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

754 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
H +
H+ H +
H +
H + H +
H + H +
H +
energy from H +
H +
sunlight
H +
H + H + H +
H + H +
H + H+ H + H +
energy
from food
Figure 14–2 Stage 1 of chemiosmotic
coupling. Energy from sunlight or the
oxidation of food compounds is captured
to generate an electrochemical proton
gradient across a membrane. The
electrochemical gradient serves as a
versatile energy store that drives energyrequiring
reactions in mitochondria,
chloroplasts, and bacteria.
photosynthesis
transmembrane
electrochemical
gradient
respiration
electron transfer
pumps protons
H +
H +
H +
H + H + H + MBoC6 n14.303/14.03
electron transfer
pumps protons
(“osmotic”). The chemiosmotic process occurs in two linked stages, both of which
are performed by protein complexes in a membrane.
MBoC6 n14.302/14.02
Stage 1: High-energy electrons (derived from the oxidation of food molecules,
from pigments excited by sunlight, or from other sources described
later) are transferred along a series of electron-transport protein complexes
that form an electron-transport chain embedded in a membrane.
Each electron transfer releases a small amount of energy that is used to
pump protons (H + ) and thereby generate a large electrochemical gradient
across the membrane (Figure 14–2). As discussed in Chapter 11, such an
electrochemical gradient provides a way of storing energy, and it can be
harnessed to do useful work when ions flow back across the membrane.
Stage 2: The protons flow back down their electrochemical gradient
through an elaborate membrane protein machine called ATP synthase,
which catalyzes the production of ATP from ADP and inorganic phosphate
(P i ). This ubiquitous enzyme works like a turbine in the membrane, driven
by protons, to synthesize ATP (Figure 14–3). In this way, the energy derived
from food or sunlight in stage 1 is converted into the chemical energy of a
phosphate bond in ATP.
Electrons move through protein complexes in biological systems via tightly
bound metal ions or other carriers that take up and release electrons easily, or by
special small molecules that pick electrons up at one location and deliver them to
another. For mitochondria, the first of these electron carriers is NAD + , a water-soluble
small molecule that takes up two electrons and one H + derived from food
molecules (fats and carbohydrates) to become NADH. NADH transfers these electrons
from the sites where the food molecules are degraded to the inner mitochondrial
membrane. There, the electrons from the energy-rich NADH are passed
from one membrane protein complex to the next, passing to a lower-energy compound
at each step, until they reach a final complex in which they combine with
molecular oxygen (O 2 ) to produce water. The energy released at each step as the
electrons flow down this path from the energy-rich NADH to the low-energy water
molecule drives H + pumps in the inner mitochondrial membrane, utilizing three
different membrane protein complexes. Together, these complexes generate the
proton-motive force harnessed by ATP synthase to produce the ATP that serves as
the universal energy currency throughout the cell (see Chapter 2).
Figure 14–4 compares the electron-transport processes in mitochondria,
which harness energy from food molecules, with those in chloroplasts, which harness
energy from sunlight. The energy-conversion systems of mitochondria and
chloroplasts can be described in similar terms, and we shall see later in the chapter
that two of their key components are closely related. One of these is the ATP
synthase, and the other is a proton pump (colored green in Figure 14–4).
Among the crucial constituents that are unique to photosynthetic organisms
are the two photosystems. These use the green pigment chlorophyll to capture
H + H+
H +
H + H+ H +
H +
H + H + H +
H + H + H +
H + ATP H +
synthase
H +
P i + ADP
ATP
Figure 14–3 Stage 2 of chemiosmotic
coupling. An atp synthase (yellow)
embedded in the lipid bilayer of a
membrane harnesses the electrochemical
proton gradient across the membrane,
using it as a local energy store to drive
ATP synthesis. The red arrows show the
direction of proton movement through the
ATP synthase.