Photosynthesis Notes
Gross photosynthesis = total amount of CO2 fixed as carbohydrate.
Net photosynthesis is difference between gross and CO2 released during respiration.
i.e. NP = GP - R
Light-dependent - requires light; produces ATP and reduced NADP i.e. NADPH
Light-independent - NADP used to reduce CO2 to carbohydrate; ATP used.
Light-Dependent Reactions:
Electrons in chlorophyll become excited by light energy (high energy electrons). These are emitted and
taken up by electron carriers.
Chlorophyll
(reduced)
light energy

Chlorophyll +
(oxidised)
+
e(electron)
Electrons are passed along a chain of electron carriers in a series of oxidation-reduction reactions (redox).
ATP is produced from ADP and Pi. This is referred to as photophosphorylation.
The electrons lost from chlorophyll need to be replaced. Enzymes catalyse the splitting of water
(photolysis) as follows:H2O
water

2H+
+
hydrogen
ions
1/2O2
oxygen
+
2eelectrons
The electrons released replace those lost from chlorophyll.
Electrons passed into two electron chains - PSI and PSII.
Non-Cyclic Photophosphorylation:electron acceptor
e.t.
chain
final electron acceptor
2eelectron acceptor
e.t.
chain
H+ + NADP+
2e-
ADP
+ Pi
PSI
ATP
H2O
2e2e-
1/2 O2 + 2H+
Light
PSII
Light
NADPH
The above processes supply reduced NADP for the light-independent reaction. Each photosystem raises
two electrons to higher energy levels and  higher chemical potential energy levels.
Cyclic Photophosphorylation:The above processes do not provide sufficient ATP to drive the light-independent reactions. Cyclic
photophosphorylation only involves PSI. Excited electrons are passed back to the electron transport chain
between PSII and PSI. Extra ATP is produced and the electrons simply pass back to PSI. No oxygen is
produced.....why??
The Light-Independent Reactions: