3.4 Lipids

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

lipid Fatty, oily, or waxy organic compoundAll 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 saturatedTend to remain solid at room temperature because their saturated tails pack tightly together

Most vegetable oils are unsaturatedKinked 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 fatsSolid at room temperature

Fatty Acids

Fig. 3.8, p. 42carboxyl group(head)hydro carbon tailB linoleic acidA stearic acidC 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 structureTwo 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. 42one layer of lipidshydrophilic headone layer of lipidstwo hydrophobic tails

Waxeswax Water-repellent mixture with long fatty-acid tails bonded to long-chain alcohols or carbon rings

Functions:Covers exposed surfaces of plantsProtects and lubricates skin and hairHoneycomb

Steroids

steroid Lipid with four carbon rings and no fatty acid tailsFound 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

LipidsLipids function as energy reservoirs and as waterproofing or lubricating substancesSome are remodeled into other compounds such as vitaminsLipids are the main structural component of all cell membranes

Animation: Fatty acidsTo play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE

Animation 2.2: Triglyceride formationTo play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE

3.5 ProteinsDiversity 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 proteinsConsists 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. 44Stepped ArtAmino 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 bondsPrimary 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 serineAdditional amino acids are added to the carboxyl end

Fig. 3.13, p. 44Stepped ArtPolypeptide Formation

Animation: Peptide bond formationTo play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE

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 domainsHemoglobin 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

Fig. 3.14.1-4, p. 45Stepped ArtPolypeptide Formation

Animation: Secondary and tertiary structureTo play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE

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 bloodstreamLow-density lipoprotein (LDL)High-density lipoprotein (HDL)

3.6 Importance of Protein Structure

A proteins 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 moleculeCaused 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 proteinsMad cow disease (bovine spongiform encephalitis, BSE)CreutzfeldtJakob disease (vCJD) in humansScrapie in sheep

prion Infectious proteinMisfolded PrPC protein

PrPC Protein Becomes a Prion

The PrPC protein misfolds into an unknown conformationPrions cause other PrPC proteins to misfoldMisfolded proteins aggregate into long fibers

Fig. 3.16, p. 46?prion proteinPrPCproteinConformational change

Variant CreutzfeldtJakob Disease

Charlene Singh:Diagnosed in 2001Died in 2004

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

Key Concepts

ProteinsStructurally and functionally, proteins are the most diverse molecules of lifeThey include enzymes and structural materialsA proteins function arises from and depends on its structure

ANIMATION: Globin and hemoglobin structureTo play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE

ANIMATION: Sickle-Cell AnemiaTo play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE

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 sugarphosphate 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 sugarphosphate backbone

ANIMATION: Nucleotide Subunits of DNATo play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE

DNA and RNA

DNA (Deoxyribonucleic acid)DNA encodes heritable information that guides the synthesis of RNA and proteinsConsists 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 triphosphateNucleotide that consists of an adenine base, a five-carbon ribose sugar, and three phosphate groups

Key Concepts

Nucleic AcidsNucleotides are the building blocks of nucleic acidsSome have additional roles in metabolismDNA and RNA are part of a cells 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

*Figure 3.8 Fatty acids. A The tail of stearic acid is fully saturated with hydrogen atoms. B Linoleic acid, with two double bonds, is unsaturated. The first double bond occurs at the sixth carbon from the end, so linoleic acid is called an omega-6 fatty acid. Omega-6 and C omega-3 fatty acids are essential fatty acids. Your body does not make them, so they must come from food.Figure 3.10 Phospholipids as components of cell membranes. Left, the head of a phospholipid is hydrophilic, and the tails are hydrophobic. Right, a double layer of phospholipidsthe lipid bilayeris the structural foundation of all cell membranes.Figure 3.10 Phospholipids as components of cell membranes. Left, the head of a phospholipid is hydrophilic, and the tails are hydrophobic. Right, a double layer of phospholipidsthe lipid bilayeris the structural foundation of all cell membranes.

Figure 3.11 Estrogen and testosterone, steroid hormones that cause different traits to arise in males and females of many species such as wood ducks (Aix sponsa), pictured at right.

Figure 3.12 Generalized structure of amino acids. Appendix V has models of all twenty of the amino acids used in eukaryotic proteins.

Figure 3.12 Generalized structure of amino acids. Appendix V has models of all twenty of the amino acids used in eukaryotic proteins.

Figure 3.13 Polypeptide formation. Chapter 9 offers a closer look at protein synthesis.

Figure 3.13 Polypeptide formation. Chapter 9 offers a closer look at protein synthesis.

Figure 3.14 Protein structure.

Figure 3.14 Protein structure.

Figure 3.14 Protein structure.Figure 3.14 Protein structure.

Figure 3.16 The PrPC protein becomes a prion when it misfolds into an as yet unknown conformation. Prions cause other PrPC proteins to misfold, and the misfolded proteins aggregate into long fibers.

Figure 3.15 Variant CreutzfeldtJakob disease (vCJD).Figure 3.17 Nucleic acid structure.