Photosynthesis!!!!
Photosynthesis is divided into two parts
The overall reaction in
photosynthesis:
6CO2 + 12 H2O + Light
energy
C6H12O6 + 6O2 + 6 H2O
Two components:
Light-dependent reactions
Light
energy
H2O
O2
Chemical
energy
(ATP,
NADPH)
Energy Harvest
Light-independent reactions
Chemical
energy
(ATP,
NADPH)
CO2
Synthesis
Chemical
energy
(C6 H12O6)
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the
Calvin cycle (Layer 3)
Photosynthesis starts in the thylakoid membrane
These aggregations
of pigment molecules
are called
photosystems
Figure 10.11 How a photosystem harvests light
The energy from light is captured and
converted in chloroplasts
• Energy from light excites an electron in chlorophyll. This
electron is then passed to acceptor molecules to be used to
make ATP and NADPH
Stroma
Light
photon
Thylakoid
Chloroplast
Electron acceptor
molecule
e-
Reaction center
(chlorophyll a )
Photosystem
Thylakoid
space
Stroma
There are 2 types of reaction centers in
green algae and plants:
Photosystem I: activated by light of wavelengths of 700nm
Photosystem II: activated by light of wavelengths of 680nm
These photosystems are named after the order in
which they were discovered, not the order in which
they act during photosynthesis
Noncyclic electron flow during the light reactions generates ATP and NADPH
The electrons in photosystem II are replaced by electrons from water
The electrons from chlorophyll are passed to an ETC which makes ATP
The electrons reach photosystem I where another photon of light excites them
once again
The electrons from photosystem II are passed down a second ETC, in which
they are used to make NADPH
Figure 10.13 A mechanical analogy for the light reactions
Figure 10.16 The light reactions and chemiosmosis: the organization of the thylakoid
membrane
Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts
Sometimes an alternate pathway happens
In cyclic electron transport, which
drives cyclic photophosphorylation,
photosystem I transfers electrons to
plastoquinone (PQ).
Summary of the Light Dependent
Reaction of Photosynthesis
• Light energy is absorbed by chlorophyll molecules,
which excites electrons
• The energy from the excited electrons is converted to
chemical energy:
– Photosystem II:
• electron transport chain produces ATP
• The excited electrons are replaced by splitting a water molecule,
releasing O2
– Photosystem I:
• The excited electron is donated to NADP+ to produce NADPH
Photosynthesis is a two-step
process
• The light independent reactions (Calvin
cycle) use the energy in ATP and
NADPH to reduce organic compounds
to form glucose.
Figure 10.17 The Calvin cycle (Layer 1)
Figure 10.17 The Calvin cycle (Layer 2)
Figure 10.17 The Calvin cycle (Layer 3)
Summary
• Light Dependent Reaction
– Light + chlorophyll --> ATP + NADPH + (O2
as waste)
• Light Independent Reaction (Calvin
Cycle)
– CO2 + ATP + NADPH --> glucose
What happens to the glucose
produced by photosynthesis?
Sucrose
CH2OH
O
H
O
HOCH2
H
H
H
OH
H
O
H
HO
CH2OH
HO
H
OH
Glucose
subunit
HO
H
Fructose
subunit
Starch
CH2OH
O H
H
O
CH2OH
H
OH
H
H
OH
Glucose
subunit
O H
H
O
CH2OH
H
OH
H
H
OH
Glucose
subunit
O H
H
O
H
OH
H
H
OH
Glucose
subunit
O
Up to 1000
or more
monomers
MMM… Starch