Chapter 7 Photosynthesis: Using to Make Food PowerPoint Lectures for Biology: Concepts and Connections, Fifth Edition Campbell, Reece, Taylor, and Simon Lectures by Chris Romero Photosynthesis is the process by which certain organisms use light energy To make sugar and oxygen gas from carbon dioxide and water energy 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2 Carbon dioxide Water PHOTOSYNTHESIS Glucose Oxygen gas As the human demand for energy grows Fossil fuel supplies are dwindling Energy plantations Are being planted to serve as a renewable energy source 1
AN OVERVIEW OF PHOTOSYNTHESIS 7.1 Autotrophs are the producers of the biosphere Plants are autotrophs Producing their own food and sustaining themselves without eating other organisms Plants, algae, and some bacteria are photoautotrophs Producers of food consumed by virtually all organisms Figure 7.1A D 7.2 Photosynthesis occurs in chloroplasts In plants, photosynthesis Occurs primarily in the leaves, in the chloroplasts, which contain stroma, and stacks of thylakoids called grana Leaf Cross Section Mesophyll Cell Leaf LM 2,600 Mesophyll Vein Stoma CO2 O 2 TEM 9,750 Figure 7.2 Grana Stroma Stroma Granum Thylakoid Thylakoid space Outer membrane Inner membrane Intermembrane space 2
7.3 Plants produce O 2 gas by splitting water The O 2 liberated by photosynthesis Is made from the oxygen in water Experiment 1 6 CO 2 + 12 H 2 O Experiment 2 6 CO 2 + 12 H 2 O + + C 6 H 12 O 6 6 H 2 O 6 O 2 Not labeled + + C 6 H 12 O 6 6 H 2 O 6 O 2 Labeled Reactants: 6 CO 2 12 H 2 O Figure 7.3A C Products: C 6 H 12 O 6 6 H 2 O 6 O 2 7.4 Photosynthesis is a redox process, as is cellular respiration In photosynthesis H 2 O is oxidized and CO 2 is reduced Reduction 6 CO 2 + 6 H2 O C 6 H 12 O 6 + 6 O 2 Oxidation Oxidation C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O Figure 7.4A, B Reduction 7.5 Overview: Photosynthesis occurs in two stages linked by and NADPH The complete process of photosynthesis consists of two linked sets of reactions The light reactions and the Calvin cycle 3
The light reactions Convert light energy to chemical energy and produce O2 The Calvin cycle assembles sugar molecules from CO2 Using and NADPH from the light reactions H2O CO2 NADP+ ADP + P CYCLE (in stroma) LIGHT REACTIONS (in thylakoids) Ele Figure 7.5 ctr on s NADPH O Sugar THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions Certain wavelengths of visible light, absorbed by pigments Drive the light reactions of photosynthesis Increasing energy 10 5 nm 10 3 nm Gamma rays 103 nm 1 nm X-rays UV 106 nm 1m Microwaves Infrared 103 m Radio waves Visible light 380 400 500 600 700 Reflected light 750 Wavelength (nm) 650 nm Figure 7.6A, B Absorbed light Transmitted light 7.7 Photosystems capture solar power Thylakoid membranes contain multiple photosystems That absorb light energy, which excites electrons Figure 7.7A 4
Each photosystem consists of -harvesting complexes of pigments A reaction center with a primary electron acceptor that receives excited electrons from a reaction-center chlorophyll Excited state e Heat Thylakoid membrane Energy of electron Photosystem (fluorescence) Ground state Primary electron acceptor To electron transport chain e Pigment molecules Chlorophyll molecule Figure 7.7B, C -harvesting Reaction center complexes Transfer of energy Chlorophyll a molecule 7.8 In the light reactions, electron transport chains generate and NADPH Two connected photosystems absorb photons of light And transfer the energy to chlorophyll P680 and P700 The excited electrons Are passed from the primary electron acceptor to electron transport chains Photosystem II Photosystem I NADP+ + H+ 1 e 2 e 4 5 P700 P680 Thylakoid space Figure 7.8A NADPH 6 Thylakoid membrane Stroma 3 H2O 1 O2 + 2 H+ 2 Electron transport chain Provides energy for synthesis of by chemiosmosis 5
e e e Electrons shuttle from photosystem II to I Providing energy to make Electrons from photosystem I Reduce NADP + to NADPH e e e NADPH Mill makes e Figure 7.8B Photosystem II Photosystem I Photosystem II regains electrons by splitting water Releasing O 2 7.9 Chemiosmosis powers synthesis in the light reactions The electron transport chain Pumps H + into the thylakoid space 6
The diffusion of H + back across the membrane through synthase Powers the phosphorylation of ADP to produce (photophosphorylation) Stroma (low H + concentration) H + H H + ADP + P + NADP + + H + NADPH H + Thylakoid membrane H 2O H + H + O 2 + 2 H + H+ H + H+ H + Photosystem II Electron Photosystem I transport chain H + H + synthase Figure 7.9 Thylakoid space (high H + concentration) THE CYCLE: CONVERTING CO 2 TO SUGARS 7.10 and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle Occurs in the chloroplast s stroma Consists of carbon fixation, reduction, release of G3P, and regeneration of RuBP Input CO 2 NADPH CYCLE Figure 7.10A Output: G3P Using carbon from CO 2, electrons from NADPH, and energy from The cycle constructs G3P, which is used to build glucose and other organic molecules Figure 7.10B Step 1 Carbon fixation. An enzym e Input: caled rubisco combines CO 2 with a five-carbon sug ar called!!!!!!!!!!!!!! ribulo se bisphosphate (abbreviated RuBP). The unstable product splits into two molecules of the three-carb on organic acid, 3-phosp hoglyceric acid (3-PGA). For three CO 2 entering, six 3-PGA result. Step 2 Reduction. Two chemical reactions (in dicated b y the two blu e arrows) consume energy from six molecules of and oxidize six molecule s of NADPH. Six m olecules of 3- PGA are reduced, producing six molecules of the energyrich three-carbon sugar, G3P Step 3 Release of one molecule of G3P. Five of the G3Ps from step 2 re main in the cycle. The sing le molecule of G3P you see leaving the cycle is the net product of photosynthesi s. A plant cell uses two G3P molecules to make one molecule of glucose. Step 4 Regeneration of RuB P. A series of chemical reactions uses energy from to rearrange the atoms in the five G3P molecu les (15 carbons total), forming three RuBP molecules (1 5 ca rbons).these can start another turn of the cycle.!!!!! 3 3 P 3 ADP RuBP 4 5 P G3P 3 CO 2 CYCLE 1 In a reaction catalyzed by rubisco, CO 2 is added to RuBP. P 6 P 3 Output: 1 P G3P 2 3-PGA 6 P G3P Glucose and other compounds 6 6 ADP + P 6 NADPH 6 NADP + 7
PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules H 2 O CO 2 NADP + ADP Photosystem II Electron transport chains Thylakoid Photosystem I membranes + P RUBP CYCLE 3-PGA (in stroma) Stroma NADPH G3P Cellular respiration Cellulose Figure 7.11 O 2 LIGHT REACTIONS Sugars CYCLE Starch Other organic compounds 7.12 C 4 and CAM plants have special adaptations that save water In C 3 plants a drop in CO 2 and rise in O 2 when stomata close on hot dry days Divert the Calvin cycle to photorespiration C 4 plants first fix CO 2 into a four-carbon compound That provides CO 2 to the Calvin cycle Mesophyll cell CO 2 4-C compound CO 2 CYCLE Figure 7.12 (left half) Sugarcane Bundle-sheath cell 3-C sugar C 4 plant 8
CAM plants open their stomata at night Making a four-carbon compound used as a CO2 source during the day CO 2 CO2 Night 4-C compound CO2 CYCLE 3-C sugar Figure 7.12 (right half) Day Pineapple CAM plant PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH S ATMOSPHERE CONNECTION 7.13 Photosynthesis moderates global warming Greenhouses used to grow plants Trap solar radiation, raising the temperature inside Figure 7.13A Excess CO2 in the atmosphere Is contributing to global warming Some heat energy escapes into space Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases Figure 7.13B 9
Photosynthesis, which removes CO 2 from the atmosphere Moderates this warming TALKING ABOUT SCIENCE 7.14 Mario Molina talks about Earth s protective ozone layer Figure 7.14A Solar radiation converts O 2 high in the atmosphere to ozone (O 3 ) Which shields organisms on the Earth s surface from the damaging UV radiation 10
Industrial chemicals called CFCs have caused dangerous thinning of the ozone layer But international restrictions on CFC use are allowing recovery Southern tip of South America Antarctica Figure 7.14B 11