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Photosynthesis
– Process that feeds the biosphere
– Process that converts solar energy into chemical energy
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• Photosynthesis
– Occurs in plants, algae, certain protists, and some prokaryotes
(a) Plants
(b) Multicellular algae
(c) Unicellular protist 10 µm
40 µm(d) Cyanobacteria
1.5 µm(e) Pruple sulfurbacteria
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Classification of Organisms
1) Photoautotrophs
– Use the energy of sunlight to make organic molecules from water and carbon dioxide
– Are the producers of the biosphere
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2. Heterotrophs
– Obtain their organic material from other organisms
– Are the consumers of the biosphere
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• The leaves of plants
– Are the major sites of photosynthesis
Vein
Leaf cross section
Mesophyll
CO2 O2Stomata
Where Photosynthesis Happens?
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Chloroplasts
– Are the organelles in which photosynthesis occurs
– Contain thylakoids and grana
Chloroplast
Mesophyll
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
ThylakoidGranumStroma
1 µm
Chloroplasts: The Sites of Photosynthesis in Plants
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• Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2 O
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Splitting of Water
• Chloroplasts split water into
– Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
6 CO2 12 H2OReactants:
Products: C6H12O66 H2O 6 O2
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Photosynthesis as a Redox Process
• Photosynthesis is a redox process
– Water is oxidized, carbon dioxide is reduced
Redox reactions - Transfer electrons from one reactant to another by oxidation and reduction
• In oxidation - A substance loses electrons, or is oxidized
• In reduction - A substance gains electrons, or is reduced
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• Examples of redox reactions
Na + Cl Na+ + Cl–
becomes oxidized(loses electron)
becomes reduced(gains electron)
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The Two Stages of Photosynthesis: A Preview
1) The Light Reactions
– Occur in the grana
– Split water, release oxygen, produce ATP, and form NADPH
2) The Calvin Cycle
– Occurs in the stroma
– Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power
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Photosynthesis
Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere
Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions
O2
CO2H2O
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP 3-Phosphoglycerate
Amino acidsFatty acids
Starch(storage)
Sucrose (export)
G3P
Photosystem IIElectron transport chain
Photosystem I
Chloroplast
Figure 10.21
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The Nature of Sunlight
• Light
– Is a form of electromagnetic energy, which travels in waves
• Wavelength
– Is the distance between the crests of waves
– Determines the type of electromagnetic energy
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• The electromagnetic spectrum
– Is the entire range of electromagnetic energy, or radiation
Gammarays X-rays UV Infrared
Micro-waves
Radiowaves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m
106 nm 103 m
380 450 500 550 600 650 700 750 nm
Visible light
Shorter wavelength
Higher energy
Longer wavelength
Lower energyFigure 10.6
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• The visible light spectrum
– Includes the colors of light we can see
– Includes the wavelengths that drive photosynthesis
• Pigments
– Are substances that absorb visible light
Photosynthetic Pigments – light receptors
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– Reflect light, which include the colors we see
Light
ReflectedLight
Chloroplast
Absorbedlight
Granum
Transmittedlight
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Types of Pigments
• Chlorophyll a
– Is the main photosynthetic pigment
• Chlorophyll b
– Is an accessory pigment
• Other accessory pigments
– Absorb different wavelengths of light and pass the energy to chlorophyll a
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Organic compounds produced by photosynthesis
– Provide the energy and building material for ecosystems