What is serum?

Plasma is a translucent, yellowish fluid. Plasma can become milkier after digesting a lot of fat or when one has high level of lipids in the blood. Plasma is about 90% water; the other 10 percent of stuff dissolved in plasma is essential for life. These life-giving substances are circulated throughout the body where they are needed. Differences in concentration cause them to diffuse to where they are needed. Waste materials flow in the opposite direction.

How do we isolate serum? It's what's left over when plasma is left to clot. Hence, we allow blood to clot, letting the cells and the clotting factors (i.e. certain proteins) fall to the bottom. What you have left is serum.

Proteins make up a huge part of the 10% of material that is dissolved in plasma. They are responsible for providing oncotic pressure. Protein molecules are much larger than water molecules; as a result, they tend to stay in the blood vessels. These proteins are bioactive and cause can cause problems when serum is used in cell culture medium.

When serum is used in culture media, all of these bioactive molecules (such as the aforementioned proteins) wreak havoc with the assays. That is why it's advisable to use serum-free media instead. This eliminates the need for costly characterizations and all that rot. (See culture media.)

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