Chylomicrons are just one class of lipoproteins, which can transport hydrophobic lipid molecules in blood plasma.
Lipoproteins are classified by their density, size and lipid composition, as measured by experimental centrifugation. Density is related to the relative content of lipids to proteins.
Proteins are more dense, so a lipoprotein with a higher ratio of proteins to lipids, will be of higher density. Chylomicrons are the least dense, with the highest ratio of lipids to proteins, then Very Low-Density Lipoproteins (VLDL), Intermediate-Density Lipoproteins (IDL), Low-Density Lipoproteins (LDL) and High-Density Lipoproteins (HDL).
Each lipoprotein has specific protein components, called apolipoproteins, which bind to and help solubilize hydrophobic lipids in the blood for easy transport of lipoprotein complexes to specific locations in the body.
Classified by density, as measured by centrifugation.
Lipids are less dense than proteins, so
the lipid content is inversely related to its density. All share common structural features with chylomicrons.
LDLs are taken up from blood by receptor-mediated endocytosis via the LDL receptor. On slide, you can see labeled LDL binding to "pits" on the surface of the cells coated with a specific protein.
Cholesterol is obtained by tissues from dietary cholesterol and liver-synthesized cholesterol. Excess LDL cholesterol accumulates on arterial walls, attracting white blood cells. If levels are too high to be removed, macrophages become engorged and harden into plaques (atherosclerosis), blocking blood vessels and can cause heart attack.
Receptors are clustered in a structure called a "coated pit", an invagination with an intracellular protein called clathrin. Clathrin is a protein that plays a major role in the formation of coated vesicles. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. Clathrin interacts with itself, forming a cage, bringing large extracellular molecules into the cell.
LDL receptors are synthesized in ER, matured in Golgi and go to cell surface in coated pits.
LDL binds via Apo B-100 to LDL receptor, is internalized endocytic vesicles, which form endosomes. Fuse with lysosome, hydrolytic enzymes and Proton pumping lowers pH and dissociates LDL from receptors. Free cholesterol, amino acids and cholesterol esters. Receptors are recycled to surface.