Photosynthesis: Using Light to Make Food Chapter 7 Photosynthesis: Using Light to Make Food

Plant Power Photosynthesis makes sugar and oxygen gas from carbon dioxide and water Carried out by plants, algae, and some bacteria The ultimate source of all food eaten by animals A major source of heat, light, and fuel

Using photosynthesis to meet growing energy demands Energy plantations of trees Renewable energy source Productive Environmentally safer than fossil fuels

AN OVERVIEW OF PHOTOSYNTHESIS 7.1 Autotrophs are the producers of the biosphere Autotrophs produce their own food Autotrophs produce the food supply for the global ecosystem Photoautotrophs produce organic molecules using light energy Plants, algae, and photosynthetic bacteria Video: Cyanobacteria (Oscillatoria)

7.2 Photosynthesis occurs in chloroplasts In plants, photosynthesis occurs primarily in leaves CO2 enters and O2 exits through stomata Chloroplasts are the site of photosynthesis Concentrated in mesophyll cells of leaves

Chloroplast structure Stroma Fluid enclosed by inner membrane Location of sugar synthesis Thylakoids Suspended in stroma Interconnected sacs Stacked in grana Contain chlorophyll in membranes

LE 7-2 Leaf Cross Section Mesophyll Cell Leaf Mesophyll LM 2,600 Vein Chloroplast Stoma CO2 O2 Outer membrane Chloroplast Inner membrane TEM 9,750 Stroma Intermembrane space Grana Stroma Granum Thylakoid Thylakoid space

7.3 Plants produce O2 gas by splitting water Experiments have made it possible to follow all the atoms in photosynthesis O2 is both a product and a reactant in photosynthesis The O2 liberated by photosynthesis is made from the oxygen in water

LE 7-3c Reactants: 6 CO2 12 H2O C6H12O6 6 H2O 6 O2 Products:

7.4 Photosynthesis is a redox process, as is cellular respiration H2O is oxidized CO2 is reduced Electrons gain energy Cellular respiration Glucose is oxidized O2 is reduced Electrons lose potential energy

7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH Light reactions Occur in thylakoid membranes Convert light energy to chemical energy as ATP and NADPH Produce O2 as a waste product Calvin cycle Occurs in stroma Assembles sugar molecules from CO2 using ATP and NADPH from light reactions

H2O CO2 O2 Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN LE 7-5 H2O CO2 Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN CYCLE (in stroma) (in thylakoids) ATP Electrons NADPH O2 Sugar

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions Sunlight is radiation, or electromagnetic energy Radiation travels as waves The distance between waves is a wavelength Visible light is only a small part of the electromagnetic spectrum Light also behaves as discrete photons Animation: Light and Pigments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m LE 7-6a Increasing energy Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light 380 400 500 600 700 750 Wavelength (nm) 650 nm

Several pigments are built into the thylakoids of chloroplasts Absorb some wavelengths of light; reflect or transmit others Chlorophyll a Absorbs blue-violet and red light, reflects green light Participates directly in the light reactions

Chlorophyll b Absorbs blue and orange light, reflects yellow-green Conveys absorbed energy to chlorophyll a Carotenoids Yellow-orange pigments that absorb mainly blue-green light May pass energy to chlorophyll a or protect it by dissipating excessive light energy

Light Reflected light Chloroplast Absorbed light Transmitted light LE 7-6b Light Reflected light Chloroplast Absorbed light Transmitted light

Light also behaves as photons A fixed quantity of energy Specific amounts of energy in photons absorbed by different pigments

7.7 Photosystems capture solar power When a pigment molecule absorbs a photon An electron is raised in energy from a ground state to an unstable excited state The electron rapidly drops back to the ground state The electron releases excess energy as heat Some pigments also emit light (fluorescence)

LE 7-7b Excited state Heat Energy of electron Photon Photon (fluorescence) Ground state Chlorophyll molecule

A reaction center that contains The thylakoid membrane contains two types of photosystems, which consist of Light-harvesting complexes of chlorophyll and other pigments and proteins A reaction center that contains A chlorophyll a molecule A primary electron acceptor molecule Receives excited electrons from the chlorophyll and transfers them to an electron transport chain

Photosystem Photon Thylakoid membrane Chlorophyll a molecule Light-harvesting complexes Reaction center Primary electron acceptor Photon To electron transport chain Thylakoid membrane Transfer of energy Pigment molecules Chlorophyll a molecule

7.8 In the light reactions, electron transport chains generate ATP and NADPH Photosystem II A pigment molecule absorbs a photon The energy is ultimately transferred to chlorophyll P680 The excited electrons from P680 are passed to the primary electron acceptor, then to an electron transport chain

Electrons shuttle down the chain from photosystem II to photosystem I, providing energy to make ATP Water is split, replacing electrons lost by P680 and releasing O2 Photosystem I A photon is absorbed and excites an electron of chlorophyll P700 The excited electron passes through a short electron transport chain, reducing NADP+ to NADPH

LE 7-8a Animation: Light Reactions Photon Photon Photosystem I Photosystem II NADP+ + H + NADPH Stroma e– e– Thylakoid membrane P700 P680 Thylakoid space Electron transport chain Provides energy for synthesis of by chemiosmosis ATP H2O 1 O2 + 2 H + Animation: Light Reactions

ATP NADPH Mill makes ATP Photosystem II Photosystem I LE 7-8b e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem II Photosystem I

7.9 Chemiosmosis powers ATP synthesis in the light reactions The electron transport chain Pumps H+ from the stroma into the thylakoid space Generates a concentration gradient across the thylakoid membrane

Photophosphorylation Energy of the concentration gradient drives H+ back across the membrane through ATP synthase ATP synthase couples flow of H+ to the phosphorylation of ADP to produce ATP Light provides the initial energy for the phosphorylation

LE 7-9 + + + + + + + + + + + + + + + + Chloroplast Stroma (low H+ concentration) Light Light H + H + +  ATP H + NADP+ + H + NADPH H + Thylakoid membrane H2O H + H + 1 2 O2 + 2 H H + + H + H + H + H + H + Photosystem II Photosystem I H + Electron transport chain ATP synthase H + Thylakoid space (high H+ concentration)

THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle makes sugar in the chloroplast Inputs Carbon from CO2 Energy from ATP High-energy electrons from NADPH Output Energy-rich G3P Can be used to make organic molecules

LE 7-10a CO2 Input ATP NADPH CALVIN CYCLE Output: G3P

Animation: Calvin Cycle Steps of the Calvin cycle Carbon fixation Reduction Release of one molecule of G3P Regeneration of RuBP Animation: Calvin Cycle

LE 7-10b Carbon fixation Input: 3 In a reaction catalyzed by rubisco, CO2 is added to RuBP. CO2 3 P P 6 P Reduction RuBP 3-PGA 6 ATP 3 ADP 6 ADP + P 3 ATP CALVIN CYCLE 6 NADPH Release of one molecule of G3P 6 NADP+ 5 P 6 P G3P G3P Regeneration of RuBP Glucose and other compounds Output: 1 P G3P

PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules The light reactions Capture light energy Generate NADPH and ATP Release O2 and water The Calvin cycle Manufactures sugar Cells use many of the same mechanisms in photosynthesis and cellular respiration

H2O CO2 + LE 7-11 Chloroplast NADP+ ADP P RUBP CALVIN CYCLE Light NADP+ ADP + P RUBP Photosystem II CALVIN CYCLE (in stroma) Electron transport chains 3-PGA Thylakoid membranes Photosystem I ATP Stroma NADPH G3P Cellular respiration Cellulose Starch O2 Sugars Other organic compounds LIGHT REACTIONS CALVIN CYCLE

7.12 C4 and CAM plants have special adaptations that save water C3 plants Corn, soybeans, wheat, rice Use CO2 directly Rate of photosynthesis decreases in dry weather Stomata close, CO2 supply reduced Calvin cycle diverted to photorespiration

Include corn and sugarcane Stomata closed in hot, dry weather C4 plants Include corn and sugarcane Stomata closed in hot, dry weather CO2 fixed into a 4-carbon compound that acts as a carbon shuttle Enables plant to continue making sugar Prevents photorespiration and water loss

Pineapple, cactus, succulents Adapted to very dry climates CAM plants Pineapple, cactus, succulents Adapted to very dry climates Open stomata and admit CO2 only at night Fix CO2 into a 4-carbon compound that banks CO2 for release to Calvin cycle during the day

LE 7-12 Mesophyll cell CO2 CO2 Night 4-C compound 4-C compound CO2 CO2 CALVIN CYCLE CALVIN CYCLE Sugarcane Pineapple Bundle-sheath cell 3-C sugar 3-C sugar Day C4 plant CAM plant

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH'S ATMOSPHERE: CONNECTION 7.13 Photosynthesis moderates global warming The greenhouse effect Like a greenhouse, the atmosphere traps CO2 and other gases that absorb heat radiated from Earth's surface Excess greenhouse gases in the atmosphere contribute to global warming Photosynthesis, which removes CO2 from the atmosphere, moderates warming Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 7-13b Some heat energy escapes into space Sunlight Atmosphere Radiant heat trapped by CO2 and other gases

TALKING ABOUT SCIENCE 7.14 Mario Molina talks about Earth's protective ozone layer Nobel Prize winner Mario Molina has studied how pollutants are affecting Earth's ozone layer Solar radiation converts O2 high in the atmosphere to ozone (O3) The ozone layer shields organisms on Earth's surface from damaging UV radiation CFCs have caused dangerous thinning of the ozone layer International restrictions on CFC use are allowing a slow recovery

LE 7-14b Southern tip of South America Antarctica