Light and Dark Reactions

Objectives

• Explain the necessity of photolysis
• Explain the role of ATP synthase

Light is the source of energy for photosynthesis.

When light strikes chlorophyll, electrons jump to higher energy levels, becoming excited. These excited electrons leave the chlorophyll.

The excited electrons are directed through electron transport chains by electron carrier molecules.

To replace the electrons that leave, water is photolyzed, which forms electrons, hydrogen ions, and oxygen gas.

Excited electrons travel through an ETC, which uses the energy to pump protons into the thylakoids.

The build-up of protons in the thylakoids is called a chemiosmotic gradient.

As protons flow back out, ATP synthase uses that energy to make ATP.

When light excites electrons again, they flow through another ETC and eventually form NADPH.

Calvin Cycle

Objectives

• Describe the function of the Calvin cycle
• Explain RuBisCo

Two Parts of the Calvin Cycle

• Carbon dioxide is fixed
• ATP and NADPH provide energy to make sugar

Carbon fixation is the process of converting low-energy carbon dioxide into higher energy organic molecules.

Carbon dioxide is combined with ribulose biphosphate (RuBP) by enzyme RuBisCo to form an unstable 6-carbon molecule.

The 6-carbon compound is so unstable it splits into two 3-carbon molecules.