What Are Lipoproteins? - Definition, Functions & Types

Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

Have you ever wondered what the doctor is measuring when he does a lipid screen? Or why a lipid screen is even necessary to begin with? In this lesson, you will learn about how fats and lipids travel through your body and what those numbers really mean.

Lipoproteins are in Your Blood

Often times, during a physical exam, your nurse takes a blood sample. Then a few days later, you get a letter in the mail with a bunch of acronyms and numbers. Some of those acronyms represent structures in your blood called lipoproteins. Like the name suggests, lipoproteins are made up of lipids and proteins. Before we get into more detail, lets first review the different types of macromolecules and what happens when we eat them.

The Molecules We Eat

All living things are made up of four main types of molecules:

  • Sugars and carbohydrates
  • Amino acids and proteins
  • Nucleotides and nucleic acids (i.e. DNA)
  • Fatty acids and lipids (aka fat)

Humans eat living or previously living things, so the foods we eat are made up of those four categories of molecules. When we eat food, our digestive tract breaks down carbohydrates into sugars, lipids into fatty acids, and so forth. Then cells lining the small intestine called epithelial cells transfer the smaller molecules to our blood stream so they can be used by the rest of the cells in our body.

Lipids are Hydrophobic

On average, our bodies are made up of 60% water. Most of that is inside our cells and in our blood. Many of the molecules we eat are hydrophilic, or water soluble, which means they easily dissolve in water. In order to be transported in our blood, the molecules we eat must be water-soluble. However, lipids and fatty acids are hydrophobic (literally water-hating) and cannot dissolve in water on their own.

Think of a vinaigrette dressing. If you set out the vinaigrette undisturbed on a table, the oil in the vinaigrette, which is made up of hydrophobic lipids, will eventually separate from the water and vinegar and rise to the top.

So the body has a problem. How does it transport fatty acids and lipids around the body to the tissues that need them, if the molecules won't dissolve in the blood?

Phospholipids and Proteins Make Lipids Soluble

Your body solves the problem above by coating fatty acids with proteins and a special lipid called a phospholipid. A phospholipid is a molecule with an identity crisis. One end is composed of fatty acids and is hydrophobic; the other end has a phosphate group, which is hydrophilic. When phospholipids are dissolved in water a bunch of them get together to form little balls with their fatty acid 'tails' toward the center and their hydrophilic 'heads' on the outside interacting with water molecules.

When fatty acids from your food reach your small intestine, they are absorbed into epithelial cells. Once inside, three fatty acids combine to form a triglyceride. Triglyceride molecules clump together and then are coated with phospholipids and a few proteins, which altogether form a chylomicron, the first type of lipoprotein. Chylomicrons also contain a fat-soluble, steroid molecule called cholesterol. Your body needs cholesterol in order to make steroid hormones and cell membranes.

Once chylomicrons are released into circulation, they travel to the liver where some of the lipids are extracted for use in the liver. Next, they travel to adipose tissue (aka fat tissue) where more lipids are extracted and stored. Then they circulate to various parts of the body where cells take what lipid and cholesterol they need to survive or divide. Finally, they return to the liver so the body can dispose of the excess cholesterol it doesn't need.

Lipoproteins have various amounts of lipid, protein, phospholipids and cholesterol. Lipoproteins start out as chylomicrons. As lipids and cholesterol are removed by parts of the body, they become VLDL, LDL and HDL.
The different forms of lipoproteins are Chylomicrons, VLDL, LDL and HDL

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