Biological Macromolecules

Carbohydrates

  • Carbohydrates: a diverse group of molecules that are mainly composed of carbon, hydrogen and oxygen in the chemical arrangement [Cn(H2O)n].
  • The number and arrangement of these atoms determines the kind or class or carbohydrate.
  • The simplest carbohydrates (monosaccharide and some disaccharides) often have free aldehyde or ketone functional groups.
  • In more complex polysaccharides these groups are no longer “free”.
  • This is a useful property of monosaccharides since it is often desirable to know the concentration of simple sugars in a biological sample.
  • Benedict’s Test (reducing sugar test): a test that can determine if a carbohydrate has a free aldehyde or ketone functional group.
    • If it does, electrons from the functional group may be donated to another chemical.
    • Reducing sugars: the carbohydrates that can donate electrons
    • The electrons from the free aldehyde can reduce another chemical.
    • The Benedict’s reagent contains sodium bicarbonate, sodium citrate and cupric sulfate.
    • Under sufficiently alkaline pH the reducing sugar will give up an electron and reduce a weak oxidizing agent like Cu2+ ion of the cupric sulfate to give the monovalent form of copper ion, Cu+, in the form of cuprous oxide (Cu2O).
    • This will form a reddish brown to green precipitate depending upon the concentration of reducing sugar.
  • Iodine Test: used to determine if a sample contains starch.
    • Starch: one of several polymers of glucose that are widely found in living organisms.
    • Other common polymers of glucose are cellulose and glycogen.
    • In each case the individual glucose units are held together by glycosidic bonds.
    • Iodine binds to the starch because of its structure

Proteins

  • Proteins serve many cellular functions.
    • They can be enzymes, help determine the shape of cells, be involved in movement of cells and the movements of material inside of cells, be involved in recognition functions, information transduction, membrane function and many other cellular functions.
  • Basic structure of proteins: polymers of amino acids where the adjacent amino acids are connected by peptide bonds.
  • Amino acids contain two functional groups: the amino group and the carboxyl group that are bonded to the central alpha carbon.
  • The side chains of the amino acids differ.
  • The structure and properties of the amino acid side chains play a very important role in determining the structure and function of proteins.
  • Biuret Test: detects and quantitate proteins; involves treating a protein sample with an alkaline cupric sulfate.
    • Compounds having at least two peptide bonds will complex with Cu++ ion and produce a blue color.

Nucleic Acids

  • Two types of nucleic acids: DNA and RNA
    • Both are polymers of nucleotides
  • DNA: deoxyribonucleic acid; the sugar is a deoxyribose
  • RNA: ribonucleic acid; the pentose sugar is a ribose sugar
  • The basic components of nucleotides: a nitrogenous base bonded to a pentose sugar, which is in turn bonded to a phosphate functional group.
  • In nucleic acids, the sugars of adjacent nucleotides are connected by phosphodiester bonds
  • The nitrogenous bases of RNA can be adenine, cytosine, guanine or uracil.
  • DNA does not normally contain uracil but instead has thymine.
  • Dische’s Test: can be used to specifically detect the presence of DNA in a sample because deoxyribose will react with the diphenylamine in the Dische’s reagent, giving a deep blue to purple color, but ribose will not.
    • In order for the reaction of deoxyribose with diphenylamine to occur, the DNA must first be degraded with very strong acid.
  • Orcinol test: the Orcinol reagent will react specifically with the ribose sugar of RNA to give a green color.
    • In order for the reaction to occur the RNA must first be degraded by strong acid.

Lipids

  • Lipids: a very structurally diverse group of molecules that are characterized by being hydrophobic.
  • Hydrophobic substances will not dissolve in water, or they dissolve very poorly.
  • Hydrocarbon region: the hydrophobic regions of lipids; generally composed of just hydrogen and carbon
  • Many lipids, although they are predominately hydrophobic, have functional groups that are hydrophilic.
    • These hydrophilic regions can be the hydroxyl functional group, the phosphate functional group, the carboxyl functional group and others.
  • Sudan III Test: takes advantage of the fact that one hydrophobic substance will dissolve in another hydrophobic substance.
    • The dye, Sudan III, is a red hydrophobic dye that will dissolve in other hydrophobic substances.