Chapter 7 Photosynthesis Using Light to Make Food

Chapter 7 Photosynthesis: Using Light to Make Food 0

DO NOW Write 5 things that you ate in the past 24 hours.

0 • Photosynthesis is the process by which certain organisms use light energy to make sugar and oxygen gas from carbon dioxide and water • http: //www. youtube. com/watch? v=C 1_uez 5 WX 1 o 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

0 • As the human demand for energy grows o Fossil fuel supplies are dwindling • Energy plantations o Are being planted to serve as a renewable energy source

AN OVERVIEW OF PHOTOSYNTHESIS 0 7. 1 Autotrophs are the producers of the biosphere • Plants are autotrophs o Producing their own food and sustaining themselves without eating other organisms

• Producers o Are autotrophs. o Manufacture the biosphere's food supply. o Make organic food molecules from simple materials. o Sustain themselves without eating. • Consumer o Must eat in order to survive. raw

0 • Plants, algae, and some bacteria are photoautotrophs o Produce organic molecules from inorganic molecules. o Utilize light as their energy source Figure 7. 1 A–D

7. 2 Photosynthesis occurs in chloroplasts • In plants, photosynthesis occurs primarily in the leaves. Mesophyll Cell Leaf Cross Section Leaf Mesophyll LM 2, 600 Chloroplast Vein Stoma CO 2 Chloroplast Outer membrane TEM 9, 750 Grana Figure 7. 2 Stroma Inner membrane Stroma Intermembrane space Granum Thylakoid space

• Stomata (Stoma)-tiny pores on the underside of a leaf that allow carbon dioxide, oxygen, and water to enter/exit. • Mesophyll-green tissue on the interior of the leaf • Chloroplast-where photosynthesis occurs • Chlorophyll-light-absorbing pigment

• Chloroplasts o Composed of two membranes with an intermembrane space o Stroma- Thick fluid where sugars are made o Thylakoid Membrane- system of interconnected membranous sacs. where chlorophyll is found o Grana- disklike membranous sacs arranged in stacks.

7. 3 Plants produce O 2 gas by splitting water • The oxygen released into the air as a product of photosynthesis comes from water. Experiment 1 6 CO 2 + 12 H 2 O Experiment 2 6 CO 2 + 12 H 2 O Products: C 6 H 12 O 6 + 6 H 2 O + 6 O 2 Labeled 6 CO 2 Reactants: Figure 7. 3 A–C C 6 H 12 O 6 + 6 H 2 O + 6 O 2 Not labeled C 6 H 12 O 6 12 H 2 O 6 O 2

0 7. 4 Photosynthesis is a redox process, as is cellular respiration • In photosynthesis H 2 O is oxidized and CO 2 is reduced • Light is the source of energy that provides the boost for electrons during photosynthesis Reduction 6 CO 2 6 H 2 O C 6 H 12 O 6 O 2 6 Oxidation Figure 7. 4 A, B C 6 H 12 O 6 O 2 6 6 CO 2 6 H 2 O Reduction

0 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 o The light reactions and the Calvin cycle

• Light Reactions o Occurs in the thylakoid membrane o Convert light energy to chemical energy produce O 2 gas as a waste product o ATP, NADPH, O 2 are produced and

• Calvin Cycle o Occurs in the stroma o Cyclic series of reactions that assemble sugar molecules using CO 2 and the energy-containing products of the light reactions. o Glucose, ADP, NADP+ are produced. o Carbon Fixation- occurs when carbon and oxygen from CO 2 are incorporated into an organic molecule.

0 • The light reactions o o o Convert light energy to chemical energy and produce O 2 The Calvin cycle assembles sugar molecules from CO 2 Using ATP and NADPH from the light reactions H 2 O CO 2 Chloroplast Light NADP+ ADP +P LIGHT REACTIONS (in thylakoids) Electrons Figure 7. 5 O CALVIN CYCLE (in stroma) ATP NADPH Sugar

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 0 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 X-rays 1 nm 103 nm UV 1 m 106 nm Microwaves Infrared 103 m Radio waves Light Visible light 380 400 500 600 700 Reflected light 750 Wavelength (nm) 650 nm Figure 7. 6 A, B Chloroplast Absorbed light Transmitted light

• Electromagnetic Energy- energy that travels in waves. o Ex. Sunlight • Chlorophyll a-Participates directly in the light reactions. o Ex. Most plants are green because Chlorophyll a reflects green light. • Chlorophyll b-Passes absorbed energy to chlorophyll a. • Photon-fixed quantity of light energy

0 7. 7 Photosystems capture solar power • Thylakoid membranes contain multiple photosystems, that absorb light energy, which excites electrons Figure 7. 7 A

0 • Each photosystem consists of o Light-harvesting complexes of pigments o A reaction center with a primary electron acceptor that receives excited electrons from a reaction-center chlorophyll § The reaction centers of photosystem I and II absorb slightly different wavelengths of light. e– Excited state Photosystem Light-harvesting Reaction Photon complexes center Heat Energy of Photon electron Figure 7. 7 B, C Photon (fluorescence) Ground state Chlorophyll molecule Thylakoid membrane Primary electron acceptor To electron transport chain e– Transfer of energy Chlorophyll a molecule Pigment molecules

0 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. • Electrons removed from water molecules pass from photosystem II to photosystem I to NADP+.

• The excited electrons o Are passed from the primary electron acceptor to electron transport chains Photon Photosystem II Stroma Photosystem I NADP+ + H+ 1 6 e– 2 e– Thylakoid membrane 4 5 P 700 P 680 Thylakoid space Figure 7. 8 A 3 H 2 O 1 O 2 + 2 H+ 2 Electron transport chain Provides energy for synthesis of by chemiosmosis ATP NADPH

• Electrons shuttle from photosystem II to I o Providing energy to make ATP • Electrons from photosystem I o Reduce NADP+ to NADPH e– ATP e– e– NADPH e– e– Mill makes ATP e– Photon Figure 7. 8 B Photosystem II Photosystem I

• As a result of the cascade of electrons down the electron transport chains of the light reactions, NADP+ is reduced to NADPH. • Photosystem II regains electrons by splitting water, releasing O 2 • Water molecules NADP+ which is reduced to NADPH, the source of electrons in the Calvin Cycle

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)

• The final electron acceptor in oxidative phosphorylation is O 2, while it is NADP+ in photophosphorylation. Chloroplast Stroma (low H+ concentration) H+ Light ADP + H+ NADP+ + H+ P H+ NADPH H+ Thylakoid membrane H 2 O 1 2 H+ O 2 2+ H+ Photosystem II Thylakoid space (high H+ concentration) Figure 7. 9 H+ Electron transport chain H+ H+ Photosystem I H+ H+ H+ ATP synthase ATP

• Chemiosmosis • http: //www. youtube. com/watch? v=e. Y 1 Reqi. Yw. Y s • Transition • http: //www. youtube. com/watch? v=Bdcf. Hn 0 z. AU&feature=related

THE CALVIN CYCLE: CONVERTING CO 2 TO SUGARS 7. 10 ATP and NADPH power sugar synthesis in the Calvin cycle • The Calvin cycle occurs in the chloroplast’s stroma Input CO 2 ATP NADPH CALVIN CYCLE Figure 7. 10 A Output: G 3 P

• The cycle constructs G 3 P, an energy-rich molecule that a plant cell can then use to make glucose or other organic molecules. 1 Input: 3 CO 2 1 3 P P P 6 Ru. BP 2 3 -PGA 6 3 ADP 3 ATP CALVIN CYCLE 4 6 ADP + 6 NADPH 6 NADP+ 2 3 5 P 6 G 3 P P G 3 P 3 4 Figure 7. 10 B Output: 1 P G 3 P ATP Glucose and other compounds P

PHOTOSYNTHESIS REVIEWED AND EXTENDED 7. 11 Review: Photosynthesis uses light energy to make food molecules • The ultimate source of all the food we eat and the oxygen we breathe is photosynthesis. H 2 O CO 2 Chloroplast Light NADP+ ADP + P Thylakoid membranes Photosystem II Electron transport chains Photosystem I CALVIN CYCLE (in stroma) Figure 7. 11 LIGHT REACTIONS 3 -PGA ATP NADPH O 2 RUBP Stroma G 3 P Sugars CALVIN CYCLE Cellular respiratio Cellulose Starch Other org compoun

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 and CAM plants help the plant conserve water and synthesize glucose efficiently under hot, dry conditions.

• 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 CALVIN CYCLE Sugarcane Figure 7. 12 (left half) Bundle-sheath cell 3 -C sugar C 4 plant

• CAM plants open their stomata at night making a four-carbon compound used as a CO 2 source during the day CO 2 Night 4 -C compound CO 2 CALVIN CYCLE 3 -C sugar Figure 7. 12 (right half) CAM plant Day Pineapple

• Do photosynthesizing plants have mitochondria? Why or why not? • Yes, to supply the plant with the ATP needed to power various cell activities.

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE CONNECTION 7. 13 Photosynthesis moderates global warming • Greenhouses used to grow plants o Trap solar radiation, raising the temperature inside Figure 7. 13 A

• Excess CO 2 in the atmosphere is contributing to global warming Some heat energy escapes into space Sunlight ATMOSPHERE Radiant heat trapped by CO 2 and other gases Figure 7. 13 B

• 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. 14 A

• Solar radiation converts O 2 high in the atmosphere to ozone (O 3) o Which shields organisms on the Earth’s surface from the damaging UV radiation

• Industrial chemicals called CFCs (chlorofluorocarbons) have caused dangerous thinning of the ozone layer o But international restrictions on CFC use are allowing recovery Southern tip of South America Antarctica Figure 7. 14 B