Photophosphorylation
Definitions
• Photosystems:- comprise of the chlorophyll and accessory
pigments that form the antenna complex or light harvesting
complex.
• Oxidation:- is the loss of electrons from or the increase in
oxidation number
• Reduction:- is the gain of electrons from or the decrease in
oxidation number
Definitions
• Electron acceptor:- is a molecule that accepts an excited
electron from a chlorophyll molecule at a higher energetic
state than it normally existed in the chlorophyll molecule
allowing it to be REDUCED.
• Electron donor :- supplies low energy electrons which replaces
the excited ones in photosynthesis allowing it to be OXIDISED.
• NADP/NADPH:- (Nicotinamide Adenine Dinucleotide
Phosphate) this is a molecule that acts as an electron carrier in
electron transfer reactions such as photosynthesis.
Photosynthesis
Three main phases:
1. Light harvesting-light energy captured by photosynthetic
pigments
2. Light dependent stage (photolysis)-flow of e- from the effect
of light on chlorophyll, splitting of water into H and O ions
3. Light independent stage-H ions used in the reduction of CO2
and sugar production
Light Harvesting
• Chlorophyll and carotenoids arranged into
groups called antenna complex
• Proteins associated with pigments help funnel
photons of light( discrete packet of light) onto
chlorophyll a – reaction centre chlorophyll
molecule
• When light strikes chlorophyll a, e- is raised to a
higher energy level
flow of e-
Photosystems
• Two types of reaction centres
1. Photosystem I (P 700) or PSI- has a reaction centre of a
chlorophyll a mol with max light absorption of 700nm
wavelength
2. Photosystem II (P 680) or PSII- has a reaction centre of a
chlorophyll a mol with max light absorption of 680nm
wavelength
Light Harvesting
• Different pigments absorb light energy of
different wavelengths
• Chlorophyll mol at reaction centre becomes
‘excited’
• High energy e- released from chlorophyll mol
• Light energy in photosystem travels from a
higher energy level to a lower energy level
through a light harvesting complex
Light Dependent Stage
• Occurs in the thylakoids of chloroplasts
• Involves splitting of water-photolysis of water
• ADP converted to ATP- phosphorylation, using lightphotophosphorylation
• Light shines of PSI and PSII
• High energy e- released by chlorophyll mol in PS
• Energy from e- used to make ATP and NADPH2
• ATP-ernergy carrier
• NADP- H carrier into light independent rxn
• e- from PSI or PSII boosted to higher energy level, acquires
excitation energy
Z Scheme
Light Dependent Stage
• e- captured by e- acceptor-light energy converted to chemical
energy
• e- acceptor reduced (addition of e-)
• Positively charged (oxidised) chlorophyll mol left in PSI and
PSII (removal of e-)
• Excited e- from PS II (P680) picked up by chain of e- carriers,
passed along travelling downhill in terms of energy in a series
of redox rxns
• Energy lost during e- flow is ‘coupled’ to rxns that synthesise
ATP from ADP and Pi
• Excitation level of e- fall back to ‘ground state’ due to loss of
energy filling the vacancy in reaction centre in PSI (P700)
Light Dependent Stage
• e- transferred from PSII (P680) to PSI (P700)
• Oxidised chlorophyll in PSII has to be filled by ground state efrom water mol
• The positively charged vacancy in PSII is enough to cause lysis
of water
• Ground state e-, H+ ions and O atoms released
• O atoms combine to form oxygen gas-given off
• H+ ions used in the reduction of NADP
• Excited e- from PSI (700) taken up by different e- acceptor
• 2 at a time they are passed to NADP; plus H+ ions from
pholysis reduces NADP to NADPH2
Light Dependent Stage
• Products of light dependent stage are:
1. ATP
2. NADPH2 (reduced NADP)
Cyclic Photophosphorylation
•
•
•
•
The cyclic pathway only uses photosystem I (P700).
Neither oxygen or NADPH are produced, only ATP
light strikes P700
an electron absorbs the solar energy and is excited to a higher
energetic state where it is captured by a primary electron acceptor.
• The excited electron then passes from this primary acceptor
downhill in terms of energy along a series of electron carriers such
that the dissipated energy is coupled to the production of ATP.
• The “energy drained electron” now returns to the ground state of
photosystem I (P700 complex) so conservation takes place.
• The cyclic component of the light dependent stage is primarily used
in times when the energetic demands of the plant are high as ATP is
the main product.
Cyclic Photophosphorylation
Non-cyclic
Photophosphorylation
• Solar energy strikes the antennae complex of PSII (P680)
• An electron from this chlorophyll molecule absorbs this energy
and becomes excited.
• The excited electron is then caught by a primary electron
acceptor at a higher energy level, while the water molecule
undergoes photolysis to replace the just, excited electron from
the oxidised P680 chlorophyll; this also generates 2H+ ions and
O2 which is lost through the stomata.
• The caught electron is passed along a system of electron
carriers (electron transport system) which reduces the energy
of the electron and dissipates it in an amount that is enough
to produce an ATP energy carrier molecule, via the ATPase
machinery, the 1st product of the light dependent stage.
Non-cyclic
Photophosphorylation
• The less excited electron (initially which was excited from
the P680) now replaces another electron that would
have been subsequently excited in the P700 chlorophyll
of photosystem I, when solar energy would have struck
the leaf.
• The excited electron from PS I (P700) is again accepted
by another primary electron acceptor.
• The 2nd excited electron from PS I is then passed from
the second primary electron acceptor along electron
carriers to combine with H+ ions and NADP+ generating
the 2nd product of the light dependent stage which is
NADPH2 (REDUCED NADP).
Comparison of Cyclic and Noncyclic photophosphorylation
NON-CYCLIC
Electrons do not come back to
the same molecule
First electron donor is water..
Involves both PSI and PSII
Last electron acceptor is NADP.
The net products are ATP,
NADPH and O2.
CYCLIC
Electrons return to the
same molecule.
First electron is the P700
(PSI).
Involves only PSI
Last electron acceptor is
P700 (PSI).
The product is ATP only