Chapter 17: Lipids and their functions in biochemical systems

Define the term glyceride.

A glyceride is a lipid ester that contains the glycerol molecule and from one to three fatty acids.

Define the term glycosphingolipid.

A glycosphingolipid or glycolipid is a lipid that is built on a ceramide backbone structure. Ceramide is a fatty acid derivative of sphingosine

Describe peripheral membrane proteins.

A peripheral membrane protein is bound to only one surface of the membrane, either inside or outside the cell

What is the difference between a saturated and an unsaturated fatty acid?

A saturated fatty acid is one in which the hydrocarbon tail has only carbon-to-carbon single bonds. An unsaturated fatty acid has at least one carbon-to-carbon double bond.

Define the term sphingolipid.

A sphingolipid is a lipid that is not derived from glycerol, but rather from sphingosine, a long-chain, nitrogen-containing (amino) alcohol. Like phospholipids, sphingolipids are amphipathic.

What do you predict the physical state would be of a triglyceride with three saturated fatty acid tails? Explain your reasoning

A triglyceride with three saturated fatty acid tails would be a solid at room temperature. The long, straight fatty acid tails would stack with one another because of strong intermolecular and intramolecular London dispersion force attractions

What is the mechanism of uptake of cholesterol from plasma?

Receptor-mediated endocytosis

What is a terpene?

They are structurally similar to the fused 4-ring system in steroids and are the metabolic precursor to this important class of biomolecules.

List the four main groups of lipids.

Fatty acids, glycerides, nonglyceride lipids, and complex lipids

What is meant by the term fused ring?

Fused rings share one or more common bonds as part of their ring backbone structures.

Distinguish among the three types of glycosphingolipids, cerebrosides, sulfatides, and gangliosides.

Glycosphingolipids include the cerebrosides, sulfatides, and gangliosides and are built on a ceramide backbone (a fatty acid derivative of sphingosine.) Cerebrosides are characterized by the presence of a single monsaccharide head group. Sulfatides are derivatives of galactocerebroside (a cerebroside having galactose as its monosaccharide head group) that contain a sulfate group. Gangliosides are glycolipids that possess oligosaccharide groups, including one or more molecules of N-acetylneuraminic acid.

How does a genetic defect in the LDL receptor contribute to atherosclerosis?

If the LDL receptor is defective, it cannot function to remove cholesterol-bearing LDL particles from the blood. The excess cholesterol, along with other substances, will accumulate along the walls of the arteries, causing atherosclerosis

How will the properties of a biological membrane change if the fatty acid tails of the phospholipids are converted from saturated to unsaturated chains?

If the fatty acid tails of the membrane phospholipids are converted from saturated to unsaturated, the fluidity of the membrane will increase.

In terms of solubility, explain why a diet that contains no lipids can lead to a deficiency of the lipid-soluble vitamins

Lipid-soluble vitamins are transported into cells of the small intestine in association with dietary fat molecules. Thus, a diet low in fat reduces the amount of vitamins A, D, E, and K that enters the body

What do you predict the physical state would be of a triglyceride with three unsaturated fatty acid tails? Explain your reasoning.

A triglyceride with three unsaturated fatty acid tails would be a liquid at room temperature. The fatty acid tails would be "kinked" because each carbon-to-carbon double bond introduces a 30o bend. Thus, the fatty acid tails could not stack with one another.

Why are sphingomyelins amphipathic?

An amphipathic molecule has both polar and nonpolar regions. Sphingomyelins are amphipathic because they have a polar head group composed of a phosphoryl group bonded to choline, as well as a nonpolar region (tail) consisting of sphingosine and a long fatty acid hydrocarbon chain.

What are emulsifying agents and what are their practical uses?

An emulsifying agent is a molecule that aids in the suspension of triglycerides in water. They are amphipathic molecules, such as lecithin, that serve as bridges holding together the highly polar water molecules and the nonpolar triglycerides

What molecules are formed from arachidonic acid?

Arachidonic acid is a starting material for the biosynthesis of prostaglandins. The prostaglandins are hormone-like molecules that exert a variety of effects on the body

Do you think that a diet higher in omega-3 fatty acids would be an effective treatment for the symptoms of arthritis? Defend your answer

Arthritis is associated with inflammation of the joints. Since omega-3 fatty acids are precursors of prostaglandins that exhibit anti-inflammatory effects and omega-6 fatty acids are precursors to prostaglandins that have inflammatory effects, it seems logical that a diet rich in omega-3 fatty acids might help alleviate some of the painful symptoms of arthritis.

As the number of carbon-carbon double bonds in fatty acids increases, what is the effect on the melting point?

As the number of carbon-to-carbon double bonds in the hydrocarbon chain of a fatty acid increases, the melting point decreases because each double bond introduces a 30 degree bend in the hydrocarbon chain. These "kinked" molecules cannot stack in an organized arrangement and thus have lower intermolecular London dispersion force attractions and lower melting points.

What is the biochemical basis for the effectiveness of aspirin in decreasing the inflammatory response?

Aspirin effectively decreases the inflammatory response by inhibiting the synthesis of all prostaglandins. Aspirin works by inhibiting cyclooxygenase, the first enzyme in prostaglandin biosynthesis. This inhibition results from the transfer of an acetyl group from aspirin to the enzyme. Because cyclooxygenase is found in all cells, synthesis of all prostaglandins is inhibited.

What is the relationship between atherosclerosis and high blood pressure?

Atherosclerosis results when cholesterol and other substances coat the arteries, causing a narrowing of the passageways. As the passageways become narrower, greater pressure is required to provide adequate blood flow. This results in higher blood pressure (hypertension)

Why do the hydrocarbon tails of membrane phospholipids provide a barrier between the inside and outside of the cell?

Because the fatty acyl tails of phospholipids are hydrophobic, they stack together forming a hydrophobic layer. Small, uncharged molecules, such as CO2, N2, and O2 may freely diffuse through this hydrophobic layer. However, ions and larger polar molecules are not able to do so. Thus, the hydrophobic region is a barrier between the inside and the outside of the cell.

How has our understanding of the steroid sex hormones contributed to the development of oral contraceptives

By understanding the role of natural steroid sex hormones in the process of pregnancy, similar compounds can be synthesized that will cause a false pregnancy and prevent pregnancy. These compounds have the advantage that they can be taken orally.

A wax found in the head of sperm whales is cetyl palmitate. What fatty acid and what alcohol are used to form this compound?

Cetyl palmitate (whale oil) is made up of the fatty acid palmitic acid and the alcohol called cetyl alcohol - CH3(CH2)14CH2OH.

What is the major effect of cholesterol on the properties of biological membranes?

Cholesterol is freely soluble in the hydrophobic layer of a biological membrane. It moderates the fluidity of the membrane by disrupting the stacking of the fatty acid tails of membrane phospholipids.

What is the role of cholesterol in biological membranes?

Cholesterol is readily soluble in the hydrophobic region of biological membranes. It is involved in regulating the fluidity of the membrane

How does cholesterol contribute to atherosclerosis?

Cholesterol, in combination with other substances, appears to coat arteries, causing atherosclerosis or hardening of the arteries. This narrows the passageway. As a result, greater pressure is needed to allow adequate blood flow.

Distinguish among the four plasma lipoproteins in terms of their composition and their function

Chylomicrons carry dietary lipids from the intestine to other tissues. They are approximately 4% phospholipids, 90% triglycerides, 5% cholesterol, and 1% protein.Very low density lipoproteins carry triglycerides synthesized in the liver to adipose tissue for storage. They are approximately 18% phospholipids, 60% triglycerides, 14% cholesterol, and 8% protein. Low density lipoproteins carry cholesterol to peripheral tissues and help regulate cholesterol levels in those tissues. They are approximately 20% phospholipids, 10% triglycerides, 45% cholesterol, and 25% protein. High density lipoproteins transport cholesterol from peripheral tissues to the liver. They are approximately 30% phospholipids, 5% triglycerides, 20% cholesterol, and 45% protein.

What is the function of each of the four types of plasma lipoproteins?

Chylomicrons carry dietary lipids from the intestine to other tissues. Very low density lipoproteins carry triglycerides synthesized in the liver to adipose tissue for storage. Low density lipoproteins carry cholesterol to peripheral tissues and help regulate cholesterol levels in those tissues. High density lipoproteins transport cholesterol from peripheral tissues to the liver.

What are the four major types of plasma lipoproteins?

Chylomicrons, high-density lipoproteins, low-density lipoproteins, and very low density lipoproteins

What is the medical application of cortisone?

Cortisone is used to treat rheumatoid arthritis, asthma, gastrointestinal disorders, and many skin conditions.

How are the structural differences between triglycerides and phospholipids reflected in their different biological functions?

In triglycerides there are three ester linkages. These are formed between the three hydroxyl groups of glycerol and the carboxyl groups of three fatty acids. As a result, the structure is nonpolar. This structure causes these molecules to readily associate with one another in water, since they are hydrophobic and will avoid water. Thus, they are found in fat droplets in the cytoplasm of adipocytes. In phospholipids, there are two ester linkages between glycerol and fatty acids. At the third hydroxyl group of glycerol there is a phosphoester linkage. The phosphoryl group is often attached to an additional molecule bearing a charge. Thus, phospholipids have a hydrophobic region (fatty acid tails) and a hydrophilic region (phosphoryl group). These tend to form lipid bilayers that serve the cell as membranes.

What are isoprenoids?

Isoprenoids are a large, diverse collection of lipids that are synthesized from the isoprene unit:

How is LDL taken into cells?

LDL is taken into cells by a process called receptor-mediated endocytosis. LDL binds to an LDL receptor on the surface of the cell. At that point, the cell surface begins to be pulled into the cell. Eventually this portion of the membrane pinches off into the cytoplasm to form a vesicle or endosome that contains the LDL.

What are the functions of thromboxane A2 and leukotrienes?

Leukotrienes cause bronchial constriction, as is associated with asthma. Thromboxane A2 is secreted by platelets and stimulates blood clotting.

Why are lipids (triglycerides) such an efficient molecule for the storage of energy in the body?

Lipids are excellent energy sources because, when oxidized, they yield more than twice the energy released by oxidation of the same amount of carbohydrates. Each gram of fat releases 9 kcal of energy.

List the biological functions of lipids.

Lipids serve as energy sources for the body. Triglycerides are used for energy storage. Phospholipids are components of cellular membranes. Steroid hormones and prostaglandins are lipids that allow tissues of the body to communicate. Vitamins A, D, E, and K are lipids. Deposits of lipids in the body serve as shock absorbers for internal organs and subcutaneous fat insulates the body from extreme cold temperatures

What is the role of lysosomes in the metabolism of plasma lipoproteins?

Lysosomes are cell organelles that consist of a membrane surrounding a collection of hydrolytic enzymes. They fuse with endosomes carrying the low density lipoprotein complex. The LDL complex is digested by certain of the hydrolytic enzymes and cholesterol is released into the cytoplasm of the cell.

A wax found in beeswax is myricyl palmitate. What fatty acid and what alcohol are used to form this compound?

Myricyl palmitate (beeswax) is made up of the fatty acid palmitic acid and the alcohol myricyl alcohol

Explain the logic behind decreasing the ratio of omega-6 to omega-3 fatty acids in the diet

Omega-3 fatty acids are precursors of prostaglandins that exhibit anti-inflammatory effects. On the other hand, omega-6 fatty acids are precursors to prostaglandins that have inflammatory effects. To reduce the inflammatory response contribution to cardiovascular disease, it is logical to increase the amount of omega-3 fatty acids in the diet and to decrease the amount of omega-6 fatty acids

Summarize the health benefits associated with omega-3 fatty acids.

Omega-3 fatty acids reduce the risk of cardiovascular disease by decreasing blood clot formation, blood triglyceride levels, and growth of atherosclerotic plaque.

Write the structures for a saturated and an unsaturated fatty acid.

Palmitic acid (hexadecanoic acid) is a saturated fatty acid that has the following structure:

Write the structure for an unsaturated fatty acid:

Palmitoleic acid is a monounsaturated fatty acid.

What is the correlation between saturated fats in the diet and atherosclerosis?

People with high levels of saturated fats in the diet tend to have high cholesterol levels. They tend to develop atherosclerosis because the excess cholesterol, along with other substances, coats the artery walls.

What are the biological functions of progesterone, testosterone, and estrone?

Progesterone is the most important hormone associated with pregnancy. Testosterone is needed for development of male secondary sexual characteristics. Estrone is required for proper development of female secondary sexual characteristics.

List four effects of prostaglandins.

Smooth muscle contraction, enhancement of fever and swelling associated with the inflammatory response, bronchial dilation, inhibition of secretion of acid into the stomach

What experimental observation shows that proteins diffuse within the lipid bilayers of biological membranes?

Specific membrane proteins on human and mouse cells were labeled with red and green fluorescent dyes, respectively. The human and mouse cells were fused into single-celled hybrids and were observed using a microscope with an ultraviolet light source. The ultraviolet light caused the dyes to fluoresce. Initially, the dyes were localized in regions of the membrane representing the original human or mouse cell. Within an hour, the proteins were evenly distributed throughout the membrane of the fused cell.

What is the biological function of sphingomyelin?

Sphingomyelins are important structural lipid components of nerve cell membranes. They are found in the myelin sheath that surrounds and insulates cells of the central nervous system.

List some important biological molecules that are terpenes

Steroids and bile salts, lipid-soluble vitamins, certain plant hormones, and chlorophyll

What is the recommendation of the National Institutes of Health for intake of DHA, EPA, linoleic acid, and linolenic acid?

The National Institutes of Health have issued recommended daily intakes of four of these fatty acids: 650 mg of EPA and DHA, 2.22 g/day of alpha-linolenic acid and 4.44 g/day of linoleic acid

What is the basic structure of a biological membrane?

The basic structure of a biological membrane is a bilayer of phospholipid molecules arranged so that the hydrophobic hydrocarbon tails are packed in the center and the hydrophilic head groups are exposed on the inner and outer surfaces.

What are the two major types of sphingolipids?

The two major types of sphingolipids are the sphingomyelins and the glycosphingolipids.

Explain the relationship between increased levels of omega-3 fatty acids and a decreased risk of cardiovascular disease.

The decrease in blood clot formation, along with the reduced blood triglyceride levels and decreased atherosclerotic plaque result in improved arterial health. This, in turn, results in lower blood pressure and a decreased risk of sudden death and heart arrhythmias.

What is the function of the essential fatty acid

The essential fatty acid linoleic acid is required for the synthesis of arachidonic acid, a precursor for the synthesis of the prostaglandins, a group of hormonelike molecules.

Describe the fluid mosaic model of membrane structure

The fluid mosaic model of membrane structure describes a fluid phospholipid bilayer with many proteins and glycoproteins embedded within it.

What is the role of prostaglandins in the inflammatory response?

The inflammatory response is a protective mechanism that is turned on when tissues are damaged. It is characterized by swelling, fever, pain, and redness. Prostaglandins promote the fever and pain caused by the inflammatory response.

As the length of the hydrocarbon chain of saturated fatty acids increases, what is the effect on the melting point?

The melting points increase. As the length of the hydrocarbon chain of saturated fatty acids increases, the melting point also increases. Saturated fatty acids only contain carbon-carbon single bonds and no double bonds. The shape of the hydrocarbon chain of saturated fatty acid is linear and full extended. These hydrocarbon chains are stacked in a regular manner which increases the intermolecular between the molecules. Thus, results increase the melting point of saturated fatty acids and they are generally solid at room temperature. Therefore, the length of the hydrocarbon chain of saturated fatty acids increases which increases melting point.

Explain the relationship between fatty acid chain length and melting points that you described in the answer to Question 17.23.

The melting points of fatty acids increase as the length of the hydrocarbon chains increase. This is because the intermolecular attractive forces, including London dispersion forces, increase as the length of the hydrocarbon chain increases

What do the terms omega-3 and omega-6 indicate about structures of the fatty acids in those classifications?

The name of these fatty acids arises from the position of the double bond nearest the terminal methyl group of the molecule. The terminal methyl group is designated omega (v). In v-3 fatty acids, the double bond nearest the v methyl group is three carbons along the chain. In v-6 fatty acids, the nearest double bond is six carbons from the end.

Why don't proteins turn around in biological membranes like revolving doors?

The phospholipid bilayer is highly structured. The nonpolar hydrocarbon tails are packed side-by-side in the center of the bilayer, forming a hydrophobic region. The polar heads form hydrophilic shells on the surfaces of the membrane. Transmembrane proteins have domains that are either hydrophobic, for effective interaction with the center of the membrane, or hydrophilic, for effective interaction with the hydrophilic regions inside and outside the cell. These interactions are very stable and many would need to be destabilized in order for the protein to "flip-flop" within the membrane.

What are the possible side effects of cortisone treatment?

The possible side-effects of taking cortisone include fluid retention, sodium retention, and potassium loss that can lead to congestive heart failure, muscle weakness, osteoporosis, gastrointestinal distress (including stomach ulcers), and neurological problems such as dizziness, headaches, and convulsions.

What foods are good sources of EPA and DHA?

The so-called "oily" fish are good sources of the ω-3 fatty acids. These include salmon, albacore tuna, sardines, lake trout, and mackerel.

There is a single, unique structure for the cholesterol molecule. What is meant by the terms good and bad cholesterol?

The terms "good" and "bad" cholesterol refer to two classes of lipoprotein complexes. The high-density lipoproteins, or HDL, are considered to be "good" cholesterol because a correlation has been made between elevated levels of HDL and a reduced incidence of atherosclerosis. Low-density lipoproteins, or LDL, are considered to be "bad" cholesterol because evidence suggests that a high level of LDL is associated with increased risk of atherosclerosis

Describe transmembrane proteins and list some of their functions

Transmembrane proteins are embedded within the membrane of the cell and extend completely through it. They are exposed both outside and inside the cell. Transmembrane proteins are generally glycoproteins and often serve as hormone receptors for cell-to-cell communication. Others are channels through which required nutrients or ions enter or leave the cell.

Why are triglycerides also referred to as triacylglycerols?

Triglycerides can also be called triacylglycerols because they consist of a glycerol molecule to which three fatty acid acyl groups are bonded via ester bonds.

What are the structural differences between triglycerides (triacylglycerols) and phospholipids?

Triglycerides consist of three fatty acids esterified to the three hydroxyl groups of glycerol. In phospholipids, there are only two fatty acids esterified to glycerol. A phosphoryl group is esterified (phosphoester linkage) to the third hydroxyl group.

What is the function of unsaturation in the hydrocarbon tails of membrane lipids ?

Unsaturation of fatty acyl tails of phospholipids contributes to increased membrane fluidity. Each carbon-to-carbon double bond that is added will introduce a "kink" into the fatty acyl tail. As a result, the tails cannot pack together as they would if they were saturated. The result is increased fluidity.

List some foods that are good sources of a-linolenic acid.

a-Linolenic acid is found in flax seed, soybeans, and canola, as well as in oil made from these plants.

Define the term phosphatidate

phosphatidate is a phosphoglyceride that consists of a glycerol molecule having a phosphoryl group bonded to carbon-3 via a phosphoester bond.