copyright © 2003 pearson education, inc. publishing as benjamin cummings chemistry part 2

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

CHEMISTRY PART 2


Chemical Bonds

• Electron shells, or energy levels, surround the nucleus of an atom

• Bonds are formed using the electrons in the outermost energy level

• Valence shell – outermost energy level containing chemically active electrons

• Octet rule – atoms usually react in a manner to have 8 electrons in their valence shell. (2 in first, 8 in second and third)


Chemically Inert and Reactive Elements

• Inert elements have their outermost energy level fully occupied by electrons

Figure 2.4a


Chemically Inert and Reactive Elements

• Reactive elements do not have their outermost energy level fully occupied by electrons

Figure 2.4b


Types of Chemical Bonds

• Ionic

• Covalent

• Hydrogen


Ionic Bonds

• Ions are charged atoms resulting from the gain or loss of electrons

• Anions have gained one or more electrons

• Cations have lost one or more electrons

• Opposite charges on anions and cations hold them close together, forming ionic bonds


Formation of an Ionic Bond

• Ionic compounds form crystals instead of individual molecules

• Example: NaCl (sodium chloride)


Formation of an Ionic Bond

Figure 2.5


Covalent Bonds

• Electrons are shared by two atoms

• Electron sharing produces molecules


Covalent Bonds

Figure 2.6a


Covalent Bonds

Figure 2.6b


Covalent Bonds

Figure 2.6c


Polar and Nonpolar Molecules

• Electrons shared equally between atoms produce nonpolar molecules

• Unequal sharing of electrons produces polar molecules

• Atoms with 6 or 7 valence shell electrons are electronegative

• Atoms with 1 or 2 valence shell electrons are electropositive


Figure 2.8

Comparison of Ionic, Polar Covalent, and Nonpolar Covalent Bonds


Hydrogen Bonds

• Too weak to bind atoms together

• Common in dipoles such as water

• Responsible for surface tension in water

• Important as intramolecular bonds, giving the molecule a three-dimensional shape

Figure 2.9


Chemical Reactions

• Occur when chemical bonds are formed, rearranged, or broken

• Are written in symbolic form using chemical equations

• Chemical equations contain:

• Number and type of reacting substances, and products produced

• Relative amounts of reactants and products

H + H H2

(reactants) (product)


Patterns of Chemical Reactions

• Combination reactions: Synthesis reactions which always involve bond formation

• A + B AB

• Decomposition reactions: Molecules are broken down into smaller molecules

• AB A + B

• Exchange reactions: Bonds are both made and broken

• AB + C AC + B


Oxidation-Reduction (Redox) Reactions

• Reactants losing electrons are electron donors and are oxidized

• Reactants taking up electrons are electron acceptors and become reduced


Energy Flow in Chemical Reactions

• Exergonic reactions – reactions that release energy

• Endergonic reactions – reactions whose products contain more potential energy than did its reactants


Reversibility of Chemical Reactions

• All chemical reactions are theoretically reversible

A + B AB

AB A + B

• If neither a forward nor reverse reaction is dominant, chemical equilibrium is reached


Factors Influencing Rate of Chemical Reactions

• Temperature – chemical reactions proceed quicker at higher temperatures

• Particle size – the smaller the particle the faster the chemical reaction

• Concentration – higher reacting particle concentrations produce faster reactions

• Catalysts – increase the rate of a reaction without being chemically changed

• Enzymes – biological catalysts


Biochemistry

• Organic compounds

• Contain carbon, are covalently bonded, and are often large

• Inorganic compounds

• Do not contain carbon

• Water, salts, and many acids and bases


Water

• High heat capacity – absorbs and releases large amounts of heat before changing temperature

• High heat of vaporization – changing from a liquid to a gas requires large amounts of heat

• Polar solvent properties – dissolves ionic substances, forms hydration layers around large charged molecules, and serves as the body’s major transport medium

• Reactivity – is an important part of hydrolysis and dehydration synthesis reactions

• Cushioning – resilient cushion around certain body organs


Salts

• Inorganic compounds

• Contain cations other than H+ and anions other than OH–

• Are electrolytes; they conduct electrical currents


Acids and Bases

• Acids release H+ and are therefore proton donors

HCl H+ + Cl –

• Bases release OH– and are proton receptors

NaOH Na+ + OH–


Acid-Base Concentration (pH)

• Acidic solutions have higher H+ concentration and therefore a lower pH

• Alkaline solutions have lower H+ concentration and therefore a higher pH

• Neutral solutions have equal H+ and OH– concentrations


Acid-Base Concentration (pH)

• Acidic: pH 0–6.99

• Basic: pH 7.01–14

• Neutral: pH 7.00

Figure 2.12


Buffers

• Systems that resist abrupt and large swings in the pH of body fluids

• Carbonic acid–bicarbonate system

• Carbonic acid dissociates reversibly releasing bicarbonate ions and protons

• The chemical equilibrium between carbonic acid and bicarbonate resists pH changes in the blood


Break


Organic Compounds

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids


Carbohydrates

• Contain carbon, hydrogen, and oxygen

• Their major function is to supply a source of cellular food

• Examples:

• Monosaccharides or simple sugars

Figure 2.13a


Carbohydrates

• Disaccharides or double sugars

Figure 2.13b


Carbohydrates

• Polysaccharides or polymers of simple sugars

Figure 2.13c


Lipids

• Contain C, H, and O, but the proportion of oxygen in lipids is less than in carbohydrates

• Examples:

• Neutral fats or triglycerides

• Phospholipids

• Steroids

• Eicosanoids


Neutral Fats (Triglycerides)

• Composed of three fatty acids bonded to a glycerol molecule

Figure 2.14a


Other Lipids

• Phospholipids – modified triglycerides with two fatty acid groups and a phosphorus group

Figure 2.14b


Other Lipids

• Steroids – flat molecules with four interlocking hydrocarbon rings

• Eicosanoids – 20-carbon fatty acids found in cell membranes

Figure 2.14c


Representative Lipids Found in the Body

• Neutral fats – found in subcutaneous tissue and around organs

• Phospholipids – chief component of cell membranes

• Steroids – cholesterol, bile salts, vitamin D, sex hormones, and adrenal cortical hormones

• Fat-soluble vitamins – vitamins A, E, and K

• Eicosanoids – prostaglandins, leukotriens, and thromboxanes

• Lipoproteins – transport fatty acids and cholesterol in the bloodstream


Amino Acids

• Building blocks of protein, containing an amino group and a carboxyl group

• Amino acid structure

Figure 2.15


Protein

• Macromolecules composed of combinations of 20 types of amino acids bound together with peptide bonds

Figure 2.16


Structural Levels of Proteins

• Primary – amino acid sequence

• Secondary – alpha helices or beta pleated sheets

• Tertiary – superimposed folding of secondary structures

• Quaternary – polypeptide chains linked together in a specific manner


Structural Levels of Proteins

Figure 2.17


Fibrous and Globular Proteins

• Fibrous proteins

• Extended and strandlike proteins

• Examples: keratin, elastin, collagen, and certain contractile fibers

• Globular proteins

• Compact, spherical proteins with tertiary and quaternary structures

• Examples: antibodies, hormones, and enzymes


Protein Denaturation

• Reversible unfolding of proteins due to drops in pH and/or increased temperature

Figure 2.19a


Protein Denaturation

• Irreversibly denatured proteins cannot refold and are formed by extreme pH or temperature changes

Figure 2.19b


Molecular Chaperones (Chaperonins)

• Help other proteins to achieve their functional three-dimensional shape

• Maintain folding integrity

• Assist in translocation of proteins across membranes

• Promote the breakdown of damaged or denatured proteins


Characteristics of Enzymes

• Most are globular proteins that act as biological catalysts

• Holoenzymes consist of an apoenzyme (protein) and a cofactor (usually an ion)

• Enzymes are chemically specific

• Frequently named for the type of reaction they catalyze

• Enzyme names usually end in -ase

• Lower activation energy


Characteristics of Enzymes

Figure 2.20


Mechanism of Enzyme Action

• Enzyme binds with substrate

• Product is formed at a lower activation energy

• Product is released

Figure 2.21


Nucleic Acids

• Composed of carbon, oxygen, hydrogen, nitrogen, and phosphorus

• Their structural unit, the nucleotide, is composed of N-containing base, a pentose sugar, and a phosphate group

• Five nitrogen bases contribute to nucleotide structure – adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U)

• Two major classes – DNA and RNA


Deoxyribonucleic Acid (DNA)

• Double-stranded helical molecule found in the nucleus of the cell

• Replicates itself before the cell divides, ensuring genetic continuity

• Provides instructions for protein synthesis


Structure of DNA

Figure 2.22a


Structure of DNA

Figure 2.22b


Ribonucleic Acid (RNA)

• Single-stranded molecule found in both the nucleus and the cytoplasm of a cell

• Uses the nitrogenous base uracil instead of thymine

• Three varieties of RNA: messenger RNA, transfer RNA, and ribosomal RNA


Adenosine Triphosphate (ATP)

• Source of immediately usable energy for the cell

• Adenine-containing RNA nucleotide with three phosphate groups

Figure 2.23

copyright © 2003 pearson education, inc. publishing as benjamin cummings chemistry part 2

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