History 1. Spontaneous generation was a belief that frogs could arise from earth, mice from rotten matter, etc. In , Louis Pasteur demonstrated sterilized broth in flasks, even exposed to air, could not spontaneously ferment. Figure 2. Life on earth depends on the properties of water. Water has a high specific heat capacity; this ability protects organisms from extreme thermal fluctuations.
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History 1. Spontaneous generation was a belief that frogs could arise from earth, mice from rotten matter, etc. In , Louis Pasteur demonstrated sterilized broth in flasks, even exposed to air, could not spontaneously ferment. Figure 2. Life on earth depends on the properties of water. Water has a high specific heat capacity; this ability protects organisms from extreme thermal fluctuations. Water has a high heat of vaporization; this allows terrestrial organisms to cool themselves by removing excess heat.
Water has a unique density behavior; ice is less dense than liquid water. Water has a high surface tension that lends to its great cohesiveness. Water has a low viscosity.
Water is an excellent solvent. Carbon 1. Organic compounds contain carbon; most are produced in living systems. Over one million carbon-based molecules have been identified. Carbohydrates provide structural elements and store energy. Glucose is commonly found in the blood of animals and is an important immediate energy source for cells.
Cellulose occurs in greater quantities than all other organic materials combined. Carbohydrates, synthesized by plants by photosynthesis, are the starting point of food chains. Monosaccharides a. Monosaccharides are simple sugars with a carbon backbone of four, five or six carbon atoms. Glucose, galactose and fructose all contain free sugar groups. The hexose glucose is particularly important in life. Disaccharides Figures 2. Disaccharides contain two monosaccharides bonded together.
Maltose is formed from binding two glucose molecules and removing one water molecule. Sucrose table sugar is a linkage of glucose to fructose. Lactose milk sugar is a linkage of glucose and galactose. Polysaccharides a. Polysaccharides are chains of glucose molecules called polymers. Most have the formula C6H10O5 n where n is the number of simple sugar subunits.
Glycogen is a polymer of glucose; found in vertebrate liver and muscle cells, it is storage carbohydrate of animals. Cellulose is the principal structural carbohydrate of plants. Lipids: Fuel Storage and Building Material 1. Lipids are fats and fat-like substances. Lipids have low polarity; therefore they are insoluble in water but soluble in organic solvents. Triglycerides Figure 2. Stored fats are derived directly or converted from carbohydrates; they are the major animal fuels. Triglycerides consist of glycerol and three molecules of fatty acids.
Neutral fats are esters, combining alcohol and an acid. When every carbon in a chain is bonded to two hydrogen atoms, it is saturated. Unsaturated fatty acids, common in plant oils, have two or more carbon atoms joined by double bonds. Phospholipids Figure 2. Phospholipids have a structural role in molecular organization of tissues and membranes. They resemble triglycerides with one fatty acid replaced by phosphoric acid and an organic base.
Lecithin is an important phospholipid of nerve membranes. The phosphate group is charged and therefore polar; the rest of the molecule is nonpolar, so phospholipids can bridge both environments. The term amphiphilic describes compounds, like phospholipids, that are polar and watersoluble on one end and non-polar on the other end. Steroids Figure 2. Steroids are complex alcohols with fat-like properties.
They are biologically important. Steroids include cholesterol, vitamin D, adrenocortical hormones and sex hormones. Amino Acids and Proteins 1. Proteins are large molecules composed of 20 kinds of amino acids. Amino acids are joined by peptide bonds.
Two amino acids and a peptide bond form a dipeptide. With one free amino group on one end and a free carboxyl on the other, additional amino acids can be joined to form a long chain of enormous variety. Levels of Protein Structure Figure 2. Primary structure is the sequence of amino acids in the polypeptide chain.
Secondary structure comes from the bond angles of the sequence: alpha-helix and beta sheets. Bending and folding of secondary structures forms the tertiary structure, often stabilized by disulfide, hydrogen, ionic and hydrophobic bonds.
Quaternary structure occurs when several polypeptide chains form subunits of a huge protein molecule, as in hemoglobin. Proteins form much of the framework of the cytoplasm and organelles. Proteins function as enzymes to catalyze most reactions; cell biology can be studied as protein biology. Nucleic Acids 1.
Nucleic acids are complex polymeric molecules. Sequence of nitrogenous bases encodes genetic information for inheritance. They store directions for synthesis of enzymes and other proteins. They are the only molecules that can replicate themselves. DNA is deoxyribonucleic acid. RNA is ribonucleic acid. Both DNA and RNA are polymers of repeated units called nucleotides, each containing a sugar, a nitrogenous base and a phosphate group.
Chemical Evolution A. Oparin-Haldane Hypothesis 1. Aleksander Oparin and J. Haldane independently proposed a hypothesis of chemical evolution. They proposed the early atmosphere consisted of simple compounds: water, carbon dioxide, hydrogen gas, methane and ammonia, but lacked oxygen. The compounds necessary for life are not synthesized outside cells nor are they stable in the presence of oxygen.
Rock evidence indicates virtually no atmospheric oxygen at earliest times; this provided a reducing atmosphere. Both methane CH4 and ammonia NH3 are fully reduced compounds. Such an atmosphere, with variations in heat and high radiation, was conducive to prebiotic synthesis but unsuited to modern life forms.
Many chemicals would not react without a continuous source of free energy to produce a reaction. Electrical discharges in lightning today produce a large amount of organic matter. Many compounds related to life were formed, including four of the amino acids, urea and several simple fatty acids. Omitting ammonia and methane resulted in smaller amounts of compounds and required longer time periods.
More recent experiments have clarified the sequences leading through formaldehyde, hydrogen cyanide, cyanoacetylene, etc. The finding of amino acids in meteorites provides additional evidence for their natural abiotic synthesis. Formation of Polymers 1. The next state required condensation of amino acids, nitrogenous bases and sugars. These polymerizations are condensation dehydration reactions 3. Water tends to drive reactions toward decomposition by hydrolysis.
In living systems, condensation reactions occur in aqueous cellular environments with enzymes. Without enzymes and ATP energy, macromolecules soon decompose. The strongest hypothesis for prebiotic assembly of biologically important polymers is that they occurred within the boundaries of semi-permeable membranes formed from small amphiphilic molecules. Amphiphiles extracted from the Murchison meteoriteform membranous vesicles in aqueous solutions Figure 2. Origin of Living Systems A. Self-replicating Systems 1.
Fossils date to 3. Protocells would have been autonomous, membrane-bound units with functional organization that permitted self-reproduction. On top of previous chemical evolution, nucleic acids were needed as simple genetic systems.
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