Module 5: Lipids 1

Question 1

Lipid structure and function overview

Answer 1

The molecules that fit the label of lipid do not follow a single structural template or share a common set of functional groups, as nucleotides and amino acids do. In fact, lipids are defined primarily by the absence of functional groups. Because they consist mainly of C and H atoms and have few, if any, N- or O-containing functional groups, they lack the ability to form hydrogen bonds and are therefore largely insoluble in water. This characteristic is why they are known as hydrophobic or “water fearing.” Although some lipids do contain polar or charged groups, the bulk of their structure is hydrocarbon (mostly C-H and C-C bonds) and nonpolar.

Question 2

How are lipids primarily defined?

Answer 2

The molecules that fit the label of lipid do not follow a single structural template or share a common set of functional groups, as nucleotides and amino acids do.

In fact, lipids are defined primarily by the absence of functional groups.

Question 3

Why are lipids largely insoluble in water?

Answer 3

Because they consist mainly of C and H atoms and have few, if any, N- or O-containing functional groups, they lack the ability to form hydrogen bonds and are therefore largely insoluble in water.

This characteristic is why they are known as hydrophobic or “water fearing.”

Question 4

What is the bulk of the structure of lipids?

bonds, polarization

Answer 4

Although some lipids do contain polar or charged groups, the bulk of their structure is hydrocarbon (mostly C-H and C-C bonds) and nonpolar.

Question 5

Fatty Acids and Triglycerides

KEY CONCEPTS
Fatty acids have two main components—a carboxylic acid and a hydrocarbon “tail.”
The two main classes of fatty acids include saturated and unsaturated fatty acids.
Unsaturated fatty acids can be monounsaturated or polyunsaturated fatty acids, and some unsaturated fatty acids must be obtained in the diet.
Triglycerides contain a glycerol molecule bound to three fatty acids.
Triglycerides are the main energy storage molecule in the body and are stored in adipose tissue.

Answer 5

Fatty acids have two main components—a carboxylic acid and a hydrocarbon “tail.”
The two main classes of fatty acids include saturated and unsaturated fatty acids.
Unsaturated fatty acids can be monounsaturated or polyunsaturated fatty acids, and some unsaturated fatty acids must be obtained in the diet.
Triglycerides contain a glycerol molecule bound to three fatty acids.
Triglycerides are the main energy storage molecule in the body and are stored in adipose tissue.

Question 6

What are the two main components of Fatty acids?

Answer 6

Fatty acids have two main components—a carboxylic acid and a hydrocarbon “tail.”

Question 7

What are the two main classes of fatty acids?

Answer 7

The two main classes of fatty acids include saturated and unsaturated fatty acids.

Question 8

What are the types of unsaturated fatty acids?

Answer 8

Unsaturated fatty acids can be monounsaturated or polyunsaturated fatty acids, and some unsaturated fatty acids must be obtained in the diet.

Question 9

What is the structure of triglycerides?

Answer 9

Triglycerides contain a glycerol molecule bound to three fatty acids.

Question 10

How are triglycerides used in the body?

Answer 10

Triglycerides are the main energy storage molecule in the body and are stored in adipose tissue.

Question 11

Fatty acid structure

Answer 11

The simplest lipids are the fatty acids, which are long-chain carboxylic acids (at physiological pH, they are ionized to the carboxylate, -COO_ ,form). These molecules may contain up to 42 carbon atoms, but the most fatty acids in plants and animals have an even number of carbons with carbon chains between 8 to 36 carbons in length. Short-chain fatty acids range from 4 to 7 carbons in length. They remain liquid at colder temperatures. For example, the short-chain fatty acids in whole milk remain liquid even in the refrigerator. Medium-chain fatty acids, such as those in coconut oil, range from 8 to 12 carbons. They solidify in the refrigerator but remain liquid at room temperature. Long-chain fatty acids (greater than 12 carbons), such as those in beef fat, usually remain solid at room temperature.

Question 12

What are the simplest lipids?

Answer 12

Fatty acids

Question 13

What are fatty acids?

Answer 13

The simplest lipids are the fatty acids, which are long-chain carboxylic acids (at physiological pH, they are ionized to the carboxylate, -COO_ ,form).

Question 14

How many Carbon Atoms are contained in fatty acids?

Answer 14

These molecules may contain up to 42 carbon atoms, but the most fatty acids in plants and animals have an even number of carbons with carbon chains between 8 to 36 carbons in length.

Question 15

Short-chain fatty acids

Answer 15

Short-chain fatty acids range from 4 to 7 carbons in length. They remain liquid at colder temperatures. For example, the short-chain fatty acids in whole milk remain liquid even in the refrigerator.

Question 16

Medium-chain fatty acids

Answer 16

Medium-chain fatty acids, such as those in coconut oil, range from 8 to 12 carbons. They solidify in the refrigerator but remain liquid at room temperature. Long-chain fatty acids (greater than 12 carbons), such as those in beef fat, usually remain solid at room temperature.

Question 17

Fatty Acids Carbon bond overview.

Answer 17

Each carbon atom must form four bonds to be stable. These four bonds can each be single bonds that link carbon to four other atoms.

A fatty acid in which each carbon in the hydrocarbon chain is bound to four atoms, including at least two hydrogen atoms, is called a saturated fatty acid because the carbons are “saturated” with hydrogen—there are no double bonds between the carbons.

Unsaturated fatty acids contain one or more pairs of carbons that are not saturated with hydrogen atoms. The carbon pairs within the chain that form carbon-carbon double bonds are bound to only one hydrogen.

The positions of carbons and bonds within the fatty acid chain are often named using Greek letters beginning at the carbon closest to the carboxylic acid.

That is the alpha carbon, and its bond to the carboxylic acid is called the alpha bond.

Similarly, the next carbon in the chain is the beta carbon, and its bond to the alpha carbon is known as the beta bond. This bond is very important in fatty acid metabolism as we will see in the next chapter.

Question 18

How many carbon bonds in fatty acids?

Answer 18

Each carbon atom must form four bonds to be stable. These four bonds can each be single bonds that link carbon to four other atoms.

Question 19

Saturated fatty acid

Answer 19

A fatty acid in which each carbon in the hydrocarbon chain is bound to four atoms, including at least two hydrogen atoms, is called a saturated fatty acid because the carbons are “saturated” with hydrogen—there are no double bonds between the carbons.

Question 20

Unsaturated fatty acid

Answer 20

Unsaturated fatty acids contain one or more pairs of carbons that are not saturated with hydrogen atoms. The carbon pairs within the chain that form carbon-carbon double bonds are bound to only one hydrogen.

Question 21

Naming of the positions of carbons and bonds within a fatty acid.

Answer 21

The positions of carbons and bonds within the fatty acid chain are often named using Greek letters beginning at the carbon closest to the carboxylic acid.

Question 22

Alpha bond

Answer 22

That is the alpha carbon, and its bond to the carboxylic acid is called the alpha bond.

Question 23

Beta bond

Answer 23

Similarly, the next carbon in the chain is the beta carbon, and its bond to the alpha carbon is known as the beta bond. This bond is very important in fatty acid metabolism as we will see in the next chapter.

Question 24

Abbreviated structural formulas and examples.

Answer 24

The last carbon in the chain is known as the omega carbon, regardless of the length of the chain.

The bonds that are closer to the omega carbon than to the carboxylic acid are often referred to based on their position relative to the omega carbon. For example, in the figure above, the bond third from the omega carbon is known as the omega-3 bond. Fatty acids with omega-3 and omega-6 double bonds are important to health, as discussed below.

Fatty acid structures are often indicated by abbreviated structural formulas for convenience.

For example, the structural formula for lauric acid, a 12 carbon saturated fatty acid, is shown as CH3(CH2)10COOH with the 10 identical CH2 groups indicated with the parentheses and the two terminal carbons, the omega CH3 carbon and the carboxylic acid (COOH) carbon, indicated at each end. Similarly, structural formulas for unsaturated fatty acids indicate the location of the double bond by giving the number of CH2 groups on each side. For example, palmitoleic acid, which has a double bond between the 9 and 10 carbons, is shown as CH3(CH2)5CH=CH(CH2)7COOH (Figure 5-3).

Question 25

Omega carbon

Answer 25

The last carbon in the chain is known as the omega carbon, regardless of the length of the chain.

Question 26

monosaturated fatty acid

Answer 26

A fatty acid containing one double bond in its carbon chain is called a monounsaturated fatty acid (Figure 5-4).

In our diets, the most common monounsaturated fatty acid is oleic acid, which is prevalent in olive and canola oils.

A fatty acid with more than one double bond in its carbon chain is called a polyunsaturated fatty acid.

The most common polyunsaturated fatty acid is linoleic acid, found in corn, safflower, and soybean oils.

Unsaturated fatty acids melt at cooler temperatures than saturated fatty acids of the same chain length. ]

Therefore, the more unsaturated bonds a fatty acid contains, the more likely it is to be liquid at room temperature.

Question 27

monosaturated fatty acid

Answer 27

A fatty acid containing one double bond in its carbon chain is called a monounsaturated fatty acid (Figure 5-4).

Question 28

Most common monosaturated fatty acid in our diets.

Answer 28

In our diets, the most common monounsaturated fatty acid is oleic acid, which is prevalent in olive and canola oils.

Question 29

polyunsaturated fatty acid

Answer 29

A fatty acid with more than one double bond in its carbon chain is called a polyunsaturated fatty acid.

Question 30

Most common polyunsaturated fatty acid

Answer 30

The most common polyunsaturated fatty acid is linoleic acid, found in corn, safflower, and soybean oils.

Question 31

fatty acids melting temperature

Answer 31

Unsaturated fatty acids melt at cooler temperatures than saturated fatty acids of the same chain length. ]

Question 32

Fatty acids that are liquid at room temperature.

Answer 32

Therefore, the more unsaturated bonds a fatty acid contains, the more likely it is to be liquid at room temperature.

Question 33

Fatty acids that are liquid at room temperature.

Answer 33

Therefore, the more unsaturated bonds a fatty acid contains, the more likely it is to be liquid at room temperature.

Question 34

Essential fatty acids

Answer 34

There are different categories of unsaturated fatty acids depending on the location of the first double bond in the carbon chain.

If the first double bond occurs between the third and fourth carbons, counting from the omega end (CH3) of the chain, the fat is said to be an omega-3 fatty acid (see Figure 5-4). Alpha- linolenic acid, found in vegetable oils, and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), found in fish oils, are omega-3 fatty acids.

If the first double bond occurs between the sixth and seventh carbons (from the omega end), the fatty acid is called an omega-6 fatty acid; this type of fatty acid is obtained from vegetable oils such as corn and safflower oil.

Although human cells can synthesize a variety of unsaturated fatty acids, they are unable to make any with double bonds past carbon 9 (counting from the carboxylate end), including omega-3 and omega-6 fatty acids. These fatty acids are essential to human health and must be obtained through the diet, which is why they are known as essential fatty acids.

Question 35

Categories of unsaturated fatty acids

Answer 35

There are different categories of unsaturated fatty acids depending on the location of the first double bond in the carbon chain.

Question 36

omega-3 fatty acids structure

examples

Answer 36

If the first double bond occurs between the third and fourth carbons, counting from the omega end (CH3) of the chain, the fat is said to be an omega-3 fatty acid (see Figure 5-4). Alpha- linolenic acid, found in vegetable oils, and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), found in fish oils, are omega-3 fatty acids.

Question 37

Omega-6 fatty acids structure

examples

Answer 37

If the first double bond occurs between the sixth and seventh carbons (from the omega end), the fatty acid is called an omega-6 fatty acid; this type of fatty acid is obtained from vegetable oils such as corn and safflower oil.

Question 38

What are the essential fatty acids and why are they needed?

Answer 38

Although human cells can synthesize a variety of unsaturated fatty acids, they are unable to make any with double bonds past carbon 9 (counting from the carboxylate end), including omega-3 and omega-6 fatty acids. These fatty acids are essential to human health and must be obtained through the diet, which is why they are known as essential fatty acids.

Question 39

What are the essential fatty acids and why are they needed?

Answer 39

Although human cells can synthesize a variety of unsaturated fatty acids, they are unable to make any with double bonds past carbon 9 (counting from the carboxylate end), including omega-3 and omega-6 fatty acids. These fatty acids are essential to human health and must be obtained through the diet, which is why they are known as essential fatty acids.

Question 40

Cis vs. Trans Double Bonds in Unsaturated Fatty Acids

Answer 40

The position of the hydrogen atoms around a double bond also affects the properties of unsaturated fatty acids.

Most unsaturated fatty acids found in nature have both hydrogen atoms on the same side of the double bond, called the cis configuration. The asymmetry forces a kink or bend in the carbon chain, making it difficult for the fatty acids to pack together (Figure 5-5).

When the hydrogen atoms are on opposite sides of the double bond, called the trans configuration, the fatty acid is a trans fatty acid. Trans fatty acids can pack tightly like saturated fatty acids and so have a higher melting point than the same fatty acid in the cis configuration.

Question 41

Hydrogen atoms affect on the properties of unsaturated fatty acids

Answer 41

The position of the hydrogen atoms around a double bond also affects the properties of unsaturated fatty acids.

Question 42

cis configuration

Answer 42

hydrogen atoms on the same side of the double bond, called the cis configuration.

The asymmetry forces a kink or bend in the carbon chain, making it difficult for the fatty acids to pack together (Figure 5-5).

Question 43

Trans configuration

Answer 43

When the hydrogen atoms are on opposite sides of the double bond, called the trans configuration, the fatty acid is a trans fatty acid. Trans fatty acids can pack tightly like saturated fatty acids and so have a higher melting point than the same fatty acid in the cis configuration.

Question 44

Trans configuration

Answer 44

When the hydrogen atoms are on opposite sides of the double bond, called the trans configuration, the fatty acid is a trans fatty acid. Trans fatty acids can pack tightly like saturated fatty acids and so have a higher melting point than the same fatty acid in the cis configuration.

Question 45

Trans fats overview

Answer 45

Trans fatty acids are found in small amounts in nature, but most of the trans fat we eat comes from products that have undergone hydrogenation.

Hydrogenation is a process in which hydrogen gas is bubbled into liquid oil. This causes some of the double bonds in the oil to accept hydrogen atoms and become saturated. The resulting fat has more of the properties of a saturated fat, such as increased stability against rancidity and a higher melting point. However, during hydrogenation, only some of the bonds become saturated. Some of those that remain unsaturated are altered, converting them from the cis to the trans configuration, producing more trans fatty acids than the original oil contained.

Hydrogenated or partially hydrogenated vegetable oils are a primary ingredient in margarine and vegetable shortening because they raise the melting point of the products, making them more solid at room temperature. They are also used to lengthen shelf life in other processed foods, such as cookies, crackers, breakfast cereals, and potato chips.

When consumed in the diet, trans fats raise blood cholesterol levels and increase the risk of heart disease.

Question 46

Trans fats in nature

Answer 46

Trans fatty acids are found in small amounts in nature, but most of the trans fat we eat comes from products that have undergone hydrogenation.

Question 47

Hydrogentation

result

Answer 47

Hydrogenation is a process in which hydrogen gas is bubbled into liquid oil. This causes some of the double bonds in the oil to accept hydrogen atoms and become saturated.

The resulting fat has more of the properties of a saturated fat, such as increased stability against rancidity and a higher melting point. However, during hydrogenation, only some of the bonds become saturated. Some of those that remain unsaturated are altered, converting them from the cis to the trans configuration, producing more trans fatty acids than the original oil contained.

Question 48

Hydrogenated products in our food

Answer 48

Hydrogenated or partially hydrogenated vegetable oils are a primary ingredient in margarine and vegetable shortening because they raise the melting point of the products, making them more solid at room temperature. They are also used to lengthen shelf life in other processed foods, such as cookies, crackers, breakfast cereals, and potato chips.

Question 49

Affect of trans fats to our health.

Answer 49

When consumed in the diet, trans fats raise blood cholesterol levels and increase the risk of heart disease.

Question 50

Affect of trans fats to our health.

Answer 50

When consumed in the diet, trans fats raise blood cholesterol levels and increase the risk of heart disease.

Question 51

Triglycerides and Fatty Acids

Answer 51

Free fatty acids are relatively scarce in biological systems. Instead, they are usually attached to another molecule, especially glycerol. The fats and oils found in animals and plants are triglycerides, also known as triacylglycerols, and consist of a backbone of glycerol with three fatty acids.

Triglycerides are created in a dehydration reactions (removal of a water molecule) as shown in Figure 5-6. If only one fatty acid is attached to the glycerol, the molecule is called a monoglyceride or monoacylglycerol, and when two fatty acids are attached, it is a diglyceride or diacylglycerol. This is as close as lipids come to forming polymers: They cannot be linked end-to-end to form long chains, as the other types of biological molecules can.

Question 52

Triglycerides

Answer 52

Triglycerides may contain any combination of fatty acids: long, medium, or short chain; saturated or unsaturated; cis or trans.

The types of fatty acids in triglycerides determine their texture, taste, and physical characteristics.

For example, the amounts and types of fatty acids in chocolate allow it to remain solid at room temperature, snap when bitten into, and then melt quickly and smoothly in the mouth.

The triglycerides in red meat contain predominantly long-chain, saturated fatty acids, so the fat on a piece of steak is solid at room temperature.

The triglycerides in olive oil contain predominantly mono-unsaturated fatty acids, whereas those in corn oil are mostly polyunsaturated. These fats are liquid at room temperature.

Question 53

Triglycerides

Answer 53

Triglycerides may contain any combination of fatty acids: long, medium, or short chain; saturated or unsaturated; cis or trans.

The types of fatty acids in triglycerides determine their texture, taste, and physical characteristics.

For example, the amounts and types of fatty acids in chocolate allow it to remain solid at room temperature, snap when bitten into, and then melt quickly and smoothly in the mouth.

The triglycerides in red meat contain predominantly long-chain, saturated fatty acids, so the fat on a piece of steak is solid at room temperature.

The triglycerides in olive oil contain predominantly mono-unsaturated fatty acids, whereas those in corn oil are mostly polyunsaturated. These fats are liquid at room temperature.

Question 54

Triglycerides Store Energy

Answer 54

Fatty acids, like carbohydrates, may be oxidized to produce ATP, a topic explored further in the chapter on lipid metabolism.

If the body has no immediate need to use fatty acids in this way, they are stored as triglycerides in adipose tissue (fat deposits) throughout the body and in the liver.

Triglycerides stored in adipose tissue constitute 98% of all body energy reserves because they can store more energy in less space compared to glycogen due to their compact structure and low degree of oxidation (discussed more in the chapter on lipid metabolism).

Adipose tissue also insulates and protects various parts of the body.

Triglycerides in adipose tissue are continually broken down and resynthesized.

Thus, the triglycerides stored in adipose tissue today are not the same molecules that were present last month because they are continually released from storage, transported in the blood, and redeposited in other adipose tissue cells.

Question 55

Triglycerides Store Energy

Answer 55

Fatty acids, like carbohydrates, may be oxidized to produce ATP, a topic explored further in the chapter on lipid metabolism.

If the body has no immediate need to use fatty acids in this way, they are stored as triglycerides in adipose tissue (fat deposits) throughout the body and in the liver.

Triglycerides stored in adipose tissue constitute 98% of all body energy reserves because they can store more energy in less space compared to glycogen due to their compact structure and low degree of oxidation (discussed more in the chapter on lipid metabolism).

Adipose tissue also insulates and protects various parts of the body.

Triglycerides in adipose tissue are continually broken down and resynthesized.

Thus, the triglycerides stored in adipose tissue today are not the same molecules that were present last month because they are continually released from storage, transported in the blood, and redeposited in other adipose tissue cells.

Question 56

Phospholipid Structure and Function

KEY CONCEPTS

Answer 56

Phospholipids have a glycerol backbone attached to two fatty acids and a phosphate head group.
The dual solubility of phospholipids allows them to spontaneously form several important structures in the cell.

The lipid bilayers formed by phospholipids are the primary structural foundation of cellular membranes.

The fluid mosaic model includes the role of phospholipids, cholesterol, proteins, and carbohydrates in cellular membranes.

Micelles are formed by phospholipids and aid in the absorption of hydrophobic nutrients, such as fat-soluble vitamins.

Question 57

Phospholipid Structure

Answer 57

Like triglycerides, phospholipids have a backbone of glycerol. However, they have only two fatty acids attached rather than three. In place of the third fatty acid is a phosphate group, which is then attached to a variety of other molecules (Figure 5-7 shows one example). The fatty-acid end of a phospholipid is lipid-soluble (hydrophobic), whereas the phosphate end (often referred to as the “head group”) is water-soluble (hydrophilic). This allows phospholipids to mix in both water and lipids—a property that makes them important for many functions in the body.

Question 58

amphipathic

Answer 58

Phosphlipids are amphipathic molecules (amphi = “both”) because they are both lipid-soluble and water-soluble. This dual nature allows them to play vital roles in the body, including their role in cell membrane and micelle formation.

Question 59

The Lipid Bilayer

Answer 59

The foundational structure of a biological membrane is the lipid bilayer, a two-dimensional array of phosphlipids whose nonpolar tails associate with each other, out of contact with water, and whose polar head groups interact with the watery environment.

Question 60

lipid bylayer

Answer 60

The beauty of the bilayer as a barrier for biological systems is that it forms spontaneously due to the hydrophobic effect, which drives hydrophobic fatty acid tails to aggregate together in order to minimize their contact with water. In addition, a bilayer is self-sealing and, despite its thinness, it can enclose a relatively vast compartment or an entire cell. Once it has formed, a bilayer is quite stable and flexible..

Question 61

lipid bylayer

Answer 61

The beauty of the bilayer as a barrier for biological systems is that it forms spontaneously due to the hydrophobic effect, which drives hydrophobic fatty acid tails to aggregate together in order to minimize their contact with water. In addition, a bilayer is self-sealing and, despite its thinness, it can enclose a relatively vast compartment or an entire cell. Once it has formed, a bilayer is quite stable and flexible..

Question 62

The Fluid Mosaic Model

Answer 62

Biological membranes consist of both proteins and lipids that diffuse about in the fluid membrane environment. Because there are multiple different components, this is often referred to as the fluid mosaic model of membrane structure. According to this model, membrane proteins are like icebergs floating in a lipid sea, though their movement can be restricted by the proximity of cholesterol or attachment to intracellular structures, such as the cytoskeleton.

Question 63

Phospholipids Form Micelles

Answer 63

In addition to the formation of the cellular membranes, phospholipids also form micelles. Micelles have a fat-soluble center surrounded by a coating of bile acids and polar head groups. They facilitate the absorption of lipids, including dietary fats, into the mucosal cells of the small intestine. The fat-soluble vitamins (A, D, E, and K) must also be incorporated into micelles to be absorbed. The amounts absorbed can be reduced if dietary fat is very low or if disease interferes with fat absorption (Figure 5-12).

Question 64

Structure and Function of Other Lipids

KEY CONCEPTS

Answer 64

Sterols are lipids with four hydrocarbon rings, such as cholesterol and steroid hormones.
Fat soluble vitamins are all isoprenoid structures with various important roles in the body. Deficiencies in these vitamins can lead to disease.
Eiconsanoids are hormones that act locally to regulate pain, fever, inflammation, blood pressure, etc.

Question 65

Structure and Function of Other Lipids

KEY CONCEPTS

Answer 65

Sterols are lipids with four hydrocarbon rings, such as cholesterol and steroid hormones.
Fat soluble vitamins are all isoprenoid structures with various important roles in the body. Deficiencies in these vitamins can lead to disease.
Eiconsanoids are hormones that act locally to regulate pain, fever, inflammation, blood pressure, etc.

Question 66

Sterols

Answer 66

Like most other lipids, sterols do not dissolve well in water. Unlike triglycerides and phospholipids, their structure consists of multiple chemical rings (Figure 5-13). Sterols are found in both plants and animals. Cholesterol, probably the best-known sterol, is only found in animals. Cholesterol is necessary in the body, but because the liver manufactures it, it is not essential in the diet. More than 90% of the cholesterol in the body is found in cell membranes, where it helps maintain constant membrane fluidity.

Question 67

Fat Soluble Vitamins

Answer 67

The molecules known as vitamins A, D, E, and K are all isoprenoids (made from 5-carbon units called “isoprenes”) that perform a variety of physiological roles not related to membrane structure. They are required in relatively small amounts but are vital to good health.

Question 68

Eicosanoids

Answer 68

As shown thus far, many lipids act as hormones, including the steroid hormones and vitamin D. Eicosanoids are another class of lipids derived from essential fatty acids that also act as local hormones. These molecules are not always present, but are generated as needed from the different phospholipids and fatty acids in cellular membranes. Eicosanoids act at very low concentrations and are involved in the production/regulation of pain and fever, blood pressure, blood coagulation, and reproduction. In humans, derivatives of the C20 fatty acid arachidonic acid (an omega-6 fatty acid) are made into different eiconsanoids, such as prostaglandins. Aspirin blocks the first step in the conversion of arachidonic acid to prostaglandin H2, a moleule that serves as the precursor to several other eicosanoids involved in inflammation (Figure 5-15). Blocking this step decreases physiological responses such as fever, inflammation, and blood clotting, the reduction of which are all well-known results of aspirin treatment.

Question 69

Eicosanoids

Answer 69

As shown thus far, many lipids act as hormones, including the steroid hormones and vitamin D. Eicosanoids are another class of lipids derived from essential fatty acids that also act as local hormones. These molecules are not always present, but are generated as needed from the different phospholipids and fatty acids in cellular membranes. Eicosanoids act at very low concentrations and are involved in the production/regulation of pain and fever, blood pressure, blood coagulation, and reproduction. In humans, derivatives of the C20 fatty acid arachidonic acid (an omega-6 fatty acid) are made into different eiconsanoids, such as prostaglandins. Aspirin blocks the first step in the conversion of arachidonic acid to prostaglandin H2, a moleule that serves as the precursor to several other eicosanoids involved in inflammation (Figure 5-15). Blocking this step decreases physiological responses such as fever, inflammation, and blood clotting, the reduction of which are all well-known results of aspirin treatment.

Question 70

Summary

Answer 70

Lipids are hydrophobic molecules that include a highly diverse group of structures performing many important functions in the body. Fatty acids are both an important fuel source as well as building blocks for both triglycerides and phospholipids. Triglycerides store vast amounts of energy for the body, and phospholipids provide the structual foundation of cell membranes. Lipids include important signaling molecules, such as steroid hormones and eicosanoids, as well as essential vitamins.

Question 71

Lipid Metabolism

Answer 71

Breakdown of Fats by Beta Oxidation

Triglycerides are carried by lipoproteins to tissues where they are broken down to fatty acids and glycerol and used to make ATP.

Question 72

Animation

Answer 72

The many roles of lipids in the body are done both by lipids that we take in with our diets as well as lipids that our bodies can make on their own. Much of this chapter focuses on the opposing pathways of fatty acid synthesis and degradation. A summary of the context of these reactions is shown in the following figure:

Question 73

Fatty Acid Synthesis

KEY CONCEPTS

Answer 73

Acetyl-CoA is the starting material to manufacture fatty acids.
Acetyl-CoA is transported out of the mitochondria to the cytosol for fatty acid synthesis.
Fatty acids are built by extending the chain two carbon units at a time.

Question 74

Acetyl-CoA is Used for Fatty Acid Synthesis

Answer 74

When there is little demand for ATP in the body, the citric acid cycle and oxidative phosphorylation will reduce their output, and acetyl-CoA molecules become available for other uses. Acetyl-CoA is used in the biosynthesis of several lipids, including cholesterol, which is used to make steroid hormones such as testosterone and cortisol, and fatty acids.

Fatty acid synthesis occurs in the cytosol of a cell but acetyl-CoA is produced in the mitochondria (where it usually enters the citric acid cycle) and cannot be transported across the mitochondrial membrane directly. For this reason, the acetyl-CoA arrives in the cytoplasm in the form of citrate (recall the first step of the citric acid cycle where acetyl-CoA combines with oxaloacetate to make citrate). Once in the cytosol, citrate can undergo the reverse reaction to produce oxaloacetate and acetyl-CoA.

Question 75

Acetyl-CoA is Converted to Malonyl-CoA in the First Step of Fatty Acid Synthesis

Answer 75

The first committed step in fatty acid biosynthesis is the conversion of acetyl-CoA, a two-carbon molecule, to malonyl-CoA, a three-carbon molecule. This reaction involves the donation of a carboxylate group from biotin, a coenzyme used in this enzymatic reaction, to acetyl-CoA, as shown in the reaction below:

Question 76

Fatty Acid Synthase Produces Fatty Acids

Answer 76

Fatty acid synthase is the enzyme that catalyzes successive reactions by which the fatty acid is built. This process is cyclical with malonyl-CoA donating the two-carbon unit in each round until the final length of the fatty acid is achieved. Before each cycle, the malonyl group of the malonyl-CoA is transferred to a carrying protein (known as ACP) that facilitates the reaction with fatty acid synthase.

The first step shown in Figure 5-19 is the addition of a malonyl-ACP to acetyl-CoA and the loss of CO2 to generate acetoacetyl-ACP, which is then converted to butyryl-ACP—a fatty acid with four carbons. The cycle then repeats itself with another malonyl-ACP being added to the butyryl-ACP to create a six-carbon chain. With each round of the cycle, two carbons are added to the fatty acid chain until the desired length is acheived. For this reason, the vast majority of naturally occurring fatty acids have an even number of carbons in their chain.

Question 77

Catabolism of Fatty Acids: Beta Oxidation

KEY CONCEPTS

Answer 77

Lipids are absorbed in the body by micelles, and are transported in the form of lipoproteins, with varying combinations of lipids and proteins.
Fatty acids are catabolized by beta oxidation in the mitochondria of cells.
Beta oxidation produces acetyl-CoA molecules that can enter cellular respiration.

Question 78

Absorption and Transport of Dietary Lipids

Answer 78

Mammals can obtain fats from their diet or internally synthesized fatty acids. Dietary fats as well as other lipids, such as fat-soluble vitamins and cholesterol, are initially packaged into micelles (Figure 5-20) by the addition of bile salts, derivatives of cholesterol that aid in emulisfying and absorbing lipids. The fats are then digested by enzymes known as lipases that convert the triglycerides to smaller components, such as diglycerides, monoglycerides, fatty acids, and glycerol. These smaller components diffuse through the intestinal lining and enter the mucosal (epithelial) cells of the intestinal wall. Within these cells, the smaller fat components are repackaged as chylomicrons. The chylomicrons are a combination of lipids (triglycerides, cholesterol, fat soluble vitamins) and specialized proteins, and they are commonly referred to as lipoproteins (lipid + protein).

Question 79

Deficiencies in Beta Oxidation Can Lead to Disease

Answer 79

The enzymes that participate in beta oxidation are often specific to varying lengths of fatty acids. Recall that fatty acid hydrocarbon chains can range anywhere from 4 to 36 carbons. Often, fatal disorders appear when certain enzymes specific for oxidizing a particular length of fatty acid chain are defective, resulting in an inability to use fats containing fatty acids with those specific lengths for ATP production. One such example is a defect in medium-chain (carbons 4-12) acyl-CoA dehydrogenase, which catalyzes the first step of beta oxidation for medium chain fatty acids. A defect in this enzyme is called medium-chain acyl-CoA dehydrogenase deficiency (MCADD), which usually presents in infants as vomiting and/or lack of energy due to their inability to utilize medium-chain fatty acids. The disease is often fatal if not detected early, and newborn infants are routinely screened for this disease. Individuals with MCADD must avoid fasting for prolonged periods and need diets rich in slow-release carbohydrates to maintain their energy levels.

Question 80

Catabolism of Fatty Acids: Ketone Bodies

KEY CONCEPTS

Answer 80

Acetyl-CoA produced by beta-oxidation can also be converted to ketone bodies for energy.
Insulin and glucagon impact fatty acid metabolism as well as carbohydrate metabolism.
Dietary as well as disease conditions can result in states of ketosis or diabetes ketoacidosis.

Question 81

Acetyl-CoA Can Be Converted to Ketone Bodies

Answer 81

During a prolonged fast, when glucose is unavailable from the diet and liver glycogen has been depleted, many tissues depend on fatty acids released from stored triglycerides to meet their energy needs. Beta-oxidation of fatty acids produces acetyl-CoA molecules that can enter cellular respiration to make ATP for these cells. However, some tissues, such as the brain, do not use fatty acids for energy and must be supplied with other energy sources in order to produce the ATP they need. The liver fulfills this role using gluconeogenesis to produce glucose for the dependent tissues as well as ketone bodies to supplement gluconeogenesis. The ketone bodies—acetoacetate and 3-hydroxybutyrate—are synthesized from acetyl-CoA in liver mitochondria by a process called ketogenesis. Ketogenesis involves three enzymatic steps that convert acetyl-CoA molecules into acetoacetate as well as an additional step to form 3-hydroxybutyrate (Figure 5-24). Because ketogenesis uses acetyl-CoA derived from fatty acids, it helps spare amino acids that would otherwise be diverted to gluconeogenesis (see Figure 4-17 in Module 4 - Carbohydrate Metabolism).