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

THE MITOCHONDRION
755
(A)
MITOCHONDRION
(B)
CHLOROPLAST
H + gradient
H + gradient
fats and
carbohydrate
molecules
e
e – pump
e
H +
– H
light
pump
H +
light
NADPH
NADH
– H +
pump
citric
acid
cycle
CO 2
H 2 O
O 2
photosystem I
photosystem II
carbonfixation
CO 2
H 2 O
cycle
carbohydrate
O 2 molecules
products
products
Figure 14–4 Electron-transport processes. (A) The mitochondrion converts energy from chemical fuels. (B) The chloroplast
converts energy from sunlight. In both cases, electron flow is indicated by blue arrows. Each of the protein complexes (green)
is embedded in a membrane. In the mitochondrion, fats and carbohydrates from food molecules are fed into the citric acid
cycle and provide electrons to generate the energy-rich compound NADH from NAD + . These electrons then flow down an
energy gradient as they pass from one complex to the next in the electron-transport chain, until they combine with molecular
O 2 in the last complex to produce water. The energy released at each stage is harnessed to pump H + across the membrane.
In the chloroplast, by contrast, electrons are extracted from water through the action of light in the photosystem II complex and
molecular O 2 is released. The electrons pass on to the next complex in the chain, which uses some of their energy to pump
protons across the membrane, before passing to photosystem I, where sunlight generates high-energy electrons that combine
with NADP + to produce NADPH. MBoC6 NADPH m14.03/14.04
then enters the carbon-fixation cycle along with CO 2 to generate carbohydrates.
light energy and power the transfer of electrons, not unlike a photocell in a solar
panel. The chloroplasts drive electron transfer in the direction opposite to that in
mitochondria: electrons are taken from water to produce O 2 , and these electrons
are used (via NADPH, a molecule closely related to the NADH used in mitochondria)
to synthesize carbohydrates from CO 2 and water. These carbohydrates then
serve as the source for all other compounds a plant cell needs.
Thus, both mitochondria and chloroplasts make use of an electron-transfer
chain to produce an H + gradient that powers reactions that are critical for the cell.
However, chloroplasts generate O 2 and take up CO 2 , whereas mitochondria consume
O 2 and release CO 2 (see Figure 14–4).
THE MITOCHONDRION
Mitochondria occupy up to 20% of the cytoplasmic volume of a eukaryotic cell.
Although they are often depicted as short, bacterium-like bodies with a diameter
of 0.5–1 μm, they are in fact remarkably dynamic and plastic, moving about
the cell, constantly changing shape, dividing, and fusing (Movie 14.1). Mitochondria
are often associated with the microtubular cytoskeleton (Figure 14–5),
which determines their orientation and distribution in different cell types. Thus,
in highly polarized cells such as neurons, mitochondria can move long distances
(up to a meter or more in the extended axons of neurons), being propelled along
the tracks of the microtubular cytoskeleton. In other cells, mitochondria remain
fixed at points of high energy demand; for example, in skeletal or cardiac muscle
cells, they pack between myofibrils, and in sperm cells they wrap tightly around
the flagellum (Figure 14–6).
Mitochondria also interact with other membrane systems in the cell, most
notably the endoplasmic reticulum (ER). Contacts between mitochondria and ER
define specialized domains thought to facilitate the exchange of lipids between
the two membrane systems. These contacts also appear to induce mitochondrial