PinkMonkey Online Study Guide-Biology
3.2 Structure of cell
Both prokaryotic and eukaryotic cells possess the basic
features of a plasma membrane and cytoplasm. The plasma membrane is the
outermost surface of the cell which separates the cell from the environment.
The cytoplasm is the aqueous content within the plasma membrane.
Plasma membrane : It is like any other
membrane in the cell but it plays a very important function. It
forms the border of a cell, so it is also called the cell membrane.
It is primarily composed of proteins and phosphalipid. The phospholipids
occur in two layers referred to as a bilayer. Protein is
embedded within the lipid layer, or attached to the surface of it.
The plasma membrane is elastic and very fluid because of protein
and lipid. Normally the function of the plasma membrane is that
of a gate-keeper. It allows certain important substances to enter
and exit the cell.
Cytoplasm and organelles : The cytoplasm
is a semi-solid substance which is present in the cell and which gives
structure, size, shape and foundation to the cell. It is enclosed by the
plasma membrane. Within the cytoplasm are a number of microscopic bodies
called organelles that perform various functions essential for the survival
of the cell.
Figure 3.2 Endoplasmic reticulum with the nucleus and
the Golgi complex
Endoplasmic reticulum (ER) : is one of the important
organelles present in the cytoplasm. Endoplasmic reticulum is a series
of membranes which extend throughout the cytoplasm in eukaryotic cells.
In certain cases in ER there are submicroscopic bodies called ribosomes
which are involved in production of protiens.
Rough ER : In this kind of ER the ribosomes are presenton
the surface.The endoplasmic reticulum is responsible for protein synthesis
in a cell. Ribosomes are suborganelles in which the amino acids are actually
bound together to form proteins. There are spaces within the folds of
ER membrane are known as cisternae.
Smooth ER : This type of ER does not have ribosomes.
Another organelle is the Golgi body or Golgi apparatus
(G.A.). The Golgi body is a series of flattened sacs usually curled
at the edges. Proteins which have formed in ER are processed in G.A. After
processing, the final product is discharged form the G.A. At this time
the G.A. bulges and breaks away to form a dropline vesicle known as secretory
vesicles. The vesicles move butward to the cell membrane and either insert
their protien contents in the membrane, or release their contents outside
There is another organelle which is related to the Golgi
apparatus called the lysosome. The lysosome is derived from the
Golgi body. It is a sac of enzymes in the cytoplasm, used for digestion
within the cell. These enzymes break down particles of food taken into
the cell and make the food product available for use. There are also cytoplasmic
organelles called peroxisomes in the cell which produces the enzymes to