19.
CELLULAR PROTEIN SYNTHESIS
Protein synthesis takes place on the surface of the ribosomes. The coded
message is transcribed from the DNA to form messenger RNA (mRNA) in a process
called transcription. The mRNA leaves the nucleus and attaches itself to the ribosomes.
Each mRNA molecule is coded to establish the sequence of amino acids in at least one
polypeptide chain. However, a smaller type of RNA, called transfer RNA, carries a
specific amino acid and is capable of reading the amino acid code of the mRNA, is
required to position each amino acid in the sequence dictated by the mRNA. This
process is called translation. Once the amino acids are in the proper position, they
combine to form peptide bonds with one another and at the same time detach
themselves from the transfer RNA (tRNA) molecules. These tRNA molecules are then
free to attach themselves to new amino acids that, in turn, are assembled in sequence
on the mRNA.
110. DIFFUSION
The continuous movement of molecules among each other in liquids or in gases
is called diffusion; the diffusion of solute particles is from an area of greater
concentration to an area of lower concentration. The following are related concepts:
a. Brownian Movement. All molecules and ions in the fluids of the body are in
constant motion. Each particle in solution moves in its own particular way. In 1828,
Robert Brown observed under the microscope that during germination, pollen grains
were in constant motion. The motion is now called Brownian movement, and it occurs
in all types of fluids including those of the human body. Brownian movement is due to
the collisions of the molecules of the dispersion medium (usually a fluid), against
colloidal particles. This motion can sometimes be seen occurring inside a living cell.
Brownian movement depends on the size of the particles and the viscosity of the
medium. Rapidity of the movement is proportional to the temperature; the higher the
temperature, the more rapid is the agitation and the greater the movement of the
molecules.
b. Molecular Collisions. The motion of molecules never ceases. When a
moving molecule hits another, it propels the second molecule. This striking of a second
molecule decreases the energy of the first molecule and adds energy to the second
molecule. Now the second molecule moves more rapidly than before, while the first
molecule slows down in speed of movement. Molecules can be propelled first in one
direction and then immediately in another. Figure 14 shows the movement of a single
molecule during a fraction of a second. It shows the molecule being bounced off other
molecules. Ions, molecules, and colloids diffuse in a similar manner; however, heavier
particles diffuse more slowly than lighter ones.
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