Working Of The Electron Microscope Essay, Research Paper
The diagram below shows a picture
of an electron microscope. This is a type of microscope that makes use of a
beam of electrons rather than visible light. Because the wavelength of
electrons is much smaller than the wavelength of visible light, an electron
microscope not only gives a high magnification but it also has high resolution.
This means that details can be seen clearly. An electron microscope is very
similar to a light microscope in the ways in which it works but, instead of
using glass lenses to focus a beam of light, it uses magnets to focus a beam of
electrons. Electrons are very small so they are scattered if they hit molecules
in the air. Because of this they must travel through a vacuum. Specimens that
are to be examined must be cut into very thin sections, and these must be
treated so that they can be examined in these conditions. The variety of
chemical processes that are carried out in this preparation may change the
appearance considerably. Features which have been introduced in this way are
known as artefacts and care needs to be taken in interpreting electron
micrographs because of the possible presence of artefacts. The main differences
between a light microscope and an electron microscope are shown in the table
below.FEATURE?????????????????? LIGHT MICROSCOPE????????????????????? ELECTRON
MICROSCOPESource of illumination??? Visible light?????????????????????????????????????????? Beam of electrons Method of focusing?????? Glass lenses????????????????????????????????????????? Magnets Specimens which can ?? Live or dead???????????????????????????????????????? Dead Be examined Magnification??????????????? Student microscopes magnify up?????????? Up to 500000 times ??????????????????????????????????? To
about 400 times but the ??????????????????????????????????? Maximum
magnification that can ??????????????????????????????????? Be
achieved with a light ??????????????????????????????????? Microscope
is about 1500 times Resolution??????????????????? About 0.2 ??????????????????????????????????????????? About
1?? or 0.001??? .(B) Outline
the structure and functions of the following organelles: endoplasmic reticulum. ??????????? A network
of membranes found in the cytoplasm of a cell. It consists of a complex system
of pairs of membranes arranged parallel to each other enclosing flattend,
fluid-filled spaces. These membranes may be covered with ribosomes, in which
case they form rough endoplasmic reticulum. The ribosomes produce proteins
which are transported through the spaces between the membranes. Smooth endoplasmic
reticulum does not have ribosomes on its surface. Its main function is the
production and transport of lipids. Golgi apparatus ??????????? An
organelle which is responsible for the processing and packaging of substances
produced by a cell. The golgi apparatus consists of a series of flattened sacs,
each one enclosed by a membrane. These sacs are continually being formed on one
side and pinched of to form small vesicles at the other. Mitochondria ??????????? A
cell organelle in which the biochemical reactions associated with aerobic
respiration take place. Each mitochondrion is surrounded by an envelope
consisting of two membranes. The inner one is folded to form structures
cristae. Numbers of mitochondria vary considerably from cell to cell. Cells
which contain particularly large amounts of these organelles usually require
large amounts of ATP. Ribosomes ??????????? A very
small organelle which plays an important part in making proteins. It is made of
a mixture of protein and a special sort of RNA called ribosomal RNA. During
protein synthesis a molecule of messenger RNA (mRNA) carries the genetic code
for a particular protein from the nucleus into the cytoplasm of the c
Ribosomes now move along this mRNA molecule. This results in amino acids being
assembled in the correct order to form a molecule of the protein concerned.
There are two basic types of ribosome. Those found in the cytoplasm of
eukaryotic cells are slighlt larger than those found in the prokaryotic cells. Lysosomes ??????????? An
organelle containing digestive enzymes. These enzymes are seperated from the
rest of the cell contents by the membrane which surrounds the lysosome. This is
essential otherwise these enzymes would digest the proteins and lipids normally
found in the cell and destroy them. Lysosomes are usually associated with
animal cells where they have a number of different functions. Chloroplasts ??????????? A
chlorophyll containing organelle found in the cells of plants and algae. It is
where photosynthesis takes place. The chloroplasts found in plant cells are
small, flattened discs, each about five micrometers in diameter. They are found
in the cytoplasm but can change their position in response to differences in
light intensity. Chloroplasts are surrounded by an outer envelope consisting of
two cell membranes. Plasma membrane ??????????? A highly
selective filter and a protective boundary. A new permeable membrane of ~5nm in
thickness termed the plasma membrane surrounds the cell, similar membranes
surround each organelle. Nuclear envelope ??????????? The outer
membrane of the nuclear envelope is continuous with the endoplasmic reticulum.
The nuclear envelope is conspicuously perforated by the nuclear pores. Centriole ??????????? Structures
found in the animal cells which are associated with the separation of
chromosomes during mitosis. They are small, hollow cylinders each containing a
ring of microtubules. Nucleus ??????????? The
nucleus is the largest cell organelle. It is surrounded by two membranes known
as the nuclear envelope. The nuclear envelope is conspicuously perforated by
the nuclear pores. These allow exchange between the nucleus and the cytoplasm. Nucleolus ??????????? A dark
staining body found in the nucleus of a cell. It contains the DNA with the
copies of the genes that code for the special sort of RNA found in ribosomes,
ribosomal RNA. It is in the nucleolus that ribosomal RNA is produced and the
early stages of ribosomes formation take place. Cilia ??????????? Tiny,
hair-like organelles found on the cell surface membranes of certain cells. They
have a distinctive internal structure. Each has a ring on nine pairs of tubules
towards the outside and a single pair in the centre, the so-called 9+2
structure. (C) Compare and contrast the structure of prokaryotic and
eukaryotic cells. Eukaryotic cells found in
members?????????????????????????? Prokaryotic
cells found in????? of the kingdoms Protoctista,????????????????????????????????????? members of
the kingdom Fungi, Animalia and Plantae????????????????????????? Prokaryotae Large cells up to 50? m in diameter??????????????????????????????? Cells small with a mean diameter ??????????????????????????????????????????????????????????????????????????????????? Under
5? m?????????????????????????????????????????? DNA linear and associated with
proteins?????????????????????? Circular
strands of DNA not to form a true chromosome.
Chromosomes ????????????????? associated
with proteins and found found within a nucleus.????????????????????????????????????? In the
cytoplasm. No nucleus present Many membrane-surrounded
organelles??????????????????????? Few
organelles present and none are such as mitochondria present.??????????????????????????????????????? Surrounded
by a plasma membraneFlagella
(correctly known as undulipodia)????????????????????? Flagella
lack system of microtubules have an internal arrangement of
microtubules.