biological molecules: the carbon compounds of life
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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
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