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 molecule1. Monosaccharides

• Simple carbohydrates with one sugar residue• Elements C, H and O in ratio CH2O• 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

B. Lipids• 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

C. Proteins• Polymers of amino acids• Abundant: Account for > 50% of dry weight in

most cells• Functions

1. Structural support2. Storage of amino acids3. 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

D. Nucleic Acids1. 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

II. Early Earth

• 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 Life1. Little or no free oxygen (O2)

• O2 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 Stages1. 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

C. Pre-Biotic Chemistry• 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

C. Pre-Biotic Chemistry1. Urey-Miller Apparatus

• Formation of amino acids, complex hydrocarbons, sugars, lipids, nucleotides

• Questions: Enough NH3 and CH4 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

III. Origin of Life on Earth

C. Pre-Biotic Chemistry2. 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

III. Origin of Life on Earth

C. Characteristics of Early Life1. 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 (H2S) as

hydrogen source (modern purple & green sulfur bacteria still get H from H2S)

• First autotrophs to split water for H probably ancestors of modern cyanobacteria (3.1 – 3.5 bya)

• Production of O2 had profound effects