CHAPTER 4CARBON AND THE MOLECULAR DIVERSITY OF LIFE

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Overview: Carbon—The Backbone of Life Aside from water, the most biologically important

molecules are carbon-based (organic).

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

Carbon is unparalleled in its ability to form large, complex, and diverse molecules

Proteins, DNA, carbohydrates, and other molecules that distinguish living matter are all composed of carbon compounds

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Concept 4.1: Organic chemistry is the study of carbon compounds

A. Organic chemistry is the study of compounds that contain carbon1. Organic compounds range from simple molecules to colossal ones2. Most organic compounds contain hydrogen atoms in addition to carbon atoms

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Concept 4.2: Carbon atoms can form diverse molecules by bonding to four other atoms

Electron configuration is the key to an atom’s characteristics

Electron configuration determines the kinds and number of bonds an atom will form with other atoms

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A. The Formation of Bonds With Carbon1. Carbon atom• Atomic number 6 (6 protons, 6 electrons, 6

neutrons)• 4 valance electrons• Forms 4 covalent bonds usually with H, O, N, C• Can form large, complex molecules

2. Carbon Dioxide (OCO)• Source of carbon for all organic molecules

found in organisms

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B. Molecular Diversity Arising from Carbon Skeleton Variation1. Covalent bonds link carbon atoms together in long chains that form the skeletal framework for organic molecules.2. Carbon skeletons may vary in:•Length•Shape (straight chain, branched, ring)•Number and location of double bonds•Other elements covalently bonded to available

sites3. Variation contributes to complexity and diversity

of organic molecules

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C. Hydrocarbons1. Hydrocarbons are organic molecules consisting of

only carbon and hydrogen2. Many organic molecules, such as fats and fossil

fuels, have hydrocarbon components3. Hydrocarbons can undergo reactions that release

a large amount of energy4. Hydrocarbon chains are hydrophobic because C

—C and C—H bonds are nonpolar.

Hydrocarbon

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D. Isomers1. Isomers are compounds with the same molecular

formula but different structures and properties.2. Three types of isomers:

a. Structural Isomers•Have different covalent arrangements of their atoms•Number of possible isomers increases as C skeleton size increases•Ex: C5 H13—3 isomers

C8 H18—18 isomersC20 H42—366,319 isomers

•May differ in location of double bonds

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Structural Isomers

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b. Geometric isomers • Have the same covalent arrangements but

differ in spatial arrangements• Result from the fact that double bonds will not

allow the atoms they join to rotate freely about the axis of the bonds• Subtle differences between isomers affects

their biological activity

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Geometric Isomers

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c. Enantiomers•Are isomers that are mirror images of each other•Can occur when 4 different atoms or groups of atoms are bonded to the same carbon (asymmetric carbon)•Usually one form is biologically active and its mirror image is not•Important in the pharmaceutical industry•Two enantiomers of a drug may have different effects

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Enantiomers

Enantiomer

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Concept 4.3: A small number of chemical groups are key to the functioning of biological molecules

Distinctive properties of organic molecules depend not only on the carbon skeleton but also on the molecular components attached to it

A number of characteristic groups are often attached to skeletons of organic molecules

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A. The Chemical Groups Most Important in the Process of Life1. Functional groups are the components of

organic molecules that are most commonly involved in chemical reactions

2. The number and arrangement of functional groups give each molecule its unique

properties 3. Functional group replaces an H bonded to the carbon of a hydrocarbon

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4. The seven functional groups that are most important in the chemistry of life:

Hydroxyl groupCarbonyl groupCarboxyl groupAmino groupSulfhydryl groupPhosphate groupMethyl group

5. The Seven Functional Groups a. Hydroxyl Group (-OH)

Consists of a hydrogen atom bonded to an oxygen atom, which is bonded to carbon

H H | |

H—C—C—OH R—OH | |H H

Ethanol Polar group, therefore it makes the molecule attached to water solubleOrganic compounds with hydroxyl groups are called alcoholsNames end in –ol 23

b. Carbonyl Group (-CO)Consists of a carbon atom double-bonded to

oxygenPolar group, therefore it makes the molecule

attached to water solubleFound in sugarsIf the carbonyl group is on the end of the carbon

skeleton, the compound is an aldehyde OH OH O O | | // //H—C—C—C R—C | | \ \

H H H H

Glyceraldehyde 24

If the carbonyl group is in the internal part of a carbon skeleton, the compound is a ketone

H O H O | || | ||

H—C—C—C—H R—C—R | | H H

Acetone

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c. Carboxyl Group (-COOH)Consists of a carbon atom which is both double-bonded to an oxygen and single bonded to the oxygen of a hydroxyl groupPolar group and water solubleHave acidic propertiesCompounds with this group are called carboxylic acids (organic acids)

H O O | // //

H—C—C R—C | \ \

H OH OH Acetic Acid

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d. Amino Group (-NH2)Consists of a nitrogen atom bonded to two hydrogens and to the carbon skeletonPolar group and water solubleWeak baseOrganic compounds with this functional group are called amines

H H H | / /

H—C—N R—N | \ \ H H H

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B. Amino Acids

Contain both carboxyl and amino groups

H H O \ | // N—C—C / | \ H R OH

amino carboxyl

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e. Sulfhydryl Group (-SH)Consists of an atom of sulfur bonded to an atom of hydrogenHelps stabilize the structure of proteinsOrganic compounds with this functional group are called thiols

H H | | H—C—C—SH R—SH

| | H H

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f. Phosphate Group (H3PO4)Functional group which is the dissociated form of phosphoric acidPolar group and water solubleAcid propertiesForm organic phosphatesOrganic phosphates are important in cellular energy storage and transfer (ATP)

OH OH H O O | | | || ||

H—C—C—C—O—P—O- R—O—P—O-

| | | | | H H H O- O-

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ATP

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g. Methyl Group (-CH3) Consists of a carbon bonded to 3 hydrogensMay be attached to a carbon or to a different atomNot really a functional group because they are not reactive but may serve as important markers on organic molecules Can affect expression of genes

H / —C—H \ H