I.
Biological Molecules
•
•
•
Affected by natural selection
Based on carbon skeletons
Mostly polymers (made of monomers )
A.
Carbohydrates
•
• Sugars, starches, cellulose
May be classified by number of sugar residues in molecule
1.
Monosaccharides
•
•
•
•
Simple carbohydrates with one sugar residue
Elements C, H and O in ratio CH
2
O
Nutrients for cells; building blocks for complex carbohydrates
Ex: Glucose, fructose, galactose
2.
Disaccharides
•
• Two linked monosaccharides
Ex: Maltose (glucose + glucose), lactose (glucose + galactose), sucrose (glucose + fructose)
3.
Polysaccharides
•
• Polymers containing multiple monosaccharides
Serve two important types of functions a.
Energy storage ( Ex: starch, glycogen) b.
Structural support ( Ex: cellulose, chitin)

I.
Biological Molecules
• Fats and fat-like substances
• Chemically diverse; all insoluble in water but soluble in polar solvents
1.
Fats
• Used for energy storage, insulation, buoyancy
2.
Phospholipids
• Important components of cell membranes
3.
Steroids
• Common components of cell membranes
• Building blocks for hormones and other, complex biological molecules

I.
Biological Molecules
• Polymers of amino acids
• Abundant: Account for > 50% of dry weight in most cells
• Functions
1.
Structural support
2.
Storage of amino acids
3.
Transport of other substances ( Ex: hemoglobin)
4.
Signaling ( Ex: chemical messengers)
5.
Cellular response to chemical stimuli ( Ex: receptors)
6.
Movement ( Ex: contractile proteins)
7.
Defense against foreign substances and pathogens
( Ex: antibodies)
8.
Catalysis of biochemical reactions ( Ex: enzymes)

I.
Biological Molecules
1.
Deoxyribonucleic Acid (DNA)
• Can self-replicate (with help from enzymes)
• Passed from one generation to the next
• Primarily in nuclei of eukaryotic cells
• Comprises genes
2.
Ribonucleic Acid (RNA)
• Functions in protein synthesis

Fig. 17.3

• Earth ~4.6 billion years old
• First water ~3.9 bya
• First evidence of life ~3.8 bya (controversial)
• First fossils (bacteria-like) ~3.5 bya
• First eukaryotes ~2.2 bya
• First animal fossils ~700 mya
A.
Conditions
• High energy environment
1.
Violent thunderstorms
2.
Widespread volcanic activity
3.
Intense radiation (including uv) from early sun
• No ozone layer and sun more intense then vs.
now
4.
Bombardment by meteors and other objects from space

III.
Origin of Life on Earth
A.
Requirements for Origin of Life
1.
Little or no free oxygen (O
2
•
)
O
2 highly reactive; would have destroyed reduced compounds
(chemical building blocks)
2.
Source of energy
•
Intense radiation, volcanism, bombardment
3.
Availability of chemical building blocks
•
Elements
– C, H, N, O, P, S
4.
Time
• End of bombardment to first evidence of life >100 million years
B.
First Cells May Have Arisen through Four Stages
1.
Abiotic synthesis of small organic molecules ( e.g.
amino acids, nucleotides)
2.
Assembly of small organic molecules into polymers ( e.g.
proteins, nucleic acids)
3.
Packaging of macromolecules into protobionts
•
•
Membranes
Internal environment different from external environment
4.
Development of self-replicating molecules

III.
Origin of Life on Earth
• A.I. Oparin & J.B.S. Haldane (1920’s): Organic molecules necessary to life on earth might have arisen spontaneously from inorganic raw materials
• First empirical studies in 1950’s by Harold Urey
& Stanley Miller (U. of Chicago)

III.
Origin of Life on Earth
1.
Urey-Miller Apparatus
• Formation of amino acids, complex hydrocarbons, sugars, lipids, nucleotides
• Questions: Enough NH
3 and CH
4 in earth’s early atmosphere? Reducing atmosphere?
• Alternative: Origin of life at hydrothermal vents
• Sources of hot water and minerals, including S &
Fe compounds
• Alternative: Panspermia
• Amino acids found in meteorites

C.
Pre-Biotic Chemistry
2.
Abiotic Synthesis of Polymers
• Clays may have helped
• Negatively charged (bind positively charged organic molecules)
• Contain minerals that might have catalyzed polymerization
• Amino acid solution + hot clay  AA polymers
3.
Protobionts
• Abiotically produced molecules surrounded by bilayered lipid membrane a.
Highly organized b.
Some forms reproduce when large enough c.
Internal environment chemically distinct from outside d.
Some forms carry out catalytic activity e.
Some forms produce electrochemical gradients

Fig. 25.3

C.
Characteristics of Early Life
1.
Self-Replication
• Proteins aren’t self-replicating
• RNA may carry out catalytic functions
• Ribozyme – Autocatalytic RNA
•
RNA may have been first informational molecule
• DNA more stable; may have arisen from RNA
2.
Nutrition
• First cells likely heterotrophic
• No free oxygen in atmosphere of early earth
• First heterotrophs probably used anaerobic fermentation
(less efficient than aerobic metabolism)
• First autotrophs may have used hydrogen sulfide (H
2
S) as hydrogen source (modern purple & green sulfur bacteria still get H from H
2
S)
• First autotrophs to split water for H probably ancestors of modern cyanobacteria (3.1 – 3.5 bya)
• Production of O
2 had profound effects