4.3 photosynthesis in detail key concept photosynthesis requires a series of chemical reactions

4.3 Photosynthesis in Detail

KEY CONCEPT Photosynthesis requires a series of chemical reactions.


Objectives• Describe the light-dependent reactions in which energy is

captured.• Describe the light-independent reactions in which sugar

is produced.


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.


The first stage of photosynthesis captures and transfers energy.

• The light-dependent reactions include groups of molecules called photosystems.


• 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


• 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


• The light-dependent reactions produce ATP.

– hydrogen ions flow through a channel in the thylakoid membrane

– ATP synthase attached to the channel makes ATP


• 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.


• 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


– 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.


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.


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.


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.

4.3 photosynthesis in detail key concept photosynthesis requires a series of chemical reactions

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