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The CELL (You...just a million times smaller)
Mitochondria

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 After the nucleus, the Mitochondria is the second most important organelle. It's so important that it has it's own set of DNA which accounts for up to 1% of the cell's total DNA. Just why is this organelle so important? It's because the mitochondria is the "powerhouse" of our cells. It is in this organelle that our energy (ATP) is made. The mitochondria converts the chemical energy from the food we eat to the chemical energy stored in the bonds of ATP (adenosine triphosphate). This is done by the life process of cellular respiration.
 Briefly, here is what cellular respiration is about. There are two types of respirations: anaerobic and aroebic. The former doesn't need the use of oxygen or the mitochondria. It happens in the cell's cytoplasm. We use anaroebic respiration during strenuous conditions. The latter, which we use on a far more regular basis, does need oxygen and involves the mitochondria.
 The mitochondria. like the nucleus, has two membranes. The outer mitochondrial membrane is smooth. By contrast, the inner mitochondrial membrane has many folds (called cristae) that extend into the space enclosed by the second membrane (referred to as the matrix). The intermembrane space between the two membranes is used to hold protons during respiration. It holds enzymes to break down food molecules.
 The amount of mitochondria in a cell depends on what type of cell it is. The liver cells (which are some pretty busy cells) have over 1,000 mitochondria in each cell.
 It was mentioned that the mitochondria has it's own set of DNA. It has been found that this mitochondrial DNA has far more frequent mutations than the DNA in our nucleus. For this reason, frequent mutations lead to a decrease in generating energy and this may aid in aging. The mitochondria also sometimes leaks electrons which from dangerous compounds called free radicals that disrupt normal activity. The mitochondria also play an important role in programmed cell death (apoptosis).

For a more thorough look at cellular respiration, click here