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The electron microscope
The high resolving power of the electron microscope has enabled
scientist to observe the detailed structures of prokaryotic and eukaryotic
cells .The superior resolution of the electron microscope is due to that
fact that electrons have a much shorter wave length than the photons of
white light.
There are two types of electron microscope in general use:
1- The transmission electron microscope (TEM). Which has many
features in common with the light microscope.
2- The scanning electron microscope (SEM).
The TEM was the first to be developed and employs a beam of
electrons projected from an electron gun and directed or focused by an
electromagnetic condenser lens on to a thin specimen ;some electrons
pass through the specimen and are gathered and focused by an
electromagnetic objective lens which presents an image of specimen
to the projector lens system for further enlargement .The image is
visualized by allowing it to impinge on a screen that fluoresces when
struck with the electrons .The image can be recorded on photographic
film .
TEM can resolve particles 0.001µm a part . Viruses whit
diameters of 0.01-0.2 µm can be easily resolved (i.e. The human eye
cannot see electrons ,therefore the transmission electron microscope
uses a set of magnetic lenses and electrons passing through the object
to produce an image that is projected on to a fluorescent screen or
photographic film . The magnification produced by a transmission
electron microscope is much higher than that of a light microscope
.Also the ability of this microscope to make out detail in enlarged
images is much greater.
In other words, the TEM has a higher resolving power that is
the ability to distinguish between two adjacent points .Following is a
comparison of the resolving power of the eye .
The light microscope and the TEM
Eye
0.2mm
= 200µm = 200.000nm
Light microscope 0.0002mm = 0.2µm = 200nm
TEM (50.000x) 0.00001mm = 0.01µm = 10nm
The SEM generally has a lower resolving power than the
TEM ,however, it is particularly useful for providing threedimensional images of the surface of an object .A narrow beam of
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electrons are focused by means of lenses into very fine point .The
interaction of electrons with the specimen results in the release of
different forms of radiation (secondary electrons ) from the surface of
material (or, electrons is scanned over the surface of the specimen
which
Has been coated with a thin layer of metal. The metal gives off
secondary electron) which can captured an appropriate detector,
amplified and then image on a television screen.
An important technique in electron microscopy is the use of, shadowing,.
This involves the depositing a thin layer of heavy metal (such as
platinum) on the specimen by placing it in the path of a beam of metal
ions in a vacuum. The beam is directed at allow angle to the specimen so
that it acquires a ,shadow, in the form of an uncoated area on the other
side. When an electron beam is passed through the coated preparation in
the electron microscope and a positive print is made from the ,negative,
image, a three dimensions achieved.
Cellular organization
Cells are organize was not apparent to the first microscopists. Only
the advent of the electron microscope allowed biologists to determine the
internal organization of the cell.
Today we know that a plasma membrane surrounded the cell. Keeps it
intact and regulates that enters and exits a cell. The plasma membrane is a
phospholipids bilayer that is said to be selectively permeable because it
allows certain molecules but not others to inter the cell. Proteins present
in the plasma membrane play important roles in allowing substances to
enter the cell.
The nucleus is a large often centrally located structure that can
often be seen with a light microscope. The nucleus contains the
chromosome and is the control center of the cell. It controls the metabolic
functioning and structural characteristics of the cell.
The cytoplasm is the portion of the cell between the nucleus and the
plasma membrane. The matrix of cytoplasm is a semi fluid medium that
medium that contains water and various types of molecules suspended or
dissolved in the medium. The presence of proteins a count for the semi
fluid nature of the cytoplasm.
The cytoplasm contains various organelles. Organelles are small
often membranous structures that can usually only be seen with an
electron microscope. Each type of organelle has specific function. One
type of organelle transports substance and another type produce ATP for
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the cell. Since many organelles are composed of membrane we can say
that membrane compartmentalizes the cell. Keeping the various cellular
activities separated from another.
Cells also have a cytoskeleton, a network of interconnected
filaments and microtubules in the cytoplasm. The name cytoskeleton is
convenient in that it allows us to compare the cytoskeleton to the bones
and muscles of an animal. Bones and muscle give an animal structure and
produce movement. Similarly the elements of the cytoskeleton maintain
cell shape and allow the cell and its contents to move. Some cells move
by using cilia and flagella which contain microtubules as does the
cytoskeleton
Structures in animal cells
Name
_______
composition
_____________
2- Nucleus
4-Ribosoms
nuclear envelope surrounding
nucleoplasma ,chromatin
and nucleolus
information
ribosomal formation
protein and RNA in two subunit
Name
______
6-Rough endoplasmic
protein synthesis
synthesis and /or modification
substances and transport by
rescale formation
composition
_____________
studded with ribosoms
7-Smooth ER
9-Vacuole and vesicle
storage of genetic
concentrated area of chromatin ,RNA
,and proteins
5-Endoplasmic reticulum membrane saccules and canals
8-Golgi apparatus
Selective passage of
molecules into and out of
cell
Phospholipids bilayer with
embedded proteins
1-Plasma membrane
3- Nucleolus
function
__________
no ribosomes
stack of membranous saccules
membranous sac
function
_________
protein synthesis
lipid
synthesis in some cells
processing
,packaging and
distribution
of molecules
storage and transport of
substances
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Name
_______
11-Mitochonderia
composition
_____________
inner membrane (with cristae)
function
__________
cellular respiration
W ithin outer membrane
10-Lysosom
12-Cytoskeleton
13-Cilia and flagella
14-Centriol
membranous vesicle
Containing digestive enzymes
microtubules actin filaments
And intermediate filaments
9 +2 pattern of microtubules
9+0 pattern of microtubules
intracellular digestion
shape of cell movement it's part
movement of cell
formation of basal bodies
Plasma membrane
A human cell, like all cells , is surrounded by an outer plasma membrane.
The plasma membrane marks the boundary between the out side and
inside of the cell
( i.e. Plasma membrane integrity and function are necessary to life of the
cell ,the boundaries of cells are formed by
biological membranes ,the barriers that define the inside and the out side
of a cell.
The plasma membrane is a phospholipids bilayer with attached or
embedded proteins .A phospholipids molecules has a polar head and non
polar tails. The polar being charged are hydrophilic (water-loving) and
face out ward ,to ward the cytoplasm on one side and the tissue fluid on
the other side , where they will encounter a watery environment .The non
polar tails are hydrophobic (non attracted to water )and face in ward to
ward one another where there is no water . When phospholipids are
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placed in water , they naturally form a spherical bilayer because of the
chemical properties of the head and the tails .
At body temperature phospholipids bilayer is a liquid ,it has the
consistency of olive oil ,and the proteins are able to change their position
by moving laterally .
The
fluid –mosaic model
a working
description
of
membrane
structure ,says that the protein molecules form a shifting pattern within
the fluid phospholipids bilayer.Cholesterol lends support to the
membrane .
Short chains of sugars are attached to the outer surface of some
protein and lipid molecules ( called glycoprotein and glycolipid ,
respectively ).It is believed that these CHOs chains specific to each cells ,
help mark it as belonging to a particular individual .They account for why
people have different blood types (blood group ) as example.
Other glycoprotein have a special configuration that allows them to
act as a hormone .Some plasma membrane ,proteins form channels
through which certain substances enter cells ;others are enzymes that
catalyze reactions or carries involved in the passage of molecules through
the membrane.
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