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:
The transmission electron microscope (TEM). Which has many features in common with the light microscope.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 0.001m a part . Viruses whit diameters of 0.01-0.2 m 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 = 200m = 200.000nm
Light microscope 0.0002mm = 0.2m = 200nmTEM (50.000x) 0.00001mm = 0.01m = 10nm
The SEM generally has a lower resolving power than the TEM ,however, it is particularly useful for providing three-dimensional images of the surface of an object .A narrow beam of 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 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
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 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.