cell biology

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Cell biology : is a branch of biology 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 . The body of eukaryotic cells may consist of billions of cells are different in many things such as in number, function , structure , size ,shape and life span 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 , 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 sometimes 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

An eukaryote cells contain true nucleus and numerous membrane – bounded organelles on contrary to prokaryotic cells .
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).




Substance Percentage Form
water 85 – 90% free and bound
protein 7 – 10% albumin , globulin
fatty substance 1 -2% lipid
organics material 1 – 1.5 % carbohydrates
inorganic element 1 – 1.5 % Na , K, Ca ,Mg ,Cl , SO4

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

Plasma membrane
It is an 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 bilayers of polar lipids ; phospholipids , cholesterol , glycolipid coated on both sides by proteins , the thickness of the whole structure being 80 – 100 A?? .
The molecular organization of plasma membrane against 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, certain ions and 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 are not require of energy .
2- Active transport
In this type , movement of ions and molecules against the concentration gradient from the lower to higher concentration , for this it requires the energy for example when the plasma membrane transport Na from the inside of the cell ( low Na) to outside (high Na) and at the same time the transport of K from the cell outside (low K) to interior of cell ( high K) as shown in figure .















Fig. methods of transport across the plasma membrane




Organelles are sub cellular structures with specialized functions , among the kinds of organelles are endoplasmic reticulum , mitochondria, ribosome , Golgi apparatus ,vesicles are membrane – bounded except ribosome .

Endoplasmic reticulum (ER)
ER. (Endoplasmic means within the cytoplasm , reticulum : little net ) it is a network of tubular , flattened sacs surrounded by very thin lipoprotein membrane composed of lipid bilayer embedded with proteins , it appears to be continuous with cell membrane on one hand and nuclear membrane on the other .

There are two types of ER are known :

1- Rough or granular type (RER) : It is characterized by presence of numerous minute particles ribosome are capable of protein synthesis ex. exocrine cells of pancreas and plasma cells.

2-Smooth or a granular type (SER): It lacks the ribosome , so it appears in cells which have no active in synthesis of protein but in lipid production such as the cells of adrenal cortex , adipose cells .












Fig. Endoplasmic reticulum structure




A ribosome is a tiny non–membranous organelle , nucleoprotein in which composed of rRNAs and proteins ,it is a factory for proteins synthesis using information provided by mRNAs . A ribosome consists of two ribosomal subunits that combine in presence of Mg+2 ions to form a functional ribosome of 70S in bacteria and 80S in eukaryotes .
S the Svedberg unite ( sedimentation coefficient ) is a measure of size determined by the rate at which a particle in solution sediments in the centrifuge.







Mitochondria (Mito : filament, chondron : granule) are large organelles in eukaryotic cells , they are sites of cellular respiration contains the enzymes of citric acid cycle, respiratory chain and oxidative phosphorylation . They are bounded by a double membrane , the inner mitochondrial membrane ones folds producing cristae, the crista are the sites of oxidative phosphorylation in cell . The matrix of mitochondria is the fluid filled interior of the mitochondrion and most of enzymes of citric acid cycle are contained in the ground . Mitochondria produces most of ATP in eukaryotic cells.


Fig. mitochondria structure

Chloroplast
It is a green oval organelle containing chlorophyll , it has double membrane with inner membrane modified into sacs called thylakoids this stacks is called grana .The inner most substance is gel like called stroma . Chlorophyll traps the energy of sunlight which is then used by the plant to make sugar for energy and release of oxygen to keep the balance in the environment . Chloroplast is found only in plants and algae ( Fig.).