Question

Modified from Critical Thinking Question 1 (page 49): Lipoprotein particles are relatively large clumps of protein and lipid molecules that circulate in the blood of mammals. They are like suitcases that move cholesterol, fatty acid remnants, triglycerides, and phospholipids from one place to another in the body. Consider what you know about the different types of lipids, their structures, and their solubility in water. For each component of lipoproteins, state whether they would be sheltered inside the particle or on the outside of the lipoprotein clump, bathed in the water-based fluid of blood. a) Cholesterol b) Fatty acid remnants c) Triglycerides d) Phospholipids e) Proteins

Answer 1

The subject of lipoproteins, how they are structured, the reason for such a structure, and their function, is a pretty compex, but unique one.

Essentially, these lipoproteins have the task of transporting substances, especially fats (fatty acids, triglycerides and cholesterol), that cannot simply float around freely in the bloodstream due to their non-polarity, from the tissues and organs that process them, to those that use them for their various tasks. Lipoproteins, thus are known as transport proteins composed of spherical, or globular units of proteins (known as apoproteins), esthered cholesterol and phospholipids, that surround the lipid cargo and protect it from the watery surroundings in blood plasma.

The fact of any of these units to be, or not be, exposed to the watery composition of blood, in plasma, comes explained by whether they are water soluble, and their polarity. As such, the answer to each of these components in the question, would be:

a) Cholesterol: sheltered from the blood by the lipoprotein carrier. The only exception is when the cholesterol unit has been re-estherized, which happens in the liver.

b) Fatty acids: also sheltered and carried by lipoproteins, especially when they are taken in through the diet. They are transported within chylomycrons.

c) Triglycerides: also sheltered from plasma, carried by lipoproteins and released into muscle tissue and adipose tissue for use.

d) Phospholipids: They are components of the lipoprotein, essential since their two external layers are hydrophilic and not hydrophobic. They form, with apoproteins, the protective covering for lipids.

e) Proteins: they are not sheltered, they are part of the lipoprotein covering the lipids, and therefore are not insoluble in water.