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Photosynthesis
Using light energy to synthesize biomolecules
Overview of Photosynthesis
Photosynthesis has two parts:
Light-dependent reactions convert light energy
into chemical energy by using light energy to
make ATP and NADPH.
Energy from ATP and NADPH drives lightindependent reactions that produce energy-rich
carbon compounds.
Thinking Question:
Where does wood come from?
Photosynthesis in the leaf
The Chloroplast
Use of light in
photosynthesis
Chemiosmosis in PSII
Chemiosmosis in PSII
Light-independent Reactions
Sunlight
CO2
LightDependent
Reactions
ATP
LightIndependent
Reactions
NADPH
H2O
Water
O2
C6 H12O 6 — P
The Calvin-Benson,
or C3 cycle, is a
cyclical series of
chemical
transformation that
produces a 3-carbon
compound, G3P.
Details of the
Calvin-Benson
Cycle, where
organic GP3 is
synthesized from
inorganic carbon
dioxide. GP3 is
then used to make
glucose, fatty acids,
and amino acids
(with the addition
of nitrogen).
Sunlight
CO2
LightDependent
Reactions
ATP
LightIndependent
Reactions
NADPH
H2O
Water
O2
C6 H12O 6 — P
Relationships
between the
light-dependent
and light
independent
reactions.
Relationships between photosynthesis and cellular
respiration. Both occur in a plant cell.
C3 plants and photorespiration
Plants that live where there is abundant
water use the light-independent reactions
we’ve already looked at: the C3 system.
But if plants are water stressed, their
stomata close, and they can’t get more
carbon dioxide.
Oxygen accumulates, and degrades RuBP in
a wasteful process called photorespiration.
C4 plants and photorespiration
Plants adapted to dry areas have a way of
coping with having to close their stomata
in the heat of the day.
C4 plants capture carbon dioxide and
store in in a 4-carbon compound. They
add a second cycle to the light
independent reactions to release stored
carbon so the C3 cycle can continue.
Summary
Light-dependent reactions use light energy
to produce ATP and NADPH to run the
light-dependent reactions.
Light independent reactions take in
inorganic carbon dioxide and use it to
synthesize organic compounds.