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 energycarrying 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
The second stage of photosynthesis uses energy from
the first stage to make sugars.
• Light-independent
reactions occur in the
stroma and use CO2
molecules.
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
• A molecule of glucose is formed as it stores some of the
energy captured from sunlight.
– two three-carbon molecules bond to form a sugar
– remaining molecules stay in the cycle
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 electrontransport 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 threecarbon molecules needed to form a six-carbon sugar.