Biology 12

electron

carriers

proton

pump

electron

pathway

intermembrane

space

AT P

synthase

AT P

inner

membrane

matrix

NADH NAD+ FAD^4

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+ H+

ADP+

+O 2

H 2 O

H 2 O

FADH 2

Pi

through a special protein complex is called
chemiosmosis.) Figure 3.10 shows how electron
transfer moves H+ions.
Recall that during glycolysis and the Krebs cycle,
ATP molecules are produced through substrate-
level phosphorylation. In this process, the ADP
molecule is phosphorylated. A phosphate group
is moved from another substrate (like PEP) to ADP
to make ATP. In the electron transport chain, the
carriers are reduced (accept electrons) and then
oxidized (lose electrons to the next carrier) in a
sequence. At each step, some energy is liberated and
used to pump H+(against a concentration gradient)
across the inner membrane to the intermembrane
space. In addition, some of the liberated energy is
lost to the environment as thermal energy. At the
end of this process, the spent (low energy) electrons
must be removed. Oxygen must be present in the
matrix to oxidize the last component of the electron
transport chain. When oxygen is combined with
available H+ions in the matrix, water is formed. This
allows additional electrons to enter the electron
transport chain and release the energy needed to
pump more H+ions into the intermembrane space.
ATP is produced when the high concentration of
H+ions diffuses through the channel of the AT P
synthase complexthat is embedded in the inner
membrane, as shown in Figure 3.11. Because of
the crucial role played by oxygen, this process of

chemiosmosis is also called oxidative

phosphorylation. In the next section, you will

see that a similar process is used to make ATP

in photosynthesis.

When NADH is oxidized, electrons enter the

electron transport chain. The electrons first transfer

to NADH dehydrogenase complex, a multienzyme

system that oxidizes NADH. The liberated

hydrogen ions are released into the matrix, leaving

NAD+. The electrons are passed along from one

carrier to another, as shown in Figure 3.10. Some

of these carriers are cytochromes, proteins with a

heme group containing an iron atom that can be

oxidized or reduced reversibly.

The two electrons from the FADH 2 molecules,

which have less energy than the electrons carried

by NADH, enter the electron transport chain at a

different point. As a result, the less energetic

electrons from FADH 2 are responsible for the

production of fewer ATP molecules.

Energy from the electrons powers the

multienzyme complexes, called proton pumps, to

move hydrogen ions (H+) into the intermembrane

space, as shown in Figure 3.11. As a result of this

high concentration of H+ions, the inner membrane

of the mitochondrion becomes positively charged.

At the same time, negative ions are attracted to

the exterior of the outer membrane, helping the

movement of protons through the proton pumps.

Chapter 3 Cellular Energy • MHR 75

As electrons (e−) move through the

electron transport chain, hydrogen

ions (H+) are pumped from the matrix

into the intermembrane space.

A A hydrogen ion gradient is formed,

with a higher concentration of ions

in the intermembrane space than

in the matrix.

B When hydrogen ions flow back into

the matrix down their concentration

gradient, ATP is synthesized from

ADP+Piby an ATP synthase complex.

C

Figure 3.11Mitochondria synthesize ATP by chemiosmosis. ATP production is
based on a gradient of hydrogen ion (H+) concentration. The gradient is established
by pumping hydrogen ions into the intermembrane space of the mitochondrion.