BIOL 201 - Lecture 3
Biomolecules are compounds of carbon
Relative Abundance of Atoms
In Earth's Crust In Organisms (As % Dry Weight)
Oxygen (O) 47 Carbon (C) 50
Silicon (Si) 28 Oxygen (O) 20
Aluminum (Al) 8 Hydrogen (H) 10
Iron (Fe) 4.5 Nitrogen (N) 10
Hydrogen (H) 0.22 Phosphorus (P) 4
Carbon (C) 0.19 Sulfur (S) 1
Why is carbon so prevalent in biological systems?
Carbon has an unparalleled versatility in forming stable covalent bonds by e- pair sharing
Tetrahedral carbon atom: 4 such bonds per carbon atom gives rise to an enormous variation in molecular architecture
Linear, branched and cyclic (ring) structures
Simple examples: n-butane, isobutane, benzene
Functional groups: Parts of molecules (ensembles of atoms) that are often involved in molecular interactions and biochemical reactions (see Figure 2.23)
Collectively, these properties of carbon give rise to virtually
endless possibilities for the composition and structure of organic molecules.
Despite this enormous potential, learning the structures of biomolecules is a relatively easy matter because of the hierarchical nature of biological organization.
For our purposes, we can define a"core" set of 33 or so basic biomolecules from which all others are derived and to which all others may be related:
20 Amino Acids: L-a -amino acids 20
Fundamental structure (variation is in R groups, as we shall see)
Amino acids have 4 different substituents on the a -C atoms; therefore, they come in 2 different optical isomers - D and L - that are mirror images (enantiomers) of one another.
5 Nitrogenous Bases: heterocyclic aromatic ring systems 25
3 are pyrimidines
Note the numbering system
2 are purines - Adenine (A) and guanine (G)
Note the numbering system
2 Sugars (ose endings) 27
Both are aldoses; one (glucose) is an aldohexose (6 carbons), the other is an aldopentose (5 carbons)
Glucose (Glc ) Ribose (Rib)
These sugars may form 6-membered rings (pyranose structures) or 5-membered rings (furanose) when in aqueous solution:
It is easy to see why sugars form such structures in solution.
1 Fatty Acid 28
Palmitic Acid, a C-16 fatty acid. The many other fatty acids differ in number of carbon atoms (chain length) or in the presence of one (or more) double bonds (degree of unsaturation).
1 3-Carbon Alcohol 29
Glycerol
1 N-Containing Alcohol 30
Choline - an important component of phospholipids
Phosphate (either as the free inorganic anion 31
or as a functional group)
The free anion is symbolized as Pi
Isoprene 32
A 5-carbon building block for steroids
Cholesterol 33
The most abundant lipid in animal cell membranes and the precursor to formation of steriod hormones, such as estrogen and progesterone
Hierarchy of Organization in the 4 Major Classes of Biomolecules
Carbohydrates Lipids Proteins Nucleic Acids
Polymers: Covalent assemblies of monomers
Polysaccharides (Glycans) Proteins Nucleic Acids
Membranes are noncovalent assemblies of lipid (and protein) molecules.
Biological Polymers: Unifying Rules of Biological Relevance
1. Building blocks polymerize by dehydration synthesis (condensation) reactions:
Elements of water (H- and -OH) removed on bond formation
Further, polymers are degraded by hydrolysis ("splitting by water")
2. Building blocks have structural polarity; that is,"heads" and"tails"
Polymerization involves head-to-tail condensation
3. Polymers have a sense or direction
as in the N ® C direction of peptides and proteins
4. Because of their sense and variation in monomer sequence, polymers may be informational.
sequence of monomeric units can represent encoded information, as in proteins (amino acid sequence) or nucleic acids (nucleotide sequence), but NOT polysaccharides.