The Origin and Chemistry of Life Chapter 2
Water and Life • Water makes up a large portion of living organisms. • It has several unusual properties that make it essential for life. • Hydrogen bonds lie behind these important properties.
Water and Life • High specific heat capacity – 1 calorie is required to elevate temperature of 1 gram of water 1°C. • Moderates environmental changes. • High heat of vaporization – more than 500 calories are required to convert 1 g of liquid water to water vapor. • Cooling produced by evaporation of water is important for expelling excess heat.
Water and Life • Unique density behavior – while most liquids become denser with decreasing temperature, water’s maximum density is at 4°C. • Ice floats! Lakes don’t freeze solid – some liquid water is usually left at the bottom.
Water and Life • Water has high surface tension. • Because of the hydrogen bonds between water molecules at the water-air interface, the water molecules cling together. • Water has low viscosity.
Water and Life • Water acts as a solvent – salts dissolve more in water than in any other solvent. • Result of the dipolar nature of water.
Water and Life • Hydrolysis occurs when compounds are split into smaller pieces by the addition of a water molecule. • R-R + H2O R-OH + H-R • Condensation occurs when larger compounds are synthesized from smaller compounds. • R-OH + H-R R-R + H2O
Acids, Bases, and Buffers • Acid: Substance that liberates hydrogen ions (H+) in solution. • Base: Substance that liberates hydroxyl ions (OH-) in solution. • The regulation of the concentrations of H+ and OH- is critical in cellular processes.
Acids, Bases, and Buffers • pH – A measure of the concentration of H+ in a solution. • The pH scale runs from 0 - 14. • Represents the negative log of the H+ concentration of a solution.
Acids, Bases, and Buffers Neutral solution with a pH of 7: [H+] = [OH-] Basic solution with a pH above 7: [H+] < [OH-] Acidic solution with a pH below 7: [H+] > [OH-]
Acids, Bases, and Buffers • Buffer:Molecules that prevent dramatic changes in the pH of fluids. • Remove H+ and OH- in solution and transfers them to other molecules. • Example: Bicarbonate Ion (HCO3-).
Organic Molecular Structure of Living Systems • Chemical evolution in the prebiotic environment produced simple organic compounds that ultimately formed the building blocks of cells. • Organic compounds contain carbon in the form of chains or rings and also contain hydrogen. • More than a million organic compounds are known.
Chemistry of Life • Recall the four major categories of biological macromolecules: • Carbohydrates • Lipids • Proteins • Nucleic acids
Carbohydrates • Carbohydrates are compounds of carbon (C), hydrogen (H) and oxygen (O). • Usually found 1C:2H:1O. • Usually grouped as H-C-OH. • Function as structural elements and as a source of chemical energy (ex. glucose).
Carbohydrates • Plants use water (H2O) and carbon dioxide (CO2) along with solar energy to manufacture carbohydrates in the process of photosynthesis. • 6CO2 +6H2O light C6H12O6 + 6O2 • Life depends on this reaction – it is the starting point for the formation of food.
Carbohydrates • Three classes of carbohydrates: • Monosaccharides – simple sugars • Disaccharides – double sugars • Polysaccharides – complex sugars
Monosaccharides • Monosaccharides – Single carbon chain 4-6 carbons. • Glucose C6H12O6 • Can be straight chain or a ring.
Monosaccharides • Some common monosaccharides:
Disaccharides • Disaccharides – Two simple sugars bonded together. • Water released • Sucrose = glucose + fructose • Lactose = glucose + galactose
Polysaccharides • Polysaccharides – Many simple sugars bonded together in long chains. • Starch is the common polymer in which sugar is usually stored in plants. • Glycogen is an important polymer for storing sugar in animals. • Found in liver and muscle cells – can be converted to glucose when needed. • Cellulose is the main structural carbohydrate in plants.
Lipids • Lipids are fatty substances. • Nonpolar – insoluble in water • Neutral fats • Phospholipids • Steroids
Neutral Fats • Neutral fats are the major fuel of animals. • Triglycerides – glycerol and 3 fatty acids
Neutral Fats • Saturated fatty acids occur when every carbon holds two hydrogen atoms. • Unsaturated fatty acids have two or more carbon atoms joined by double bonds.
Phospholipids • Phospholipids are important components of cell membranes. • They resemble triglycerides, except one fatty acid is replaced by phosphoric acid and an organic base. • The phosphate group is charged (polar).
Phospholipids • Amphiphiliccompounds are polar and water–soluble on one end and nonpolar on the other end. • They have a tendency to assemble themselves into semi-permeable membranes.
Steroids • Steroids are complex alcohols with fatlike properties. • Cholesterol • Vitamin D • Adrenocortical hormones • Sex hormones
Proteins • Proteins are large complex molecules composed of amino acids. • Amino acids linked by peptide bonds. • Two amino acids joined – dipeptide • Many amino acids – polypeptide chain
Proteins • There are 20 different types of amino acids.
Protein Structure • Proteins are complex molecules organized on many levels. • Primary structure – sequence of amino acids. • Secondary structure – helix or pleated sheet. Stabilized with H-bonds.
Protein Structure • Tertiary structure – 3-dimensional structure of folded chains. Eg. Disulfide bond is a covalent bond between sulfur atoms in two cysteine amino acids that are near each other. • Quaternary structure describes proteins with more than one polypeptide chain. Hemoglobin has four subunits.
Proteins • Proteins serve many functions. • Structural framework • Enzymes that serve as catalysts
Nucleic Acids • Nucleic acids are complex molecules with particular sequences of nitrogenous bases that encode genetic information. • The only molecules that can replicate themselves – with help from enzymes. • Deoxyribonucleic acid (DNA) • Ribonucleic acid (RNA)
Nucleic Acids • The repeated units, called nucleotides, each contain a sugar, a nitrogenous base, and a phosphate group.
Chemical Evolution • Life evolved from inanimate matter, with increasingly complex associations between molecules. • Life originated ~3.5 billion years ago.
Chemical Evolution • Origin of Life • Oparin-Haldane Hypothesis (1920s) • Alexander Oparin and J.B.S. Haldane proposed an explanation for the chemical evolution of life.
Chemical Evolution • Early atmosphere consisted of simple compounds: Water vapor Carbon Dioxide (CO2) Hydrogen Gas (H2) Methane (CH4) Ammonia (NH3) No free Oxygen Early atmosphere → Strongly Reducing
Chemical Evolution • Such conditions conducive to prebiotic synthesis of life. • Present atmosphere is strongly oxidizing. • Molecules necessary for life cannot be synthesized outside of the cells. • Not stable in the presence of O2
Chemical Evolution • Possible energy sources required for chemical reactions: • Lightning • UV Light • Heat from volcanoes
Simple Organic Molecules Complex Organic Molecules Cells Chemical Evolution • Simple inorganic molecules formed and began to accumulate in the early oceans. Over time:
Chemical Evolution • Prebiotic Synthesis of Small Organic Molecules • Stanley Miller and Harold Urey (1953) simulated the Oparin-Haldane hypothesis.
Chemical Evolution • Miller & Urey reconstructed the O2 free atmosphere they thought existed on the early Earth in the lab. • Circulated a mixture of H2 H2O CH4 NH3 Energy source: electrical spark to simulate lightening and UV radiation.
Chemical Evolution • Results: • In a week, 15% of the carbon in the mixture was converted to organic compoundssuch as: Amino Acids Urea Simple Fatty Acids
Chemical Evolution • Conclusion: life may have evolved in “primordial soup” of biological molecules formed in early Earth’s oceans.
Chemical Evolution • Today it is believed that the early atmosphere was only mildly reducing. • Still……if NH3 and CH4 are omitted from the mixture: • Organic compounds continue to be produced (smaller amount over a longer time period).
Chemical Evolution • More recent experiments: • Subjecting a reducing mixture of gases to a violent energy source produces: Formaldehyde Hydrogen Cyanide Cyanoacetylene • All highly reactive intermediate molecules Significance?
Chemical Evolution • All react with water and NH3 or N2 to produce a variety of organic compounds: Amino Acids, Fatty Acids, Urea, Sugars, Aldehydes, Purine and Pyrimidine Bases Subunits For Complex Organic Compounds.
Chemical Evolution • Formation of Polymers • The next stage of chemical evolution required the joining of amino acids, nitrogenous bases and sugars to form complex organic molecules. • Does not occur easily in dilute solutions. • Water tends to drive reactions toward decomposition by hydrolysis.
Chemical Evolution • Condensation reactions occur in aqueous environments and require enzymes.
Chemical Evolution • The strongest current hypothesis for prebiotic assembly of biologically important polymers suggests that they occurred within the boundaries of semi-permeable membranes. • Membranes were formed by amphiphilic molecules. • Meteorites are common sources of organic amphiphiles.
Origin of Living Systems • Life on Earth: 4 billion years ago • First cells would have been autonomous, membrane-bound units capable of self-replication requiring: Nucleic Acids • This causes a biological paradox. • How could nucleic acids appear without the enzymes to synthesize them? • How could enzymes exist withoutnucleicacids to direct their synthesis?