2.6 Organic Molecules
The molecules of life – carbohydrates, proteins, lipids, and nucleic acids – are organic molecules
Organic• Type of molecule that consists primarily of carbon
and hydrogen atoms
Building Organic Molecules
Carbon atoms bond covalently with up to four other atoms, often forming long chains or rings
Enzyme-driven reactions construct large molecules from smaller subunits, and break large molecules into smaller ones
From Structure to Function
Cells assemble large polymers from smaller monomers, and break apart polymers into component monomers
Metabolism• All the enzyme-mediated chemical reactions by
which cells acquire and use energy as they build and break down organic molecules
Monomers and Polymers
Monomers• Molecules that are subunits of polymers• Simple sugars, fatty acids, amino acids,
nucleotides
Polymers• Molecules that consist of multiple monomers• Carbohydrates, lipids, proteins, nucleic acids
Condensation and Hydrolysis
Condensation (water forms)• Process by which an enzyme builds large
molecules from smaller subunits
Hydrolysis (water is used)• Process by which an enzyme breaks a molecule
into smaller subunits by attaching a hydroxyl to one part and a hydrogen atom to the other
2.7 Carbohydrates
Cells use carbohydrates for energy and structural materials
Carbohydrates• Molecules that consist primarily of carbon,
hydrogen, and oxygen atoms in a 1:2:1 ratio
Complex Carbohydrates
Enzymes assemble complex carbohydrates (polysaccharides) from simple carbohydrate (sugar) subunits
Glucose monomers can bond in different patterns to form different complex carbohydrates• Cellulose (a structural component of plants)• Starch (main energy reserve in plants)• Glycogen (energy reserve in animals)
2.8 Lipids
Lipids are greasy or oily nonpolar organic molecules, often with one or more fatty acid tails
Lipids• Fatty, oily, or waxy organic compounds
Fatty acid• Consists of a long chain of carbon atoms with an
acidic carboxyl group at one end
Fats
Fats, such as triglycerides, are the most abundant source of energy in vertebrates – stored in adipose tissue that insulates the body
Fat• Lipid with one, two, or three fatty acid tails
Triglyceride• Lipid with three fatty acid tails attached to a
glycerol backbone
Saturated and Unsaturated Fats
Saturated fats pack more tightly than unsaturated fats, and tend to be more solid
Saturated fat• Fatty acid with no double bonds in its carbon tail
Unsaturated fat• Lipid with one or more double bonds in a fatty
acid tail
Fig. 2-14, p. 32
carboxyl group
long carbon chain
cis double bond
trans double bond
A stearic acid B linolenic acid C oleic acid D elaidic acid
Phospholipids
Phospholipids are the main structural component of cell membranes
Phospholipid• A lipid with a phosphate group in its hydrophilic
head, and two nonpolar fatty acid tails
Waxes
Waxes are part of water-repellent and lubricating secretions in plants and animals
Wax• Water-repellent lipid with long fatty-acid tails
bonded to long-chain alcohols or carbon rings
Steroids
Steroids such as cholesterol occur in cell membranes or are remodeled into other molecules (such as steroid hormones, bile salts, and vitamin D)
Steroid• A type of lipid with four carbon rings and no fatty
acid tails
2.9 Proteins
A protein’s function depends on its structure, which consists of chains of amino acids that twist and fold into functional domains
Protein• Organic compound that consists of one or
more chains of amino acids
Amino Acid
Amino acid• Small organic compound with a carboxyl group,
amine group, and a characteristic side group (R)
Peptide Bonds
Amino acids are linked into chains by peptide bonds
Peptide bond• A bond between the amine group of one amino
acid and the carboxyl group of another
Polypeptide• Chain of amino acids linked by peptide bonds
Protein Synthesis
1. Primary structure (polypeptide formation) • A linear sequence of amino acids
2. Secondary structure• Hydrogen bonds twist the polypeptide into a coil
or sheet
3. Tertiary structure• Secondary structure folds into a functional shape
Protein Synthesis
4. Quaternary structure • In some proteins, two or more polypeptide chains
associate and function as one molecule• Example: hemoglobin
5. Fibrous proteins may aggregate into a larger structure, such as keratin filaments• Example: hair
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5) Many proteins aggregate by the thousands into larger structures, such as the keratin filaments that make up hair.
Fig. 2-18, p. 35
Stepped Art
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2) Secondary structure arises when a polypeptide chain twists into a coil (helix) or sheet held in place by hydrogen bonds between different parts of the molecule. The same patterns of secondary structure occur in many different proteins.
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3) Tertiary structure occurs when a chain’s coils and sheets fold up into a functional domain such as a barrel or pocket. In this example, the coils of a globin chain form a pocket.
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4) Some proteins have quaternary structure, in which two or more polypeptide chains associate as one molecule. Hemoglobin, shown here, consistsof four globin chains (green and blue). Each globin pocket now holds a heme group (red).
lysine glycine glycine arginine
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1) A protein’s primary structure consists of a linear sequence of amino acids (a polypeptide chain).
Protein Structure
The Importance of Protein Structure
Changes in a protein’s structure may also alter its function
Denature• To unravel the shape of a protein or other large
biological molecule
Misfolded Proteins: Prion Disease
Prion • A misfolded protein that becomes infectious• Example: mad cow disease (BSE) in cattle• Example: vCJD in humans
2.10 Nucleic Acids
Nucleotide• Monomer of nucleic acids• Has a five-carbon sugar, a nitrogen-containing
base, and phosphate groups
Nucleic acids• Polymers of nucleotide monomers joined by
sugar-phosphate bonds (include DNA, RNA, coenzymes, energy carriers, messengers)
ATP
The nucleotide ATP can transfer a phosphate group and energy to other molecules, and is important in metabolism
Adenosine triphosphate (ATP)• Nucleotide that consists of an adenine base, five-
carbon ribose sugar, and three phosphate groups• Functions as an energy carrier
Functions of DNA and RNA
DNA encodes heritable information about a cell’s proteins and RNAs
Different RNAs interact with DNA and with one another to carry out protein synthesis
DNA and RNA
Deoxyribonucleic acid (DNA)• Nucleic acid that carries hereditary material• Two nucleotide chains twisted in a double helix
Ribonucleic acid (RNA)• Typically single-stranded nucleic acid• Functions in protein synthesis