CHAPTER 3

THE MOLECULES OF LIFE

POLYMERS ARE BUILT OF MONOMERS

• Organic molecules are formed by living organisms.• Carbon-based core• The core has attached groups of atoms called

functional groups.• The functional groups confer specific chemical

properties on the organic molecules.

FIVE PRINCIPAL FUNCTIONAL GROUPS

Found InGroup StructuralFormula

Ball-and-Stick Model

O–P

O–

O

O

OH

O

OH

C

H

H

N

C O

O H

O–

O

PO O–

O

O

HO

H

H

N

C

C

Lipids

Proteins

Proteins

DNA,ATP

Carbohydrates

Carboxyl

Carbonyl

Hydroxyl

Amino

Phosphate

MACROMOLECULES

• The building materials of the body are known as macromolecules because they can be very large.

• There are four types of macromolecules:1. Proteins2. Nucleic acids3. Carbohydrates4. Lipids

MACROMOLECULES

• Large macromolecules are actually assembled from many similar small components, called monomers.• The assembled chain of monomers is known as

a polymer.

DEHYDRATION SYNTHESIS

• All polymers are assembled the same way.• A covalent bond is formed by removing a

hydroxyl group (OH) from one subunit and a hydrogen (H) from another subunit.

H

HH

Energy

HO

HO HO

H2O

DEHYDRATION SYNTHESIS

• Because this amounts to the removal of a molecule of water (H2O), this process of linking together two subunits to form a polymer is called dehydration synthesis.

HYDROLYSIS

• The process of disassembling polymers into component monomers is essentially the reverse of dehydration synthesis.• A molecule of water is added to break the

covalent bond between the monomers.• This process is known as hydrolysis.

H

H HHOEnergy

HO

HO

H2O

PROTEINS

• Proteins are complex macromolecules that are polymers of many subunits called amino acids.

O

C N

H

O

C N

H

H

H N C

H

H

H2O

H C

O

C

H

H N C

H

H

C

O

C

R R

R R

Amino acidAmino acid

OH OH

OH

Polypeptide chain

PROTEINS

• The covalent bond linking two amino acids together is called a peptide bond.

• The assembled polymer is called a polypeptide.

PROTEINS

• Amino acids are small molecules with a simple basic structure, a carbon atom to which three groups are added:• an amino group (—NH2)

• a carboxyl group (—COOH)• a functional group (R)

• The functional group gives amino acids their chemical identity.• There are 20 different types of amino

acids.

PROTEINS

• Protein structure is complex.• The order of the amino acids that form the

polypeptide is important.• The sequence of the amino acids affects how

the protein folds together.

PROTEINS

• The way that a polypeptide folds to form the protein determines the protein’s function.• Some proteins are comprised of more than

one polypeptide.

PROTEINS

• There are four general levels of protein structure:

1. Primary2. Secondary3. Tertiary4. Quaternary

N

N

N

N

N

H

H

H

C

C

O

O

O

CC

C

C

CC

OC

NN

H H

C

OCC

OC

NN

H

OC

CC

H

O C

C

Secondarystructure

β-pleated sheet

α-helix

Tertiarystructure

Quaternarystructure

Amino acids

Primarystructure

O

PROTEINS

• Primary structure—the sequence of amino acids in the polypeptide chain.• Determines all other levels of protein structure.

Amino acids

Primarystructure

PROTEINS

• Secondary structure forms because regions of the polypeptide that are nonpolar are forced together; hydrogen bonds can form between different parts of the chain.• The folded structure may resemble coils,

helices, or sheets.N

N

N

N

N

H

H

H

C

C

O

O

O

CC

C

C

CC

OC

NN

H H

C

OC

C

OC

N

N

H

OC

C

C

H

OC

C

Secondarystructure

β-pleated sheet

α-helixO

PROTEINS

• Tertiary structure—the final 3-D shape of the protein.• The final twists and folds that lead to this shape

are the result of polarity differences in regions of the polypeptide.

Tertiarystructure

PROTEINS

• Quaternary structure—the spatial arrangement of proteins comprised of more than one polypeptide chain.

Quaternarystructure

PROTEINS

• The shape of a protein affects its function.• Changes to the

environment of the protein may cause it to unfold or denature.• Increased

temperature or lower pH affects hydrogen bonding, which is involved in the folding process.

• A denatured protein is inactive.

DenaturationFoldedprotein

Denatured protein

PROTEINS

• Enzymes are globular proteins that have a special 3-D shape that fits precisely with another chemical.• They cause the

chemical that they fit with to undergo a reaction.

• This process of enhancing a chemical reaction is called catalysis.

Active-sitecleft

NUCLEIC ACIDS

• Nucleic acids are very long polymers that store information.

• Comprised of monomers called nucleotides.• Each nucleotide has 3 parts:

1. a five-carbon sugar2. a phosphate group3. an organic nitrogen-containing base

NUCLEIC ACIDS

• There are five different types of nucleotides.

• Information is encoded in the nucleic acid by different sequences of these nucleotides.

N

N

OP

O

–O

O–

R

O

N

N

CC

NN

N

C

HN

C

CH

O

H

HCC

NC

HN

CN

NCH

OCC

NC

H

N

CH

H OCC

NC

H

N

C

O

HH3C

H OCC

NC

H

N

C

O

HH

H

4

5

1

3 2

28

7 6

394

51

Structure of nucleotide Nitrogenous base Nitrogenous bases

Guanine

Uracil (RNA only)Sugar Thymine (DNA only)Cytosine

Phosphate group

H in DNA

OH in RNA

Adenine

OH

CH2

NH2 NH2

NH2

NH2

(a) (b)

NUCLEIC ACIDS

• There are two types of nucleic acids:• Deoxyribonucleic acid (DNA)• Ribonucleic acid (RNA)

• RNA is similar to DNA except that • it uses uracil instead of thymine• it is comprised of just one strand• it has a ribose sugar

OO

OO

O

O

O

O

O

O

O

O

P

P

PG

C

P

P

P

P

P

P C

C

G

G

A

A

T

T

P

P

Phosphodiester bond

Hydrogen bondsbetween nitrogenousbases

Sugar-phosphate“backbone”

OH

NUCLEIC ACIDS

• The structure of DNA is a double helix because:• There are only two base pairs possible

• Adenine (A) pairs with thymine (T)• Cytosine (C) pairs with Guanine (G)

• Properly aligned hydrogen bonds hold each base pair together.

• A sugar-phosphate backbone comprised of phosphodiester bonds gives support.

A

A

G

G

C

C

T

T

G and C can align to formthree hydrogen bonds.

A and T can align to formtwo hydrogen bonds.

G and T cannot properlyalign to form hydrogenbonds.

A and C cannot properlyalign to form hydrogenbonds.

NUCLEIC ACIDS

• The structure of DNA helps it to function.• The hydrogen bonds of the base pairs can be

broken to unzip the DNA so that information can be copied.• Each strand of DNA is a mirror image so that

the DNA contains two copies of the information.

• Having two copies means that the information can be accurately copied and passed to the next generation.

CARBOHYDRATES

• Carbohydrates are monomers that make up the structural framework of cells and play a critical role in energy storage.

• A carbohydrate is any molecule that contains the elements C, H, and O in a 1:2:1 ratio.

CARBOHYDRATES

• The sizes of carbohydrates varies:• Simple carbohydrates—consist of one or two

monomers.• Complex carbohydrates—are long polymers.

CARBOHYDRATES

• Simple carbohydrates are small.• Monosaccharides consist

of only one monomer subunit.• An example is the sugar

glucose (C6H12O6).

• Disaccharides consist of two monosaccharides.• An example is the sugar

sucrose, which is formed by joining together glucose and fructose.

CARBOHYDRATES

• Complex carbohydrates are long polymer chains.• Because they contain many C-H bonds, these

carbohydrates are good for storing energy.• These bond types are the ones most often

broken by organisms to obtain energy.• The long chains are called polysaccharides.

CARBOHYDRATES

• Plants and animals store energy in polysaccharide chains formed from glucose.• Plants form starch.• Animals form glycogen.

• Some polysaccharides are structural and resistant to digestion by enzymes.• Plants form cellulose cell walls.• Some animals form chitin for

exoskeletons.

LIPIDS

• Lipids—fats and other molecules that are not soluble in water.• Lipids are nonpolar molecules.• There are many different types of lipids.

• fats• oils• steroids• rubber• waxes• pigments

LIPIDS

• Fats are converted from glucose for long-term energy storage.

• Fats have two subunits• 1. fatty acids• 2. glycerol• Fatty acids are chains of C and H atoms, known

as hydrocarbons.• The chain ends in a carboxyl (—COOH) group.

SATURATED AND UNSATURATED FATS

Because there are 3 fatty acids attached to a glycerol, another name for a fat is triglyceride

H

H

CH O

CH O

CH O C

O

H

C

O

H

C

O

H

H

C

H

C

H

C

H

H

H

H

C

H

C

H

C

H

H

H

H

C

H

C

H

C

H

H

H

H

C

H

C

H

C

H

H

H

H

C

H

C

H

C

H

H

H

H

C

H

C

H

C

H

H

H

H

C

H

C

H

C

H

H

H

H

C

H

C

H

C

H

H

H

(a) Fat molecule (triacylglycerol)

Glycerolbackbone

Fatty acids

LIPIDS

• Fatty acids have different chemical properties due to the number of hydrogens that are attached to the non-carboxyl carbons• If the maximum number of hydrogens are

attached, then the fat is said to be saturated.• If there are fewer than the maximum attached,

then the fat is said to be unsaturated.

SATURATED AND UNSATURATED FATS

C

H

H

C

H

H

C

H

C

H

(c) Oil (unsaturated): Fatty acidsthat contain double bondsbetween one or more pairsof carbon atoms

(b) Hard fat (saturated): Fattyacids with single bondsbetween all carbon pairs

PHOSPHOLIPIDS

• Biological membranes involve lipids.• Phospholipids make up the two layers of the

membrane.• Cholesterol is embedded within the

membrane.Cellmembrane

CholesterolPhospholipid

Inside of cell

Membrane proteins

Outside of cell

Carbohydrate chains