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 (see chapter 5).
Mitochondria vary in size and shape, but all have the same basic structure (fig. 3.10). Each mitochondrion is surrounded by an inner and outer membrane, separated by a narrow intermem-branous 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 the cytoplasm of a cell and are able to reproduce themselves. Indeed, mitochondria contain their own DNA. This is a more primitive form of DNA (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 separate organisms, related to bacteria, that invaded the ancestors of animal cells and remained in a state of symbiosis.
/Inner mitochondrial membrane
/Outer mitochondrial membrane
■ Figure 3.10 The structure of a mitochondrion. (a) An electron micrograph of a mitochondrion. The outer mitochondrial membrane and the infoldings of the inner membrane—the cristae—are clearly seen. The fluid in the center is the matrix. (b) A diagram of the structure of a mitochondrion.
An unfertilized ovum (egg cell) contains numerous mitochondria, and upon fertilization, gains few if any mitochondria from the sperm. The mitochondrial DNA replicates itself and the mitochondria subsequently divide by pinching off, so that mitochondria can enter the proliferating cells of the embryo and fetus. Thus, all (or nearly all) of the mitochondria in a person are ultimately inherited from that person's mother. This provides a unique form of inheritance that is passed only from mother to child. A rare cause of blindness known as Leber's hereditary optic neuropathy, as well as several other disorders, are inherited only along the maternal lineage and are known to be caused by defective mitochondrial DNA.
■ Figure 3.11 A ribosome is composed of two subunits. This is a model of the structure of a ribosome, showing the smaller (lighter) and larger (darker) subunits. The space between the two subunits accommodates a molecule of transfer RNA, needed to bring amino acids to the growing polypeptide chain.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.