Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 3Molecules of Life

(Sections 3.4 - 3.6)

3.4 Lipids

• Cells use lipids as major sources of energy and as structural materials

• lipid • Fatty, oily, or waxy organic compound• All are hydrophobic (nonpolar )

Types of Lipids

• Fats and some other lipids have fatty acid tails; triglycerides have three

• Phospholipids are the main structural component of cell membranes

• Waxes are lipids that are part of water-repellent and lubricating secretions

• Steroids occur in cell membranes, and some are remodeled into other molecules

Key Terms

• fat • Lipid that consists of a glycerol molecule with one, two, or

three fatty acid tails

• triglyceride • A fat with three fatty acid tails

• fatty acid • Organic compound that consists of a chain of carbon

atoms with an acidic carboxyl group at one end • Carbon chain of saturated types has single bonds only;

that of unsaturated types has one or more double bonds

Fats and Fatty Acids

• Unsaturated fatty acids have one or more double bonds that limit their flexibility

• These bonds are termed cis or trans, depending on the way the hydrogens are arranged around them

• A cis bond kinks the tail, and a trans bond keeps it straight

Saturated and Unsaturated Fats

• Animal fats are saturated• Tend to remain solid at room temperature because their

saturated tails pack tightly together

• Most vegetable oils are unsaturated• Kinked tails do not pack tightly, so unsaturated fats are

typically liquid at room temperature

• Partially hydrogenated vegetable oils have a trans double bond that allows them to pack tightly, like saturated fats• Solid at room temperature

Fatty Acids

Fig. 3.8, p. 42

carboxyl group(head)

hydro carbon tail

B linoleic acidA stearic acid C linolenic acid

Trans and Cis

Phospholipids

• phospholipid • Lipid with a highly polar phosphate group in its hydrophilic

head, and two nonpolar, hydrophobic fatty-acid tails • Main constituent of eukaryotic cell membranes

• Opposing properties of a phospholipid molecule give rise to cell membrane structure• Two layers of lipids (lipid bilayer)• Hydrophobic tails sandwiched between hydrophilic heads

Phospholipids and Cell Membranes

• Head is hydrophilic –tails are hydrophobic

• Lipid bilayer— the structural foundation of all cell membranes

Fig. 3.10, p. 42

one layer of lipids

hydrophilic head

one layer of lipids

two hydrophobic tails

Waxes• wax • Water-repellent mixture

with long fatty-acid tails bonded to long-chain alcohols or carbon rings

• Functions:• Covers exposed surfaces

of plants• Protects and lubricates

skin and hair• Honeycomb

Steroids

• steroid • Lipid with four carbon rings and no fatty acid tails• Found in all eukaryotic cell membranes

• Cholesterol, the most common steroid in animal tissue, is remodeled into many molecules:• Bile salts (which help digest fats) and vitamin D • Steroid hormones (estrogens and testosterone)

Estrogen and Testosterone

• Estrogen and testosterone

• Steroid hormones derived from cholesterol

Steroid Functions

• Steroid hormones cause different traits to arise in males and females of many species, such as wood ducks (Aix sponsa)

Key Concepts

• Lipids• Lipids function as energy reservoirs and as waterproofing

or lubricating substances• Some are remodeled into other compounds such as

vitamins• Lipids are the main structural component of all cell

membranes

Animation: Fatty acids

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Animation 2.2: Triglyceride formation

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3.5 Proteins—Diversity in Structure and Function

• Structurally and functionally, proteins are the most diverse molecules of life

• The shape of a protein is the source of its function

• protein • Organic compound that consists of one or more chains of

amino acids (polypeptides)

Amino Acids

• Cells make thousands of different kinds of proteins from only twenty kinds of monomers (amino acids)

• amino acid • Small organic compound that is a subunit of proteins• Consists of a carboxyl group, an amine group, and a

characteristic side group (R), all typically bonded to the same carbon atom

Amino Acids

• Generalized structure of amino acids: Twenty amino acids are used in eukaryotic proteins

Fig. 3.12, p. 44

Stepped Art

Amino Acids

Building Proteins

• Protein synthesis involves covalently bonding amino acids into a chain polypeptide linked by peptide bonds

• polypeptide • Chain of amino acids linked by peptide bonds• Primary structure of a protein

• peptide bond • Bond that joins the amine group of one amino acid and the

carboxyl group of another in a protein

Polypeptide Formation

• Condensation: A peptide bond forms between the carboxyl group of the methionine and the amine group of the serine

• Additional amino acids are added to the carboxyl end

Fig. 3.13, p. 44

glutaminemethionine serine arginine

methionine

serine

methionine serine

Stepped Art

Polypeptide Formation

Animation: Peptide bond formation

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Protein Structure

• Polypeptides (primary structure) twist into loops, sheets, and coils (secondary structure) that can pack further into functional domains (tertiary structure)

• Many proteins, including most enzymes, consist of two or more polypeptides (quaternary structure)

• Fibrous proteins aggregate into much larger structures

Primary and Secondary Structure

• Primary structure (polypeptide) twists into secondary structure

Tertiary and Quaternary Structure

• Tertiary structure forms functional domains• Hemoglobin has quaternary structure (4 globin chains)

Aggregate Proteins

• Many proteins aggregate by thousands into much larger structures, such as keratin filaments that make up hair

5 Many proteins aggregate by the thousands into much larger structures, such as the keratin filaments that make up hair. Fig. 3.14.1-4, p. 45

A protein’s primary structure consists of a linear sequence of amino acids (a polypeptide chain). Each type of protein has a unique primary structure.

1

glycinelysine glycine arginine

Stepped Art

2 Secondary structure arises as a polypeptide chain twists into a coil (helix) or sheet held in place by hydrogen bonds between different parts of the molecule. The same patterns of secondary structure occur in many different proteins.

3 Tertiary structure occurs when a chain’s coils and sheets fold up into a functional domain such as a barrel or pocket. In this example, the coils of a globin chain form a pocket.

4 Some proteins have quaternary structure, in which two or more polypeptide chains associate as one molecule. Hemoglobin, shown here, consists of four globin chains (green and blue). Each globin pocket now holds a heme group (red).

Polypeptide Formation

Animation: Secondary and tertiary structure

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Combined Proteins

• Enzymes often attach sugars or lipids to proteins

• Glycoproteins allow a tissue or body to recognize its own cells

• Lipoproteins carry fats and cholesterol through the bloodstream• Low-density lipoprotein (LDL)• High-density lipoprotein (HDL)

3.6 Importance of Protein Structure

• A protein’s structure dictates its function so, if a protein unravels (denatures), it loses its function

• denature • To unravel the shape of a protein or other large biological

molecule• Caused by shifts in pH or temperature, exposure to

detergent or some salts that disrupt hydrogen bonds, and other molecular interactions responsible for protein shape

Prions

• Prion diseases, are the result of misfolded proteins• Mad cow disease (bovine spongiform encephalitis, BSE)• Creutzfeldt–Jakob disease (vCJD) in humans• Scrapie in sheep

• prion • Infectious protein• Misfolded PrPC protein

PrPC Protein Becomes a Prion

• The PrPC protein misfolds into an unknown conformation• Prions cause other PrPC proteins to misfold• Misfolded proteins aggregate into long fibers

Fig. 3.16, p. 46

?

prion protein

PrPC

protein

Conformational change

Variant Creutzfeldt–Jakob Disease

• Charlene Singh:• Diagnosed in 2001• Died in 2004

• Brain tissue shows characteristic holes and prion protein fibers radiating from several deposits

Key Concepts

• Proteins• Structurally and functionally, proteins are the most diverse

molecules of life• They include enzymes and structural materials• A protein’s function arises from and depends on its

structure

ANIMATION: Globin and hemoglobin structure

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ANIMATION: Sickle-Cell Anemia

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3.7 Nucleic Acids

• Nucleotides are small organic molecules consisting of a sugar, a phosphate group, and a nitrogen-containing base

• nucleotide • Monomer of nucleic acids; has five-carbon sugar, nitrogen-

containing base, and phosphate groups

A Nucleotide Monomer

• ATP, a nucleotide monomer of RNA, and also an essential participant in many metabolic processes

Nucleic Acids

• Nucleotides are monomers of DNA and RNA, which are nucleic acids

• nucleic acid • Single- or double-stranded chain of nucleotides joined by

sugar–phosphate bonds; for example, DNA, RNA

A Nucleic Acid

• A chain of nucleotides is a nucleic acid

• The sugar of one nucleotide is covalently bonded to the phosphate group of the next, forming a sugar–phosphate backbone

ANIMATION: Nucleotide Subunits of DNA

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DNA and RNA

• DNA (Deoxyribonucleic acid)• DNA encodes heritable information that guides the

synthesis of RNA and proteins• Consists of two nucleotide chains twisted in a double helix

• RNA (Ribonucleic acid)• RNAs interact with DNA and with one another to carry out

protein synthesis

DNA

• DNA consists of two chains of nucleotides, twisted into a double helix

• Hydrogen bonding maintains the three-dimensional structure

Other Nucleotides

• Some nucleotides have additional functions

• Example: ATP energizes many kinds of molecules by phosphate-group transfers

• ATP Adenosine triphosphate• Nucleotide that consists of an adenine base, a five-carbon

ribose sugar, and three phosphate groups

Key Concepts

• Nucleic Acids• Nucleotides are the building blocks of nucleic acids• Some have additional roles in metabolism• DNA and RNA are part of a cell’s system of storing and

retrieving heritable information

Fear of Frying (revisited)

• Trans fatty acids are rare in unprocessed foods

• Enzymes that hydrolyze cis fatty acids have difficulty breaking down trans fatty acids, a problem that may be a factor in the ill effects of trans fats on our health