The Process of Photosynthesis
Learning Objectives
 Describe what happens during the light-dependent
reactions.
 Describe what happens during the light-independent
reactions.
 Identify factors that affect the rate at which photosynthesis
occurs.
Two Sets of Reactions
Photosynthesis involves two primary sets of reactions:
light-dependent reactions and light-independent reactions.
Light-Dependent Reactions
Light-dependent reactions use energy from sunlight to produce
ATP, NADPH, and oxygen.
Thylakoid
Thylakoid
Thylakoids contain clusters of chlorophyll and proteins known
as photosystems.
Light-Dependent Reactions
• Use energy from sunlight to produce oxygen
• Convert ADP and NADP+ into energy carriers ATP and NADPH
hydrogen ion
movement /
ATP formation
photosystem I
photosystem II
electron transport chain
Photosystem II
• Begins when light strikes the pigments.
•The electrons get energized are passed down an electron
transport chain (ETC)
• To replace the electrons that are sent to the ETC, water is used.
•Photolysis splits the water; keeps hydrogens in the thylakoids
and releases oxygen into the air.
Electron Transport Chain
• A series of proteins in the thylakoid membranes that pump
hydrogens from the stroma into the thylakoid
Photosystem I
•Electrons from the ETC reach Photosystem I with less energy.
•Pigments in PSI will reenergize them
• Second electron transport chain transfers electrons to
NADP+, producing NADPH
ATP Formation
The difference in both charge and H+ ion concentration across the
membrane provides the energy to make ATP.
• Hydrogens inside the thylakoids will leave through an
enzyme; ATP Synthase
• This movement spins the enzyme and provides energy to
join a phosphate to an ADP and produce ATP.
Light-Dependent Reactions Summary
The light-dependent reactions produce oxygen gas and convert
ADP and NADP+ into the energy carriers ATP and NADPH.
Light-Independent Reactions
• The second phase of photosynthesis does not require
light and is called the Calvin Cycle.
• The Calvin Cycle occurs in the stroma of the chloroplast.
• The Calvin Cycle uses the ATP and NADPH that was built
during the light-dependent reactions
Stroma
Carbon Dioxide Enters the Cycle
Uses Carbon Dioxide
from the air
Uses another ATP
to replenish RuBP
Builds a glucose molecule
Uses ATP and
NADPH from
light reactions
Photosynthesis Equation
Light Energy
6CO2 + 6H2O
Carbon Dioxide
from the air –
Used in the
Calvin Cycle
during the Dark
Reactions
C6H12O6 + 6O2
Glucose Made in
the Calvin Cycle
during the Dark
Reactions
Water Split during
Photolysis in the
Light Reactions
Oxygen Released
during Photolysis
in the Light
Reactions
Plants can use this glucose molecule for energy during Cellular Respiration. Plants can
also convert this glucose molecule into other organic compounds such as proteins and
fats/lipids or other carbohydrates like starch and cellulose
Factors Affecting Photosynthesis
Important factors that affect
photosynthesis include
temperature, light intensity,
and availability of carbon
dioxide and water.
Some plants have adapted to
extremely bright, hot conditions:
C4 plants and CAM plants.