Biology 12

70 MHR • Unit 1 Metabolic Processes

concentration in the intermembrane space. This

process creates a positively charged gradient in

which there is a greater concentration of hydrogen

ions on one side of the membrane than the other. The

passage of these protons to the inner mitochondrial

compartment drives the synthesis of ATP.

Mitochondria are the main source of ATP

molecules in cells. When the mitochondria are not

functioning properly, a depletion of ATP molecules

can occur in tissues, such as muscle, brain, and

heart. These tissues require large amounts of energy

to function properly. Insufficient amounts of ATP can

result in cell damage and even cell death. Eventually,

entire body systems can fail, and the health of the

organism may be severely compromised.

The intact membranes of the mitochondria

are important to the overall health of the cell.

Mitochondria are believed to be key activators for

apoptosis, or programmed cell death. When the

mitochondrion produces ATP, the membranes are

polarized by the high concentration of H+ions in

the intermembrane space. One of the early steps

towards cell death is depolarization, the loss of

this concentration gradient. Pores in both the

outer membrane and inner membrane cause ions

to escape. This compromises the electron transport

chain, which is indirectly involved in ATP prod-

uction. Aerobic respiration stops. Cytochrome c, a

key component of the electron transport chain, is

released into the cytosol. Here, cytochrome c may

combine with ATP and a protein, forming an apop-

tosome, or apoptosis activator. Endonuclease and

protease enzymes in the cytosol are activated and

break down the nucleus and the rest of the cell.

The Transition Reaction

The transition reaction, which occurs in the

matrix of the mitochondrion, is the first step in the

process of aerobic cellular respiration. This process

continues as long as sufficient levels of oxygen are

available in the mitochondrion. If little or no oxygen

is available, pyruvate in the cytosol can be oxidized

through one of two fermentation processes. These

processes will be described later in this section.

Pyruvate crosses the mitochondrion’s outer

membrane and then enters the matrix by way of a

transport protein in the inner membrane (see

Figure 3.8). Once in the matrix, the pyruvate

dehydrogenase complex(a complex of enzymes)

aids the process of oxidative decarboxylation.

NAD+removes two electrons, oxidizing pyruvate.

Carbon dioxide is removed, leaving a two-carbon

acetyl group that combines with coenzyme A to

form acetyl-CoA. Coenzyme A is a compound that

To learn more about how oxygen and nutrient levels can

affect ATP production in mitochondria, go to your Electronic

Learning Partner now.

ELECTRONIC LEARNING PARTNER

Eukaryotic cells are believed to have arisen from a

relationship, called endosymbiosis, between two prokaryotic

cells, or bacteria. In this relationship, one cell, the

endosymbiont, lived inside the other cell, called the host

cell. The endosymbiont provided the host cell with a surplus

of ATP molecules, while the host cell provided some

metabolic functions for the endosymbiont. The endosymbiont

eventually took the role of the mitochondrion found in

eukaryotic cells of today.

BIO FACT

One carbon atom and two oxygen atoms are

removed from pyruvate as a CO 2 molecule.

A

The remaining two-carbon fragment is oxidized

to form an acetate ion. Electrons from this

reaction are picked up by NAD+, which is

reduced to form NADH.

B

The acetyl group of the acetate ion is transferred

to coenzyme A, forming acetyl CoA.

C

cytosol

transport protein

pyruvate

A

C

B

mitochondrion

outer membrane inner membrane matrix

inner membrane space

Coenzyme A acetyl CoA

NAD+

CO

CO

CH 3

O−

CO

S CoA

CH 3

CO 2

NADH+H+

Figure 3.8In the transition reaction, pyruvate is

converted to acetyl-CoA. This reaction marks the

junction between glycolysis and the Krebs cycle.