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The Reactions of Photosynthesis
8-3
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Inside a Chloroplast
• Where photosynthesis takes place
• Contains thylakoids, which are sac-like photosynthetic membranes
• Thyalkoids are arranged in stacks called grana
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• Thylakoids contain proteins which organize chlorophyll and other pigments into clusters called photosystems– Photosystems are light-collecting units of
chloroplasts
• The reactions of the photosystems are divided into 2 types:– Light-dependent reactions– Light-independent reactions, or Calvin
Cycle
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• Light-dependent reactions take place in the thylakoid membranes
• Light-independent reactions take place in the stroma, or region outside the thylakoids
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Electron Carriers
• When sunlight strikes chlorophyll, it transforms the electrons into a high-energy state
• These high-energy electrons are like hot coals and need a special electron carrier
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• Carrier molecules are a compound capable of transporting these high-energy electrons, along with most of their energy to other molecules
• This process is known as electron transport
• Electron carriers are known as electron transport chain
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• The molecule NADP+ serves as an electron carrier– Its job is to hold 2 high-energy electrons along with
a hydrogen ion (H+)
• This converts NADP+ to NADPH– This conversion is one way in which some energy
from sunlight is trapped in chemical form
• NADPH then transfers the high-energy electrons to chemical reactions elsewhere in the cell
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Light-Dependent Reactions
• Require light to happen
• Converts ADP to ATP and NADP+ to NADPH
• Produces oxygen gas
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Step A
• Photosystem II absorbs light
• Light energy is absorbed by electrons, increasing energy levels
• Electrons are passed to electron transport chain (ETC)
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• Enzymes on inner surface of thylakoid membrane break up each water molecule into 2 electrons, 2 H+ ions, and 1 oxygen atom– 2 e- replace 2 high-energy e- lost to ETC– Oxygen released to air– H+ ions are released inside the thylakoid
membrane
• This reaction is source of nearly all oxygen in the atmosphere
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Step B
• High-energy electrons move through ETC from photosystem II to photosystem I
• Energy from the e- is used by ETC to transport H+ ions from stroma to inner thylakoid space
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Step C
• Pigments in photosystem I use energy from light to reenergize e-
• NADP+ picks up high-energy electrons and H+ions to become NADPH
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Step D
• More H+ ions are pumped across the membrane
• Inside of the membrane becomes positively charged and the outside of the membrane becomes negatively charged
• The differences in charges provide energy to make ATP
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Step E
• In order for H+ ions to move across the membrane, the protein ATP synthase is needed
• As the H+ ions pass through ATP synthase, the protein rotates
• As it rotates, ATP synthase binds ADP and a phosphate together to make ATP
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• Because of this system, light-dependent electron transport produces not only high-energy electrons, but ATP as well
• In summary, in light-dependent reactions:– Plants use water, ADP, and NADP+– They produce oxygen, and 2 high-energy
compounds: ATP and NADPH