The Nature of the Living State

Macromolecules of Living Systems

Cellular Life

Energy Flow in Living Systems

The Instruction Set of Life

Cell Division



Macromolecules of Living Systems

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Saturated vs. Non-saturated Lipids
Nucleic Acids

Monomers and Polymers - monomers are building blocks, many times of the same type, that join together to form larger structures (polymers) which have properties that are more than just a collection of monomers.  A good analogy is that of bricks to a building.  Bricks would be the monomers which together form the building (polymer) which is much more than just a pile of bricks.  

Macromolecules - large molecules that directly make up the structures of the living cell.
    (1) Lipids - long term energy storage and cellular membrane structure
    (2) Carbohydrates - short term energy storage
    (3) Proteins - structural elements of cells; special class of functional proteins called enzymes
    (4) Nucleic Acids - genetic information transfer
        (A) DNA (Deoxyribose Nucleic Acid) - source of genetic information
        (B) RNA (Ribose Nucleic Acid) - information processing  


Lipids:  Fats (oils, waxes) made up of glycerol (a three carbon alcohol with three hydroxyl groups) and long fatty acid chains (basically long hydrocarbon chains which terminate in a carboxyl group).  The hydroxyl and carboxyl groups give the two parts of lipids limited solubility in water.  However, when lipids form, the carboxyl and hydroxyl groups line up, a water molecule is removed and up to three fatty acids can be joined to each glycerol molecule.  The resulting simple lipid is then devoid of hydrophilic reactive groups and is non-polar or hydrophobic.  Hydrophobic literally means "the fear of water" versus hydrophilic which literally means "the love of water."  There is a special kind of lipid called a phospholipid which has only two fatty acid chains, and in the third position, a hydrophilic group called a phosphate group.  The phospholipids are important in cellular membranes.  

There is also a difference between saturated versus non-saturated lipids.  Non-saturated fats are from plant origin versus saturated fats from animal origin.  In non-saturated fats, there are carbon-carbon double bonds in the fatty acid chains, and hence, less than the maximum number of hydrogens (non-saturated).  When humans ingest non-saturated fats (plant origin), there is additional chemical processing required before they can be incorporated into the body and this is believed to be nutritionally beneficial.  


Carbohydrates:   Sugars can be classified in two different ways:  (1) how many carbons are found in their carbon-based ring structures and (2) how many rings they contain.  The first classification scheme gives rise to the --ose series where the Latin based number of carbons plus the ending --ose divides the sugars into groups.  For instance, six carbon sugars are called hexoses while five carbon sugars are called pentoses.  The second classification scheme above gives rise to the  --saccharide series depending on how many rings are in the sugars.  One ring is called a monosaccharide, two rings a disaccharide, and many rings present are called polysaccharides.  

Glucose is a hexose, monosaccharide (a single, 6 carbon ring) and is the most important sugar in energy yielding metabolism.  Most other sugars are first converted to glucose before being broken down to yield energy.  There are also two pentose, monosaccharides important in the construction of DNA and RNA -- deoxyribose sugar in DNA and ribose sugar in RNA  (see below).  

Glucose molecules (monomer) can be joined together by chemical bonding to form larger polysaccharides (polymers).  Important glucose based polysaccharides are (1) starches which temporarily store excess sugars in plants and (2) glycogen which does the same in animals.  The third (3) is cellulose which is important in constructing plant cell walls (an external, rigid structure surrounding the cells of higher plants).  


Proteins:  Proteins are polymers of a type of molecule called an amino acid.  Each amino acid has a carboxyl group on one end connected to an interior carbon (alpha carbon) and an amino group on the other end.  In addition, some other chemical group referred to as an R group, is also attached to the alpha carbon (see books for details).  There are only about 30 or so common amino acids found in nature and they vary according to differences in their R groups.  Proteins exist at at least three, and sometimes four, levels of organization.  


At the first level of organization, amino acids join together by chemically bonding (called a peptide bond) an amino group of one amino acid to the carboxyl group of another to form a long chain of amino acids called a polypeptide chain.  At the second level, the polypeptide chains fold into either an alpha helical structure or a beta pleated sheet form.  This second level folding is due largely to hydrogen bond formation internally within the molecule.  And for all proteins, the alpha helix or beta pleated sheets fold on themselves to form the maximum number of additional internal interactions including ionic, covalent, hydrogen bonding and non-polar/non-polar interactions.  The final 3D structure is largely determined by which type of amino acid appear in which position along the polypeptide chain.  This is the third level of organization of a protein.  The sequence of occurrence of amino acids along the polypeptide chain is the basis for the difference between different proteins.  

Some larger proteins (for example, hemoglobin) are made of several distinct polypeptide chains (called subunits) held together in specific ways to form an overall 3D structure.  The orientation of the subunits constitutes the fourth level of organization of those proteins which have subunits.  


Nucleic Acids:  Nucleic acids, both DNA and RNA are polymers of a type of molecule called a nucleotide.  Each nucleotide is made up of three types of molecules: (1) a sugar (deoxyribose in DNA and ribose in RNA), (2) a phosphate group and (3) heterocyclic bases (ring compounds whose rings are made up of both carbon and nitrogen).  Each type of nucleic acid has the bases adenine (symbol=A), cytosine (symbol=C) and guanine (symbol=G).  DNA has a additional base called thymine (symbol=T) while RNA has instead uracil (symbol=U).  Each nucleotide has the base joined to the sugar which in turn is joined to the phosphate group.  The nucleotides then join together by covalent chemical bonds between the sugar of one nucleotide to the phosphate group of the next forming polynucleotide chains.  

RNA has only a single polynucleotide chain.  The chain folds on itself so that it forms the maximum number of C-G pairs and A-U pairs giving an overall 3D structure.  Such pairing is known as complementary base pairing and is held together by hydrogen bonding (2/each A-U pair and 3/each C-G pair).  Once folded, the RNA molecule is complete.  

DNA has two separate polynucleotide chains situated side-by-side.  The bases of one chain point inward to oppose the bases of the other chain.  They are held together by A-T and C-G pairing, and therefore, each chain is the complementary mirror image of the other.  The final step in forming DNA is that both chains or strands twist to form the famous "alpha helix" structure (like a spring or coil).  The turns of the coil are held together by hydrogen bond formation.  

The exact sequence of specific bases along the polynucleotide chains in both DNA and RNA represent the way in which genetic information is encoded in these molecules.