Molecules of Life

Chapter 2

Part 2

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

Some Elemental Abundances

Modeling an Organic Molecule

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

Condensation and Hydrolysis

Condensation and hydrolysis

Functional groups

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)

Some Complex Carbohydrates

Animation: Structure of starch and cellulose

Animation: Examples of monosaccharides

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

Fatty Acids

Saturated, unsaturated, cis, and trans fatty acids

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

Phospholipids

Fig. 2-15b, p. 32

one layer of lipids

one layer of lipids

B a lipid bilayer

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

Steroids

Phospholipid structure

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

Polypeptide Formation

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

Variant Creutzfeldt-Jakob Disease (vCJD)

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

A Nucleotide and Nucleic Acid

2.11 Impacts/Issues Revisited

Our enzymes can’t easily break down trans fats in processed foods, which causes health problems – several countries will not import foods made in the US that contain trans fats