Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CHLOROPLASTS AND PHOTOSYNTHESIS
789
light
light-harvesting
antenna complexes
energy transferred from
one chlorophyll molecule
to another
reaction center
photosystem
chlorophyll
special pair
thylakoid
membrane
light-harvesting complex, or LHC. In addition to many chlorophyll molecules,
an LHC contains orange carotenoid pigments. The carotenoids collect light of
a different wavelength from that absorbed by chlorophylls, helping to make the
antenna complex more efficient.
MBoC6
They
e14.32/14.45
also have an important protective role in
preventing the formation of harmful oxygen radicals in the photosynthetic membrane.
Figure 14–45 A photosystem. Each
photosystem consists of a reaction
center plus a number of light-harvesting
antenna complexes. The solar energy for
photosynthesis is collected by the antenna
complexes, which account for most of the
chlorophyll in a plant cell. The energy hops
randomly by resonance energy transfer (red
arrows) from one chlorophyll molecule to
another, until it reaches the reaction center
complex, where it ionizes a chlorophyll in
the special pair. The chlorophyll special pair
holds its electrons at a lower energy than
the chlorophyll in the antenna complexes,
causing the energy transferred to it from
the antenna complex to become trapped
there. Note that it is only energy that
moves from one chlorophyll molecule
to another in the antenna complex, not
electrons (Movie 14.10).
The Thylakoid Membrane Contains Two Different Photosystems
Working in Series
The excitation energy collected by the antenna complex is delivered to the special
pair in the photochemical reaction center. The reaction center is a transmembrane
chlorophyll–protein complex that lies at the heart of photosynthesis. It harbors
the special pair of chlorophyll molecules, which acts as an irreversible trap
for excitation energy (see Figure 14–45).
Chloroplasts contain two functionally different although structurally related
photosystems, each of which feeds electrons generated by the action of sunlight
into an electron-transfer chain. In the chloroplast thylakoid membrane, photosystem
I is confined to the unstacked stroma thylakoids, while the stacked grana
thylakoids contain photosystem II. The two photosystems were named in order
of their discovery, not of their actions in the photosynthetic pathway, and electrons
are first activated in photosystem II before being transferred to photosystem
I (Figure 14–46). The path of the electron through the two photosystems can be
described as a Z-like trajectory and is known as the Z scheme. In the Z scheme,
the reaction center of photosystem II first withdraws an electron from water. The
electron passes via an electron-transport chain (composed of the electron carrier
plastoquinone, the cytochrome b 6 -f complex, and the protein plastocyanin) to
photosystem I, which propels the electron across the membrane in a second lightdriven
charge-separation reaction that leads to NADPH production.
sunlight
sunlight
energy
transfer
lightharvesting
antenna
complex
H 2 O
Figure 14–46 The Z scheme for
photosynthesis. The thylakoids of plants
THYLAKOID
and cyanobacteria contain two different
SPACE
photosystems, known as photosystem I
e – e – energy
transfer
photosystem
II
reaction
center
STROMA
NADPH
photosystem
I
reaction
center
lightharvesting
antenna
complex
and photosystem II, which work in series.
Each of the photosystem I and II reaction
centers receives excitation energy from
its own set of tightly associated antenna
complexes, known as LHC-I and LHC-II,
by resonance energy transfer. Note that, for
historical reasons, the two photosystems
were named opposite to the order in which
they act, with photosystem II passing its
electrons to photosystem I.