Bio hw

Complexity EH:

Molecular Efficiency

and Variety

When you have opportunity, watch a squirrel work its way into a sunflower seed that's fallen from a bird feeder (see Figure 4.1). Or watch an otter "on a lunch break" cracking its way into a mussel's shell with a pebble. It is fascinating to observe how elegantly the parts of the organism suit the process that is being carried out. One major purpose of this chapter is to extend your awareness. This high level of elegance in form-facilitating function stretches all the way back to the level of the biological molecules that comprise the individual cells of the squirrel or otter. Just as in organisms where the structure of organs determines their function, so it is with molecules: the way the atoms are bonded to each other determines what function the molecule will have. One molecule is built in a way that is optimal for storing information. Another molecule's structure perfectly suits it for breaking a specific covalent bond in a specific kind of molecule generating a specific product. Form determines function in molecules too.

Life Is Complex. In those living things that possess many levels of organization, the functions at one level determine what is functionally possible at higher levels. Molecular form and function determine cellular form and function and so on. The squirrel skillfully gets rid of the sunflower seed hull and ingests the starchy interior only because there is a corresponding elegance of organ, tissue, cellular, organellar, and molecular structure that support this feeding process.

Imagine that you are Mother Nature. You are given a mutant squirrel that knows how to feed on sunflower seeds but has no forelimbs. All you have to do is to design a set of forelimbs that will support this process. But you must do this by selecting individual molecules

What role does fat play in living systems? How are plant fats distinct from animal fats? How much dietary fat is good for humans? What is a phospholipid, and where are they used in cells? What features of a phospholipid's structure highly suit it to its role in the cell? · What hormones are lipids by structure? · What effect(s) does the hormone testosterone have on the male human organism? 4.5 Proteins: Structure and Function What roles do proteins play in living systems? How does their structure support such roles? What are the structural elements or building blocks of proteins? · What features of these elements suit them to the high level of variability of function observed in proteins? · What are the separate levels of complexity in protein structure, and why are such complicated concepts needed in protein study? Are all biomolecules clearly either carbohydrates, lipids, or proteins? Are there hybrid molecules that have features of more than one class? 4.6 Proteins Conceal Wisdom · What is hemoglobin, and what is its role in living systems? · How does its design wonderfully suit the challenges inherent in its role in living things? 4.7 Nucleic Acids: Structure and Function · How are nucleic acids structured? · What are the structures and some examples of nucleotides? What roles do they perform? · What are two different roles for the molecule ATP? · How is the informational molecule DNA structured? 4.8 Living Things Need Just a Few Good Molecules · How many molecules were produced in the prebiotic simulations of Miller and Urey? · What critical feature is lacking in these studies that would make the production of life a more plausible outcome?

THE CENTRALITY OF CARBON TO THE ORGANIC MOLECULES OF LIFE

Mere molecules are the stuff that constitutes a cell. Yet a cell is hundreds of thousands of times their size! So if molecules are built of atoms and if struc​ture at the cellular level is on a scale much larger than that of atoms, then it follows that at least some of the molecules we observe will be huge by com​parison with the atoms that compose them. How are these huge biomolecules to be constructed?

Of the 90 or so naturally occurring elements, only about 28 of them find their way into signifi​cant amounts of cellular structure. And of those, only four—carbon (C), oxygen (0), hydrogen (H), and nitrogen (N)—compose 95% of the mass of the cell! Among these four kinds of elements is one whose atoms are incredibly versatile for the pur​pose of building large biological molecules. That element is carbon.

Carbon has 6 protons and 6 electrons (see Fig​ure 4.2). Two of these electrons fill an inner shell leaving four electrons to populate its outer shell. As a result, carbon tends to form covalent bonds—to share its four electrons. This is useful for building biological molecules because covalent bonds are directed—giving specific shapes to the molecules formed from them. The fact that carbon forms four

Figure 4.2 The carbon atom, with four electrons in its outer shell is constructed so that it readily bonds covalently with four other atoms. This makes large molecules possible. Hydrogen's shell fills by bonding to one other atom, oxygen by bonding with two other atoms, and nitrogen, three other atoms. How convenient! These 4 kinds of atoms compose 95% of cell structure by mass.

such bonds means it can bond easily to four other atoms. And if carbon atoms readily bond to other carbon atoms, we can begin to understand how molecules of enormous size and elegantly crafted shape can be designed.

This versatility of structure can be further re​fined by using the other three kinds of elements mentioned above; all of whose atoms are light and easily form covalent bonds as well. Hydrogen forms one covalent bond, oxygen forms two, and nitrogen forms three (see Figure 4.2). So virtually any shape of molecule can be designed with these wonderful and versatile subunits.

From life's vast diversity of species, we may infer the existence of millions of different sorts of mol​ecules out there supporting all of it. And now we can see how the versatile bonding potentials of car​bon, hydrogen, nitrogen, and oxygen atoms well serve the production of that diversity. This infer​ence causes us to wonder (fearfully!) how many of these diverse molecules we will have to study before even a rudimentary understanding of life is possible! Yes, biochemical life is complex, but we are in for a pleasant surprise.

It is true that huge biological molecules are composed of thousands of individual atoms. But most of them are built from a small collection—perhaps 40 or so distinct kinds—of simpler molecules containing only 10 to 100 atoms. We call these simpler building block molecules monomers (mono in Greek = one). The much larger molecules assembled from these monomers are called polymers (poly in Greek = many). Some of the common monomers are pictured in Figure 4.3. Once we've learned the names and structures of a few of the most important monomers, we start finding them in the polymers

monomer—any chemical compound that can be used as a building block or structural subunit in the assembly of a much larger molecule called a polymer.

polymer—a large chemical compound formed by assembly of repeating structural units called monomers.

IOH 1 1 CH3 SH C IH3

H CH3 CH2 HO—CH CH2 H3C —CH

I I I I

HO —C —CH–NH2 HO —0 —CH–NH2 HO —C —CH–NH2 HO —0 —CH–NH2 HO —C —CH–NH2 HO —0 —CH–NH2

II 11 11 11 11 11

0 0 0 0 0 0

Glycine Alanine Serine Threonine Cysteine Valine

NH2 NH2 HN =C

I I

NH2 CH2 NH

I 1 1

0= CH2 CH2 NH2 HN‑

I I I

1

CH2 C 1H2 C IH2 0 =C I N N

CH2 CH2 CH2 CH2 CH2

I 1 1 1

HO —C —CH–NH2 HO —C —CH–NH2 HO —C —CH–NH2 HO —C —CH–NH2 HO —C —CH–NH2

11 ll ll 11 ll

0 0 0 0 0

Glutamine Lysine Arginine Asparagine Histidine

Aspartate Glutamate Isoleucine Leucine Phenylalanine Methionine

CH2

HO —C —CH–NH2

II

0 Tryptophan

HN

I IH2

C 0 ,C,

C C C' 'N

1 1 II I

HNC --NH C C

C N o

11 H 0

Thymine Cytosine

0 NH2

11 1

,C, N , N

C' -NH // C

II I C II I

C \ ,C C

Nc‑

0 N N--

H

Adenine

0 CI NNH C1 II \ C C NH2 Guanine

Amino acids Purines Pyrimidines Sugars Fatty acids

Figure 4.3 The monomers of life. These 30 monomers plus a few others comprise much of the molecular structure of life. The big trick: putting them together in sequences that support life!

OHHH

II I I I

H—O—C—C C C I I I H H H

H H H H H H H H H H H H H

1111111111111

C C C—C—C—C—C—C—C—C—C—C—C—H

H H H H H H H H H H H H H

Stearic acid

H

H—C— OH H—C— OH H—C--OH

H

Glycerol

CH3

HOCH2CH2— N+— CH3

CH3

Choline

HOCH2 .OOH

C H c

I I

CIIIIIIMC H

IV I

H

OH OH

Ribose

CH2OH

H CH 0

1 Z I

C OH H c

I ,IV I

OH 1 Hy 0

I I

H OH

Glucose

Figure 4.3 (Continued)

of virtually all the organisms we study. So even at the humble level of biological molecules, we discover a glorious unity concealed within diversity. There is a unity structurally in the set of monomers used to create life but an incredible

diversity of polymers that can be built from them. We are seeing here the same sort of Genius that can take a few basic oil colors on a palette and generate an infinite variety of colors and paintings using the same few pigments.

IN OTHER WORDS

1. Most biomolecules are huge in size compared to the atoms that compose them.

2. Only four kinds of atoms—carbon, nitrogen, oxygen, and hydrogen—compose 95% of the biomass of all living things.

3. Because carbon has four electrons in its outer shell, it is well suited to bind covalently to four other atoms. This predicts its utility for constructing large biomolecules.

4. Most large biological molecules are polymers that are constructed from a limited number of monomers commonly found in all living systems.

&INSTRUCTION AND DEGRADATION OF ORGANIC MOLECULES

A major portion of the cell's metabolism is the set of chemical reactions involved in synthesiz​ing, changing, and degrading biological molecules. A cell living in the sheltered environment of your body receives the raw materials it needs for mo​lecular synthesis in the form of monomers (simple sugars, fatty acids, amino acids, or nucleotides). A single, independent cell like a bacterium living in a pond must build even these monomers from still simpler molecules like ammonia, carbon dioxide, and water (see Figure 4.4).

Once any cell has the monomers needed for growth, metabolic reactions link these monomers together into polymers. These reactions are essentially the same in all cells. In each case, a large catalytic molecule called an enzyme (see Section 4.5) attaches to two monomers. It removes an oxygen and hydrogen atom from one monomer and a hydrogen from the other. Then it covalently bonds the two monomers together. The two hydrogens and the oxygen are combined to form water. The entire process is termed a condensation reaction (see Figure 4.5).

Consider, for example, the formation of the polymer starch—the major sort of molecule that was in your breakfast cereal this morning. The cereal plant cell uses an enzyme called starch synthase to take two simple sugar molecules, called glucose, and bond them together to form a dimer (di in Greek = two). The synthase enzyme continues to add glucose monomers to the dimer to lengthen the growing chain until a large polymer of starch is assembled.

And if you like observing genius in design, isn't it exceedingly efficient that the molecular by-product of forming all the various monomers into polymers

is simply water—the universal solvent of the cell? If a system is very, very carefully designed, there is no categorical waste anywhere in it. Everything is useful somewhere!

At times, polymers need to be degraded back to monomers. Again, across legions of kinds of cells, the process is essentially the same. An enzyme binds to a polymer and breaks a covalent bond between two monomers. It then takes an oxygen and a hydrogen that were once part of a water molecule and binds them to one of the monomers. Another hydrogen from a water molecule is bound to the other monomer. The two monomers, now bound to new atoms, float freely and stably in solution (see Figure 4.6).

This process, the reverse of condensation, is termed hydrolysis because the atoms from a water molecule are used to stabilize the breaking (lysing) of a bond in a polymer molecule. When a portly gentlemen goes on a diet, deep within his adipose tissues, the process of hydrolysis degrades fat molecules—polymers—into monomers. These monomers can be further broken down to generate cellular energy.

condensation—a chemical reaction in which two molecules combine to form one with loss of a small molecule—usually water in biological systems.

dimer—the result of a condensation reaction in which two monomers are bonded together.

hydrolysis—a chemical reaction in which water is split (lysed) into a hydrogen ion and an —OH ion. In polymer degradation, these ions are added to each product stabilizing the resulting monomer and smaller polymer.

Monomers supplied to cell directly.

Simple compounds built into monomers.

H 0

/ \ H H

H—C—H H H

Figure 4.4 Biosynthesis starts with what is available. For the bacterial cell in a pond (left-hand side) more enzymatic machinery is needed because starting materials are simpler. A cell in your brain (right-hand

side) has an easier time of it with monomers supplied directly in the bloodstream.

CH2OH

H

V CH 0 H

OH

N I H c

I N I /

OH C, C, OH

I I

H OH

Glucose

H2O

CH2OH

H CH 0 H

V OHH N I

6 IV

OH 1 C, OH

I

H OH

Glucose

C/ I

OH HNc

I I IV I

OH 7 c OH

OH

CH2OH CH2OH

I

H CH 0 OH H CH 0 H

1 / O I V OH N I

C 1 H H c

N6H I N / c C

OH C 0 C y OH

1 I I I

H OH H OH

Maltose

Figure4.5 Condensation of two monomers to form a dimer. Addition of further monomers will generate a polymer, starch. An enzyme carries out this reaction.

CH2OH CH2OH

I I

H

I / CH 0 N I I / OH H CH 0 H

N I

c OH H c c OH H c

iv N z I IV

OH y c NO/ c C OH I I I

H OH H OH

Maltose

Figure 4.6 Hydrolysis of a dimer to form two monomers. Water is needed. It is split into a hydrogen and an —OH group which are added to the resulting monomers to chemically stabilize them. The enzyme that carries out this reaction is often not the same enzyme that performed the (reverse) condensation reaction shown in Figure 4.5.

IN OTHER WORDS

1. Building, altering, and degrading the cell's biological molecules is a major component of metabolism.

2. Building cell structure requires either the acquisition or construction of molecular monomers followed by condensation reactions that polymerize these monomers into large polymeric molecules.

3. Hydrolysis reactions are used by the cell to disassemble polymers for subsequent reuse or transport of the resulting monomers.

CARBOHYDRATES: STRUCTURE AND FUNCTION

Biological molecules or biomolecules, though legion in variety, have been organized into about four broad classes based on their structural fea​tures (Table 4.1). Since the function of a molecule is the direct result of its structure, these four broad classes tend to differ from each other functionally as well. The class of biomolecules called carbohy​drates got its name from the three kinds of atoms that comprise all of its molecules: carbon, hydro​gen, and oxygen. Further, the atoms are present in a ratio of one carbon to two hydrogens to one oxy​gen (CH2O)n, where n can be any whole number. Usually, the hydrogens and oxygens in the mole​cules are peripherally bonded to carbon atoms that are more centrally arranged within the molecule.

CH2OH

H CH 0 H

C OH H c

V

N

OH

OH

OH Glucose

CH2OH

OHCH 0 H

C OH H c IIV

H ymmis• c, OH

OH

Galactose

Sugars

The monomeric molecules (the building blocks) among the carbohydrates are the simple sugars or monosaccharides (saccharo in Greek = sugar). By far, the most important of these in all living systems is glucose, the form of sugar found in the human bloodstream. Its molecular formula is C6I-11206 (see Figure 4.7). Notice the 1:2:1 ratio of carbons, hydrogens, and oxygens that make it a carbohydrate.

Table4.1 Classes of Biomolecules

Class carbohydrates	Diagnostic Features molecules contain atoms of carbon, hydrogen, and oxygen in a ratio of 1:2:1
lipids	molecules are hydrophobic, insoluble in water, oils, fats
proteins	polymers of amino acids, linked in linear chains, contain nitrogen
nucleic acids	polymers composed of nucleotide monomers, includes informational molecules DNA and RNA

Figure 4.7 Three common monosaccharides with identical molecular formulas but very different structural formulas. Carbon atoms are represented by apices at corners of each polygon.

The milk sugar galactose and the sugar fructose, found in fruit and in honey, both have the same molecular formula as glucose, C6F11206. The -OH groups (hydroxyl groups) that form common parts of the structure of these sugars make them very sol​uble in water and, therefore, in bodily fluids, such as blood (glucose) or milk (galactose). Look again at Figure 4.7. Though the molecular formulas are identical for these three sugars, the structural for​mulas and their resulting shapes are quite distinct. The manner in which the atoms are bonded to each other makes a difference in living systems. Enzymes

monosaccharide—a simple sugar built on a structure of any​where from three to seven carbon atoms with associated hydrogen and oxygen atoms.

glucose—a monosaccharide sugar that is central to cellular metabolism, the form of sugar found in the human bloodstream.

fructose—a monosaccharide sugar that is found in many foods; fruits rich in the disaccharide sucrose have high levels of fructose, a component of sucrose.

in the body encounter these differences in shape between sugars and will only utilize the one they fit and bind to. Throughout nature, simple sugars are a ready source of energy for driving biological pro​cesses. They also serve as building blocks in larger polymers for energy storage and structural purposes.

The most widespread sugar found in nature is a disaccharide or double sugar called sucrose (C12H22011)• Sugarcane stalks and sugar beet roots are loaded with it. It is our common table sugar. Planet Earth generates over 1 billion tons of it per year. Plant cells manufacture it enzymatically by doing a condensation reaction. They link together the monosaccharides glucose and fructose, discarding a water molecule in the process (see Figure 4.8). Sucrose is an efficiently transportable form of energy in plants.

In the cuboidal cells of human female breast tissue, yet another disaccharide is formed. There, the monosaccharides glucose and galactose are

CH2OH

H CH 0 H

1 / N I

C OH H c

IV

H

0 11..4114i C O

1

H OH

Glucose Fructose

H2O
CH2OH
H CHI 0	H	HOCH2
1 /	1	1
c OH H	C	C
1 1 IV		0/
OH ClimmC
1 1
H OH

0 Sucrose

O Lactose

Figure 4.8 (a) Condensation reaction between glucose and fructose to generate sucrose and water. (b) the structure of lactose, the sugar found in mammalian milk.

condensed enzymatically to form the disaccharide lactose (C121-122011), or milk sugar (see Figure 4.8b). Happily, in your infantile digestive tract you had an enzyme that hydrolyzed lactose to glucose and galactose. You were able to absorb these sugars and use their energy to generate neural tissues that would eventually enable you to read this sentence.

Carbohydrate Polymers

Simple sugars can be covalently bonded together to form disaccharides or they can be linked to each other in larger numbers to become much longer polymeric molecules. The sugar glucose, when po​lymerized in this way, gives rise to molecules con​taining thousands of individual atoms. The most common of these polymers are starch, glycogen, and cellulose. Though all three of these polymers are composed exclusively of identical glucose monomers, they are quite distinct in structure and in solubility because of the way in which the glu​coses are bonded to each other in each of these molecules.

Starch (see Figure 4.9a) can be a relatively simple straight-chained amylose polymer up to several hundred glucose units in length. Or it can be a branched-chain amylopectin polymer in which, at about every 30th glucose, a side chain of additional glucoses branches off. Starch polymers fold into spiral coil arrangements that render them insoluble in water. This makes them an excellent immobile storage form of energy. Plant cells within a potato tuber (see Figure 4.9b) are loaded with granules composed entirely of starch. When you ingest potatoes, rice, wheat, or oats, you receive that stored energy. In your digestive tract, an enzyme

disaccharide—two monosaccharide sugars covalently bonded together by a condensation reaction.

sucrose—a disaccharide sugar; energy storage form in plants; table sugar.

lactose—a simple sugar or monosaccharide found in mamma​lian milk; milk sugar.

starch—a polysaccharide polymer of glucose sugar units; energy storage form in plants; major portion of human diet.

amylose—a linear polymer of glucose subunits, a component of plant starch molecules.

amylopectin—a highly branched polysaccharide polymer of glucose units in starch; product of plant metabolism.

Amylose grains (purple) in plant root tissue

0

Glycogen, formed from glucose units joined in chains by a(1—.4) linkages; side branches are linked to the chains by a(1—.6) linkages (boxed in blue).

0

Cellulose, formed from glucose units joined end to end by g(1— 4) linkages. Hundreds to thousands of cellulose chains line up side by side, in an arrangement reinforced by hydrogen bonds between the chains, to form cellulose microfibrils in plant cells.

Figure 4.9 Polysaccharides (a) amylose, a straight-chain form of starch. Covalent linkages in the chain cause it to coil up as the chain grows in size. (b) starch (amylose) grains in plant tissue (c) a branched chain of glycogen (d) glycogen particles (magenta) in liver cells. (e) chain in a cellulose molecule. Many separate chains bond to each other within a cellulose microfibril. (f) microfibrils are visible under an electron microscope as the warp and woof of a plant cell wall. All subunits in all of these molecules are glucose. Bonds between the glucoses vary however.

called amylase hydrolyzes starch molecules down to their component glucose monomers so that their energy is more readily available to you.

Sometimes animals need to store energy efficiently, as for example, when an excess of glucose is present in the blood. In your liver (and in skeletal muscle), excess glucose from your breakfast starch load is removed from the blood. By condensation, it is assembled to form a highly branched polymer called glycogen (see Figure 4.9c). Later in the morning, as circulating glucose gets used for energy, glycogen is enzymatically degraded and the

resulting glucose is released into the bloodstream to keep blood sugar levels within an acceptable range. The most abundant organic molecule in the world is the polymeric carbohydrate, cellulose (see

glycogen—a polysaccharide formed from glucose primarily in muscle and liver tissue; serves as a temporary storage form of energy. cellulose—a polymeric carbohydrate whose subunits are mono​mers of glucose. It is probably the most common organic molecule on the face of the earth.

Figure 4.9e). It makes up most of the supportive structure of plant tissue. Wood is approximately 65% cellulose. Cotton is 91% cellulose. A single cellulose molecule can have anywhere from 300 to 15,000 glucose monomers in its structure depending on the species of organism it comes from. Plants need tough, sturdy cell walls both to support their aerial growth and to protect them from cellular bursting when they are submerged in water (see Figure 4.9f). The distinct bonding of glucose units in cellulose supports these functions.

Each successive glucose monomer in cellulose is bonded such that it is "upside down" from the one next to it. This arrangement allows additional covalent bonds to form between strands of the polymer, generating a strong, net-like structure (note the interior of the microfibril in Figure 4.9e). Humans have taken this wonderfully designed molecule and modified it for use in everything from explosives, to movie film, to building insulation, where it is proving safer for humans than traditional fiberglass insulation.

IN OTHER WORDS

1. Carbohydrates are biomolecules composed of carbon, hydrogen, and oxygen atoms in a ratio of 1 carbon to 2 hydrogens to 1 oxygen.

2. The simplest carbohydrates are the monosaccharides or simple sugars. Two examples are the six-carbon molecules glucose and fructose.

3. Although glucose and fructose have identical molecular formulas, their structural formulas and resulting recognition by cellular enzymes are distinct, giving them distinct roles to play in life.

4. Sucrose and lactose are two examples of disaccharides or double sugars that function as temporary trans​port and storage forms of energy.

5. The carbohydrate polymer starch is a plant polysaccharide composed of many glucose units that serves as a more permanent form of energy storage within plant tissues.

6. A corresponding form of energy storage in animals is the polysaccharide glycogen whose glucose sub​units can be transported to tissues where immediate chemical energy needs exist.

7. The most abundant polysaccharide in the world is cellulose. Its major role is structural support in plant tissues.

UCTURE AND FUNCTION

A second broad class of biomolecules, the lipids, derives its name from the term lipos in Greek, which referred to animal fat or vegetable oil. Un​like other classes of biomolecules, lipids are defined by their insolubility in water. Instead, they dissolve in solvents whose molecules are composed of non-polar covalent bonds—solvents like benzene or chloroform. Lipids and their solvents are said to be hydrophobic. If you pour oil into water, it will sep​arate itself from the water and form a discrete layer above the water. The large number and arrange​ment of nonpolar (H–C–H) bonds in the oil predict that this will happen. Polar molecules like water tend to attract each other and to exclude nonpolar oil molecules from their intervening spaces.

Lipids, then, include all substances that feel greasy or oily (see Figure 4.10). But this single solubility criterion means that the lipids include a wide diversity of molecules structurally and functionally. All fats, oils, waxes, and steroids are lipids. Functionally, lipids participate in membrane structure as protective outer coatings on the surfaces of many organisms, as storage forms of metabolic energy, and as signal molecules that diffuse toward, and recognize molecules on the cell surfaces of many organisms. Rather than classify the lipids into subgroups, we will examine three specific examples of lipids noting a truly pleasing correspondence between structure and function.

The Wonderfully Functional Fat Molecule

Many Americans are fat. This means that their tissues harbor a disproportionately large amount of a lipid polymer whose technical name is triglyceride (or triacylglycerol). This polymer (see Figure 4.11) is composed of three monomers of fatty acid, covalently bonded to a three-carbon glycerol molecule. Fatty acids are long chains of carbon atoms bonded to (and surrounded by) hydrogen atoms. They become attached to the glycerol molecule by the condensation reaction discussed in Section 4.2. A hydroxyl group (-O-H) from each fatty acid and a hydrogen atom from

hydrophobic—water-fearing—descriptive of any molecule that water effectively excludes from its own surroundings due to the extensive hydrogen bonding that occurs between water molecules.

triglyceride—a glycerol bonded to three fatty acids; the main constituent of plant and animal fats.

fatty acid—a long chain of carbons and hydrogens that is a monomer from which fat polymers are constructed.

glycerol—a three-carbon molecule with three —OH side groups to which fatty acids are attached in triglyceride synthesis.

Figure 4.10 The oil foods are fried in (a), the waxy surface of a cactus plant (b), and the fat found in the droplets within these adipose cells (c) are all examples of lipids.

	OHHHH
11		I
	H—C—O—C—C	C		C C	C	CH
	H	H	H	H	H
		OHHHHH
		1111111

H—C—O—C—C—C—C—C—C---- —C—H

H H H H H

OHHHHH H—C-0—C—C—C—C—C—C---- —C—H

H H H H H H

Glycerol Fatty Acids

Figure 4.11 A triglyceride or fat molecule possesses only three kinds of atoms. But notice that there are far fewer oxygen atoms than would be present in a carbohydrate molecule this size. The dotted lines within the fatty acids represent many more —CH2 groups than are shown here.

Unsaturated fatty acid

Saturated

co

Figure 4.12 (a) Linoleic acid, an unsaturated fatty acid with two double bonds adding to its structural rigidity. (red atoms = oxygen, black = carbon, white = hydrogen) (b) Palmitic acid, a saturated fatty acid; all carbon atoms are singly bonded to hydrogens or to each other giving free rotation of the molecule around any covalent bond in the chain.

the glycerol molecule are removed to form water, and the fatty acid is linked to one of the carbons on the glycerol molecule. So then, three separate condensation reactions take us from three fatty acids and one glycerol to a single fat molecule.

Let's consider those monomers in more detail. Fatty acids vary in structure. They range in length from as few as 4 carbons to as many as 24. They are produced in many kinds of plant and animal cells. Two of them, linoleic acid and alpha-linolenic acid, have been shown to be essential to humans. This means that we must have them for our normal metabolic processes, but we can't synthesize them so they must be supplied in our diets. (You don't need a triple cheeseburger to get them—in fact, you get far more of them in nuts and grains.)

Fatty acids from plant tissue often have one to several double covalent bonds between their carbon atoms (see Figure 4.12). Since this involves binding fewer hydrogen atoms, these fatty acids are said to be unsaturated (with hydrogen atoms).

Saturated fatty acids, by contrast, have carbon atoms singly bonded to each other and thus to a full complement of surrounding hydrogen atoms (see Figure 4.12). A double bond inhibits free rotation around itself for the atoms to either side of it. So it adds a certain rigidity to the fatty acid containing it. Fat molecules containing unsaturated fatty acids in their structure are termed unsaturated fats. Their rigidity tends to inhibit their tangling

through each other, so they remain liquids (oily) at room temperature and at body temperature.

Saturated fats, by contrast, are more frequently the product of animal tissue. Since their fatty acids have free rotation around every single carbon in their chain, they tend to rotate their way through other nearby fatty acids forming a wonderfully messy network that's a pasty solid at room temperature. Dairy products, creams, cheeses, and animal fats are high in saturated fats (see Figure 4.13). There's a positive correlation between a diet high in saturated fat and atherosclerosis with accompanying heart disease. On the other hand, a diet more respective of the Biblical (Genesis 1) mandate to eat a variety of fruits and vegetables is healthier for the heart and blood vessel walls. The Biblical text also inveighs against ingestion of animal fat. Fascinating! What other life wisdom might this amazing resource contain?

What function do fatty acids serve in the tissues where they are found? They are a wonderfully designed and highly concentrated source of energy. Energy? Yes. Carbon-hydrogen bonds are fairly easy to break—little energy is required to do so. And when

atherosclerosis—a thickening of the walls of arterial blood ves​sels as a result of the accumulation of fatty materials being trans​ported in the blood.

Figure 4.13 (a) Diets high in saturated fats can result in buildup of fibrous and lipid material called "plaque"on the inner lining of arteries.

(b) Plaque appears as zones of lighter color in this coronary artery.

Survey Questions

4.1 The Centrality of Carbon to the

Organic Molecules of Life

How prominent is carbon in the structure of living things?

What features of the carbon atom make it so useful for the design of biological molecules?

How many molecules of life are

there, and how are they organized for discussion and study's sake?

4.2 Construction and Degradation of Organic Molecules

How important is construction and degradation of organic molecules to the living cell?

How is construction of a large organic molecule carried out?

Why would organic molecules

need to be degraded? How is this accomplished?

4.3 Carbohydrates: Structure and Function

What are carbohydrates? What does that term denote?

Why is sugar considered to be

a carbohydrate? What are some examples of carbohydrates?

How are larger carbohydrates built from smaller ones?

What are some important examples of large carbohydrates?

4.4 Lipids: Structure and Function

What are the defining characteristics of a lipid? What are some examples of lipids?

Life Is Complex—one of the 12 principles of life on which this text is based.

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CI

OH

OH

Polymers built into cell structure.

Monomers built into polymers.

H\ /H�N

H C

On

0 = P

O

/H

1\1*

�

�

CH2OH

CH2OH

I

H CH 0 H CH 0 H

C OH H c

0 1-.. C OH

IV

1

H OH

HOCH2 O OH

SCI

Cumm•C H

OH OH

HOCH2

OH

CH2OH

0 c�V1

COH

H

CH2OH 0c

II

y OH

H

H

I�C

I OH

CH2OH OH CH

I /

c OH

H

H

CH2OH 1

0 H CH

1 /

V

HCOC OH

1 1

C H �1

OH H

ON OH

1

H c

1

C, OH

OH OH

OH

CH2OH CH2OH

H H 0

1 4

O�

H H

1 4

—O

CH2OH

O

0

Amylose, formed from�a-glucose units joined�end to end in a(1—> 4)�linkages. The coiled�structures are induced�by the bond angles in�the a-linkages.

• 11111:\

N')V1Pjli 871

C\H H

H

1 4 \ 1 4

Glycogen particles (magenta) in liver cell

CH2OH

OH OH

CH2OH 6 CH2

O�

—0—

OH

OH

OH

0

0

CH2OH

CH2OH OH

H9 \ —0--/ K\FI

— H H\r.....0/-0 —

OH CH2OH

CH2OH

O�

OH —

OH

LC

Glucose subunit

Cellulose�microfibril

Cellulose microfibrils in plant cell wall

0

co

, 6

i lit' 'I

t -- -1 A j'+

.1.

JP .( P , lki 'V

' 1%W , ' 4', 7i.'k.

VI k i 1 Lipids

' Irri;' Vi I

t ir,

11014

vf

O

n

0

Coronary artery --Mr

Atherosclerotic

plaques

Cardiac muscle —(heart muscle tissue)

a