Chapter 10: Lipids

Question 1

Biological lipids defining feature is

Answer 1

their insolubility in water

Question 2

fatty acids

Answer 2

• are hydrocarbon derivatives
• at about the same low oxidation state (highly reduced) as the hydrocarbons in fossil fuels
• cellular oxidation of fatty acids (to CO₂ and H₂O) is highly exergonic
• fats and oils used almost universally as stored forms of energy in living organisms
• carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (C₄ to C₃₆). In some
  
  • in some this chain is unbranched and fully saturated (contains no double bonds)
  • in others the chain contains one or more double bonds
  • few contain three-carbon rings, hydroxyl groups, or methyl-group branches

Question 3

lipid nomenclature

two types

Answer 3

1. Standard nomenclature
   
   • chain length and number of double bonds, separated by a colon: 16:0
   • number 1 is assigned to the carboxyl carbon (C-1), and α to the carbon next to it
   • position of any ═ bond(s) is indi
   • cated by Δ followed by a superscript number indicating the lower-numbered carbon in the ═ bond
2. polyunsaturated acids
   • of special importance in human nutrition.
   • physiological role of PUFAs is related to the position of the first ═ bond near the methyl end of the chain, not the carboxyl end
   • numbers the carbons in the opposite direction
   • assign the number 1 to the methyl carbon at the other end of the chain and this carbon as ω (omega; last letter in the Greek alphabet)
   • The positions of the double bonds are indicated relative to the ω carbon

Question 4

most commonly occurring fatty acids have even numbers of carbon atoms in an unbranched chain of _____ to _____ carbons

Answer 4

• 12
• 24

Question 5

common pattern in location of double bonds

Answer 5

• in most monounsaturated fatty acids the double bond is between C-9 and C-10 (Δ⁹)
• double bonds of polyunsaturated fatty acids
  
  • generally D12 and D15
  • Arachidonic acid is an exception
  • almost never conjugated (alternating single and double bonds
• separated by a methylene group: —CH═CH—CH2—CH═CH—

Question 6

physical properties of the fatty acids, and of compounds that contain them, are largely determined by the _____ and degree of ______ of the hydrocarbon chain.

Answer 6

• length
• unsaturation

Question 7

The _____ ______ chain accounts for the poor solubility of fatty acids in water. The _____ the fatty acyl chain and the fewer the ____ _____, the lower is the solubility in water. The _____ _____ ____ is polar (and ionized at neutral pH) and accounts for the slight solubility of short-chain fatty acids in water

Answer 7

• nonpolar hydrocarbon
• longer
• double bonds
• carboxylic acid group

Question 8

Melting points are also strongly influenced by the ______ and degree of ______ of the hydrocarbon chain. At room temperature (25 °C), the ______ fatty acids from 12:0 to 24:0 have a waxy consistency, whereas ______ fatty acids of these lengths are oily liquids

Answer 8

• length
• unsaturation
• saturated
• unsaturated

Question 9

Differences in melting points are due to different degrees of packing of the fatty acid molecules. Explain.

Answer 9

Fully saturated compounds

• In fully saturated compounds packing around each C–C bond gives the hydrocarbon flexibility
• The fully free rotation is the stable conformation
• steric hindrance of neighboring atoms is minimized
• can pack together tightly in nearly crystalline arrays, w/atoms along their lengths in van der Waals contact with the atoms of neighboring molecules

Unsaturated fatty acids

• a cis double bond forces a kink in the hydrocarbon chain
• Fatty acids with kinks cannot pack together as tightly as fully saturated fatty acids
• interactions w/each other are weaker
• less thermal energy is needed to disorder these poorly ordered arrays of unsaturated fatty acids = lower melting points

Question 10

ester functional group

Answer 10

Question 11

phosphodiester bond

Answer 11

Question 12

amide

Answer 12

Question 13

ether

Answer 13

Question 14

glycosidic link

Answer 14

Question 15

triacylglycerols, triglycerides, fats, or neutral fats

Answer 15

• simplest lipids constructed from fatty acids
• composed of three fatty acids each in ester linkage with a single glycerol
• Those containing the same kind of fatty acid in all three positions are called simple triacylglycerols
• Most naturally occurring triacylglycerols are mixed; they contain two or three different fatty acids
• nonpolar hydrophobic molecules, insoluble in water because the polar hydroxyls of glycerol and the polar carboxylates of the fatty acids are bound in ester linkages

The mixed triacylglycerol shown here has three different fatty acids attached to the glycerol backbone

Question 16

lipases

Answer 16

enzymes that catalyze the hydrolysis of stored triacylglycerols

Question 17

There are two significant advantages to using triacylglycerols as stored fuels, rather than polysaccharides such as glycogen and starch:

Answer 17

1. carbon atoms of fatty acids are more reduced than those of sugars, and oxidation of triacylglycerols yields more than twice as much energy, gram for gram, as the oxidation of carbohydrates
2. because triacylglycerols are hydrophobic and therefore unhydrated, the organism that carries fat as fuel does not have to carry the extra weight of water of hydration that is associated with stored polysaccharides

Question 18

Vegetable oils such as corn and olive oil are composed largely of triacylglycerols with ______ fatty acids and thus are liquids at room temperature. Triacylglycerols containing only ______ fatty acids, such as tristearin, the major component of beef fat, are white, greasy solids at room temperature.

Answer 18

• unsaturated
• saturated

Question 19

partial hydrogenation

Answer 19

• process that convert many of the cis double bonds in the fatty acids to single bonds
• increases melting temperature of the oils so that they are more nearly solid at room temperature

Question 20

partyial hydrogenation has an undesirable effect

Answer 20

• some cis double bonds are converted to trans double bonds
• strong evidence that dietary intake of trans fatty acids (“trans fats”) leads to a higher incidence of cardiovascular disease
• acids raise the level of triacylglycerols and of LDL (“bad”) cholesterol in the blood, and lower the level of HDL (“good”) cholesterol
• increase the body’s inflammatory response
• deleterious effects of trans fats occur at intakes of 2 to 7 g/day

Question 21

Biological waxes

Answer 21

• esters of long-chain (C14 to C36) saturated and unsaturated fatty acids with long-chain (C16 to C30) alcohols
• melting points (60 to 100 8C) are generally higher than those of triacylglycerols
• water-repellent properties
• firm consistency

Question 22

• Membrane lipids are ______ : one end of the molecule is hydrophobic, the other hydrophilic.
• Their hydrophobic interactions with each other and their hydrophilic interactions with water direct their packing into sheets called ______ _____

Answer 22

• amphipathic
• membrane bilayers

Question 23

five general types of membrane lipids:

triacylglycerols

Answer 23

• storage lipids (neutral)
• glycerol backbone (red)
• backbone is attached to one or more longchain alkyl groups (yellow) and a polar head group (blue)
• the alkyl groups are fatty acids in ester linkage

Question 24

five general types of membrane lipids:

phospholipids

Answer 24

• membrane lipids (polar)
• backbone (red)
  
  • glycerophospholipids: glycerol backbone
  • sphingolipids: sphingosine backbone
• backbone is attached to one or more longchain alkyl groups (yellow) and a polar head group (blue)
• the alkyl groups are fatty acids in ester linkage
• the polar head group is joined through a phosphodiester

Question 25

five general types of membrane lipids: glycolipids

Answer 25

* membrane lipids (polar) * backbone is attached to one or more longchain alkyl groups (yellow) and a polar head group (blue) * galactolipids (sulfolipids): glycerol backbone * alkyl groups are fatty acids in ester linkage * sphingolipids: sphingosine backbone * single alkyl group is a fatty acid in amide linkage * the polar head group is joined through a direct glycosidic linkage

Question 26

five general types of membrane lipids: archaeal tetraether lipids

Answer 26

* membrane lipids (polar) * different kinds * glycerol backbone (red) * backbone is two very long, branched alkyl chain (yellow) and a polar head group (blue) * the alkyl groups are fatty acids ether-linked to glycerol at both ends * the polar head group is joined through a direct glycosidic linkage

Question 27

Glycerophospholipids, phosphoglycerides

Answer 27

* membrane lipids * two fatty acids are attached in ester linkage to the first and second carbons of glycerol * a highly polar or charged group is attached through a phosphodiester linkage to the third carbon * Glycerol is prochiral * has no asymmetric carbons * but attachment of phosphate at one end converts it into a chiral compound * phosphate group bears a negative charge at neutral pH * polar alcohol may be negatively charged, neutral or positively charged * these charges contribute greatly to the surface properties of membranes

Question 28

ether lipids

Answer 28

* one of the two acyl chains is attached to glycerol in ether, rather than ester, linkage * The ether-linked chain may be saturated, as in the alkyl ether lipids, or may contain a double bond between C-1 and C-2, as in plasmalogens

Question 29

galactolipids

Answer 29

* predominate in plant cells * one or two galactose residues are connected by a glycosidic linkage to C-3 of a 1,2-diacylglycerol * localized in the thylakoid membranes of chloroplasts * probably the most abundant membrane lipids in the biosphere * Phosphate is a limiting plant nutrient in soil, perhaps evolutionary pressure to conserve phosphate for more critical roles favored plants that made phosphate-free lipids * Plant membranes also contain sulfolipids * a sulfonated glucose residue is joined to a diacylglycerol in glycosidic linkage * sulfonate group bears a negative charge like that of the phosphate group in phospholipids

Question 30

Archaea Contain Unique Membrane Lipids

Answer 30

* membrane lipids containing long-chain (32 carbons) branched hydrocarbons linked at each end to glycerol through ether bonds * more stable to hydrolysis at low pH and high temperature than are the ester bonds found in the lipids of bacteria and eukaryotes * twice the length of phospholipids and sphingolipids * can span the full width of the plasma membrane * At each end of the extended molecule is a polar head consisting of glycerol linked to either phosphate or sugar residues * glycerol dialkyl glycerol tetraethers (GDGTs) have a unique structure * The glycerol moiety of the archaeal lipids is not the same stereoisomer as that in the lipids of bacteria and eukaryotes; the central carbon is in the R configuration in archaea, in the S configuration in bacteria and eukaryotes

Question 31

Sphingolipids

Answer 31

* fourth large class of membrane lipids * contain no glycerol * backbone is composed of one molecule of the long-chain amino alcohol sphingosine or one of its derivatives * have a polar head group and two nonpolar tails * polar head group is joined by a glycosidic linkage in some cases and a phosphodiester in * ceramide * When a fatty acid attaches via amide linkage to the —NH₂ on C-2 * structural parent of all sphingolipids * 3 subclasses * Sphingomyelins * classified as phospholipids because they contain phosphocholine or phosphoethanolamine as their polar head group * no net charge on their head groups * present in the plasma membranes of animal cells * prominent in myelin, a membranous sheath that surrounds and insulates the axons of some neurons—thus the name “sphingomyelins.” * Glycosphingolipids * occur largely in the outer face of plasma membranes * head groups with one or more sugars connected directly to the —OH at C-1 of the ceramide moiety * do not contain phosphate * Cerebrosides * have galactose have a single sugar linked to ceramide * found in plasma membranes of cells in neural tissue * Globosides * have glucose in the plasma membranes of cells in nonneural tissues * two or more sugars * Gangliosides * most complex * have oligosaccharides as their polar head groups and one or more residues of N-acetylneuraminic acid * Sialic acid gives gangliosides the negative charge at pH 7 that distinguishes them from globosides * Gangliosides with one sialic acid residue are in the GM (M for mono-) series, those with two are in the GD (D for di-) series, and so on

Question 32

Sphingolipids \_\_\_\_\_ ______ of certain sphingolipids define the human blood groups and therefore the blood type of individuals

Answer 32

carbohydrate moieties

Question 33

Gangliosides are concentrated in the outer surface of cells, serving as _____ \_\_\_\_\_ of recognition for extracellular molecules or surfaces of neighboring cells. The kinds and amounts of gangliosides in the plasma membrane change dramatically during embryonic development. Tumor formation induces the synthesis of a new complement of gangliosides, and very low concentrations of a specific ganglioside have been found to induce ______ of cultured neuronal tumor cells.

Answer 33

* recognition sites * differentiation

Question 34

Phospholipids and sphingolipids are degraded in \_\_\_\_\_

Answer 34

Lysosomes

Question 35

The _____ \_\_\_\_\_\_ of membranes undergo constant metabolic turnover, the rate of their synthesis normally counterbalanced by the rate of breakdown. The breakdown of lipids is promoted by _____ \_\_\_\_\_ in lysosomes, each enzyme capable of hydrolyzing a specific bond. When sphingolipid degradation is impaired by a defect in one of these enzymes, partial breakdown products accumulate in the tissues, causing _____ \_\_\_\_\_

Answer 35

* polar lipids * hydrolytic enzymes * serious disease

Question 36

Most cells continually degrade and replace their membrane lipids. For each hydrolyzable bond in a glycerophospholipid, there is a specific hydrolytic enzyme in the lysosome. ______ of the A type remove one of the two fatty acids, producing a lysophospholipid. (These esterases do not attack the ether link of plasmalogens.) ______ remove the remaining fatty acid. Gangliosides are degraded by a set of lysosomal enzymes that catalyze the stepwise removal of sugar units, finally yielding a \_\_\_\_\_\_

Answer 36

* Phospholipases * Lysophospholipases * ceramide

Question 37

Sterols

Answer 37

* structural lipids present in the membranes of most eukaryotic cells * characteristic structure is the steroid nucleus * four fused rings, three with six carbons and one with five * almost planar and is relatively rigid * fused rings do not allow rotation about COC bonds * serve as precursors for a variety of products with specific biological activities * regulate gene expression

Question 38

Cholesterol

Answer 38

* major sterol in animal tissues * amphipathic * has a polar head group (the hydroxyl group at C-3) and a nonpolar hydrocarbon body (the steroid nucleus and the hydrocarbon side chain at C-17 * Bacteria cannot synthesize sterols but they can incorporate exogenous sterols into their membranes

Question 39

Bile acids

Answer 39

* polar derivatives of cholesterol * act as detergents in the intestine, emulsifying dietary fats to make them more readily accessible to digestive lipases

Question 40

membrane lipids play a _____ role in the cell; lipid fuels are stored until _____ by enzymes, and membrane lipids form impermeable barriers around cells and cellular compartments. others play an active roles such as

Answer 40

* passive * oxidized * active roles * roles in the metabolic traffic as metabolites and messengers * potent signals—as hormones, carried in the blood from one tissue to another, or as intracellular messengers generated in response to an extracellular signal (hormone or growth factor) * cofactors in electron-transfer reactions in chloroplasts and mitochondria or the transfer of sugar moieties in a variety of glycosylation reactions * some have a system of conjugated double bonds: pigment molecules that absorb visible light (vision and photosynthesis) * others produce natural colorations

Question 41

Phosphatidylinositols and Sphingosine Derivatives Act as _____ \_\_\_\_\_\_. Some are involved in \_\_\_\_\_\_\_. Membrane sphingolipids also can serve as sources of _____ \_\_\_\_\_\_

Answer 41

* Intracellular Signals * exocytosis * intracellular messengers

Question 42

Eicosanoids

Answer 42

* paracrine hormones * act only on cells near the point of hormone synthesis instead of being transported in the blood to act on cells in other tissues or organs * involved in reproductive function; in the inflammation, fever, and pain associated with injury or disease; in the formation of blood clots and the regulation of blood pressure; in gastric acid secretion; and in various other processes important in human health or disease * derived from arachidonic acid * 3 classes * Prostaglandins (PG) * contain a five-carbon ring originating from the chain of arachidonic acid * functions: * stimulate contraction of the smooth muscle of the uterus during menstruation and labor * affect blood flow to specific organs * wake-sleep cycle * rsponsiveness of certain tissues to hormones such as epinephrine and glucagon * elevates body temperature (producing fever) and cause inflammation and pain * thromboxanes * have a six-membered ring containing an ether * produced by platelets * act in * formation of blood clots and the reduction of blood flow to the site of a clot * some inhibit the enzyme prostaglandin H2 synthase * Leukotrienes * contain three conjugated double bonds * powerful biological signals * induces contraction of the smooth muscle lining the airways to the lung * Overproduction of leukotrienes causes asthmatic attacks

Question 43

Steroids

Answer 43

* oxidized derivatives of sterols * they have the sterol nucleus but lack the alkyl chain attached to ring D of cholesterol * more polar than cholesterol * move through the bloodstream (on protein carriers) to target tissues * they enter cells, bind to highly specific receptor proteins in the nucleus, and trigger changes in gene expression and thus metabolism * because they have very high affinity for their receptors, very low concentrations of hormones are sufficient to produce responses in target tissues * major groups: male and female sex hormones, cortisol and aldosterone

Question 44

Plants produce literally thousands of different lipophilic compounds, volatile substances that are used to

Answer 44

attract pollinators, to repel herbivores, to attract organisms that defend the plant against herbivores, and to communicate with other plants

Question 45

Vitamins A and D Are _____ \_\_\_\_\_

Answer 45

Hormone Precursors

Question 46

vitamins

Answer 46

* compounds that are essential to the health of humans and other vertebrates * cannot be synthesized by these animals and must therefore be obtained in the diet * two general classes * fat-soluble vitamins * soluble in nonpolar organic solvents * A, D, E, and K * isoprenoid compounds synthesized by the condensation of multiple isoprene units. * Two of these (D and A) serve as hormone precursors. * water-soluble vitamins: can be extracted from foods with aqueous solvents

Question 47

Vitamin D₃, cholecalciferol

Answer 47

* formed in the skin from 7-dehydrocholesterol in a photochemical reaction driven by the UV component of sunlight * not itself biologically active * it is converted by enzymes in the liver and kidney to 1␣,25-dihydroxyvitamin D3, a hormone that regulates calcium uptake in the intestine and calcium levels in kidney and bone * Deficiency leads to defective bone formation and rickets

Question 48

Vitamin A (retinol)

Answer 48

* functions as a hormone and as the visual pigment of the vertebrate eye * derivative retinoic acid regulates gene expression in the development of epithelial tissue, including skin * Retinoic acid is the active ingredient in the drug tretinoin (Retin-A), used in the treatment of severe acne and wrinkled skin * Retinal, another vitamin A derivative, is the pigment that initiates the response of rod and cone cells of the retina to light, producing a neuronal signal to the brain

Question 49

Vitamins E and K and the Lipid Quinones Are

Answer 49

Oxidation-Reduction Cofactors

Question 50

Vitamin E

Answer 50

* group of closely related lipids called tocopherols * contain a substituted aromatic ring and a long isoprenoid side chain * hydrophobic * associate with cell membranes, lipid deposits, and lipoproteins in the blood * biological antioxidants * aromatic ring reacts with and destroys the most reactive forms of oxygen radicals and other free radicals, protecting unsaturated fatty acids from oxidation and preventing oxidative damage to membrane lipids * found in eggs and vegetable oils and are especially abundant in wheat germ

Question 51

vitamin K

Answer 51

* aromatic ring under goes a cycle of oxidation and reduction during the formation of active prothrombin * a blood plasma protein essential in blood clottin * proteolytic enzyme that splits peptide bonds in the blood protein fibrinogen to convert it to fibrin * insoluble fibrous protein that holds blood clots together * found in green plant leaves * a related form, vitamin K₂ is formed by bacteria living in the vertebrate intestine

Question 52

Ubiquinone (coenzyme Q) and plastoquinone

Answer 52

* isoprenoids * function as lipophilic electron carriers in the oxidation-reduction reactions that drive ATP synthesis in mitochondria and chloroplasts, respectively * can accept either one or two electrons and either one or two protons

Question 53

Dolichols Activate _____ \_\_\_\_\_\_ for Biosynthesis

Answer 53

Sugar Precursors

Question 54

During assembly of the complex carbohydrates of bacterial cell walls, and during the addition of polysaccharide units to certain proteins (glycoproteins) and lipids (glycolipids) in eukaryotes, the sugar units to be added are chemically activated by attachment to isoprenoid alcohols called _____ . These compounds have strong _______ interactions with membrane lipids, anchoring the attached sugars to the membrane, where they participate in sugar-transfer reactions.

Answer 54

* dolichols * hydrophobic *

Question 55

Many Natural Pigments Are Lipidic Conjugated Dienes \_\_\_\_\_ _____ have carbon chains with alternating single and double bonds; this structural arrangement allows the ______ of electrons, the compounds can be excited by low-energy electromagnetic radiation (visible light), giving them _____ visible to humans and other animals. These pigments are synthesized from

Answer 55

* Conjugated dienes * delocalization * colors * five-carbon isoprene derivatives

Question 56

Polyketides

Answer 56

* a diverse group of lipids with biosynthetic pathways similar to those for fatty acids * secondary metabolites, compounds that are not central to an organism’s metabolism but that serve some subsidiary function that gives their producers an advantage in some ecological niche * Many polyketides find use in medicine as antibiotics (erythromycin), antifungals (amphotericin B), or inhibitors of cholesterol synthesis

Question 57

Because lipids are insoluble in water, their extraction and subsequent fractionation require the use of _____ \_\_\_\_\_\_ and some techniques not commonly used in the purification of water-soluble molecules such as proteins and carbohydrates. In general, complex mixtures of lipids are separated by differences in _____ or solubility in _____ \_\_\_\_\_. Lipids that contain ester- or amide-linked fatty acids can be hydrolyzed by treatment with

Answer 57

* organic solvents * polarity * nonpolar solvents * acid or alkali or with specific hydrolytic enzymes

Question 58

Lipid Extraction Requires Organic Solvents * Neutral lipids (triacylglycerols, waxes, pigments, and so forth) are readily extracted from tissues with ethyl ether, chloroform, or benzene, solvents that do not permit lipid clustering driven by ____ \_\_\_\_\_\_ . * Membrane lipids are more effectively extracted by more ____ \_\_\_\_\_ solvents, such as ethanol or methanol, which reduce the hydrophobic interactions among lipid molecules while also weakening the hydrogen bonds and electrostatic interactions that bind membrane lipids to membrane proteins * After tissue is homogenized in this solvent to extract all lipids, more _____ is added to the resulting extract and the mixture separates into two phases: * The lipids remain in the ______ layer, and the more polar molecules such as proteins and sugars partition into the _______ layer

Answer 58

* hydrophobic interactions * polar organic * water * methanol/water (top phase) and chloroform (bottom phase) * chloroform * methanol/water

Question 59

Complex mixtures of tissue lipids can be fractionated by chromatographic procedures based on the different ______ of each class of lipid

Answer 59

polarities

Question 61

adsorption chromatography

Answer 61

* an insoluble, polar material such as silica gel is packed into a glass column * lipid mixture (in chloroform solution) is applied to the top of the column * In high-performance liquid chromatography, the column is of smaller diameter and solvents are forced through the column under high pressure. * polar lipids bind tightly to the polar silicic acid, but the neutral lipids pass directly through the column and emerge in the first chloroform wash * Polar lipids are eluted, in order of increasing polarity, by washing the column with solvents of progressively higher polarity * Uncharged but polar lipids (cerebrosides) are eluted with acetone * very polar or charged lipids (glycerophospholipids) are eluted with methanol

Question 62

Thin-layer chromatography

Answer 62

* A thin layer of silica gel is spread & adheres onto a glass plate * A small sample of lipids dissolved in chloroform is applied near one edge of the plate, dipped in a shallow container of an organic solvent or solvent mixture * the entire setup is enclosed in a chamber saturated with the solvent vapor * As the solvent rises on the plate by capillary action, it carries lipids with it * less polar lipids move farthest, as they have less tendency to bind to the silicic acid * separated lipids can be detected by spraying the plate with a dye (rhodamine) that fluoresces when associated with lipids, or by exposing the plate to iodine fumes. * Iodine reacts reversibly with the double bonds in fatty acids, such that lipids containing unsaturated fatty acids develop a yellow or brown color. * Several other spray reagents are also useful in detecting specific lipids. * regions containing separated lipids can be scraped from the plate and the lipids recovered by extraction with an organic solvent.

Question 63

* Certain classes of lipids are susceptible to degradation under specific conditions. For example, all ester-linked fatty acids in triacylglycerols, phospholipids, and sterol esters are released by * somewhat harsher hydrolysis conditions release _____ \_\_\_\_\_ fatty acids from sphingolipids

Answer 63

* mild acid or alkaline treatment * amide-bound

Question 64

To establish unambiguously the length of a hydrocarbon chain or the position of double bonds, _____ \_\_\_\_\_\_ analysis of lipids or their volatile derivatives is invaluable

Answer 64

mass spectrometric