Photosynthesis - The Light Reactions
Photosynthesis - The Light Reactions
Remember, there are two types of reactions in photosynthesis.
1. The Light Reactions ("photo")
- produce ATP and NADPH so the "Dark Reactions" can
occur
- need light
- occur in the thylakoid membrane of chloroplast
2. The Dark Reactions ("synthesis")
- ATP and NADPH produced in Light Reactions are used to
produce carbohydrate from CO2
- occur in the stroma of chloroplast
For the Light Reactions to occur light must be absorbed
• Photons of light are absorbed by molecules of Chlorophyll
(a "family" of several pigments)
• There are of two sets of pigments
-Photosystem I (P-700) and Photosystem II (P-680)
• Photosystem I is most excited by light at 700 nm
• Photosystem II is most excited by light at 680 nm
• P-680 is more powerful than P-700 because P-680 can capture higher energy light
• P-680 contains chlorophyll a, chlorophyll b, chlorophyll c, chlorophyll d, carotenes, xanthophylls and anthocyanins
• P-700 only contains chlorophyll a
The Light Reactions can be carried out by two different
photosystems.
a. Non-cyclic Electron Pathway
- electrons from water are moved from P-680 to P-700 then
to NADP+
b. Cyclic Electron Pathway
- in the P-700 photosystem electrons are recycled after their
energy is used to form ATP molecules
Non-Cyclic Electron Pathway
-green plants, algae and cyanobacteria
1. P-680 becomes
excited by a photon of
light and electrons are
shed. This is an
unstable state. A
molecule of water is
oxidized and reduces
the P-680. (A coupled
reaction)
2. P-680 passes
electrons on to a series
of electron carriers in
the thylakoid
membrane.
3. As electrons pass from P-680 to P-700 the energy of electron
transport may be used to produce ATP.
4. Electrons are then passed along to P-700. When P-700 becomes
"excited" after absorption of light the electrons are passed to
ferredoxin. Ferredoxin passes the electrons on to reduce NADP+ to
NADPH.
ATP Production in the Light Reactions
1. Electron transport establishes a proton gradient
2. Electrons are donated by water molecules, leaving H+ ions
in the intrathylakoid space (2 H+ for every H2O)
3. Also, electron carrier Plastoquinone (PQ) transports
electrons between P-680 (PSII) and the cytochrome b6/f
-- as PQ is reduced and re-oxidized during non-cyclic
transport, H+ may be pumped from stroma into intrathylakoid
space
4. Large proton-motive force favors the return of protons to the
stroma via ATP synthase complex in the thylakoid
membrane and allows for production of ATP
Cyclic Electron Transport
-some photosynthetic bacteria
1. P-700 (PS I) may operate
alone, without interacting
with P-680 (PS-II)
2. NO access to external
source of electrons
ie: no H2O as a donor
-- instead electrons are
returned to P-700 via
cytochrome b6/f and
plastocyanin (PC)
3. Cyclic electron transport
may produce ATP, but does NOT produce NADPH
ATP Production in Cyclic Electron Transport
In cyclic electron transport, the proton gradient is established by pumping protons through the cytochrome complex from the stroma to the intrathylakoid space as the electron goes to
P-700
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