Phospholipids commonly serve which of the following functions

However, fat is probably the reason we are all here. Throughout history, there have been many instances when food was scarce. Our ability to store excess caloric energy as fat for future usage allowed us to continue as a species during these times of famine.

So, normal fat reserves are a signal that metabolic processes are efficient and a person is healthy. Lipids are a family of organic compounds that are mostly insoluble in water. Composed of fats and oils, lipids are molecules that yield high energy and have a chemical composition mainly of carbon, hydrogen, and oxygen. Lipids perform three primary biological functions within the body: they serve as structural components of cell membranes, function as energy storehouses, and function as important signaling molecules.

The three main types of lipids are triacylglycerols also called triglycerides , phospholipids, and sterols. Triacylglycerols also known as triglycerides make up more than 95 percent of lipids in the diet and are commonly found in fried foods, vegetable oil, butter, whole milk, cheese, cream cheese, and some meats. Naturally occurring triacylglycerols are found in many foods, including avocados, olives, corn, and nuts. As with most fats, triacylglycerols do not dissolve in water.

The terms fats, oils, and triacylglycerols are discretionary and can be used interchangeably. In this chapter when we use the word fat, we are referring to triacylglycerols. Phospholipids make up only about 2 percent of dietary lipids. They are water-soluble and are found in both plants and animals. In fact, phospholipids are synthesized in the body to form cell and organelle membranes.

In blood and body fluids, phospholipids form structures in which fat is enclosed and transported throughout the bloodstream. Sterols are the least common type of lipid. Cholesterol is perhaps the best well-known sterol. Though cholesterol has a notorious reputation, the body gets only a small amount of its cholesterol through food—the body produces most of it. Cholesterol is an important component of the cell membrane and is required for the synthesis of sex hormones, vitamin D, and bile salts.

Later in this chapter, we will examine each of these lipids in more detail and discover how their different structures function to keep your body working. The excess energy from the food we eat is digested and incorporated into adipose tissue, or fatty tissue.

Most of the energy required by the human body is provided by carbohydrates and lipids. As discussed in Chapter 3 "Carbohydrates" , glucose is stored in the body as glycogen. While glycogen provides a ready source of energy, lipids primarily function as an energy reserve. As you may recall, glycogen is quite bulky with heavy water content, thus the body cannot store too much for long.

Alternatively, fats are packed together tightly without water and store far greater amounts of energy in a reduced space. A fat gram is densely concentrated with energy—it contains more than double the amount of energy than a gram of carbohydrate. Energy is needed to power the muscles for all the physical work and play an average person or child engages in. Unlike other body cells that can store fat in limited supplies, fat cells are specialized for fat storage and are able to expand almost indefinitely in size.

An overabundance of adipose tissue can result in undue stress on the body and can be detrimental to your health. A serious impact of excess fat is the accumulation of too much cholesterol in the arterial wall, which can thicken the walls of arteries and lead to cardiovascular disease.

Thus, while some body fat is critical to our survival and good health, in large quantities it can be a deterrent to maintaining good health. Triacylglycerols also help the body produce and regulate hormones. For example, adipose tissue secretes the hormone leptin, which regulates appetite. In the reproductive system, fatty acids are required for proper reproductive health; women who lack proper amounts may stop menstruating and become infertile.

Omega-3 and omega-6 essential fatty acids help regulate cholesterol and blood clotting and control inflammation in the joints, tissues, and bloodstream. In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic or nonpolar region because of the fatty acid tails.

This region has no attraction for water or other polar molecules. Proteins make up the second major chemical component of plasma membranes. Integral proteins are embedded in the plasma membrane and may span all or part of the membrane.

Integral proteins may serve as channels or pumps to move materials into or out of the cell. Peripheral proteins are found on the exterior or interior surfaces of membranes, attached either to integral proteins or to phospholipid molecules.

Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins forming glycoproteins or to lipids forming glycolipids. These carbohydrate chains may consist of 2—60 monosaccharide units and may be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.

How Viruses Infect Specific OrgansSpecific glycoprotein molecules exposed on the surface of the cell membranes of host cells are exploited by many viruses to infect specific organs. For example, HIV is able to penetrate the plasma membranes of specific kinds of white blood cells called T-helper cells and monocytes, as well as some cells of the central nervous system.

The hepatitis virus attacks only liver cells. These viruses are able to invade these cells, because the cells have binding sites on their surfaces that the viruses have exploited with equally specific glycoproteins in their coats.

Figure 3. The cell is tricked by the mimicry of the virus coat molecules, and the virus is able to enter the cell. Antibodies are made in response to the antigens or proteins associated with invasive pathogens. These same sites serve as places for antibodies to attach, and either destroy or inhibit the activity of the virus. A second methylene group is attached by a single bond to the first methylene group, and to an oxygen molecule that is part of the phosphate group.

The phosphate group is comprised of a phosphate molecule attached by single bonds to four oxygen molecules in total. One of these oxygen molecules is attached by a single bond to a terminal methylene group of a glycerol molecule.

The glycerol molecule is a 3-carbon molecule. The central carbon is attached to a hydrogen molecule by a single bond, and the two terminal carbon molecules are both attached to two hydrogen molecules. One fatty acid tail is attached to the glycerol's terminal carbon that is not attached to the phosphate head, and a second fatty acid tail is attached to the glycerol's central carbon.

Each fatty acid is comprised of a terminal carboxyl group COO- that is attached to a long carbon chain. The carbon of each carboxyl group forms a double bond with one oxygen molecule and a single bond with the other oxygen molecule, which is connected by a single bond to the carbon of the glycerol backbone, and a single bond with a carbon from the backbone of the long carbon chain.

In phosphatidylcholine, each fatty acid tail contains 18 carbons, including the carbon of the carboxyl group. The carbons that make up the first tail are attached to each other by single bonds.

In the fatty acid chain bound to the glycerol's central carbon, the 9 th carbon in the chain is bound to the 10 th carbon in the chain by a double bond, causing a kink. Glycerophospholipids are by far the most abundant lipids in cell membranes.

Like all lipids, they are insoluble in water, but their unique geometry causes them to aggregate into bilayers without any energy input. This is because they are two-faced molecules, with hydrophilic water-loving phosphate heads and hydrophobic water-fearing hydrocarbon tails of fatty acids.

In water, these molecules spontaneously align — with their heads facing outward and their tails lining up in the bilayer's interior. Thus, the hydrophilic heads of the glycerophospholipids in a cell's plasma membrane face both the water-based cytoplasm and the exterior of the cell.

Altogether, lipids account for about half the mass of cell membranes. Cholesterol molecules, although less abundant than glycerophospholipids, account for about 20 percent of the lipids in animal cell plasma membranes. However, cholesterol is not present in bacterial membranes or mitochondrial membranes. Also, cholesterol helps regulate the stiffness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition.

In addition to lipids, membranes are loaded with proteins. In fact, proteins account for roughly half the mass of most cellular membranes. Many of these proteins are embedded into the membrane and stick out on both sides; these are called transmembrane proteins. The portions of these proteins that are nested amid the hydrocarbon tails have hydrophobic surface characteristics, and the parts that stick out are hydrophilic Figure 2.

At physiological temperatures, cell membranes are fluid; at cooler temperatures, they become gel-like. Scientists who model membrane structure and dynamics describe the membrane as a fluid mosaic in which transmembrane proteins can move laterally in the lipid bilayer. Therefore, the collection of lipids and proteins that make up a cellular membrane relies on natural biophysical properties to form and function. In living cells, however, many proteins are not free to move.

They are often anchored in place within the membrane by tethers to proteins outside the cell, cytoskeletal elements inside the cell, or both. Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly.

Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly. On the other hand, cell membranes restrict diffusion of highly charged molecules, such as ions, and large molecules, such as sugars and amino acids.

The passage of these molecules relies on specific transport proteins embedded in the membrane. Figure 3: Selective transport Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell.

Membrane transport proteins are specific and selective for the molecules they move, and they often use energy to catalyze passage. Also, these proteins transport some nutrients against the concentration gradient, which requires additional energy.

The ability to maintain concentration gradients and sometimes move materials against them is vital to cell health and maintenance. Thanks to membrane barriers and transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment and, conversely, dispose of waste products Figure 3. Other transmembrane proteins have communication-related jobs. These proteins bind signals, such as hormones or immune mediators, to their extracellular portions. Binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules.