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The light-dependent
reactions occur in the thylakoid membranes inside chloroplasts
...
For one set of light-dependent
reactions to happen, two photosystems are used: photosystem I (PSI), which is found mainly on the intergranal lamellae,
and photosystem II (PSII), which is found almost exclusively on the granal lamellae
...


Photolysis of water
The photosystem which is involved in the first part of this stage is actually photosystem II, as photosystem I is involved
later on
...

2H2O → 4H

+

-

+ 4e + O2

It is the protons and electrons produced which are of importance here
...
However, this photolysis (the splitting of a molecule using photons, or light particles) of water from the lightdependent reactions is seen as one of the most important reactions in life: the oxygen produced is the source of almost
all of the oxygen in the atmosphere, which many organisms require to respire, and therefore live
...


Formation of ATP: chemiosmosis
The protons, or hydrogen ions, have two purposes
...
4 Electron transport chain for more details)
...

The protons then are accepted by the coenzyme NADP (nicotinamide adenine dinucleotide phosphate), alongside
electrons, to form reduced NADP, or NADPH (also sometimes written NADPH2)
...

NADP
+2H

+

NADPH

-

2e
light

light

electron
carriers
2H

stroma

PSII

ADP + Pi
ATP

+

PSI
-

2e
thylakoid space

ATP synthase

-

2e

2H
H2O

+

2H

+

½O2

The diagram above shows the photosystems I and II and the enzyme ATP synthase embedded in a thylakoid membrane
...
The oxygen may
be used for aerobic respiration, but will usually leave the leaf through the stomata
...

electron
acceptor
electron
acceptor
2e

ATP

-

NADP
2e

NADP reductase

-

NADPH

+

2H
2e

-

ADP + Pi
Key:

ATP

light

PSI
P700

cyclic phosphorylation
non-cyclic phosphorylation

PSII
P680

-

2e

H2O

2H

+

1 In the presence of light, a molecule of water is split using an enzyme found within photosystem II, with a peak
absorption of light wavelength 680nm (written P680), into an atom of oxygen, as well as two electrons and two
protons
...
As proteins flow through the channel, the enzyme is activated, as the transport of the proton drives the
rotation of the enzyme so that one ADP molecule and one phosphate group form a molecule of ATP – this is
photophosphorylation (production of ATP using light energy)
There are two types of photophosphorylation which take place during the light-dependent reactions
...


Non-cyclic photophosphorylation
Essentially, the entire light-dependent stage is the process of non-cyclic photophosphorylation
...
Non-cyclic photophosphorylation involves the steps 1-4 as seen above
...

The electrons lost by photosystem I are replaced by the electrons from oxidised photosystem II, and the electrons
donated from photolysed water replace those then lost by PSII
...


Cyclic photophosphorylation
Another type, cyclic photophosphorylation also occurs, but only at photosystem I
...
This movement of electrons does generate small amounts of ATP, but does not involve photolysis of
water or reduce any NADP
...


The table below compares the different types of photophosphorylation to provide a summary:
non-cyclic photophosphorylation

cyclic photophosphorylation

Which photosystems are involved?

PSI and PSII

PSI only

Does the reaction involve photolysis of water?

yes

no

Which molecule is the electron donor?

water

chlorophyll a (P700) in PSI

Which molecule is the final electron acceptor?

NADP

chlorophyll a (P700) in PSI

What are the products?

reduced NADP, ATP and oxygen

ATP only

Products of the light-dependent stage
Both cyclic and non-cyclic photophosphorylation drive the synthesis of ATP, so we know this is one product of the lightdependent stage
...
3 Light-independent reactions)
...




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