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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). 1 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 2 ( 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 . 3