Download - Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings CHEMISTRY PART 2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
CHEMISTRY PART 2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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)
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Chemically Inert and Reactive Elements
• Inert elements have their outermost energy level fully occupied by electrons
Figure 2.4a
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Chemically Inert and Reactive Elements
• Reactive elements do not have their outermost energy level fully occupied by electrons
Figure 2.4b
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Types of Chemical Bonds
• Ionic
• Covalent
• Hydrogen
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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
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Formation of an Ionic Bond
• Ionic compounds form crystals instead of individual molecules
• Example: NaCl (sodium chloride)
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Formation of an Ionic Bond
Figure 2.5
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Covalent Bonds
• Electrons are shared by two atoms
• Electron sharing produces molecules
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Covalent Bonds
Figure 2.6a
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Covalent Bonds
Figure 2.6b
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Covalent Bonds
Figure 2.6c
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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
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Figure 2.8
Comparison of Ionic, Polar Covalent, and Nonpolar Covalent Bonds
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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
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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)
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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
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Oxidation-Reduction (Redox) Reactions
• Reactants losing electrons are electron donors and are oxidized
• Reactants taking up electrons are electron acceptors and become reduced
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Energy Flow in Chemical Reactions
• Exergonic reactions – reactions that release energy
• Endergonic reactions – reactions whose products contain more potential energy than did its reactants
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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
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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
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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
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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
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Salts
• Inorganic compounds
• Contain cations other than H+ and anions other than OH–
• Are electrolytes; they conduct electrical currents
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Acids and Bases
• Acids release H+ and are therefore proton donors
HCl H+ + Cl –
• Bases release OH– and are proton receptors
NaOH Na+ + OH–
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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
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Acid-Base Concentration (pH)
• Acidic: pH 0–6.99
• Basic: pH 7.01–14
• Neutral: pH 7.00
Figure 2.12
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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
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Break
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Organic Compounds
• Carbohydrates
• Lipids
• Proteins
• Nucleic Acids
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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
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Carbohydrates
• Disaccharides or double sugars
Figure 2.13b
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Carbohydrates
• Polysaccharides or polymers of simple sugars
Figure 2.13c
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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
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Neutral Fats (Triglycerides)
• Composed of three fatty acids bonded to a glycerol molecule
Figure 2.14a
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Other Lipids
• Phospholipids – modified triglycerides with two fatty acid groups and a phosphorus group
Figure 2.14b
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Other Lipids
• Steroids – flat molecules with four interlocking hydrocarbon rings
• Eicosanoids – 20-carbon fatty acids found in cell membranes
Figure 2.14c
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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
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Amino Acids
• Building blocks of protein, containing an amino group and a carboxyl group
• Amino acid structure
Figure 2.15
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Protein
• Macromolecules composed of combinations of 20 types of amino acids bound together with peptide bonds
Figure 2.16
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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
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Structural Levels of Proteins
Figure 2.17
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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
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Protein Denaturation
• Reversible unfolding of proteins due to drops in pH and/or increased temperature
Figure 2.19a
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Protein Denaturation
• Irreversibly denatured proteins cannot refold and are formed by extreme pH or temperature changes
Figure 2.19b
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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
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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
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Characteristics of Enzymes
Figure 2.20
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Mechanism of Enzyme Action
• Enzyme binds with substrate
• Product is formed at a lower activation energy
• Product is released
Figure 2.21
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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
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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
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Structure of DNA
Figure 2.22a
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Structure of DNA
Figure 2.22b
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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
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Adenosine Triphosphate (ATP)
• Source of immediately usable energy for the cell
• Adenine-containing RNA nucleotide with three phosphate groups
Figure 2.23