How Are Lipids Shuttled Around in Our Blood

Not only will our liver make a fair amount of cholesterol and fat on a daily basis, but it will also receive these nutrients from diet-derived chylomicrons. Like fat, most cholesterol is housed in the liver for only a short period of time as it is destined for other tissues throughout the body. Once cholesterol reaches other tissues, it can be used to make some of the substances listed previously or to become part of cell membranes. Some of the cholesterol in our liver is also used to make bile salts, a key component of bile.

Whether they are coming from the digestive tract or the liver special transportation vehicles or lipoproteins are needed to circulate lipids. Generally speaking, lipoproteins are a protein-containing shell encasing the lipid substances in need of transportation (Figure 5.9). Lipoproteins can be divided into four general classes based upon their densities (see Figure 5.9). In order of increasing density lipoproteins are chylomicrons, very low density lipoproteins (VLDLs), low density lipoproteins (LDLs), and high density lipoproteins (HDLs). Looking at the composition of these lipoproteins in Figure 5.9, we see that the greater the lipid to protein ratio, the lower the density. This makes perfect sense because lipids are less dense than proteins.

Protein rich outer "shell"

Protein rich outer "shell"

  • Protein
  • Phospholipid
  • Cholesterol ■ Triglyceride

Chylomicron VLDL

Figure 5.9 (a) Lipoproteins are lipids encased in a water-soluble protein shell.

(b) Our blood contains several types of lipoproteins, which can be separated based upon their density (lipid to protein ratio). Chylo-microns are the biggest and have the highest ratio, opposite to HDL.

The proteins that help make up the lipoprotein shell are called apopro-teins. Not only do they make the lipoprotein more soluble in water, but they will also function in helping the lipoprotein be recognized by specific tissues throughout our body. This allows a lipoprotein either to unload some of its lipid cargo or to be removed from the blood and broken down. For instance, the receptor for LDLs is located in the liver tissue and also in other tissue throughout the body. When a specific apoprotein on an LDL docks on the LDL receptor, this allows the LDL to be removed from the blood.

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