Photophosphorylation

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 light-

photophosphorylation• 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 e-

from 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. ATP2. 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 Non-cyclic photophosphorylation

NON-CYCLIC CYCLIC

Electrons do not come back to the same molecule

Electrons return to the same molecule.

First electron donor is water..First electron is the P700 (PSI).

Involves both PSI and PSII Involves only PSI

Last electron acceptor is NADP.Last electron acceptor is P700 (PSI).

The net products are ATP, NADPH and O2. The product is ATP only