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By Root 455 0

the inside from the outside permits certain molecules to go in and out of the cell while others remain only on the outside or inside. The principal components of these cell membranes are phospholipids. A phospholipid is composed of two hydrocarbon chains, typically 16 carbons long, connected at one end by a head group structure that contains both a positive and a negative charge. The charges of the head group make the head groups highly hydrophilic (attracted to water). The hydrocarbon chains are very hydrophobic (repelled by water). Cells are surrounded by water and also contain a great deal of water on the inside. The charged head groups want to be in water, while the hydrocarbon tails want to avoid water. To satisfy both the hydrophilic-charged head groups and the hydrophobic hydrocarbon tails, the phospholipids arrange themselves into a bilayer, as shown schematically in Figure 16.8.

The figure shows a section of a phospholipid bilayer membrane that completely surrounds and encloses cells. The balls are the charged head groups, and the wavy lines represent the hydrocarbon chains. An actual cell membrane is much more complex than illustrated in Figure 16.8. The membrane contains a large number of proteins that perform specific functions, such as allowing certain ions or molecules to pass into the cell while preventing others from entering.

FIGURE 16.8. Schematic of a portion of a phospholipid bilayer with two cholesterol molecules. The head groups (balls) are charged and want to be in water. The hydrocarbon tails avoid water by formation of the bilayer. The cholesterol hydroxyl is at the water interface.

In addition to phospholipids, a major component of cell membranes is cholesterol. Cholesterol comprises as much as 30% of cell membranes. Figure 16.8 shows a schematic of two cholesterol molecules replacing two of the phospholipids. Cholesterol is important because it controls the mechanical properties of the bilayers. Without cholesterol, cell membranes would not function. Therefore, cholesterol is essential. The human body produces a great deal of cholesterol, and only a limited fraction of the necessary cholesterol is taken in through food. The take-home message is that if you could eliminate all of the cholesterol from your body, you would die.

The Problem with Cholesterol

The problem with cholesterol is not that you take some in when you eat, but rather how it behaves in the body. Harmful effects on health from cholesterol are related to fat, but not because cholesterol is a fat or even because fatty foods may contain cholesterol. Cholesterol moves through the bloodstream associated with very large biomolecular aggregates called lipoproteins. These are composed of very large proteins, phospholipids, fats, cholesterol, and other molecules. The lipoproteins can be divided into at least two classes: low-density lipoproteins (LDL) and high-density lipoproteins (HDL). They are somewhat egg shaped, with an approximate diameter of 200 Å (200 × 10-10 meters). The volume of these particles is ap proximately 5,000,000 Å3. In contrast, the volume of a cholesterol Å3 molecule is approximately 200 Å3. So, an LDL or HDL particle is about 20,000 times larger than a cholesterol molecule and carries many cholesterol molecules in the bloodstream. High levels of LDL relative to HDL are strongly correlated with coronary artery disease and atherosclerosis. The mechanism is not well understood, but LDL-carrying cholesterol leads to deleterious deposits in the arteries, while HDL does not. These high levels of LDL, when compared to HDL (a high LDL to HDL ratio), are produced by saturated fats and, even worse, trans fats. Saturated fats increase the level of LDL. Trans fats not only increase LDL levels, but they also decrease HDL levels, exacerbating the problem. So eating fat does matter, but not because fatty foods can contain cholesterol. What matters is the nature of the fat we eat. Oils that are high in polyunsaturated fats, which have not been processed in a manner that produces large quantities of trans fats, are