- Lipids

We have seen that Phospholipids form an unique structure when exposed to water. The polar heads all turn outward to form H-bonds with the water molecules, while the hydrophobic lipid tails are hidden in the inside. This Phospholipid bilayer structure forms the membrane that surrounds each of your cells and plays an important role in regulating cellular function.

Cholesterol is just another lipid found in the plasma membrane. It is an important part of a healthy body since cholesterol is used as part of the cell membranes, and also as part of some hormones. But a high level of cholesterol in the blood - hypercholesterolemia- is a major risk factor for coronary heart disease, which can lead to heart attacks.

Cholesterol and other fats can’t dissolve in the blood (since they are Hydrophobic). They have to be transported to and from the cells by special lipid carriers called lipoproteins. There are several kinds, but the ones to be most concerned about are low density lipoprotein (LDL) and high density lipoprotein (HDL).

Low density lipoprotein is the major cholesterol carrier in the blood. When a person has too much LDL cholesterol circulating in the blood, it can slowly build up within the walls of the arteries feeding the heart and brain. Together with other substances it can form plaque, a thick, hard deposit that can clog those arteries. This condition is known as arteriosclerosis.

Trans Fatty Foods

Nutritional labels are not currently listing a possibly dangerous fat in the foods we eat. The culprit is called trans fat, which is structurally different from saturated fat or cholesterol. Also called stealth or phantom fat, is created during the process called partial hydrogenation, which involves turning liquid vegetable oils to solid shortening. Partially hydrogenated oils are used to make a wide variety of foods on supermarket shelves including some cookies and snacks. Although some of these foods claim to be low in calories or fat or cholesterol, the numbers are not listed for trans fat, which could be as bad or worse. Research indicates that in some cases trans fat raises blood cholesterol and increases the risk of heart disease. Both saturated fat and trans fat raise the amount of LDL or bad cholesterol, but trans fat also lowers the amount of HDL or good cholesterol.

The formation of a clot (or thrombus) in the region of this plaque can block the flow of blood to part of the heart muscle and cause a heart attack. If a clot blocks the flow of blood to part of the brain, the result is a stroke. A high level of LDL cholesterol reflects an increased risk of arteriosclerosis and heart disease. That is why LDL cholesterol is often called "bad" cholesterol.

About one-third to one-fourth of blood cholesterol is carried by high density lipoprotein or HDL. Medical experts think HDL tends to carry cholesterol away from the arteries and back to the liver, where it’s passed from the body.

Some experts believe HDL removes excess cholesterol from arteriosclerosis plaques and thus slows their growth. HDL is known as "good" cholesterol because a high level of HDL seems to protect against heart attack. The opposite is also true: a low HDL level indicates a greater risk.

Lipid Hormones - Testosterone is responsible for sexual maturation at all stages of male development throughout life. Women also secrete small amounts of testosterone from their ovaries. Synthetically, testosterone is prepared from cholesterol, the molecules are fairly similar. Anabolic steroids, derivatives of testosterone, have been used illicitly and are now controlled substances. Testosterone is also schedule C-III controlled substance.

Hydrocortisone is a steroid hormone secreted by the adrenal cortex. The anti-inflammatory effects of Hydrocortisone are believed to be due to modification of enzyme action rather than to a direct hormone-induced action. Hydrocortisone has no anabolic effects.

More membrane stuff, as we said: Phospholipids and cholesterol form the Plasma membrane (membrane around each cell). The same type of membrane structures formed by phospholipids and cholesterol are found inside the cell as well as around it. The nucleus, mitochondria and endomembrane system all are surrounded by their own phospholipid-bilayers.

The membranes around these internal structures compartmentalize the biochemical reactions that occur in each organelle". The Nucleus contains DNA. The mitochondria perform catabolic reactions releasing the Energy from sugars. The endomembrane system synthesizes proteins, manufactures lipids, and transports them to various places within the cell.

Proteins are embedded in the Phospholipid-bilayer of the plasma membrane. These proteins regulate the passage of molecules into, and out of, the cell. You can compare the plasma membrane to a fence around the cell. Actually, it is more like a chain-link fence, in that small molecules like water can sneak through the fence. Larger molecules, however, have to enter through gates in the fence, protein gates.

Like gates in a fence, these proteins control what goes in and out of the cell. Glucose uptake, salt and Ion balance, amino acids, and nucleotides,every large molecule that enters or exits the cell, has to pass through these protein gates.

Cells also have membrane proteins that attach the various cells together, identify each cell, and receive signals from other cells (e.g. hormones).

The plasma membrane and its associated proteins are one of the most important elements of cell structure and Biology. The proteins found in the plasma membrane regulate growth and development, the immune system, nerve impulses, and play a major part when cells become Cancerous.

Let's take a closer look at these "most important molecules" of cells, the Proteins.