Molecules of Life

Four major classes of biological molecules:

Class of Molecule Composition Role in Human Body
Lipids C, H, O (N, P) Membrane Structure, Fuel, Fuel Storage
Carbohydrates (CHO's) C, H, O (N) Fuel, Fuel Storage, Cell-Cell Recognition
Proteins C, H, O, N (S) Enzymes, Tissue Structure, Fuel
Nucleic Acids (DNA, RNA) C, H, O, N, P Genetic Information Storage and Transfer

Lipids, Carbohydrates, Proteins, Nucleic Acids


1. Soluble only in hydrophobic (non-polar) solvents.

2. Ester bonds....

There are several subclasses of lipids:

1. Triacylglycerols (fats)

Triglycerols are formed via Ester Bonds

2. Phospholipids

Have a tendency to form lipid bilayers (biological membranes).

Phosphoester and Phospholipid Formation

Biological membranes (products of phospholipid formation). Like attracts like. "Hydrophobic" tails inside together, "hydrophilic" heads outside (H-bonding w/ water). Proteins: membrane spanning (channels), surface.

3. Steroids- Cholesterol (rings, hydroxyl)

Carbohydrates: or sugars

1. Water soluble

2. Glucose is the most abundant, occurs as a free sugar or as a polymer (glycogen) for storage

3. Structure of D-glucose (open chain, ring, H-bonding)


1. Polymers of amino acids

2. Enzymes are proteins, catalysts of biological reactions

Nucleic Acids:

1. DNA / RNA

2. Information storage and transfer

More about Water:

Can water be considered an acid because of its ability to shed a proton:

H-O-H --> H+ + OH-

The frequecy with which this occurs in pure water is very fact only one water molecule in 500 million will be ionized at any time. Thus,

the molar concentrations of H+ and OH- are approx. 10-7 (divide the total molar concentration of water, which is ~55M by 500 million)

Special mathematical notation, by convention, the concentrations of H+ and OH- are expressed as:

-log [H+] = -log [10-7] = 7 = pH of pure water also,

[H+] [OH-] = 10-14 this allows us to calculate the [OH-] if [H+] is known


Compounds capable of giving up a proton in solution. Strong acids such as HCl almost always exist in ionized form.

Most biological acids are significantly weaker than HCl. A good example is carboxylic acids. Since the difference in electronegativity is not as dramatic as in HCl, the tendancy of a carboxylic acid to give up its proton is much less.

Because of the great variety of craboxylic acids their tendancies to donate protons to solution also vary, so a mathematical decription like that for pH was developed called pKa.

The pKa is the pH at which the acid is half protonated and half deprotonated.


CH3COOH(acetic acid) ----> CH3COO-(acetate ion) + H+, pKa = 4.8,

meaning that at pH = 4.8, the pool of acetic acid molecules exists half ionized.

In general, a way to describe all weak acids:

HA ---> A- + H+, pKa = ?? (dependent on electronegativity of A)

At pH values other than the pKa, the ratio of protonated to deprotonated or ionized form is something other than one. A convenient equation to calculate this ratio given the pH is the Henderson-Hasselbach equation:

pH = pKa + log [A-] / [HA]

This equation fits the definition of pKa ;

if pH = pKa, then log [A-] / [HA] = 0, and [A-] / [HA] = 1.

Other solutions to the Henderson-Hasselbach equation over a range of pH values can be displayed graphically.

-More than a pH unit below the pKa value, the acid molecules are mostly protonated. (charge = 0).

-More than a pH unit above the pKa, the acid molecules are mainly deprotonated. (charge = -1).

-Within a pH unit of the pKa value, the acid molecules are partially charged.

-Within a pH unit of the pKa value, the acid tends to resist a change in pH (the solution is buffered).

© Dr. Noel Sturm 2020

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