lipids are a diverse group of hydrophobic molecules lipids are the one class of large biological...
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Lipids are a diverse group of hydrophobic molecules
• Lipids are the one class of large biological molecules that do not form polymers
• Lipids are hydrophobic (water fearing) becausethey consist mostly of hydrocarbons, which form nonpolar covalent bonds
• The most biologically important lipids are fats, phospholipids, and steroids
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Fats-store energy
• Fats are constructed from two types of smaller molecules: glycerol and fatty acids
• Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon
• A fatty acid consists of a carboxyl group attached to a long carbon skeleton
• Triglyceride- glycerol + 3 fatty acids• Saturated fatty acids have the maximum number of hydrogen atoms
possible and no double bonds• Unsaturated fatty acids have one or more double bonds
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Figure 5.10
(a) One of three dehydration reactions in the synthesis of a fat
(b) Fat molecule (triacylglycerol)
Fatty acid(in this case, palmitic acid)
Glycerol
Ester linkage
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Figure 5.11
(a) Saturated fat(b) Unsaturated fat
Structuralformula of asaturated fatmolecule
Space-fillingmodel of stearicacid, a saturatedfatty acid
Structuralformula of anunsaturated fatmolecule
Space-filling modelof oleic acid, anunsaturated fattyacid
Cis double bondcauses bending.
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Figure 5.12
Choline
Phosphate
Glycerol
Fatty acids
Hydrophilichead
Hydrophobictails
(c) Phospholipid symbol(b) Space-filling model(a) Structural formula
Hyd
roph
ilic
head
Hyd
roph
obic
tails
Phospholipids In a phospholipid, two fatty acids and a phosphate group are attached to glycerol. •Fatty acid tails are hydrophobic•Phosphate group and its attachments are hydrophilic head
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• When phospholipids are added to water, they self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior
• The structure of phospholipids results in a bilayer arrangement found in cell membranes
• Phospholipids are the major component of all cell membranes
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Figure 5.13
Hydrophilichead
Hydrophobictail
WATER
WATER
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Steroids
• Steroids are lipids characterized by a carbon skeleton consisting of four fused rings
• Cholesterol, an important steroid, is a component in animal cell membranes
• Although cholesterol is essential in animals, high levels in the blood may contribute to cardiovascular disease
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Figure 5.14
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Nucleic acids store, transmit, and help express hereditary information
• The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene
• Genes are made of DNA, a nucleic acid made of monomers called nucleotides
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The Roles of Nucleic Acids
• There are two types of nucleic acids– Deoxyribonucleic acid (DNA)– Ribonucleic acid (RNA)
• DNA provides directions for its own replication• DNA directs synthesis of messenger RNA
(mRNA) and, through mRNA, controls protein synthesis
• Protein synthesis occurs on ribosomes
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Figure 5.25-3
Synthesis ofmRNA
mRNA
DNA
NUCLEUSCYTOPLASM
mRNA
Ribosome
AminoacidsPolypeptide
Movement ofmRNA intocytoplasm
Synthesisof protein
1
2
3
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The Components of Nucleic Acids
• Nucleic acids are polymers called polynucleotides• Each polynucleotide is made of monomers called
nucleotides• Each nucleotide consists of a nitrogenous base, a
pentose sugar, and one or more phosphate groups• The portion of a nucleotide without the phosphate
group is called a nucleoside
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Figure 5.26
Sugar-phosphate backbone5 end
5C
3C
5C
3C
3 end
(a) Polynucleotide, or nucleic acid
(b) Nucleotide
Phosphategroup Sugar
(pentose)
Nucleoside
Nitrogenousbase
5C
3C
1C
Nitrogenous bases
Cytosine (C) Thymine (T, in DNA) Uracil (U, in RNA)
Adenine (A) Guanine (G)
Sugars
Deoxyribose (in DNA) Ribose (in RNA)
(c) Nucleoside components
Pyrimidines
Purines
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Components of Nucleic Acids
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Double Helix and Bonding
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• Nucleoside = nitrogenous base + sugar• There are two families of nitrogenous bases
– Pyrimidines (cytosine, thymine, and uracil) have a single six-membered ring
– Purines (adenine and guanine) have a six-membered ring fused to a five-membered ring
• In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose
• Nucleotide = nucleoside + phosphate group
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Nucleotide Polymers
• Nucleotide polymers are linked together to build a polynucleotide
• Adjacent nucleotides are joined by covalent bonds that form between the —OH group on the 3 carbon of one nucleotide and the phosphate on the 5 carbon on the next
• These links create a backbone of sugar-phosphate units with nitrogenous bases as appendages
• The sequence of bases along a DNA or mRNA polymer is unique for each gene
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The Structures of DNA and RNA Molecules
• RNA molecules usually exist as single polypeptide chains
• DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a double helix
• In the DNA double helix, the two backbones run in opposite 5→ 3 directions from each other, an arrangement referred to as antiparallel
• One DNA molecule includes many genes
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• The nitrogenous bases in DNA pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)
• Called complementary base pairing• Complementary pairing can also occur between two
RNA molecules or between parts of the same molecule
• In RNA, thymine is replaced by uracil (U) so A and U pair
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Figure 5.27
Sugar-phosphatebackbones
Hydrogen bonds
Base pair joinedby hydrogen bonding
Base pair joinedby hydrogen
bonding
(b) Transfer RNA(a) DNA
5 3
53
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DNA and Proteins as Tape Measures of Evolution
• The linear sequences of nucleotides in DNA molecules are passed from parents to offspring
• Two closely related species are more similar in DNA than are more distantly related species
• Molecular biology can be used to assess evolutionary kinship
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The Theme of Emergent Properties in the Chemistry of Life: A Review
• Higher levels of organization result in the emergence of new properties
• Organization is the key to the chemistry of life
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