Structure of chlorophyll
- Chlorophyll a, the pigment that participates directly in the light
reactions of photosynthesis, has a “head” called a porphyrin ring, with a
magnesium atom at its center.
- Attached to the porphyrin is a hydrocarbon tail which interacts with
hydrophobic regions of proteins in the thylakoid membrane
- Chlorophyll b differs from chlorophyll a only in one of the functional
groups bonded to the porphyrin
Photo excitation of isolated chlorophyll
- A) absorption of a photon causes a transition of the chlorophyll
molecule from its ground state to its excited state
o The photon boosts an electron to an orbital where it has more
o If isolated chlorophyll is illuminated, its excited electron
immediately drops back down to the ground-state orbital, giving
off its excess energy as heat and fluorescence (light)
- B) A chlorophyll solution excited with ultraviolet light with fluoresces, giving off a red-orange
How photosystem harvest light
- Chlorophyll a, chlorophyll b and the carotenoids are assembled into photosystems located
within the thylakoid membrane. Each photosystem is composed of:
o Antenna complex
Several hundred chlorophyll a, chlorophyll b and carotenoid molecules are light-
gathering antennae that absorb photons and pass the energy from molecule to
molecule. This process of resonance energy transfer is called inductive
Different pigments within the antennal complex have slightly different
absorption spectra, so collectively they can
absorb photons from a wider range of the light
spectrum than would be possible with only one
type of pigment molecule.
o Reaction-center chlorophyll Only one of the many chlorophyll a molecules in each complex can actually
transfer an excited electron to initiate the light reactions.
This specialized chlorophyll a is located in the reaction center.
o Primary electron adaptor
A primary electron adapter molecule traps excited state electrons released from
the reaction center chlorophyll
The transfer of excited state electrons from chlorophyll to primary electron
acceptor molecules is the first step of the light reactions
The energy stored in the trapped electrons powers the synthesis of ATP and
NADPH in subsequent steps
- Two types of photosystems are located in the thylakoid membranes, photosystem I and
o The reaction center of photosystem I has a specialized chlorophyll a molecules known as
P700, which absorbs best at 700 nm (the far red portion of the spectrum)
o The reaction center of photosystem II has a specialized chlorophyll molecule known as
P680 which absorbs best at 680nm
o P700 and P680 are identical chlorophyll a molecules, but each is associated with a
different protein. This affects their electron distribution and results in slightly different
- Photosystems are the light harvesting units of the thylakoid membrane. Each photosystems is a
complex of proteins and other kinds of molecules
- When a photon strikes a pigment molecule, the energy is passed from molecule to molecule
until it reaches the reaction center
- At the reaction center, the energy divides an oxidation-reduction reaction
- An excited electron from the reaction-center chlorophyll is captured by a specialized molecule
called the primary electron acceptor
Noncyclic electron flow
- There are two possible routes for electron flow during light reactions: Noncyclic flow and cyclic
- Both photosystem I and II function and cooperate in noncyclic electron flow, which transforms
light energy to chemical energy stored in the bonds of NADPH and ATP
- This process:
o Occurs in the thylakoid membrane
o Passes electrons continuously from
water to NADP+
o Produces ATP by noncyclic
o Produces NADPH
o Produces O 2 - Initially, the excited state electrons are transferred from P700 to the primary electron acceptor
for photosystem I
- The primary electron acceptor passes these excited state electrons to ferredoxin (Fd), and iron-
- NADP+ reductase catalyzes the redox reaction that transfers these electron from ferredoxin to
NADP+, producing reduced coenzyme- NADPH
- The oxidized P700 chlorophyll becomes an oxidizing agent as its electron “holes” must be filled;
photosystem II supplies the electron to fill these holes.
- When antenna assembly of photosystem II absorbs light, the energy is transferred to the P680
o Electrons ejected from the P680 are trapped by photosystem II primary electron
o The electrons are then transferred to an electron transport chain in the thylakoid
o Plastoquinone (Pq) receives the electron from the primary electron acceptor.
- IN cascade of redox reactions, the electron travel from Pq to a complex of two Cytochromes to
plastocynanin (Pc) to P700 of photosystem I.
o As these electrons pass down to ETC, they lose potential energy until they reach the
ground state of P700.
o These electrons then fill the electron valences left in photosystem I when NADP+ was
- Electron from P680 flow to P700 during noncyclic electron flow, restoring the missing electron in
- This however leaves P680 reaction center of photosystem II with missing electrons, the oxidized
P680 becomes a strong oxidizing agent.
o A water-splitting enzyme extracts electrons from water and passes them to oxidized
P680, which has a high affinity for electrons.
o As water is oxidized, the removal of electrons splits water into two hydrogen ions and
an oxygen atom
o The oxygen atom is immediately combined with a second oxygen atom to form O 2
- As excited electrons give up energy along the transport chain to P700, the thylakoid membrane
couples the exergonic flow of electrons to the endergonic reactions that phosphorylate ADP to
o This coupling mechanism is called chemiosmosis
o Some electron carriers can only transport electrons in the company of protons
o The protons are picked up on one side of the thylakoid membrane and deposited on the
opposite side as the electrons move to the next member of transport chain. Cyclic electron flow
- Photo-excited electrons from photosystem I are occasionally
shunted back from ferredoxin (Fd) to chlorophyll via the
Cytochromes complex and plastocyanin (Pc). This cyclic electron
flow supplements the supply of ATP but produces no NADPH.
- It is cyclic because excited electron that leave from chlorophyll at the reaction center return to
the reaction center
- As photons are absorbed by phtotosystem I, the P700 reaction center chlorophyll releases
excited-state electron to the primary electron acceptor; from there the electron take an
alernative path that sends tehm tubling down to P700.
- The exergonic flow of electrons is coupled to ATP production by the process of chemiosmosis-
- Its function is to produce additional ATP.
o It does do without the production of NADPH or O 2
o It supplements the ATP supply requires for the Calvin cycle and other metabolic
o NADPH concentration might influcence whether electrons flow through cyclic or
Chemiosmosis in mitochondria and chloroplasts
- The inner membrane of the mitochondrion pumps