Essential Cell Biology 5th edition

Chloroplasts and Photosynthesis
479
chloroplasts
chlorophyll-containing
thylakoid membrane
thylakoids
grana
stroma
vacuole
cell wall
(A)
(B) inner outer (C)
10 µm membrane membrane
0.5 µm
photosynthesis are all contained in the thylakoid membrane. This third
membrane is folded to form a set of flattened, disclike sacs, called the
thylakoids, which are arranged in stacks called grana (Figure 14–29).
The interior of each thylakoid is thought to be connected with that of
other thylakoids, creating the thylakoid ECB5 space—a e14.28/14.29 compartment that is
separate from the chloroplast stroma.
Photosynthesis Generates—and Then Consumes—ATP
and NADPH
The chemistry carried out by photosynthesis can be summarized in one
simple equation:
light energy + CO 2 + H 2 O → sugars + O 2 + heat energy
On its surface, the equation accurately represents the process by
which light energy drives the production of sugars from CO 2 . But this
superficial accounting leaves out two of the most important players in
photosynthesis: the activated carriers ATP and NADPH. In the first stage
of photosynthesis, the energy from sunlight is used to produce ATP and
NADPH; in the second stage, these activated carriers are consumed to
fuel the synthesis of sugars.
1. Stage 1 of photosynthesis resembles the oxidative phosphorylation
that takes place on the mitochondrial inner membrane. In this stage,
an electron-transport chain in the thylakoid membrane harnesses
the energy of electron transport to pump protons into the thylakoid
space; the resulting proton gradient then drives the synthesis of ATP
by ATP synthase. What makes photosynthesis very different is that
the high-energy electrons donated to the photosynthetic electrontransport
chain come from a molecule of chlorophyll that has
absorbed energy from sunlight. Thus the energy-producing reactions
of stage 1 are sometimes called the light reactions (Figure 14–30).
Another major difference between photosynthesis and oxidative
phosphorylation is where the high-energy electrons ultimately wind
up: those that make their way down the photosynthetic electrontransport
chain in chloroplasts are donated not to O 2 but to NADP + ,
to produce NADPH.
Figure 14–29 Chloroplasts, like
mitochondria, are composed of a
set of specialized membranes and
compartments. (A) Light micrograph
showing chloroplasts (green) in the cell of
a flowering plant. (B) Drawing of a single
chloroplast showing the organelle’s three
sets of membranes, including the thylakoid
membrane (dark green), which contains
the light-capturing and ATP-generating
systems. (C) A high-magnification view
of an electron micrograph shows the
thylakoids arranged in stacks called grana;
a single thylakoid stack is called a granum
(Movie 14.9). (A, courtesy of Preeti Dahiya;
C, courtesy of K. Plaskitt.)
QUESTION 14–8
Chloroplasts have a third internal
compartment, the thylakoid
space, bounded by the thylakoid
membrane. This membrane contains
the photosystems, reaction centers,
electron-transport chain, and ATP
synthase. In contrast, mitochondria
use their inner membrane for
electron transport and ATP
synthesis. In both organelles,
protons are pumped out of the
largest internal compartment
(the matrix in mitochondria and
the stroma in chloroplasts). The
thylakoid space is completely sealed
off from the rest of the cell. Why
does this arrangement allow a larger
H + gradient in chloroplasts than can
be achieved for mitochondria?