25. Self-Sustaining Chemical        Systems: Living Cells   Previous PageNext Page

Another important organelle also is topologically outside the cell, although it floats in the cytoplasm. This is the mitochondrion, which is illustrated in the electron micrograph at the right.

Mitochondria are the sites of the citric acid cycle, respiration, and ATP synthesis. More than a thousand mitochondria are found in a typical rat liver cell. They are highly variable in size and shape in different cells and organisms, but typically measure 5000 by 20,000 , or about the size of a bacterium. They are "outside" the cell because they are completely surrounded by a smooth outer membrane, which re sembles the cell membrane and which completely separates them from the cytoplasm. Within this outer membrane is an inner membrane that is highly convoluted and folded, with deep incursions into the heart of the mitochondrion.

If a mitochondrion were the size of an ordinary two-cell flashlight, its inner membrane would have the total surface area of eight 6ft x 6ft tablecloths - an impressive feat of folding! These deep infoldings of the inner membrane, called cristae, resemble the ER of the cell, and may perform similar functions of increasing surface area and access to the interior. The mitochondrion inside the inner membrane is filled with a gelatinous matrix, which is a semifluid with 50% protein content.

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