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
783
LEAF
upper epidermis
chloroplast
grana
thylakoids
air space
nucleus
vacuole
cell wall
cell
wall
lower epidermis
cytosol
chloroplast
mitochondrion
chloroplast
envelope
(A)
(B) (C) (D)
10 mm
5 µm
0.5 µm
not folded into cristae and does not contain electron-transport chains. Instead,
the electron-transport chains, photosynthetic light-capturing systems, and ATP
synthase are all contained in the thylakoid membrane, a separate, distinct membrane
that forms a set of flattened, disc-like sacs, the thylakoids. The thylakoid
membrane is highly folded into numerous local stacks of flattened vesicles called
MBoC6 n14.318,14.34/14.37
grana, interconnected by nonstacked thylakoids. The lumen of each thylakoid is
connected with the lumen of other thylakoids, thereby defining a third internal
compartment called the thylakoid space. This space represents a separate compartment
in each chloroplast that is not connected to either the intermembrane
space or the stroma.
Chloroplasts Capture Energy from Sunlight and Use It to Fix
Carbon
We can group the reactions that occur during photosynthesis in chloroplasts into
two broad categories:
1. The photosynthetic electron-transfer reactions (also called the “light
reactions”) occur in two large protein complexes, called reaction centers,
embedded in the thylakoid membrane. A photon (a quantum of light)
knocks an electron out of the green pigment molecule chlorophyll in the
first reaction center, creating a positively charged chlorophyll ion. This
electron then moves along an electron-transport chain and through a second
reaction center in much the same way that an electron moves along
the respiratory chain in mitochondria. During this electron-transport
process, H + is pumped across the thylakoid membrane, and the resulting
Figure 14–37 Chloroplasts in the cell.
(A) Schematic cross section through the
leaf of a green plant. (B) Light microscopy
of a plant leaf cell—here, a mesophyll cell
from Zinnia elegans—shows chloroplasts
as bright green bodies, measuring several
micrometers across, in the transparent
cell interior. (C) The electron micrograph
of a thin, stained section through a wheat
leaf cell shows a thin rim of cytoplasm—
containing chloroplasts, the nucleus, and
mitochondria—surrounding a large, waterfilled
vacuole. (D) At higher magnification,
electron microscopy reveals the chloroplast
envelope membrane and the thylakoid
membrane within the chloroplast that
is highly folded into grana stacks
(Movie 14.9). (B, courtesy of John Innes
Foundation; C and D, courtesy of
K. Plaskitt.)
thylakoid membrane
inner membrane
outer membrane
intermembrane space
matrix stroma
thylakoid space
crista
grana
crista membrane thylakoids
2 µm
MITOCHONDRION
CHLOROPLAST
Figure 14–38 A mitochondrion and
chloroplast compared. Chloroplasts
are generally larger than mitochondria. In
addition to an outer and inner envelope
membrane, they contain the thylakoid
membrane with its internal thylakoid space.
The chloroplast thylakoid membrane,
which is the site of solar energy conversion
in plants and algae, corresponds to the
mitochondrial cristae, which are the sites
of energy conversion by cellular respiration.
Unlike the crista membrane, which is
continuous with the inner mitochondrial
membrane at cristae junctions, the thylakoid
membrane is not connected to the inner
chloroplast membrane at any point.