The lungs serve to exchange the two gases in the blood. Oxygen enters the blood from the lungs and carbon dioxide is expelled out of the blood into the lungs. The blood serves to transport both gases. Oxygen is carried to the cells. Carbon dioxide is carried away from the cells.
What Do They Look Like?
Mitochondria can be a variety of shapes, but you usually see them drawn like a peanut. Well, a peanut with a funny squiggly blob thing inside it, actually. This represents a double membrane: a mitochondrion has a smooth outer membrane and an inner membrane which is very folded and wrinkly. The stuff inside both membranes is called 'the matrix' and the bit in between both membranes is called 'the intermembrane space'. (It would seem that certain biologists don't have much imagination when it comes to naming things). The folds of the inner membrane are called cristae.
Mitochondria have two membranes (not one as in other organelles). The outer membrane covers the organelle and contains it. The inner membrane folds over many times (cristae). That folding increases the surface area inside the organelle. Many of the chemical reactions happen on the inner membrane of the mitochondria. The increased surface area allows the small organelle to do as much work as possible. If you have more room to work, you can get more work done. Similar surface area strategies are used by microvilli in your intestinal cells. The fluid inside of the mitochondria is called the matrix.
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