BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Neil A. Campbell Jane B. Reece Lawrence G. Mitchell Martha R. Taylor From PowerPoint ® Lectures for Biology: Concepts & Connections CHAPTER 7 Photosynthesis: Using Light to Make Food

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Light is central to the life of a plant Photosynthesis is the most important chemical process on Earth –It provides food for virtually all organisms Plant cells convert light into chemical signals that affect a plant’s life cycle Life in the Sun

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Light can influence the architecture of a plant –Plants that get adequate light are often bushy, with deep green leaves –Without enough light, plants become tall and spindly with small pale leaves Too much sunlight can damage a plant –Chloroplasts and carotenoids help to prevent such damage

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water AN OVERVIEW OF PHOTOSYNTHESIS Carbon dioxide WaterGlucoseOxygen gas PHOTOSYNTHESIS

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Plants, some protists, and some bacteria are photosynthetic autotrophs –They are the ultimate producers of food consumed by virtually all organisms 7.1 Autotrophs are the producers of the biosphere

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings On land, plants such as oak trees and cacti are the predominant producers Figure 7.1AFigure 7.1B

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In aquatic environments, algae and photosynthetic bacteria are the main food producers Figure 7.1CFigure 7.1D

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts A chloroplast contains: –stroma, a fluid –grana, stacks of thylakoids The thylakoids contain chlorophyll –Chlorophyll is the green pigment that captures light for photosynthesis 7.2 Photosynthesis occurs in chloroplasts

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The location and structure of chloroplasts Figure 7.2 LEAF CROSS SECTION MESOPHYLL CELL LEAF Chloroplast Mesophyll CHLOROPLAST Intermembrane space Outer membrane Inner membrane Thylakoid compartment Thylakoid Stroma Granum StromaGrana

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The O 2 liberated by photosynthesis is made from the oxygen in water 7.3 Plants produce O 2 gas by splitting water Figure 7.3A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.3B Figure 7.3C Experiment 1 Experiment 2 Not labeled Labeled Reactants: Products:

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Water molecules are split apart and electrons and H + ions are removed, leaving O 2 gas –These electrons and H + ions are transferred to CO 2, producing sugar 7.4 Photosynthesis is a redox process, as is cellular respiration Figure 7.4A Figure 7.4B Reduction Oxidation Reduction

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The complete process of photosynthesis consists of two linked sets of reactions: –the light reactions and the Calvin cycle The light reactions convert light energy to chemical energy and produce O 2 The Calvin cycle assembles sugar molecules from CO 2 using the energy-carrying products of the light reactions 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings An overview of photosynthesis Figure 7.5 Light Chloroplast LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) Electrons H2OH2O O2O2 CO 2 NADP + ADP + P Sugar ATP NADPH

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Certain wavelengths of visible light drive the light reactions of photosynthesis 7.6 Visible radiation drives the light reactions THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY Gamma rays X-raysUVInfrared Micro- waves Radio waves Visible light Wavelength (nm) Figure 7.6A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.6B Light Chloroplast Reflected light Absorbed light Transmitted light

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Each of the many light-harvesting photosystems consists of: –an “antenna” of chlorophyll and other pigment molecules that absorb light –a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll 7.7 Photosystems capture solar power

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.7C Primary electron acceptor Photon Reaction center PHOTOSYSTEM Pigment molecules of antenna

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Fluorescence of isolated chlorophyll in solution Figure 7.7A Heat Photon (fluorescence) Photon Chlorophyll molecule

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.7B Excitation of chlorophyll in a chloroplast Primary electron acceptor Other compounds Chlorophyll molecule Photon

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons The excited electrons are passed from the primary electron acceptor to electron transport chains –Their energy ends up in ATP and NADPH 7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O 2

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Where do the electrons come from that keep the light reactions running? In photosystem I, electrons from the bottom of the cascade pass into its P700 chlorophyll

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Photosystem II regains electrons by splitting water, leaving O 2 gas as a by-product Figure 7.8 Primary electron acceptor Electron transport chain Electron transport Photons PHOTOSYSTEM I PHOTOSYSTEM II Energy for synthesis of by chemiosmosis

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H + through that membrane –The flow of H + back through the membrane is harnessed by ATP synthase to make ATP –In the stroma, the H + ions combine with NADP + to form NADPH 7.9 Chemiosmosis powers ATP synthesis in the light reactions

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The production of ATP by chemiosmosis in photosynthesis Figure 7.9 Thylakoid compartment (high H + ) Thylakoid membrane Stroma (low H + ) Light Antenna molecules Light ELECTRON TRANSPORT CHAIN PHOTOSYSTEM IIPHOTOSYSTEM IATP SYNTHASE

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle occurs in the chloroplast’s stroma –This is where carbon fixation takes place and sugar is manufactured 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle THE CALVIN CYCLE: CONVERTING CO 2 TO SUGARS INPUT Figure 7.10A OUTPUT: CALVIN CYCLE

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle constructs G3P using –carbon from atmospheric CO 2 –electrons and H + from NADPH –energy from ATP Energy-rich sugar is then converted into glucose

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.10B Details of the Calvin cycle INPUT: Step Carbon fixation. In a reaction catalyzed by rubisco, 3 molecules of CO 2 are fixed. 1 1 Step Energy consumption and redox. 2 3P P P6 6 2 ATP 6 ADP +P 6 NADPH 6 NADP + 6P G3P Step Release of one molecule of G3P. 3 CALVIN CYCLE 3 OUTPUT: 1P Glucose and other compounds G3P Step Regeneration of RuBP. 4 G3P 4 3 ADP 3ATP 3 CO 2 5P RuBP3-PGA

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A summary of the chemical processes of photo- synthesis 7.11 Review: Photosynthesis uses light energy to make food molecules PHOTOSYNTHESIS REVIEWED AND EXTENDED Figure 7.11 Light Chloroplast Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma Electrons LIGHT REACTIONSCALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Many plants make more sugar than they need –The excess is stored in roots, tuber, and fruits –These are a major source of food for animals

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Most plants are C 3 plants, which take CO 2 directly from the air and use it in the Calvin cycle –In these types of plants, stomata on the leaf surface close when the weather is hot –This causes a drop in CO 2 and an increase in O 2 in the leaf –Photorespiration may then occur 7.12 C 4 and CAM plants have special adaptations that save water

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Photorespiration in a C 3 plant CALVIN CYCLE 2-C compound Figure 7.12A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Some plants have special adaptations that enable them to save water CALVIN CYCLE 4-C compound Figure 7.12B –Special cells in C 4 plants—corn and sugarcane—incorporate CO 2 into a four-carbon molecule –This molecule can then donate CO 2 to the Calvin cycle 3-C sugar

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism CALVIN CYCLE 4-C compound Figure 7.12C –They open their stomata at night and make a four-carbon compound –It is used as a CO 2 source by the same cell during the day 3-C sugar Night Day

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Due to the increased burning of fossil fuels, atmospheric CO 2 is increasing –CO 2 warms Earth’s surface by trapping heat in the atmosphere –This is called the greenhouse effect 7.13 Human activity is causing global warming; photosynthesis moderates it PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.13A & B Sunlight ATMOSPHERE Radiant heat trapped by CO 2 and other gases

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Because photosynthesis removes CO 2 from the atmosphere, it moderates the greenhouse effect –Unfortunately, deforestation may cause a decline in global photosynthesis

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer His research focuses on how certain pollutants (greenhouse gases) damage that layer 7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone layer Figure 7.14A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The O 2 in the atmosphere results from photosynthesis –Solar radiation converts O 2 high in the atmosphere to ozone (O 3 ) –Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer Figure 7.14B Sunlight Southern tip of South America International restrictions on these chemicals are allowing recovery Antarctica