Biology (Holt)

Step Carbon dioxide is released from the five-carbon compound,

resulting in a four-carbon compound. A molecule of ATP is

made, and a molecule of NADH is also produced.

Step The existing four-carbon compound is converted to a new

four-carbon compound. Electrons are transferred to an

electron acceptor called FAD, making a molecule of FADH 2.

is another type of electron carrier.

Step The new four-carbon compound is then converted to the

four-carbon compound that began the cycle. Another mol-
ecule of NADH is produced.
After the Krebs cycle, NADH and FADH 2 now contain much of the
energy that was previously stored in glucose and pyruvate. When the
Krebs cycle is completed, the four-carbon compound that began the
cycle has been recycled, and acetyl-CoA can enter the cycle again.

Electron Transport Chain

In aerobic respiration, electrons donated by NADH and FADH 2 pass
through an electron transport chain, as shown in Figure 13.In
eukaryotic cells, the electron transport chain is located in the inner
membranes of mitochondria. The energy of these electrons is used to
pump hydrogen ions out of the inner mitochondrial compartment.
Hydrogen ions accumulate in the outer compartment, producing a
concentration gradient across the inner membrane. Hydrogen ions
diffuse back into the inner compartment through a carrier protein
that adds a phosphate group to ADP, making ATP. At the end of the
electron transport chain, hydrogen ions and spent electrons combine
with oxygen molecules, O 2 , forming water molecules, H 2 O.

FADH 2

SECTION 3 Cellular Respiration 107

http://www.scilinks.org

Topic:Aerobic Respiration

Keyword:HX4004

In the inner membranes of mitochondria, electron transport chains (represented by the red lines) make ATP.

Figure 13 Electron transport chain of aerobic respiration

H+

H+

H+

H+

H+

H+ H+

H+

H+

H+

H+

H+

P

e-

e-

Inner compartment

Outer compartment

ATP-producing

carrier protein

Inner

mitochondrial

membrane

+

NAD+

2H 2 O

4H+ + O 2

ADP +

NADH

AT P

1. The electron transport
   chain pumps hydrogen
   ions, H+, out of the
   inner compartment.
   2. At the end of the chain,
   electrons and hydrogen
   ions combine with
   oxygen, forming water.
   3. ATP is produced as hydrogen ions
   diffuse into the inner compartment
   through a channel protein.