Ideas about cell structure have changed considerably over the years.
Early biologists saw cells as simple membranous sacs containing
fluid and a few floating particles. Today's biologists know that
cells are infinitely more complex than this.
There are many different types, sizes, and shapes of cells in the
body. For descriptive purposes, the concept of a "generalized cell"
is introduced. It includes features from all cell types. A cell consists
of three parts: the cell membrane, the nucleus, and between the two,
the cytoplasm. Within the cytoplasm lie intricate arrangements of
fine fibers and hundreds or even thousands of miniscule but distinct
structures called organelles.
Cell membrane
Every cell in the body is enclosed by a cell (Plasma)
membrane. The cell membrane separates the material outside the cell,
extracellular, from the material inside the cell, intracellular.
It maintains the integrity of a cell and controls passage of materials
into and out of the cell. All materials within a cell must have
access to the cell membrane (the cell's boundary) for the needed
exchange.
The cell membrane is a double layer of phospholipid molecules.
Proteins
in the cell membrane provide structural support, form channels for
passage of materials, act as receptor sites, function as carrier
molecules, and provide identification markers.
Nucleus and Nucleolus
The nucleus, formed by a nuclear membrane around a fluid nucleoplasm,
is the control center of the cell. Threads of chromatin
in the nucleus contain deoxyribonucleic acid (DNA), the genetic
material of the cell. The nucleolus is a dense region of ribonucleic
acid (RNA) in the nucleus and is the site of ribosome formation.
The nucleus determines how the cell will function, as well as the
basic structure of that cell.
Cytoplasm
The cytoplasm is the gel-like fluid inside the cell. It is the
medium for chemical reaction. It provides a platform upon which
other organelles can operate within the cell. All of the functions
for cell expansion, growth and replication are carried out in the
cytoplasm of a cell. Within the cytoplasm, materials move by diffusion,
a physical process that can work only for short distances.
Cytoplasmic organelles
Cytoplasmic organelles are "little organs" that are suspended in
the cytoplasm of the cell. Each type of organelle has a definite
structure and a specific role in the function of the cell. Examples
of cytoplasmic organelles are mitochondrion,
ribosomes,
endoplasmic
reticulum, golgi
apparatus, and lysosomes.
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