THE CELL
Essentials of Histology 
Life on Earth may have emerged from an ancestor cell more than 3 billion
years ago. Through evolutionary processes this cell gave rise to thousands
of different types of others cells, that together, changed all the environment.
Changes in the water, soil and atmosphere took place and made Earth a suitable
place to live in. Approximately 1.5 billion years ago, a very important
event came about: the very simple cells that once have changed all the
surrounding - the procaryotic cells, such as bacteria - turned through
different kinds of processes into more complexes cells, with several internal
structures - the eucaryotic cells- that now are the constituents of higher
animals and plants.
Bacteria
are the simplest organisms found in nature. They are spherical or rod-shaped
and their size varies from 1 to several micrometers in linear dimension.
They possess a protective coat called cell wall, beneath which a plasma
membrane encloses a cytoplasm compartment containing DNA, RNA, proteins
and small molecules. The bacterial genetic material is not surrounded by
membranes, like in the eucaryotic cells. Besides, the procaryotic cells
do not have membrane bounded organelles.
The Plasma
Membrane is the responsible by the delimitation of the boundaries of each
cell, helping in the maintenance of its individuality and allowing its
communication with neighboring cells. This last characteristic is very
important to the support of systems and tissues of animal and higher plants.
It is composed mainly by amphipatic phospholipids and proteins.
The eucaryotic
cells, in contrast with the procaryotics, present a nucleus (caryon in
Greek), which contains most of the cell's DNA, enclosed by a double layer
of membrane. This membrane contains pores (pore complex) that allow the
selective communication through the nucleous and the cytoplasm.
The cytoplams
(or cytosol) is where most of the cell's metabolic processes occur. In
the cytoplasm of eucaryotic cells one may find several organelles that
realize specific functions. And to do that, they count on different chemical
composition and enzymes.
The mithocondrion
and the chloroplast are delimited by a double layer of membrane. The mithocondrion
is found in most of the eucaryotic cells and is the responsible for the
oxidation of small molecules and the subsequent energy production in the
form of ATP. The chloroplast is found only in eucaryotic organisms capable
of photosynthesis, that is, it is not found in fungus and animals.
The cytoplasm
of eucaryotic cells has a complex system of membranes, that will constitute
others organelles. The Endoplasmic Reticullum, for example, is a membrane
bounded organelle where occurs the protein and lipids synthesis. The Rough
Endoplasmic Reticullum (RER) presents as a collection of flatted vesicles
with ribosomes adhered in its surface, responsible for the protein synthesis.
The Smooth Endoplasmic Reticullum, is tubular and does not presents ribosomes.
It is involved in the synthesis and metabolism of lipids. The Golgi Coplex
or Golgi Apparatus is formed by a system of stacked, membrane bounded vesicles,
which participate in the modification and transport of molecules.
Others membrane bounded organelles, are the lysosomes, responsible for
the intracellular digestion and the peroxisomes and glyoxisomes, responsible
for the metabolism of hydrogen peroxide. The membranes present specific
characteristics depending on the kind of organelle they will bound. They
still can form small vesicles and huge vacuoles, such as those found in
plant cells.
The plant cells
present a thick cell wall, surrounding a plasma membrane. The cell
wall is composed by cellulose and polysaccharides and is completely different
of bacterial cell wall.
To constitute
a complex organism such ours, with systems and organs completely different,
the cells underwent differentiation, to say, they had to adapt to different
functions. For example, the plasma membrane of the renal epithelium presents
different characteristics from the membrane of cells of the nervous system.
Not only the plasma membrane, but all the organelles work in different
ways depending on which cell and tissue they are.
