Human Physiology, 14th edition (2016)

Cell Structure and Genetic Control 59

This symbiosis might not always benefit the host; for exam-

ple, mitochondria produce superoxide radicals that can provoke

an oxidative stress (chapters 5 and 19), and some scientists

believe that accumulations of mutations in mitochondrial DNA

may contribute to aging. There are more than 150 mutations

of mitochondrial DNA presently known to contribute to dif-

ferent human diseases. Examples include Leber’s hereditary

neuropathy, where patients undergo sudden loss of vision as

young adults, and MELAS (an acronym for myopathy, encepha-

lopathy, lactic acidosis, and stroke-like episodes), a disorder

affecting many organ systems. A mitochondrial DNA has only

37 genes that code for only 13 proteins (as well as 2 rRNAs and

22 tRNAs). The proteins are needed for oxidative phosphoryra-

tion (chapter 5, section 5.2), an essential part of aerobic respira-

tion performed by mitochondria. However, each mitochondrion

contains approximately 1,500 proteins, most of which are coded

by DNA in the cell nucleus. Because of this, mitochondrial dis-

eases may be produced by mutations in nuclear as well as mito-

chondrial DNA.

Neurons obtain energy solely from aerobic cell respiration

(a process that requires oxygen, described in chapter 5), which

occurs in mitochondria. Thus, mitochondrial fission (division)

and transport over long distances is particularly important in

neurons, where axons can be up to l meter in length. Mito-

chondria can also fuse together, which may help to repair those

damaged by “reactive oxygen species” generated within mito-

chondria (chapters 5 and 19).

Although mitochondria are needed for aerobic cell respira-

tion and are thus essential for the life of the cell, the produc-

tion of reactive oxygen species by mitochondria can kill the

Mitochondria

All cells in the body, with the exception of mature red blood

cells, have from a hundred to a few thousand organelles called

mitochondria (singular, mitochondrion ). Mitochondria

serve as sites for the production of most of the energy of cells

(chapter 5, section 5.2).

Mitochondria vary in size and shape, but all have the same

basic structure ( fig.  3.9 ). Each mitochondrion is surrounded

by an inner and outer membrane, separated by a narrow inter-

membranous space. The outer mitochondrial membrane is

smooth, but the inner membrane is characterized by many folds,

called cristae, which project like shelves into the central area

(or matrix ) of the mitochondrion. The cristae and the matrix

compartmentalize the space within the mitochondrion and have

different roles in the generation of cellular energy. The structure

and functions of mitochondria will be described in more detail

in the context of cellular metabolism in chapter 5.

Mitochondria can migrate through a cell, combine together

in a process called fusion, and reproduce themselves in a pro-

cess called fission. Indeed, mitochondria contain their own

DNA. All of the mitochondria in a person’s body are derived

from those inherited from the mother’s fertilized egg cell.

Thus, all of a person’s mitochondrial genes are inherited from

the mother. Mitochondrial DNA is more primitive (consisting

of a circular, relatively small, double-stranded molecule) than

that found within the cell nucleus. For this and other reasons,

many scientists believe that mitochondria evolved from sepa-

rate organisms, related to bacteria, that invaded the ancestors

of animal cells and remained in a state of symbiosis.

Figure 3.9 The structure of mitochondria. (a) Electron micrograph showing a few mitochondria. Notice the cristae formed

from the inner mitochondrial membrane. (b) A diagram of a mitochondrion.

Matrix

Inner mitochondrial

membrane

Outer mitochondrial

membrane

Cristae

(a) (b)

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