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Biological Molecules:The Carbon Compounds of Life

Why It Matters

CO2 and photosynthesis

Fig. 3-1, p. 42

Carbon—The Backbone of BiologicalMolecules

• Although cells are 70–95% water, the rest consistsmostly of carbon-based compounds

• Carbon is can form large, complex, and diversemolecules

• Proteins, DNA, carbohydrates, lipids and othermolecules are all composed of carbon compounds

• All organic compounds contain carbon, most of themcontain hydrogen atoms in addition

Carbon Bonding

Organic molecules based on carbon• Each carbon atoms forms 4 bonds• Allows for a great variety of molecular shapes

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Hydrocarbons

Hydrocarbons• Molecules of carbon linked only to hydrogen• Methane is the simplest hydrocarbon• CH4 = 1 carbon + 4 hydrogens

Hydrocarbons

Hydrocarbon linear chains• Ethane = C2H6

• Propane = C3H8

• Butane = C4H10

Hydrocarbon branched chain

Hydrocarbons

Hydrocarbon rings.• Cyclohexane = C6H12

Hydrocarbons

Hydrocarbons can also have double or triplebonds between the carbons

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Hydrocarbons

Hydrocarbons are usually the wastes ordecomposition products of living systems

Other organic molecules in living organismscontain elements in addition to C and H• Carbohydrates• Lipids• Proteins• Nucleic Acids

Functional Groups in BiologicalMolecules

The hydroxyl group is a key component of alcohols

The carbonyl group is the reactive part of aldehydes and ketones

The carboxyl group forms organic acids

The amino group acts as an organic base

The phosphate group is a reactive jack-of-all-trades

The sulfhydryl group works as a molecular fastener

Functional Groups

Small, reactive groups of atoms attached toorganic molecules

Their covalent bonds are more easily broken orrearranged than other parts of the molecules

Functional Groups

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Functional Groups Dehydration Synthesis or Condensation

The components of a water molecule are removed as subunits join into alarger molecule.

Hydrolysis

The components of a water molecule are added as molecules are split intosmaller subunits.

Carbohydrates

Monosaccharides are the structural units ofcarbohydrate molecules

Two monosaccharides link to form adisaccharide

Monosaccharides link in longer chains to formpolysaccharides

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Carbohydrates

Most abundant biological molecules

Contain carbon, hydrogen, and oxygen• Usually 1 carbon:2 hydrogens:1 oxygen

Important as fuel sources and for energy storage• Glucose, sucrose, starch, glycogen

Important as structural molecules• Cellulose, chitin

Monosaccharides

Monosaccharides (“one sugar”)• Usually three to seven carbons

Monosaccharides

The position of the side groups determine thecharacteristics of different monosaccharides

Monosaccharide Isomers

Asymmetric carbons can lead to two moleculeswith different structures but the same formula• Enantiomers or optical isomers

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Monosaccharide Isomers

Monosaccharides with five or more carbons canchange from the linear form to a ring form

Monosaccharide Isomers

Asymmetric carbons in 5- and 6-carbonmonosaccharides can form α- and β-ringisomers

Polysaccharides withα- or β-ring subunitscan have vastly differentchemical properties

Disaccharides

Disaccharides (“two sugars”)• Two monosaccharides linked by a dehydration

reaction to form a glycosidic bond

a. Formation of maltose

Glucose Glucose Maltose

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c. Lactose

Glucose unitGalactose unit

Polysaccharides

Polysaccharides (“many sugars”)

• Macromolecules formed by the polymerizationof many monosaccharide subunits (monomers)

• Two common energy storage polysaccharides:• Starch and glycogen

• Two common structural polysaccharides:• Cellulose and chitin

Storage Polysaccharides

Starch is made by plants to store energy• Amylose = linear, unbranched• Amylopectin = branched

Storage Polysaccharides

Glycogen is made by animals to store energy,usually in liver and muscle tissues• Highly branched

Fig. 3-7b, p. 49

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Structural Polysaccharides

Cellulose is made by plants as a structural fiberin cell walls• Unbranched chain of glucoses connected by β-

linkages• Extremely strong

Structural Polysaccharides

Cellulose is called fiber in human nutrition• Indigestible by most animals• Termites and ruminant mammals have micro-

organisms in their digestive tract that can breakdown cellulose into glucose subunits

Structural Polysaccharides

Chitin is tough and resilient, used for cell wallsof fungi and exoskeletons of arthropods• Similar structure to cellulose, but glucose sub-

units modified with nitrogen-containing groups

Lipids

Neutral lipids are familiar as fats and oils

Phospholipids provide the framework ofbiological membranes

Steroids contribute to membrane structure andwork as hormones

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Lipids

Lipids are mostly nonpolar, water-insolublemolecules because they contain manyhydrocarbon parts• Neutral lipids are important energy-storage

molecules• Phospholipids help form membranes• Steroids contribute to membrane structure or

function as hormones

Neutral Lipids

Neutral lipids are nonpolar, with no chargedgroups at cellular pH• Triglycerides are used for energy storage.• Glycerol (3-carbon alcohol) + fatty acids

Neutral Lipids

Fats are semisolid at biological temperatures

Saturated fatty acid chains:• Usually 14 to 22 carbons long• Contain only single bonds between the carbons• Maximum number of hydrogen atoms (“saturated”)

Neutral Lipids

Oils are liquid at biological temperatures• Unsaturated fatty acid chains:

• Contain one or more double bonds• Fewer hydrogen atoms (“unsaturated”)• Fatty acid chains bend or “kink” at double bond

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Neutral Lipids

Triglycerides store twice as much energy perweight as carbohydrates• Excellent energy

source in the diet• Animals store fat

rather than glycogento carry less weight

• Triglycerides are usedby some birds to maketheir feathers waterrepellent

Neutral Lipids

Fatty acids combined with long-chain alcohols orhydrocarbons form insoluble waxes• Honeybees use wax to build their combs• Plants use waxes for the cuticle, a protective

exterior coating to reduce water loss and to resistviruses and bacteria

Phospholipids

Phospholipids provide the framework ofbiological membranes• Glycerol + 2 fatty acids + polar phosphate group

Phospholipids

Phospholipids in polar environments, like water,cluster together in special arrangements

Bilayers: two phospholipid layers with polargroups facing the water and fatty acids packedtogether in interior to exclude water

The attraction and repulsion of water creates astable, strong structure

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Steroids

Steroids have a common framework of fourcarbon rings with various side groups attached

Steroids

Cholesterol (animals) and phytosterols(plants) alter characteristics in membranes

Steroids

Steroid hormones:important regulatorymolecules

Estradiol, an estrogen

Testosterone

Proteins

Cells assemble 20 kinds of amino acids intoproteins by forming peptide bonds

Proteins have as many as four levels of structure

Primary structure is the fundamental determinantof protein form and function

Twists and other arrangements of the aminoacid chain form the secondary structure of aprotein

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Proteins (cont.)

The tertiary structure of a protein is its overallthree-dimensional conformation

Multiple amino acid chains form quaternarystructure

Combinations of secondary, tertiary, andquaternary structure form functional domains inmany proteins

Proteins combine with units derived from otherclasses of biological molecules

Amino Acids

Amino acids: building blocks of proteins• All amino acids contain an amino group (—NH2),

a carboxyl group (—COOH), and a hydrogenaround the central carbon

• The fourth “R” group represents the variety ofside groups in different amino acids

R|

H2N—C—COOH|H

Amino Acids

Nonpolar amino acids:

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Amino Acids

Uncharged polar amino acids:

Amino Acids

Negatively and positively charged amino acids:

Amino Acids

Methionine andcysteine containsulfur side groups

—SH groups intwo cysteines canbond together toproduce a disulfidebridge (—S—S—)that helps stabilizethe structure ofproteins

Amino Acids

Peptide bonds are covalent bonds that joinamino acids to form polypeptides

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Primary Protein Structure

Primary structure: Sequence of amino acidsthat characterizes a specific protein

Secondary Protein Structure

Secondary structure: Amino acids interact withtheir neighbors to bend and twist protein chain

Some secondary structures have distinctiveshapes and have been named

Secondary Protein Structure

Alpha helix(α-helix)

Stabilized withhydrogen bonds

Secondary Protein Structure

Beta sheet (β-sheet) stabilized with H bonds

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Tertiary Protein Structure

Tertiary structure is the overall conformationor three-dimensional shape of a protein

Tertiary Protein Structure

Stabilized to maintain the protein’s shape• Disulfide linkages • Positive/negative attractions• Hydrogen bonds • Polar/nonpolar associations

Tertiary Protein Structure

Denaturation: Loss of protein structure andfunction; may be permanent or reversible

Tertiary Protein Structure

Chaperone proteins (chaperonins) help somenew proteins fold into their correct conformation

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Quaternary Protein Structure

Quaternary structure: Two or more proteinsjoined together into a larger complex protein

Protein Domains

Combinations of secondary, tertiary, andquaternary structure can form functionaldomains in many proteins• Some proteins may have evolved by mixing

domains into new combinations

Protein Motifs

Motifs: Highly specialized regions with specialfunctions, within or between domains

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Nucleotides and Nucleic Acids

Nucleotides consist of a nitrogenous base, a five- carbon sugar, and one or more phosphate groups

Nucleic acids DNA and RNA are the informational molecules of all organisms

DNA molecules consist of two nucleotide chains wound together

RNA molecules usually consist of single nucleotide chains

Nucleic Acids

Nucleic acids are long polymers of nucleotidebuilding blocks• DNA (deoxyribonucleic acid) stores hereditary

information• RNA (ribonucleic acid) is used in various forms to

help assemble proteins

Phosphate groupsNitrogenous base(adenine shown)

Sugar (riboseor deoxyribose)

in ribosein deoxyribose

Nucleoside (sugar +nitrogenous base)

Nucleoside monophosphate (adenosineor deoxyadenosine monophosphate)

Nucleoside diphosphate (adenosineor deoxyadenosine diphosphate)

Nucleoside triphosphate (adenosineor deoxyadenosine triphosphate)

Nucleotides

Nucleotidesvary in sugar(ribose ordeoxyribose)and innitrogenousbase:

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Nucleic Acids

DNA and RNApolynucleotidechains are formedby linking thephosphate groupof one nucleotideto the sugar of thenext one

Phosphodiesterbond

DNA

DNA forms a double helix when two strands aretwisted together

DNA

Two strands of DNA are joined by hydrogenbonds between the nitrogenous bases followingbase-pairing rules: A–T and C–G

DNA

Because of the base-pairing rules, thenucleotide sequence of one DNA chain iscomplementary to the other chain

NewOld

New

Old

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RNA

RNA usually exists as single strands

Ribose instead of deoxyribose sugar

RNA nucleotide sequences are distinctivebecause Uracil replaces Thymine

Follows the same base-pairing rules:• A–U instead of A–T• G–C