Biology (Holt)

Mitochondria

Nearly all eukaryotic cells contain many (miet uh
KAHN dree uh), like the one shown in Figure 16.A mitochondrion is
an organelle that harvests energy from organic compounds to make
ATP, the main energy currency of cells. Although some ATP is made
in the cytosol, most of a cell’s ATP is made inside mitochondria.
Cells that have a high energy requirement, such as a muscle cell,
may contain hundreds or thousands of mitochondria. Figure 16
shows that a mitochondrion has two membranes. The outer mem-
brane is smooth. The inner membrane is greatly folded, however,
and its surface area is large. The two membranes form two com-
partments, one inside and one outside the mitochondrion’s inner
membrane. It is here that the chemical reactions that produce ATP
during cell metabolism take place.

Mitochondrial DNA

The nucleus is not the only organelle in the cell that contains nucleic
acids. Mitochondria also have DNA and ribosomes, and mitochon-
dria make some of their own proteins. However, most mitochondrial
proteins are made by free ribosomes in the cytosol. Mitochondrial
DNA is independent of nuclear DNA and similar to the circular DNA
of prokaryotic cells. This fact supports the widely accepted theory
that primitive prokaryotes are the ancestors of mitochondria. You
will learn more about the origin of mitochondrial DNA later in
this book.

mitochondria

SECTION 3 Cell Organelles 65

Inner

membrane

Outer

membrane

In a eukaryotic cell, mitochondria make most of the ATP.

Figure 16 Mitochondrion

The word mitochondrionis

from the Greek words

mitos,meaning “a thread”

and chrondros,meaning

“grain.” Knowing this makes

it easier to remember that a

mitonchondrion is a small,

elongated cell organelle.