Chapter 23:Lipids

eicosanoids

a group of compounds derived from 20-carbon unsaturated fatty acids (eicosanoic acids) and synthesized thoroughout the body. They function as short-lived chemical messengers that act near their points of synthesis ("local hormones").

Cerebrosides contain

a single monosaccharide. They are abundant in nerve cell membranes in the brain with the monosaccharide D-galactose. In other membranes, the sugar unit is D-glucose.

glycolipids (2)

also derived form sphingsine, but have attached a carbohydrate as a monosaccharide or a short polysaccharide chain

sphingomyelins

amides derived from an amino alcohol (sphingosine), also contain charged phosphate-diester groups. Essential to the structure of cell membranes, especially abundant in heme cell membranes.

Cell membranes separate the

aqueous interior of cells (cytoplasm) from the aqueous environment surrounding the cells.

Prostaglandins and leukotrienes are synthesized from

arachidonic acid, which is itself synthesized from linolenic acid.

bile acids

are essential for the emulsifcation of fats during diegstion

peripheral proteins

associated with just one face of the bilayer and are held by noncovalent interactions with the hydrophobic lipid tails or the hydrophilic head groups

Leukotriene release has been found to trigger the

asthmatic response, severe allergic reactions, and inflammation.

Soap-making (saponification) involves the

base-catalyzed hydrolysi of triacylglycerols. Any mixture of triacylglycerols can be used; the second ingredient needed is lye or potash.

Lipid-soluble substances can be incorporated into the

bilayer.

waxes

esters with long, straight hydrocarbon chains in both R groups. Secreted by sebaceous glands in the skin of animals and perform most external protective functions.

estrogens

estradiol and estrone; synthesized from testosterone

integral proteins

extend completely through the membrane and are anchored by hydrophobic regions that extend through the bilayer

Excess energy from dietary fats and oils is stored as

fat in adipose tissue.

The 2nd tail is from a

fatty acid acyl group connected by an amide link to the -NH₂ in sphingosine.

Prostaglandins and leukotrienes differ somewhat in their structure; prostaglandins contain a

five-membered ring, which leukotrienes lack.

Membrane components do not

flip from the inside of the membrane to the outside (and vice versa).

Glycolipids reside in cell membranes with their carbohydrate segments extending into the

fluid surrounding the cells.

SUMMARY: Cell membranes are composed of a

fluid-like phospholipid bilayer.

The overall structure of cell membranes is represented by the

fluid-mosaic model.

All fats and oils are composed of

glycerol (1,2,3-propanetriol; aka glycerine) with 3 fatty acids. They are named chemically as triacylglycerols, but are often called triglycerides.

Naturally occurring fats and oils are triesters formed between

glycerol and fatty acids.

gangliosides

glycolipids in which the carbohydrate is a small polysaccharide chain rather than a monosaccharide

polyunsaturated fatty acids

have more than one carbon-carbon double bond

glucocorticoids

help to regulate glucose metabolism and inflammation. Anti-inflammatory ointments contain hydrocortisone to reduce swelling and itching.

Many lipids have

hydrocarbon or modified hydrocarbon stucture, properties, and behavior

Hydrogenation. The carbon-carbon double bonds in vegetable oils can be

hydrogenated to yield saturated fats. Margarine and solid cooking fats are produced commercially by hydrogenation of vegetable oils.

The ionic end is

hydrophilic (water-loving); it tends to dissolve in water. The long hydrocarbon chain portion of the molecule is hydrophobic (water-fearing). Because of these opposing tendencies, soap molecules are attracted to both grease and water.

Like soaps, synthetic detergent molecules have

hydrophobic tails and hydrophilic heads and cleanse by forming micelles around greasy dirt. All substances that function in this manner are described as surfactants. The hydrophilic heads by may be anionic, cationic, or non-ionic.

When soap is dispersed in water, the big organic anions cluster together so that their long, hydrophobic hydrocarbon tails are

in contact. By doing so, they avoid disrupting the strong hydrogen bonds of water and create a nonpolar environment.

The anti-inflammatory and fever-reducing (antipyretic) action of aspiring results in part from its

inhibition of prostaglandin synthesis.

The carbohydrate parts of glycoproteins and glycolipids mediate the

interactions of the cell with outside agents.

steroids

lipids whose structure is based on a tetracylic (4-ring) carbon skeleton

When phospholipids are shaken vigorously with wter, they spontaneously form

liposomes, small spherical vesicles with a lipid bilayer surrounding an aqueous center.

Unsaturated acids have

rigid kinks wherever they contain cis double bonds. The kinks make it difficult for such chains to pack next to each.

because the bilayer membrane is fluid rather than rigid, it is not easily

ruptured.

Animal fats contain a large percentage of

saturated fatty acids. The difference in composition is the primary reason for the different melting points of fats and oils.

Those without double bonds are

saturated.

Phospholipids are

similar to soap and detergent molecules in having ionic hydrophilic heads and hydrophobic tails. They differ, however, in having 2 tails instead of 1.

Small nonpolar molecules cross by

simple diffusion through the lipid bilayer.

Some solutes enter and leave cells by simple diffusion:

small, nonpolar molecules and lipid-soluble substances including steroid hormones, move through the hydrophobic lipid bilayer in this way

Energy from the conversion of ATP to ADP is used to change the shape of the

sodium/potassium pump.

Animal fats are

solid, whereas vegetable oils are liquid, but their structures are closely related.

The more bonds there are in a triacylglycerol, the harder it is for it to

solidify.

Glycolipids are also derived from

sphingosine. They have a carbohydrate group at C1 instead of a phosphate group bonded to an amino alcohol.

Cholesterol is the most abundant animal steroid. It is a component of cell membranes and the

starting material for all other steroids.

Cholesterol is a

steroid, a class of biomolecules that are characterized by a system of 4 fused rings.

active transport

substances cross the membrane only when energy is supplied because they go from lower to higher concentration regions

passive transport

substances move across the membrane freely by diffusion from higher to lower concentration regions

glycerophospholipids or phosphoglycerides

Triesters of glycerol 3-phosphate; these are the most abundant membrane lipids. Two of the ester bonds are with fatty acids. The 3rd ester bond is with a phosphate group, which is itself linked to ethanol amine, choline, or serine.

short hand nomenclature for fatty acids

Uses C for carbon followed by the number of C atoms present, a colon, and the number of unsaturated bonds present. Palmitic acid=C16:0. Oleic acid=C18:1.

mimeralcorticocoids (such as aldosterone)

regulate the cellular fluid balance between Na⁺ and K⁺ ions

This pump simultaneously brings two

K⁺ ions the cell and moves three Na⁺ ions out of the cell.

Properties of the Triacylglycerols in Natural Fats and Oils

Nonpolar and hydrophobic. No ionic charges. Solid tracylglycerols (fats)-high proportion of saturated fatty acid chains. Liquid triacylglycerols (oils)-high proportion of unsaturated fatty acid chains

Prostaglandins can lower

blood pressure, influence platelet aggregation during blood clotting, stimulate uterine contractions, and lower the extent of gastric secretions. In addition, they are responsible for some of the pain and sweating that accompany inflammation.

phospholipids

built up from either glycerol (to give glycerophospholipids) or sphingosine (to give sphingomyelins).

(wax) The acids usually have an even number of

carbon atons, generally from 16-36 carbons. The alcohols have an even number of C atoms ranging from 24-36.

Liposomes are potentially useful as

carriers for drug deficiency because they can fuse with cell membranes and empty their contents into the cell.

Water-soluble substances can be trapped in the

center of liposomes.

The bilayer incorporates

cholesterol, proteins (including glycoproteins), and glycolipids.

In naturally occuring fats and oils, the double bonds are usually

cis rather than trans.

lipids

class of biological macromolecules defined by solubility in nonpolar solvents (a physical property) rather than by chemical structure

Phospholipids provide the basic structure of cell membranes, where they aggregate in a

closed, sheetlike, double leaflet structure-the lipid bilayer. The lipid bilayer is highly ordered and stable, but still flexible.

Proteins move sideways in the membrane layers

continuously (similar to floating on a pond); this is an energetically neutral motion.

glycolipids

different amides derived from sphingosine, contain polar carbohydrate groups. On cell surfaces, the carbohydrate portion is recognized and interacts with intercellular messengers.

Micelles are essential for the

digestion of dietary fat. The two most common bile acids are cholic acid and chenodeoxycholic acid. In the intestinal tract these acids are ionized to anions and referred to as bile salts.

Hydrophobic tails and hydrophilic head groups make glycerophospholipids

emulsifying agents.

Na⁺, K⁺, and other substances that maintain concentration gradients across the cell membrane cross with expenditure of

energy and the aid of proteins (active transport).

Maintaing concentration gradients requires an expenditure of

energy.

Small nonpolar molecules can easily

enter the cell through the membrane and individual lipid or protein molecules can diffuse rapidly from place to place.

Hydrolysis of Triacylglycerols. In the body, this hydrolysis is catalyzed by

enzymes (hydrolases) and is the first reaction in the digestion of dietary fats and oils.

wax

ester between a fatty acid and a long-chain alcohol.

fatty acids

long, unbranched hydrocarbon chains with a carboxylic acid group at one end; most have even numbers of carbon atoms.

The primary funciton of triacylglycerols is

long-term storage of energy.

Because they are more rigid than the hydrophobic tails, the cholesterol molecules help to

maintain the structural rigidity of the membrane.

The membrane is a mosaic because it contains

many kinds of molecules.

The more double bonds a fatty acid has, the lower its

melting point.

In facilitated diffusion, solutes are helped across the membrane by

membrane proteins. The molecule binds to a membrane protein, which changes shape so that the transported molecule is released on the other side of the membrane.

Solubility of bile acids is increased by conjugatoin with wither taurine, a cysteine derivative, or glycine. This enhances the formation of

micelles.

The model is fluid because molecules can

move around within it.

In sphingolipids, the amino alcohol sphingosine provides

one of the hydrophobic tails.

Hydrophilic ionic heads on the surface of the cluster stick

out into the water. The resulting spherical clusters are called micelles. Grease and dirt become coated by the nonpolar tails of the soap molecules and trapped in the center of the micelles as they form. One suspended within micelles, the grease and dirt can be rinsed away with water.

Sphingomyelins are sphingosine derivatives with a

phosphate ester group at C1 of sphingosine. The sphingomyelins are major components of the coating around nerve fibers.

Glycerophospholpids are named as derivatives of

phosphatidic acids.

Lipids with a phosphate ester to choline are known as

phosphatidyl cholines or lecithins.

The 3 major kinds of cell membrane lipids in animals are

phospholipids, glycolipids, and cholesterol.

Synthesized in liver cells from cholesterol and stored in the gall bladder, these molecules have a

polar end and a nonpolar end.

Small ions and polar molecules diffuse across the membrane via

protein pores (simple diffusion).

Glucose and certain other substances (including amino acids) cross with the aid of

proteins and without the energy input (facilitated diffusion).

Oxidation causes

rancidity, the decomposition of fats and oils to products with unpleasant odors or flavors.

Detergent is a term usually applied to

synthetic materaials made from petroleum chemicals.

Within a cell membrane, nearly-flat cholesterol molecules are distributed among the

tails of phospholipids.

androgens (sex hormones)

testosterone and androsterone

Adipose tissue serves to provide

thermal insulation and protective padding.

glycerophospholipids

triesters between glycerol and charged phosphate diester groups and fatty acid. Abundant in cell membranes. Together with lipids, they help to control the flow of molecules in and out of cells.

triacylglycerols (fats)

triesters between glycerol, a three-carbon trialcohol and fatty acids, carboxylic acids with long hydrocarbon chains. Found in most dietary fats and oils. These are the fat storage molecules in our bodies and a major source of biochemical energy.

Triacylglycerols are

uncharged, nonpolar, hydrophobic molecules that coalesce when stored in fatty tissue.

The hydrocarbon chains in saturated acids are

uniform in shape with identical angles at each carbon atom, and flexible chains, allowing them to pack together.

Vegetable oils consist of mainly

unsaturated fatty acids.

Those with double bonds are

unsaturated.