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4.3 Photosynthesis in Detail
KEY CONCEPT Photosynthesis requires a series of chemical reactions.
4.3 Photosynthesis in Detail
Objectives• Describe the light-dependent reactions in which energy is
captured.• Describe the light-independent reactions in which sugar
is produced.
4.3 Photosynthesis in Detail
Vocabulary• Photosystem
– Series of light-absorbing pigments and proteins that capture and transfer energy in the thylakoid membrane.
• Electron Transport Chain– Series of proteins in the thylakoid and mitochondrial
membranes that aid in converting ADP to ATP by transferring electrons.
• ATP synthase– Enzyme that catalyzes the reaction that adds a high-energy
phosphate group to ADP to form ATP.• Calvin Cycle
– Process by which a photosynthetic organism uses energy to synthesize simple sugars from CO2.
4.3 Photosynthesis in Detail
The first stage of photosynthesis captures and transfers energy.
• The light-dependent reactions include groups of molecules called photosystems.
4.3 Photosynthesis in Detail
• Photosystem II captures and transfers energy.
– chlorophyll absorbs energy from sunlight
– energized electrons enter electron transport chain
– water molecules are split
– oxygen is released as waste
– hydrogen ions are transported across thylakoid membrane
4.3 Photosynthesis in Detail
• Photosystem I captures energy and produces energy-carrying molecules.
– chlorophyll absorbs energy from sunlight
– energized electrons are used to make NADPH
– NADPH is transferred to light-independent reactions
4.3 Photosynthesis in Detail
• The light-dependent reactions produce ATP.
– hydrogen ions flow through a channel in the thylakoid membrane
– ATP synthase attached to the channel makes ATP
4.3 Photosynthesis in Detail
• Light-independent reactions occur in the stroma and use CO2 molecules.
The second stage of photosynthesis uses energy from the first stage to make sugars.
4.3 Photosynthesis in Detail
• A molecule of glucose is formed as it stores some of the energy captured from sunlight.
– carbon dioxide molecules enter the Calvin cycle– energy is added and carbon molecules are rearranged– a high-energy three-carbon molecule leaves the cycle
4.3 Photosynthesis in Detail
– two three-carbon molecules bond to form a sugar– remaining molecules stay in the cycle
• A molecule of glucose is formed as it stores some of the energy captured from sunlight.
4.3 Photosynthesis in Detail
Question/Answer• Where do the light-dependent reactions occur?
– Thylakoid membrane of chloroplast• What is the source of the electrons in the electron-
transport chain?– Chlorophyll
• How are the electrons released from the water molecules used?– To replace energized electrons that leave the
chlorophyll. What role do these electrons play?• What role do these electrons play?
– They provide energy to move hydrogen ions into the thylakoid and to produce molecules of NADPH.
4.3 Photosynthesis in Detail
Question/Answer• What two energy carriers are produced?
– NADPH and ATP• In what step is active transport occurring? Passive transport?
– Step 3– Step 6
• Where in the chloroplast do light-independent reactions occur?– Stroma
• Where are ATP and NADPH coming from?– Light-dependent reactions
• Why are these reactions called a cycle?– The five-carbon molecule of the process is regenerated, so
the cycle continues.
4.3 Photosynthesis in Detail
Question/Answer• How does the Calvin Cycle build sugar molecules?
– Carbon dioxide is added to five-carbon molecules in the cycle. Energy from ATP and NADPH is used in a series of chemical reactions that build the three-carbon molecules needed to form a six-carbon sugar.