Answer to Question 1
When necessary, the liver can assemble different lipoproteins, which are known as very-low-density lipoproteins (VLDL). As the body's cells remove triglycerides from the VLDL, the proportions of their lipid and protein contents shift. As this occurs, VLDL become cholesterol-rich low-density lipoproteins (LDL). Cholesterol returning to the liver from other parts of the body for metabolism or excretion is packaged in lipoproteins known as high-density lipoproteins (HDL). HDL are synthesized primarily in the liver. The density of lipoproteins varies according to the proportion of lipids and protein they contain. The more lipids in the lipoprotein molecule, the lower the density; the more protein, the higher the density. Both LDL and HDL carry lipids around in the blood, but LDL are larger, lighter, and filled with more lipid; HDL are smaller, denser, and packaged with more protein. LDL deliver cholesterol and triglycerides from the liver to the tissues; HDL scavenge excess cholesterol from the tissues and return it to the liver for metabolism or disposal. These different functions explain why some people refer to LDL as bad cholesterol and HDL as good cholesterol. Keep in mind, though, that there is only one kind of cholesterol molecule; the differences between LDL and HDL reflect proportions of lipids and proteins within themnot the type of cholesterol. The distinction between LDL and HDL has implications for the health of the heart and blood vessels. High concentrations of LDL in the blood are associated with an increased risk of heart disease, as are low concentrations of HDL.
Answer to Question 2
b
Composition of lipoproteins: (a) HDL; (b) LDL; (c) VLDL
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