Although this description of cellular components is about
a general eukaryotic cell, it applies to most cells. Of course,
there are some differences between liver cells and fat cells
and other cells. The chemical composition by weight in the cell
is: water (70%), proteins (15%), nucleic acid (7%), carbohydrates
(3%), lipid (2%), inorganic minerals (1%), and miscellaneous
organic molecules (2%).
Eukaryotic cells are composed of distinct subcellular particulate
bodies called organelles surrounded by a membrane. These organelles
include the cell nucleus, mitochondria, lysosomes, peroxisome,
endoplasmic reticulum, and Golgi apparatus. A simplified sketch
of the cell is shown in Figure 2.
The nucleus of the cell is the largest and most conspicuous
organelle. The nuclear membrane is a double membrane with pores
which allow the exit of large RNA molecules. Inside of the nucleus
in the nucleoplasm, there are areas rich in RNA and other areas
containing the DNA chromosomes which store genetic information.
The endoplasmic reticulum is a netlike system localized in
the cytoplasm of the cell. Two types are known: rough endoplasmic
reticulum (RER) with small granules attached and smooth endoplasmic
reticulum (SER) with no granules. The RER contains ribosomes
of RNA and enzymes necessary for the synthesis of proteins that
are destined to be used outside of the cell. The SER participates
in packaging of the proteins in small vesicles and transporting
them out of the cell.
The Golgi apparatus also packages proteins to be transported
out of the cell. In addition, this may be the site for the biosynthesis
of complex carbohydrate materials.
Lysosomes contain enzymes which can degrade or hydrolyze other
proteins so that the cell can use the products. Peroxisomes contain
enzymes which catalyze the conversion of toxic hydrogen peroxide
to water and oxygen.
The cell membrane regulates the passage of substances into
and out of the cell. Most membrane functions involve the participation
of highly specific receptor proteins which operate by binding
a specific substance on one side of the membrane. This event
triggers other events in the membrane and the cell which in turn
signals the cell to do (or not do) something.