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Light-dependent reactions
3.2 The `light' stage of photosynthesis: photolysis of water and photophosphorylation
The first stage, or set of reactions, involved in photosynthesis is called the light-dependent stage. The light-dependent
reactions occur in the thylakoid membranes inside chloroplasts. It is the photosystems (which are made up of many
photosynthetic pigments) embedded in these membranes which are of importance. 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. The light-dependent stage is focused
on converting light energy (trapped by pigments in the photosystems) into chemical energy.
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. PSII has an enzyme which, in the presence of light, splits a molecule of water into protons, electrons and oxygen.
+ -
2H2O 4H + 4e + O2
It is the protons and electrons produced which are of importance here. The oxygen is actually a waste product for this
reaction. However, this photolysis (the splitting of a molecule using photons, or light particles) of water from the light-
dependent 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. Although a waste
product in this reaction, plants use some of the oxygen produced to aerobically respire, but most of it leaves the leaves
through the stomata (pores in the leaf surface where gaseous exchange may take place in plants).
Formation of ATP: chemiosmosis
The protons, or hydrogen ions, have two purposes. Firstly, they are used to generate ATP molecules by chemiosmosis
(see 4.4 Electron transport chain for more details). In summary, protons diffuse across the thylakoid membrane through
channel proteins which are associated with the enzyme ATP synthase, driving the rotation of part of the ATP synthase
molecule, which combines a molecule of ADP (adenosine diphosphate) and an inorganic phosphate to form ATP.
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). Reduced NADP is then used in the second
stage of photosynthesis, the light-independent reactions.
light light
electron ADP + Pi
thylakoid space
- ATP synthase
2e +
The diagram above shows the photosystems I and II and the enzyme ATP synthase embedded in a thylakoid membrane.
The blue area shows the photolysis of water, and the pathways of the electrons and protons produced. The oxygen may
be used for aerobic respiration, but will usually leave the leaf through the stomata.

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The Z-pathway
The series of reactions involved in the light-dependent stage are known collectively as the `Z-pathway' because of the
movement of electrons across the photosystems and thylakoid membrane.…read more

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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…read more


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