Download Dentistry college - first class Medical biology

Dentistry college - first class
Medical biology- Lec.8
Cell biology : is a branch of biology that deals with the study of cells from
morphology , structure , function and biochemical point of views . The term of cell
was first introduced by English scientist, Robert Hook in 1665 and in 1957 cells
were classified into prokaryotic and eukaryotic cells , the body of eukaryotic cells
may consist of millions of cells are different in many things such as in size ,shape,
life span , number, function and structure as fallow :
Size of cell
The cells vary widely in size from minute as in bacterial cell that reach
microns to large cells like egg of fishes and birds which can easily seen with
naked eyes , the largest egg cell is that of Ostrich which can be as long as 170mm
, but certain nerve cells have been found to have the length of 3 -3.5 feet, among
the reasons which leads to this huge variation is the difference in the amount of
non living material contained such as stored food products and cellular
excretions .
Shape of cell
The shape of the cells are highly variable , the bacterial cell could be rod ,
cocci or spiral shape ,the different cells in multicellular organisms are flat or
sequamous as in endothelium of the artery ,cuboidal as in kidney tubules or bile
ducts of the liver , columnar as in mucosa of the alimentary canal , spherical as in
red blood cells of man , fusiform as the smooth muscle cells and irregular like
macrophage cells . The cells as a whole may appear more or less smooth or
provided with certain structures like cilia and appendages , these structures are in
correlation with the function of the cell .
Life span of cells
Life span is highly variable between different cells types , it is about 120
days for red blood corpuscles or only few hours for some leucocytes and for
epithelial lining of the intestine about 3 days . On the other hand , nerve cells that
are not capable for division have life span of many years , similarly , cardiac
muscle and certain kidney cells .
Eukaryotic cell structure
Unlike prokaryotic cells ,an eukaryote cells contain true nucleus and
numerous membrane – bounded organelles .
The cell : is defined as a mass of protoplasm surrounded by a thin membrane
and having one or more nuclei at least during a stage of its development.
The protoplasm : it is a living substance of the cells composed of a mixture of
organic and inorganic materials, it is colloidal in nature , the relative amount of
these constitutes vary from cell to cell and from tissue to tissue against the age ,
kind and degree of differentiation of cells ( show table).
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Substance
water
protein
fatty substance
organics material
inorganic element
Percentage
85 – 90%
7 – 10%
1 -2%
1 – 1.5 %
1 – 1.5 %
Form
free and bound
albumin , globulin
lipid
carbohydrates
Na , K, Ca ,Mg ,Cl , SO4
Plasma membrane
It is the interface between the cell and it is environment , it is so thin that can
be visualized by electron microscope , it appears to be made of bimolecular layers
of polar lipids ; phospholipids , cholesterol , glycolipid coated on both sides by
proteins , the thickness of the whole structure being 80 – 100 A۫۫ .
The molecular organization of plasma membrane against the fluid mosaic model of
Singer and Nicholson 1972 ( Fig.) . In the membrane a certain invagination could be
recognized , they serve to increase cellular surface area to facilitate transport of
materials in and out of the cell called desmosomes and acts as an intercellular
bridges. Plasma membrane regulate the entry and exit of molecules and ions in
addition to contains receptors for various external signals .
Fig. Plasma membrane structure
Transport across the plasma membrane
There are several methods of transport mechanisms across the plasma membrane as
fallow :
1- Passive transport
The spread of molecular and ions through the plasma membrane from region of
higher concentration to a region of lower concentration this method is called diffusion
and when the transport of molecular and certain ions , amino acids occurs through the
specific carriers proteins or channels ( pores ) in plasma membrane this type of method
is called facilitated transport ( Fig.) both of methods not require of energy .
2- Active transport
In this type the movement of ions and molecules are transport against the
concentration gradient from the lower to higher conc. , for this it requires the energy
for example when the plasma membrane transport of Na from the inside of cell
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( low Na) to outside (high Na) and at the same time the transport of K from the
outside (low K) to interior of cell ( high K) as shown in figure .
Fig. methods of transport across the plasma membrane
Cell wall
It is an outer layer of cell made up of cellulose giving the cell the rigidity , strong ,
protection and allows the molecules such as H2O , O2 ,CO2 to diffuse in and out of
cells . It is found in plant , fungi but not in animal cells ( Fig.).
Fig. cell wall in plant
Cell organelles
Organelles are sub cellular structures with specialized functions , among the
kinds of organelles are ribosome, mitochondria, Golgi apparatus , endoplasmic
reticulum are membrane – bounded except ribosome .
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