The first step in cellular respiration is called glycolysis. This is where the 6-carbon glucose breaks down into two 3-carbon molecules (pyruvic acid), two ATP, and NAHD ( a co-enzyme). The amino acids and fatty acids, however, get turned into acetyl CoA. After this the two molecules of pyruvic acid or acetyl CoA go though what is either called the Citrus Acid Cycle or the Kreb Cycle . In this cycle, which takes place in the matrix of the mitochondria, the pyruvic acid or Acetyl CoA reacts to water, and in turn makes carbon dioxide, 10 hydrogen atoms, and two more ATP.(3) These hydrogen atoms are then transported to the cristae by co-enzymes. Here in the mitochondria, the cristae use the hydrogen's electrons in the electron transport chain. The electrons are pulled away from the hydrogen atoms and sent through the electron transport chain where some will combine with other protons and oxgyen to make water. This is called oxidation. If they do not make water, those electrons will combine with trapped protons in the inner membrane of the mitochondria. Here in the inner membrane are ATP synthetases. The ATP synthetase will then add a phosphate, called phosphorylation, to the molecules going by, hence creating ATP.(1) In this final step, up to 32 molecules of ATP can be produced. This combines with the two produced in glycolysis and the two produced from the citrus acid to make a total of 36 molecules of ATP produced.(3) ATP is used all over the cell, because it is the main source of energy for the cell. It can be used for things like active transport in the plasma membrane . (1) "Mitochondria," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. (2) Campbell, Neil A. Biology Third Edition. Redwood City, CA: The Benjamin/Cummings Publishing Company, Inc. 1993. 133-134
(3) (No date). Cellular Respiration Introduction. [Online]. Available: http://step.sdsc.edu/personal/vanderschaegen/outlines/respiration.html[2001, August 17].
The mitochondrion is an organelle found in the eukayotic cell. Under a microscope it is shaped as a kidney bean. The more energy a cell needs, the more mitochondria it has in it. For example, the muscle cell has more mitochondria then a blood cell. The mitochondrion has two membranes, the outer houses the proteins which help in ATP production. The inner membrane incloses cristae, where the actual synthesis of ATP occurs. Inside the cristae is a fluid called the matrix where there are enzymes which take part in the ATP synthesis. In the walls of the inner membrane are ATP sythetases which are used in cellular respiration.(1)The mitochondria also has small amounts of DNA and free ribosomes .
So, let's just say you went to Outback Steakhouse and had a nice, juicy steak. What does the mitochondria do to your now digested steak to give you energy? First, the mitochondria will receive the steak as glucose. The mitochondria can also recieve proteins in the steak in the form of amino acids, or the fat from the steak as glycerol and fatty acidsThese will probably be received from peroxisome which breaks big things like fat, into small particles like fatty acids that the mitchondria can use more efficiently(2). From there the mitochondria will assist in a process known as cellular respiration to turn the glucose, amino acid, or fatty acid into a usable form of energy called ATP (adenosine triphosphate). 
