The light dependent reaction

1. The light dependent reaction
Photosynthesis

2. One can think of the light dependent reactions as
a way to increase the free energy of the system
and the light independent reactions as a way to
convert that new free energy into the bonds of
glucose.

3. Photosystems
Photosystems
•The light-absorbing pigments of thylakoid
membranes and their associated electron
carriers are arranged in functional sets or
clusters.
•These clusters are called photosystems.
•These photosystems can absorb light over the
entire visible spectrum but especially well
between 400 to 500 nm and 600 to 700 nm.

4. When a chlorophyll molecule in the thylakoid membrane is excited by
light, the energy level of an electron in its structure is boosted The
packet of excitation energy now migrated rapidly through the light
harvesting pigment molecules to the reaction centre of the
photosystem where it causes an electron to acquire the large amount
of energy.

5. Photosystem I and II
•The thylakoid
membranes of plant
chloroplasts have
two different kinds of
photosystems each
with its own set of
light harvesting
chlorophyll and
carotenoid molecules
and the
photochemical
reaction centre.
•Photosystem I - is
maximally excited
by light at longer
wavelengths.
(P700)
•Photosystems II - is
maximally excited
by shorter
wavelengths. (Less
the 680)

6. The light reaction
These increase the free energy made available to the
system. This free energy can be used in three ways:
1) To build the chemiosmotic or proton gradient.
2) Generate ATP.
3) Reduce NADP+ to NADPH.
•There are two ways to generate ATP
•1) Non cyclic photophosphoraltion.
•2) Cyclic photophosphoralation.
•These two systems differ in the route taken by the "light
activated" electrons and in some of the products formed.

7. Photosystem II
•Light energy is trapped in
photosystem II and
boosts electron to a
higher level.
•The electrons are
received by an electron
acceptor. Thus leaving
the chlorophyll molecule
positive.
•It then splits water by
taking 2e
-
from it.
•The electrons are passed
from one acceptor
(Plastoquinone and
Cyctochrome) to the next
to photosystem I.
•Along the way the energy
lost reduces ADP-> ATP

9. Photosystem I
•The light energy in
Photosystem I boosts the
electrons to an even higher
level.
•The electrons are received
by another acceptor and
these combine with the
protons from the split water
to reduce NADP called non
cyclic
photophosphorylation.
•Any left over electrons will
return to Photosystem I (via
the electron transport
chain) to produce more
ATP. This is called cyclic
photophosphorylation.
(This is the common
pathway in prokaryotes)

11. The ‘z’ scheme
The products so far….
-O
2
-H+ ions
-ATP
-NADPH