Реферат: Explain With Aid Of A Diagram The

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 cell.

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


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