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

To make sugar and oxygen gas from carbon dioxide and water Photosynthesis is the process by which certain organisms use light energy To make sugar and oxygen gas from carbon dioxide and water Light energy PHOTOSYNTHESIS 6 CO2 + H2O Carbon dioxide Water C6H12O6 O2 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

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 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 Leaf Mesophyll Cell Mesophyll Vein Stoma CO2 O2 Chloroplast Grana Stroma TEM 9,750  Granum Thylakoid space Outer membrane Inner Intermembrane LM 2,600  Figure 7.2

7.3 Plants produce O2 gas by splitting water The O2 liberated by photosynthesis Is made from the oxygen in water Reactants: Products: 6 CO2 12 H2O C6H12O6 6 H2O 6 O2 Labeled Experiment 1 Experiment 2 Not labeled + Figure 7.3A–C

7.4 Photosynthesis is a redox process, as is cellular respiration 7.4 Photosynthesis is a redox process, as is cellular respiration In photosynthesis H2O is oxidized and CO2 is reduced Reduction Oxidation 6 O2 6 H2O 6 CO2  C6H12O6 Figure 7.4A, B

7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH The complete process of photosynthesis consists of two linked sets of reactions The light reactions and the Calvin cycle

The Calvin cycle assembles sugar molecules from CO2 The light reactions Convert light energy to chemical energy and produce O2 The Calvin cycle assembles sugar molecules from CO2 Using ATP and NADPH from the light reactions Figure 7.5 Light CO2 H2O Chloroplast LIGHT REACTIONS (in thylakoids) CALVIN CYCLE (in stroma) NADP+ ADP + P ATP NADPH O Sugar Electrons

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 1 nm 103 nm 106 nm 1 m 103 m Gamma rays X-rays UV Infrared Micro- waves Radio Visible light 400 500 600 700 750 650 nm Wavelength (nm) Transmitted light Absorbed Reflected Light Chloroplast 380 Figure 7.6A, B

Absorption Spectra of Photosynthetic Pigments Chapter 5 Section 2 Photosynthesis Absorption Spectra of Photosynthetic Pigments

7.7 Photosystems capture solar power 7.7 Photosystems capture solar power Thylakoid membranes contain multiple photosystems That absorb light energy, which excites electrons Figure 7.7A

Chlorophyll a molecule Each photosystem consists of Light-harvesting complexes of pigments A reaction center with a primary electron acceptor that receives excited electrons from a reaction-center chlorophyll Energy of electron Photon Excited state Heat (fluorescence) Ground state Chlorophyll molecule e– Photosystem Light-harvesting complexes Reaction center Primary electron acceptor To electron transport chain Pigment molecules Chlorophyll a molecule Transfer of energy Thylakoid membrane Figure 7.7B, C

7.8 In the light reactions, electron transport chains generate ATP and NADPH Two connected photosystems absorb photons of light And transfer the energy to chlorophyll P680 and P700

Electron transport chain The excited electrons Are passed from the primary electron acceptor to electron transport chains Thylakoid space Photon Stroma Thylakoid membrane 1 Photosystem I Photon P700 e– 5 Photosystem II e– P680 2 + NADP+ H+ NADPH 6 ATP Electron transport chain Provides energy for synthesis of by chemiosmosis 4 H2O 1 2 + 2 O2 H+ 3 Figure 7.8A

Electrons shuttle from photosystem II to I Providing energy to make ATP Electrons from photosystem I Reduce NADP+ to NADPH Figure 7.8B e– ATP Mill makes Photon Photosystem II Photosystem I NADPH

Photosystem II regains electrons by splitting water Releasing O2

7.9 Chemiosmosis powers ATP synthesis in the light reactions The electron transport chain Pumps H+ into the thylakoid space

The diffusion of H+ back across the membrane through ATP synthase Powers the phosphorylation of ADP to produce ATP (photophosphorylation) Chloroplast Stroma (low H+ concentration) Light NADP+ + H+ NADPH ATP P ADP + Thylakoid membrane H2O 1 2 O2 Photosystem II Electron transport chain Photosystem I ATP synthase Thylakoid space (high H+ concentration) Figure 7.9

THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS 7.10 ATP 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 Figure 7.10A Input CO2 ATP NADPH CALVIN CYCLE G3P Output:

Using carbon from CO2, electrons from NADPH, and energy from ATP The cycle constructs G3P, which is used to build glucose and other organic molecules 3 P CO2 Step Carbon fixation. An enzyme called rubisco combines CO2 with a five-carbon sugar called ribulose bisphosphate (abbreviated RuBP). The unstable product splits into two molecules of the three-carbon organic acid, 3-phosphoglyceric acid (3-PGA). For three CO2 entering, six 3-PGA result. 1 Input: In a reaction catalyzed by rubisco, CO2 is added to RuBP. 6 RuBP 3-PGA 3 G3P 6 P 2 Step Reduction. Two che- mical reactions (indicated by the two blue arrows) consume energy from six molecules of ATP and oxidize six molecules of NADPH. Six molecules of 3-PGA are reduced, producing six molecules of the energy-rich three-carbon sugar, G3P ATP ADP + NADPH NADP+ 3 3 ADP ATP 4 Step Regeneration of RuBP. A series of chemical reactions uses energy from ATP to rearrange the atoms in the five G3P molecules (15 carbons total), forming three RuBP molecules (15 carbons).These can start another turn of the cycle. 5 P G3P CALVIN CYCLE 3 Step Release of one molecule of G3P. Five of the G3Ps from step 2 remain in the cycle. The single molecule of G3P you see leaving the cycle is the net product of photosynthesis. A plant cell uses two G3P molecules to make one molecule of glucose. Output: 1 P G3P Glucose and other compounds Figure 7.10B

PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules Light H2O CO2 NADP+ Photosystem II Photosystem I Electron transport chains ADP P + RUBP CALVIN CYCLE (in stroma) 3-PGA Stroma G3P NADPH ATP O2 LIGHT REACTIONS CALVIN CYCLE Sugars Cellular respiration Cellulose Starch Other organic compounds Thylakoid membranes Chloroplast Figure 7.11

7.12 C4 and CAM plants have special adaptations that save water In C3 plants a drop in CO2 and rise in O2 when stomata close on hot dry days Divert the Calvin cycle to photorespiration

C4 plants first fix CO2 into a four-carbon compound That provides CO2 to the Calvin cycle Figure 7.12 (left half) Sugarcane C4 plant CALVIN CYCLE 3-C sugar CO2 4-C compound Mesophyll cell Bundle-sheath cell

CAM plants open their stomata at night Making a four-carbon compound used as a CO2 source during the day CO2 Figure 7.12 (right half) CAM plant Day CALVIN CYCLE 3-C sugar CO2 4-C compound Night Pineapple

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 Sunlight ATMOSPHERE Some heat energy escapes into space Radiant heat trapped by CO2 and other gases Figure 7.13B

Photosynthesis, which removes CO2 from the atmosphere Moderates this warming

7.14 Mario Molina talks about Earth’s protective ozone layer TALKING ABOUT SCIENCE 7.14 Mario Molina talks about Earth’s protective ozone layer Figure 7.14A

Solar radiation converts O2 high in the atmosphere to ozone (O3) Which shields organisms on the Earth’s surface from the damaging UV radiation

But international restrictions on CFC use are allowing recovery 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