Photosynthesis
The Light Dependent Reactions
Formula
Chlorophyll
6 CO2 + 6 H2O + Light Energy
[CH2O] + 6O2
 There are 3 stages to Photosynthesis:
Stage 1: Capture of light
energy
Stage 2: Energy is used to
make ATP and
reduced NADP+
Stage 3: Carbon Fixation
LIGHT
REACTIONS
-Take place
on the
thylakoid
membrane
-Takes place in
the stroma
The Light Reactions

Begin when photons strike a
photosynthetic membrane.

Can be divided into three parts:
1. PHOTOEXCITATION
2. ELECTRON TRANSPORT
3. CHEMIOSMOSIS
1. Photoexcitation …
 Is the absorption of a photon by an
electron of chlorophyll.
 Before a photon of light strikes a
chlorophyll molecule, the chlorophyll
electrons are at the lowest possible
energy level – the ground state
Excitation
 When the photon is absorbed by a
chlorophyll electron, the electron gains
energy and jumps to a higher energy
level. This process is called
EXCITATION.
 The excited electron is unstable and will
return back to its ground state.
 But it has to release the energy it
absorbed from the photon.
 The energy will be released in the form of
heat and light (photons).
 This rapid loss of energy (in the form of
light) is called FLUORESCENCE.
 Like other pigments, chlorophyll emits a
photon of light when one of its electrons
return to its ground states.
 However, this only happens when the
chlorophyll molecule is separated from
the photosynthetic membrane in which it
is normally embedded in.
 Most chlorophyll molecules do not
fluoresce when associated with a
photosynthetic membrane because the
excited electron is captured by a special
primary electron acceptor molecule.
Photosystems
 In a functioning chloroplast, light is NOT
absorbed by independent pigment
molecules.
 Light is absorbed by chlorophyll or
accessory pigment molecules that are
associated with proteins in clusters called
photosystems.
PHOTOSYSTEM
 A photosystem consists of several
pigment molecules (chlorophylls and
acessory pigments) and a chlorophyll a
molecule embedded in the thylakoid
membrane.
 The pigments absorbs photons and
transfers the energy from pigment to
pigment until it reaches a chlorophyll a
molecule.
 An electron in this chlorophyll a absorbs
the energy, becomes excited, and jumps
to a higher energy level.
 But instead of transferring the energy to
another pigment, the excited electron is
transferred to the primary electron
acceptor.
 This is a redox
reaction.
 Chlorophyll is oxidized
(it loses an electron)
 The primary acceptor
is reduced (it gains an electron).
* Independent chlorophyll molecules fluoresce
because there is not primary electron acceptor to
receive the excited electron.
 The primary electron acceptor then
passes the electron off into the ETC
chain embedded in the thylakoid
membrane
 There are 2 types of photosystems in the
thylakoid membrane.
 Photosystem I (P700): which has a
chlorophyll a in the reaction centre which
absorbs wavelengths of 700nm.
 Photosystem II (P680): which has a
chlorophyll a in the reaction centre which
absorbs wavelengths of 680nm.
2. Electron Transport…
 Is the transfer of the excited electron
through a series of membrane-bound
electron carriers, resulting in the pumping
of a proton through the photosynthetic
membrane, creating a H+ reservoir and
reducing an electron acceptor.
 This a noncyclic electron flow.
Photosystem I
Photosystem II
 Energized electrons from photosystem I
are passed down an electron transport
chain containing the protein ferredoxin
(Fd) and added to NADP+ to form
NADPH.
 Meanwhile, energized electrons from
photosystem II are captured by a primary
electron acceptor called plastoquinone
(Q) and are passed through another
electron transport chain.
 Their energy is used to pump hydrogen
ions (H+) from the stroma into the
thylakoid compartment, creating a
concentration gradient.
 Electrons leaving this electron transport
chain enter photosystem I, replenishing
its lost electrons.
 Photosystem II replenishes its electrons
by splitting water with a Z protein
associated with the thylakoid membrane.
 Hydrogen ions and oxygen are released
into the thylakoid compartment. This is
where the oxygen gas generated by
photosynthesis comes from.
 The electrons are used to replenish
photosystem II.
 The protons drive Chemiosmosis.
 The oxygen is released into the
atmosphere.
Noncyclic Electron Flow
 The process is non-cyclic because once
an electron is lost by a reaction centre
chlorophyll within a photosystem, it does
not return to that system.
 The electron ends up in NADPH.
 NOTE: 2 electrons are required to reduce
NADP+ to NADPH.
 (A pair of electrons will move through the
ETC chain together)
Noncyclic Electron Flow
Cyclic Electron Flow
 Occasionally, excited electrons can take
a cyclic pathway called cyclic electron
flow that only uses photosystem I (P700).
 In this pathway, the electron released
from photosystem I is passed to
ferredoxin, and the goes to the Q cycle
and back to P700.
Cyclic Electron Flow
 The cyclic pathway generates a proton
gradient for chemiosmotic ATP synthesis,
but does not release electrons to
generate NADPH.
 NADPH is required for carbon fixation.
3. Chemiosmosis …
 Is the movement of protons through
ATPase complexes to drive the
phosphorylation of ADP to ATP.
 The protons that accumulate in the
thylakoid space contribute to an
electrochemical gradient that drives this
process.
 Since light is required to create the
proton gradient, the process is called
photophosphorylation.
Chemiosmosis
Goal of Light Dependent
Reactions
 To transfer the energy of light to ATP and
NADPH.
 Both of these substance will play a
critical role in the next stage of
photosynthesis: CARBON FIXATION.