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Chapter 7 Photosynthesis: Using Light to Make Food PowerPoint® Lectures for Campbell Essential Biology, Fifth Edition, and Campbell Essential Biology with Physiology, Fourth Edition – Eric J. Simon, Jean L. Dickey, and Jane B. Reece Lectures by Edward J. Zalisko © 2013 Pearson Education, Inc. Biology and Society: Biofuels • Industrialized societies replaced wood with fossil fuels including – coal, – gas, and – oil. • To limit the damaging effects of fossil fuels, researchers are investigating the use of biomass (living material) as efficient and renewable energy sources. © 2013 Pearson Education, Inc. Biology and Society: Biofuels • There are several types of biofuels. – Bioethanol is a type of alcohol produced by the fermentation of glucose made from starches in crops such as grains, sugar beets, and sugar cane. – Bioethanol may be used – directly as a fuel source in specially designed vehicles or – as a gasoline additive. – Cellulosic ethanol is a type of bioethanol made from cellulose in nonedible plant material such as wood or grass. – Biodiesel is made from plant oils or recycled frying oil. © 2013 Pearson Education, Inc. THE BASICS OF PHOTOSYNTHESIS • Photosynthesis – plants, algae (protists), and some bacteria, – transforms light energy into chemical energy, and – carbon dioxide and water as starting materials. • The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules. • Organisms that use photosynthesis are – photosynthetic autotrophs and – the producers for most ecosystems. © 2013 Pearson Education, Inc. Chloroplasts: Sites of Photosynthesis • Chloroplasts are – the site of photosynthesis and – found mostly in the interior cells of leaves. • Inside chloroplasts are interconnected, membranous sacs called thylakoids, which are suspended in a thick fluid called stroma. • Thylakoids are concentrated in stacks called grana. © 2013 Pearson Education, Inc. Chloroplasts: Sites of Photosynthesis • The green color of chloroplasts is from chlorophyll, a light-absorbing pigment that plays a central role in converting solar energy to chemical energy. • Stomata are tiny pores in leaves where – carbon dioxide enters and – oxygen exits. © 2013 Pearson Education, Inc. Figure 7.2-1 Photosynthetic cells Vein CO2 O2 Stomata Leaf cross section Figure 7.2-2 Chloroplast Photosynthetic cells Vein O2 Stomata LM CO2 Leaf cross section Interior cell Figure 7.2-3 Chloroplast Photosynthetic cells Inner and outer membranes Vein Stroma O2 Granum Stomata Leaf cross section Interior cell Colorized TEM LM CO2 Thylakoid space Figure 7.2a Vein (transports Photosynthetic water and nutrients) cells CO2 O2 Stomata Leaf cross section Figure 7.2b Chloroplast Thylakoid space Granum LM Stroma Inner and outer membranes Colorized TEM Interior cell Figure 7-UN01 Light energy 6 CO2 Carbon dioxide C6H12O6 6 H2O Water Photosynthesis Glucose 6 O2 Oxygen gas The Simplified Equation for Photosynthesis • In the overall equation for photosynthesis, notice that the reactants of photosynthesis are the waste products of cellular respiration. • In photosynthesis, – sunlight provides the energy, – electrons are boosted “uphill” and added to carbon dioxide, and – sugar is produced. © 2013 Pearson Education, Inc. The Simplified Equation for Photosynthesis • During photosynthesis, water is split into – hydrogen and – oxygen. • Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar. • Oxygen escapes through stomata into the atmosphere. © 2013 Pearson Education, Inc. A Photosynthesis Road Map • Photosynthesis occurs in two multistep stages: 1. the light reactions convert solar energy to chemical energy and 2. the Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide. © 2013 Pearson Education, Inc. A Photosynthesis Road Map • The initial incorporation of carbon from the atmosphere into organic compounds is called carbon fixation. – This lowers the amount of carbon in the air. – Deforestation reduces the ability of the biosphere to absorb carbon by reducing the amount of photosynthetic plant life. BioFlix Animation: Photosynthesis © 2013 Pearson Education, Inc. Figure 7.3-1 H2O Chloroplast Light Light reactions ATP – – NADPH O2 Figure 7.3-2 H2O Chloroplast CO2 Light NADP+ ADP P Calvin cycle Light reactions ATP – – NADPH O2 Sugar THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY • Chloroplasts – are chemical factories powered by the sun and – convert sunlight into chemical energy. © 2013 Pearson Education, Inc. Figure 7-UN02 Light CO2 H 2O NADP Light reactions ADP + P Calvin cycle ATP – – NADPH O2 Sugar The Nature of Sunlight • Sunlight is a type of energy called radiation, or electromagnetic energy. • The distance between the crests of two adjacent waves is called a wavelength. • The full range of radiation is called the electromagnetic spectrum. © 2013 Pearson Education, Inc. Figure 7.4 Increasing wavelength 10–5 nm 10–3 nm 1 nm Gamma rays X-rays 103 nm 106 nm Infrared UV 103 m 1m Microwaves Radio waves Visible light 380 400 500 600 Wavelength (nm) Wavelength 580 nm 700 750 Figure 7.5a Light Reflected light Chloroplast Absorbed light Transmitted light (detected by your eye) The Process of Science: What Colors of Light Drive Photosynthesis? • Observation: In 1883, German biologist Theodor Engelmann saw that certain bacteria tend to cluster in areas with higher oxygen concentrations. • Question: Could this information determine which wavelengths of light work best for photosynthesis? • Hypothesis: Oxygen-seeking bacteria will congregate near regions of algae performing the most photosynthesis. © 2013 Pearson Education, Inc. The Process of Science: What Colors of Light Drive Photosynthesis? • Experiment: Engelmann – laid a string of freshwater algal cells in a drop of water on a microscope slide, – added oxygen-sensitive bacteria to the drop, and – used a prism to create a spectrum of light shining on the slide. © 2013 Pearson Education, Inc. The Process of Science: What Colors of Light Drive Photosynthesis? • Results: Bacteria – mostly congregated around algae exposed to redorange and blue-violet light and – rarely moved to areas of green light. • Conclusion: Chloroplasts absorb light mainly in the blue-violet and red-orange part of the spectrum. Animation: Light and Pigments © 2013 Pearson Education, Inc. Figure 7.6 Light Prism Number of bacteria Microscope slide Bacteria Bacteria Algal cells 400 500 600 Wavelength of light (nm) 700 Chloroplast Pigments • Chloroplasts contain several pigments: 1. Chlorophyll a – absorbs mainly blue-violet and red light and – participates directly in the light reactions. 2. Chlorophyll b – absorbs mainly blue and orange light and – participates indirectly in the light reactions. © 2013 Pearson Education, Inc. Chloroplast Pigments • Carotenoids – absorb mainly blue-green light, – participate indirectly in the light reactions, and – absorb and dissipate excessive light energy that might damage chlorophyll. • The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids. © 2013 Pearson Education, Inc. Figure 7.7 How Photosystems Harvest Light Energy • Light behaves as photons, a fixed quantity of light energy. • Chlorophyll molecules absorb photons. – Electrons in the pigment gain energy. – As the electrons fall back to their ground state, energy is released as heat or light. © 2013 Pearson Education, Inc. Figure 7.8 Excited state Absorption of a photon excites an electron. e– The electron falls to its ground state. Heat Light Light (fluorescence) Photon Chlorophyll molecule (a) Absorption of a photon Ground state (b) Fluorescence of a glow stick Figure 7.8a Excited state Absorption of a photon excites an electron. e– The electron falls to its ground state. Heat Light Light (fluorescence) Photon Chlorophyll molecule (a) Absorption of a photon Ground state How Photosystems Harvest Light Energy • In the thylakoid membrane, chlorophyll molecules are organized with other molecules into photosystems. • A photosystem is a cluster of a few hundred pigment molecules that function as a lightgathering antenna. © 2013 Pearson Education, Inc. How Photosystems Harvest Light Energy • The reaction center of the photosystem consists of chlorophyll a molecules that sit next to another molecule called a primary electron acceptor, which traps the light-excited electron from chlorophyll a. • Another team of molecules built into the thylakoid membrane then uses that trapped energy to make – ATP and – NADPH. © 2013 Pearson Education, Inc. Figure 7.9 Chloroplast Cluster of pigment molecules Photon e– Electron transfer Primary electron acceptor Reactioncenter chlorophyll a Pigment molecules Thylakoid membrane Transfer of energy Photosystem Reaction center Figure 7.9b Photon e– Electron transfer Primary electron acceptor Reactioncenter chlorophyll a Pigment molecules Transfer of energy Photosystem Reaction center How the Light Reactions Generate ATP and NADPH • Two types of photosystems cooperate in the light reactions: 1. the water-splitting photosystem and 2. the NADPH-producing photosystem. © 2013 Pearson Education, Inc. Figure 7.10-1 Primary electron acceptor 2e – Light 1 Reactioncenter chlorophyll H2O 2e – 2 H 1 2 O2 Water-splitting photosystem Figure 7.10-2 Primary electron acceptor Energy to make ATP 2 2e – Light 1 Reactioncenter chlorophyll H2O 2e – 2 H 1 2 O2 Water-splitting photosystem Figure 7.10-3 Primary electron acceptor Primary electron acceptor Energy to make ATP 2e – 2e– 3 2 – – NADPH 2e – Light Light Reactioncenter chlorophyll 1 Reactioncenter chlorophyll H2O 2e – 2 H 1 2 O2 NADP Water-splitting photosystem NADPH-producing photosystem How the Light Reactions Generate ATP and NADPH • The light reactions are located in the thylakoid membrane. • An electron transport chain – connects the two photosystems and – releases energy that the chloroplast uses to make ATP. © 2013 Pearson Education, Inc. Figure 7.11 To Calvin cycle Light Light – – NADPH H ATP NADP H Stroma Thylakoid membrane Photosystem Electron transport chain Photosystem ATP synthase Inside thylakoid Electron flow 2e– H2O H 1 2 Thylakoid membrane O2 ADP P H H H H Figure 7.11a H Stroma Electron transport chain Photosystem Thylakoid membrane Inside thylakoid 2e – H2O H 1 2 Thylakoid membrane O2 Figure 7.11b To Calvin cycle Light – – H NADPH ATP NADP H Electron transport chain Photosystem ADP P ATP synthase Electron flow H H H H H Figure 7.12 e– ATP e– e– – – NADPH e– e– e– e– Water-splitting photosystem NADPH-producing photosystem THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE • The Calvin cycle – functions like a sugar factory within a chloroplast and – regenerates the starting material with each turn. Blast Animation: Photosynthesis: Light-Independent Reactions © 2013 Pearson Education, Inc. Figure 7-UN03 Light CO2 H2O NADP Light reactions ADP + P Calvin cycle ATP – – NADPH O2 Sugar Figure 7.13-1 CO2 (from air) 1 P RuBP sugar Three-carbon molecule P P Calvin cycle Figure 7.13-2 CO2 (from air) 1 P RuBP sugar Three-carbon molecule ATP P P ADP P Calvin cycle – – NADPH NADP G3P sugar P 2 Figure 7.13-3 CO2 (from air) 1 P RuBP sugar Three-carbon molecule ATP P P ADP P Calvin cycle – – NADPH NADP G3P sugar 3 G3P sugar P P G3P sugar P 2 Glucose (and other organic compounds) Figure 7.13-4 CO2 (from air) 1 P RuBP sugar Three-carbon molecule 4 ATP P P ADP P ADP P Calvin cycle – – NADPH ATP NADP G3P sugar 3 G3P sugar P P G3P sugar P 2 Glucose (and other organic compounds) Evolution Connection: Solar-Driven Evolution • C3 plants - use CO2 directly from the air – are very common and widely distributed. • C4 plants - close their stomata to save water during hot and dry weather – can still carry out photosynthesis. • CAM plants - are adapted to very dry climates – open their stomata only at night to conserve water. © 2013 Pearson Education, Inc. Figure 7.14 ALTERNATIVE PHOTOSYNTHETIC PATHWAYS C4 Pathway (example: sugarcane) Cell type 1 Cell type 2 CAM Pathway (example: pineapple) CO2 CO2 Four-carbon compound Four-carbon compound CO2 CO2 Calvin cycle Calvin cycle Sugar Sugar C4 plant CAM plant Night Day Figure 7.14a C4 Pathway (example: sugarcane) Cell type 1 Cell type 2 CAM Pathway (example: pineapple) CO2 CO2 Four-carbon compound Four-carbon compound CO2 CO2 Calvin cycle Calvin cycle Sugar Sugar C4 plant CAM plant Night Day Figure 7-UN04 Light energy 6 CO2 6 H2O C6H12O6 6 O2 Glucose Oxygen gas Photosynthesis Carbon dioxide Water Figure 7-UN05 Chloroplast Light CO2 H2O Stack of thylakoids NADP Light reactions ADP P Stroma Calvin cycle ATP – – NADPH O2 Sugar Sugar used for • cellular respiration • cellulose • starch • other organic compounds Figure 7-UN06 ADP e– acceptor ATP e– 2e– acceptor 2e– – – NADPH 2e – Photon Photon Chlorophyll H2O Chlorophyll 2e– 2H + 1 O 2 2 NADP Water-splitting photosystem NADPH-producing photosystem Figure 7-UN07 CO2 ADP ATP Calvin cycle – – NADPH G3P P P NADP Glucose and other compounds (such as cellulose and starch)
