David Snow
Photosystem 2
1. noncyclic
2. occurs/resides in thylakoid membrane
3. reaction center chlorophyll is P680
Photosystem 1
1. can be cyclic, if required
2. occurs/resides in thylakoid membrane
3. reaction center chlorophyll is p700
4. passes photoexcited electrons to
photosystem 1 via an electron transport
chain
5. Light enters chlorophyll and excites an
electron. The energy from this electron is
passed from electron to electron until an
electron in the reaction center is excited
and physically removed to the primary
acceptor
4. uses ‘spent’ electrons from photosystem 2
to make up the number of electrons in its
own system
5. ‘Passes’ spent electrons back to prior
electron transport chain to create extra
ATP if needed (this is the cyclic process).
6. This excited electron is passed down the
electron transport chain to photosystem 1
6. Passes its own photo excited electrons to
an electron transport chain, which
transmits them to Fd, an iron-containing
protein
7. NADP+ reductase transfers the electrons
from Fd to NADP+ creating NADPH
7. Splits water into oxygen and electrons.
Water is the source of electrons for this
process
8. ATP produced during noncyclic electron
flow is called noncyclic
photophosphorylate
9.
8. This NADPH provides reducing power for
the synthesis of sugar in the calvin cycle
9.
Calvin Cycle
1. occurs in stroma
2. CO2 enters and leaves as sugar
3. cycle spends ATP as energy and consumes
NADPH as reducing power for adding
high-energy electrons to make sugar
4. returns NADP+ to the light reactions
5. 3 phases:
 carbon fixation: each CO2 is fixed into a
5-carbon sugar, using RuBP
 reduction: each 3-phosphoglycerate
receives a phosphate from ATP. A pair of
electrons from NADPH then reduces it to
G3P
 regeneration: RuBP is regenerated, so that
CO2 can again be accepted, and cycle can
turn again
6. G3P is the product of this cycle, not
glucose. G3P is a sugar
7. Cycle must turn 3 times, fixing 3 carbons,
to create one net G3P
8. For net synthesis of one G3P, the cycle
consumes 9 ATP and 6 NADPH, these
molecules are regenerated by the light
reactions
9. G3P is starting material for metabolic
pathways that synthesize other organic
compounds