Photosynthesis
General Formula
• CO2 + H2O + light  O2 + C6H12O6
• Photosynthesis is a endothermic
reaction requiring an external source of
energy to drive the production of
organic molecules existing at a higher
energy state than the reactants.
• Entropy
The Chloroplast
• The chloroplast has two
outer membranes and an
inner most thylakoid
membrane.
• The thylakoid membrane
form grana, and are the site
of the light reactions (photo).
• The thylakoid membrane
segregates the part of the
chloroplast space into the
outer stroma and the inner
thylakoid space.
• The dark reactions occur in
the stroma (synthesis).
Light Absorption
• Photosystems are complex protein
molecules that have imbedded
within them a variety of pigments.
• Some of these pigments act as the
light harvesting complex while
others make up the reaction center.
• The light harvesting pigments
increase the range of wavelengths
that the system can absorb (some
pigments are protective as well).
• The reaction center contains
special chlorophyll a pigments that
become oxidized, losing electrons
to a primary electron acceptor.
Oxygen Production or
Photolysis
• Within photosystem II,
radiation is also used to split
water into:
1) Electrons (e-) which replace
those lost by Chlorophyll a (it
becomes reduced again)
2) Hydrogen ions (H+) which
build up inside the thylakoid,
and
3) Oxygen (O2) which is lost
through stomata to the
atmosphere
Electron Transport
• In photosynthesis there are actually two paths for electrons lost
by chlorophyll electrons.
• The predominant path is a non-cyclic pathway where electrons
pass from photosystem II though a series of more electronegative
electron acceptors (specifically plastoquinone (Pq), the
cytochrome complex, and plastocyanin (Pc)), eventually to be
passed through photosystem I, an electron acceptor called
ferredoxin (Fd) where they are passed to the final electron
acceptor NADP+ (nicotinamide adenine dinucleotide phosphate)
creating NADPH. While this is occurring hydrogen ions are being
actively transported inside the thylakoid space through the
cytochrome complex.
• In the non-cyclic pathway the electrons cycle between
photosystem I and portions of the electron transport chain without
the production of NADPH but with the generation of an H+
gradient.
Electron Transport
ATP Formation
•
•
•
Ions can’t diffuse through the
lipid membrane of the thylakoid.
During photosynthesis a H+
gradient is generated that is
1000 time more concentrated
inside the thylakoid (pH or 5
versus 8) producing a source of
potential energy.
Inside the thlakoid membrane
there are proteins called ATP
synthase that use the
movement of hydrogen ions to
power particular conformational
changes that result in the
production of ATP.
The Dark Reactions
•
•
•
The dark reactions are also
called the light independent
reaction or the Calvin-Benson
Cycle.
In these series of reactions
NADPH and ATP from the light
reactions provide the electrons
(and H+) and energy, for
converting CO2 in C6H12O6 and
other organic molecules.
The series of six reactions is
categorized into three steps,
carbon fixation, reduction, and
the regeneration of RuBP.