Thermal Molecular Movement

The disadvantage of the method of illustrating structures of organic molecules by using mechanical models composed of spheres and connecting wire axes, as well as of computer simulations of molecular structures, is their suggestion of a more or less static point of view. In reality, however, in liquids as well as liquid-crystalline phases, the molecules are always in a state of vigorous thermal motion. The following three forms of molecular movements are to be considered: vibrations, rotations, and translations.

Vibrations are oscillations in the binding distances between the atoms in a molecule. The term rotations means not only the rotation of the whole molecule, but additionally, the spin of individual atoms or atomic groups around the axes of their bonds. Of course in this case the intramolecular interactions of the atoms are to be considered. So, for example, considering a simple molecule of ethane, the rotation around the CH₃—CH₃ bond cannot be homogeneous. Energy minima of rotations occur periodically every 120°. Hence, rotation is "locked" at certain angels of low energy levels.

A covalent bound, rotating around its angle relative to another, consequently describes a cone. The next bond rotates, as well, but it does so quasi on the spot, moving itself in an orbit of that lateral cone surface. In this way chain molecules can assume a stochastic orientation unless other strong attracting or repelling interactions between the monomers are preventing this. The molecular structures resulting in this way can be calculated by the laws of statistical thermodynamics.

The so-called random flight model allows the simplest description of the behavior of a long molecule. In this case, the real molecule is described as a sequence of segments with length l. These segments are connected to each other in such a way that the orientation of each is independent from the orientation of the former segment. This random flight chain is characterized by the length (l) and the number (n) of its segments. The positions of the end points of the segments can be expressed as vectors in a polar coordinate system which is based on the center of gravity of the molecule. The distance between the endpoints of such a molecule (d_max) at maximal elongation is: d_max = nl.

Put the following words and word combinations into the gaps

disadvantages / composed of / wires / axis / liquid / spin / homogeneous / angles / repel

In the pns, both myelinated and unmyelinated axons are bundled together, much like ______________ in a cable, to form nerves.

Plant cell suspension culture involves the growth of single or small groups of plant cells in a ______________ growth medium.

One of the ______________ of external gills is that they must constantly be moved or the surrounding water becomes depleted in oxygen.

Clearly polymorphism can appear within an initially uniform bacterial population growing in a simple ______________ environment.

Hemoglobin is a protein ______________ four polypeptide chains and four organic compounds called heme groups.

The polar water molecules ______________ the long nonpolar tails of the phospholipids as the water molecules seek partners for hydrogen bonding.

Electrons ______________ around the nucleus a far distance away from the nucleus.

An imaginary plane perpendicular to the ______________ of the spindle that passes through this circle is called the metaphase plate.

Centrioles occur in pairs, usually located at right ______________ to each other near the nuclear membranes.