The Structure and Function of

MacromoleculesChapter 5

Biology – Campbell • Reece

Polymers Polymers are large molecules

consisting of many identical or similar subunits connected together› Monomer – single building block

Macromolecules (large organic polymers)› Carbohydrates› Lipids*› Proteins› Nucleic acids

Polymers in Nature

Condensation Reaction Specifically a dehydration

reaction One monomer loses a hydroxyl and

the other loses a hydrogen This reaction is repeated as each

monomer is added Requires energy Requires enzymes

Condensation Reaction

Hydrolysis Process that breaks covalent bonds

between monomers by the addition of water molecules› Essentially the reverse of the dehydration

reaction› A hydrogen from the water bonds to one

monomer, and the hydroxyl bonds to another› Example: Digestive enzymes catalyze

hydrolytic reactions which break apart large food molecules into monomers that can be absorbed by the bloodstream

Hydrolysis

Carbohydrates Carbohydrates are organic

molecules made of sugars and polymers of sugars

Provide energy and act as building material

Building block molecules are simple sugars called monosaccharides

Classified by the number of simple sugars

Monosaccharides Single sugar (CH2O) Major nutrients for cells (glucose

most common) Can be produced by photosynthetic

organisms from CO2, H2O and light Store energy in their chemical bonds

which is harvested by cellular respiration

Monosaccharides Their carbon skeletons are raw

materials for other organic molecules

Carbon skeleton varies from 3 to 7 carbons

Can be incorporated as monomers into disaccharides and polysaccharides

Monosacharides

Dissaccharides A double sugar that consists of two

monosaccharides joined by a glycosidic linkage

Glycosidic linkage = covalent bond formed by a condensation reaction between two sugar monomers› Maltose (glucose + glucose)› Lactose (glucose + galactose)› Sucrose (glucose + fructose)

Dissacharides

Polysaccharides Polymers of a few hundred or thousand

monosaccharides Important biological functions:

› Energy storage (starch and glycogen) Starch = glucose polymer that is a storage

polysaccharide in plants Glycogen = glucose polymer that is a storage

polysaccharide in animals› Structural support (cellulose and chitin)

Cellulose = major structural component of plant cell walls

Chitin=carbohydrate used by arthropods for exoskeletons

Storage Polysaccharides

Lipids Generally not big enough to be

considered macromolecules Hydrophobic – little or no affinity for

water Consist mostly of hydrocarbons Includes waxes, pigments, fats,

phospholipids, and steroids

Fats Main function is energy storage; also

provides cushioning and insulation Triacylglycerol or triglyceride Made of glycerol and 3 fatty acids

› glycerol = alcohol with three carbons› fatty acid = a long hydrocarbon chain with

a carboxyl group saturated = no double bonds between carbons

in the fatty acid tail (animal fats) unsaturated = one or more double bonds in the

fatty acid tail (vegetable and fish oils)

Fat Molecules

Phospholipids Make up cell membranes Similar to fats, but they have only

two fatty acids rather than three The hydrocarbon tail is hydrophobic

but the phosphate head is hydrophilic

Micelle = a phospholipid cluster with the phosphate heads facing outward and the hydrophobic tails inward

Phospholipids

Phospholipids When placed in water, they self-

assemble into a bilayer Phospholipid Bilayer = hydrophilic

heads are in contact with water, whereas the hydrophobic tails are in contact with each other and remote from water.

Phospholipid bilayer

Steroids Lipids characterized

by a carbon skeleton consisting of four fused rings› Cholesterol is a

precursor from which other steroids (including sex hormones) are produced High levels may

contribute to atherosclerosis

Proteins Account for more than 50% of the

dry mass of most cells Used for structural support, storage,

transport of other substances, cell communication, movement, defense against foreign substances, and catalysis (enzymes)

Proteins Made up of 20 amino acids Polypeptides = polymers of amino acids Proteins consist of one or more

polypeptides folded and coiled into specific three-dimensional structures

Amino acids have a carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable group symbolized by R› The R group (or side chain) differs with each

amino acid

Hydrophobic Amino Acids

Hydrophilic Amino Acids

Proteins Amino acids are joined by a dehydration

reaction (removal of water) forming a peptide bond between the carboxyl group of one amino acid and the amino group of the other

Each polypeptide has a unique sequence of amino acids

The sequence of amino acids in a protein is determined by inherited genetic information (DNA)

Protein Structure A functional protein is one or more

polypeptides precisely twisted, folded, and coiled into a unique shape

Many proteins are roughly spherical (globular proteins)

Others are like long fibers (fibrous proteins)

A protein’s structure determines its function

Primary Structure Basic sequence of amino acids The order is crucial to the function of

the protein › Sickle cell anemia results from the

substitution of one amino acid in hemoglobin

› Frederick Sanger discovered insulin’s primary structure in the late 1940’s

Protein Primary Structure

Secondary Structure Segments of the polypeptide chain are

repeatedly coiled or folded as a result of hydrogen bonds at regular intervals along the polypeptide backbone› Alpha (α) helix – delicate coil held together

by hydrogen bonding between every fourth amino acid (found in fibrous proteins)

› Beta (β) pleated sheet – parallel regions are held together by hydrogen bonds between adjacent polypeptides

Secondary Structure

Tertiary Structure The overall shape of a polypeptide

resulting from interactions between the R groups› Hydrophobic interaction – the nonpolar

side chains cluster at the core of the protein away from water (van der Waals interactions hold them together)

› Disulfide bridges – the sulfur of one cysteine amino acid binds to the sulfur of another (-S-S-)

Tertiary Structure

Quaternary Structure Some proteins

consist of two or more polypeptides

Structure in proteins that results from interactions between and among several polypeptide chains

Denaturation of Proteins Denaturation is a process that alters

a protein’s native conformation and biological activity.

Occurs when:› Proteins are transferred to an organic

solvent (ex. ether or chloroform)› Chemical agents disrupt hydrogen

bonds, ionic bonds, and disulfide bridges

› Excessive heat disrupts weak interactions

Denaturation of Proteins

Predicting Protein Folding Predicting protein conformation based on

amino acid sequence is difficult because most proteins pass through intermediate stages in the folding process

Chaperone proteins (chaperonins) temporarily brace a protein while it folds spontaneously

Knowledge of protein folding would allow the design of proteins for specific purposes

Nucleic Acids DNA - deoxyribonucleic acid

› Contains coded information that programs all cell activity

› Contains directions for its own replication› Is copied and passed from one generation of

cells to another› Found primarily in the nucleus in eukaryotic

cells› Makes up genes that contain instructions for

protein synthesis (direct the synthesis of mRNA)

Nucleic Acids RNA - ribonucleic Acid

› Functions in the actual synthesis of proteins coded for by DNA

› Sites of protein synthesis are on ribosomes

› Messenger RNA (mRNA) carries encoded genetic message from the nucleus to the cytoplasm

› Flow of genetic information goes from DNA to RNA to protein

Nucleotides Nucleic acids are polymers of nucleotides

1. A five carbon sugar (ribose or deoxyribose)2. A phosphate group 3. A nitrogenous base

Purines – adenine (A) and guanine (G) Pyrimidines – cytosine (C), uracil (U), and

thymine (T) Base Pairings: A-T, C-G (in DNA) and A-U,

C-G (in RNA)

Nucleotides

DNA - The Double Helix James Watson and Francis Crick

proposed the double helix structure of DNA in 1953› DNA consists of two nucleotide chains

held together by hydrogen bonds between paired bases and van der Waals attraction between stacked bases and wound in a double helix (twisted ladder)

DNA