangerous thinning of the ozone layer, but international restrictions on CFC use are allowing a slow recovery. © 2012 Pearson Education, Inc.

Figure 7. 14 A The ozone hole in the Southern Hemisphere, spring 2006 Southern tip of South America Antarctica

Learning from plants

Figure 7. UN 02 NADP P NADPH