The Reactions of Photosynthesis
8-3
Inside a Chloroplast
• Where photosynthesis takes place
• Contains thylakoids, which are sac-like
photosynthetic membranes
• Thyalkoids are arranged in stacks called
grana
• Thylakoids contain proteins which
organize chlorophyll and other pigments
into clusters called photosystems
– Photosystems are light-collecting units of
chloroplasts
• The reactions of the photosystems are
divided into 2 types:
– Light-dependent reactions
– Light-independent reactions, or Calvin
Cycle
• Light-dependent reactions take place in
the thylakoid membranes
• Light-independent reactions take place
in the stroma, or region outside the
thylakoids
Electron Carriers
• When sunlight strikes chlorophyll, it
transforms the electrons into a highenergy state
• These high-energy electrons are like
hot coals and need a special electron
carrier
• Carrier molecules are a compound
capable of transporting these highenergy electrons, along with most of
their energy to other molecules
• This process is known as electron
transport
• Electron carriers are known as electron
transport chain
• The molecule NADP+ serves as an
electron carrier
– Its job is to hold 2 high-energy electrons along
with a hydrogen ion (H+)
• This converts NADP+ to NADPH
– This conversion is one way in which some energy
from sunlight is trapped in chemical form
• NADPH then transfers the high-energy
electrons to chemical reactions
elsewhere in the cell
Light-Dependent Reactions
• Require light to happen
• Converts ADP to ATP and NADP+ to
NADPH
• Produces oxygen gas
Step A
• Photosystem II absorbs light
• Light energy is absorbed by electrons,
increasing energy levels
• Electrons are passed to electron
transport chain (ETC)
• Enzymes on inner surface of thylakoid
membrane break up each water
molecule into 2 electrons, 2 H+ ions,
and 1 oxygen atom
– 2 e- replace 2 high-energy e- lost to ETC
– Oxygen released to air
– H+ ions are released inside the thylakoid
membrane
• This reaction is source of nearly all
oxygen in the atmosphere
Step B
• High-energy electrons move through
ETC from photosystem II to
photosystem I
• Energy from the e- is used by ETC to
transport H+ ions from stroma to inner
thylakoid space
Step C
• Pigments in photosystem I use energy
from light to reenergize e-
• NADP+ picks up high-energy electrons
and H+ions to become NADPH
Step D
• More H+ ions are pumped across the
membrane
• Inside of the membrane becomes positively
charged and the outside of the membrane
becomes negatively charged
• The differences in charges provide energy to
make ATP
Step E
• In order for H+ ions to move across the
membrane, the protein ATP synthase is
needed
• As the H+ ions pass through ATP
synthase, the protein rotates
• As it rotates, ATP synthase binds ADP
and a phosphate together to make ATP
• Because of this system, light-dependent
electron transport produces not only
high-energy electrons, but ATP as well
• In summary, in light-dependent
reactions:
– Plants use water, ADP, and NADP+
– They produce oxygen, and 2 high-energy
compounds: ATP and NADPH