# Lipoproteins --- **Lipoproteins** are spherical compounds that are used to transport [[polar and nonpolar|hydrophobic]] lipids though the bloodstream and other places in the body. Lipoproteins are not to be confused with [[Gram negative bacteria|bacterial lipoproteins]], which are sometimes just referred to as "lipoproteins", but are distinct structures. They are basically a droplet of fat surrounded by a single layer of [[phospholipid|phospholipids]] (usually). They also have [[membrane proteins]] in the membrane, and [[cholesterol|cholesterols]] dispersed throughout ![[lipoprotein.png]] Lipoproteins come in four classes, which vary according to their makeup (percent triglyceride vs cholesterol vs protein) and their roles. ![[lipoprotein types.png]] %% Caption %%<center> CH = cholesterol, CE = cholesterol ester, TG = triglyceride, PL = phospholipid</center> ![[lipoprotein makeups.png]] The more triglyceride is in the composition, the less dense they are. The more protein, cholesterol and phospholipid they are, the more dense they are. ## Chylomicrons **Chylomicrons** are the largest and least dense of the lipoproteins. They are created in the [[enterocytes]] of the large intestine, and their main function is to transport freshly [[nutrient absorption|absorbed]] lipids to the [[liver]], first by traveling through the lymph, then the blood. As they are [[lymphatic system#Direct Entry|dumped wholesale]] into the blood stream, bypassing the [[hepatic portal system]], they can be absorbed by cells that need the lipids. These are only present directly after a meal, which is why you need to fast for a cholesterol blood draw. ## VLDLs **Very-low-density lipoproteins**, or **VLDLs** are formed mainly in the [[liver]], but also sometimes in the [[intestines]] in order to transport endogenous lipids (mainly [[triglycerides]]) to various tissues, including [[adipose tissue|adipocytes]] to put into storage. They are more dense than chylomicrons but still over half triglyceride. Once they drop off some of their payload, they become LDLs. ## LDLs **Low-density lipoproteins**, or **LDLs** (commonly known as "bad cholesterol") are formed from VLDLs once they drop off some of their [[triglycerides]]. They also circulate the blood, looking to drop off their [[cholesterol|cholesterols]] at special LDL-receptors. If they are unable to drop those off, they are broken down by the liver. This unfortunately frees up the cholesterols just straight into the blood, which increases the risk of cardiovascular disease. Eating saturated fats and trans fats block the special LDL receptor sites and thereby increase the level of cholesterols in the bloodstream. ## HDLs **High-density lipoproteins**, or **HDLs** (commonly known as "good cholesterol") are actually formed from scratch in the [[liver]]. Appropriately enough they are the smallest and densest of the lipoproteins in the blood. Their job is to circulate around and pick up extra [[cholesterol]] dropped by dying cells or other lipoproteins. They the bring these excess cholesterols to the [[liver]] where they can be recycled or used to make [[bile]], which is eventually expelled from the body. Bringing cholesterol back to the liver is called *reverse cholesterol transport* ![[lipoprotein action.png]] ___