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As we discussed in Chapter 4, ATP is the unit of energy in biological systems. This molecule is generated via the process of respiration in the mitochondria. Like the nucleus, mitochondria have two membranes (see Figure 10.2). The outer membrane is smooth and faces the cytoplasm. However, unlike the nucleus, the inner membrane is highly convoluted and forms “folds” in the interior of the organelle. These folds are called cristae. Two distinct compartments are bounded by the membranes: an outer compartment is formed between the two membranes, and an inner compartment (called the matrix) is bounded by the inner membrane. Most of the processes of respiration critical for the synthesis of ATP occur across the inner membrane, and therefore take place between the matrix and the outer compartment.
The structure of mitochondria is very similar to some bacteria. They contain their own circular DNA and divide on their own. It has been theorized that these organelles evolved from bacteria that were engulfed, but not digested, by other cells. This is known as the endosymbionic hypothesis.

A mitochondrion

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