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10.1 What is photosynthesis?
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• Light is used by plants, algae, and some bacteria, in a process called photosynthesis, to convert atmospheric carbon (CO2) into carbohydrate.
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1. Where do the oxygen atoms in the O2 produced during photosynthesis come from?
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10.2 Learning about photosynthesis: An experimental journey.
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• A
series of simple experiments demonstrated that plants capture energy from
light and use it to convert the carbon atoms of CO2 and the hydrogen atoms
of water into organic molecules.
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2. How did van Helmont determine that plants do not obtain their food from the soil?
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10.3 Pigments capture energy from sunlight.
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• Light consists of energy packets called photons; the shorter the wavelength of light, the more its energy. When photons are absorbed by a pigment, electrons in the pigment are boosted to a higher energy level.
• Photosynthesis channels photon excitation energy into a single pigment molecule. In bacteria, that molecule then donates an electron to an electron transport chain, which drives a proton pump and ultimately returns the electron to the pigment.
• Plants employ two photosystems. Light energy is first absorbed by photosystem II and passed to photosystem I, driving a proton pump and bringing about the chemiosmotic synthesis of ATP.
• When the electron arrives at photosystem I, another
photon of light is absorbed, and energized electrons are channeled to
a primary electron acceptor, which reduces NADP+ to NADPH.
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3. How is the energy of light captured by a pigment molecule? Why does light reflected by the pigment chlorophyll appear green?
4. What is the function of the reaction center chlorophyll? What is the function of the primary electron acceptor?
5. Explain how photosynthesis in the sulfur bacteria is a cyclic process. What is its energy yield in terms of ATP molecules synthesized per electron?
6. How do the two photosystems in plants and algae work? Which stage generates ATP and which generates NADPH?
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10.4 Cells use the energy and reducing power captured by the light reactions to make organic molecules.
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• The ATP and reducing power produced by the light reactions are used to fix carbon in a series of reactions called the Calvin cycle.
• RuBP carboxylase, the enzyme that fixes carbon in the Calvin cycle, also carries out an oxidative process called photorespiration.
• Many tropical plants inhibit photorespiration by expending
ATP to increase the intracellular concentration of CO2. This process,
called the C4 pathway, nearly doubles the energetic cost of synthesizing
glucose.
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7. In a C3 plant, where do the light reactions occur? Where does the Calvin cycle occur?
8. What is photorespiration? What advantage do C4 plants have over C3 plants with respect to photorespiration? What disadvantage do C4 plants have that limits their distribution primarily to warm regions of the earth?
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