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Light microscopes commonly used in the undergraduate college laboratory magnify up to approximately 400 times. Two parameters that are important in microscopy are magnification and resolving power. Magnification is the process of enlarging an object in appearance. Resolving power is the ability of a microscope to distinguish two adjacent structures as separate: the higher the resolution, the better the clarity and detail of the image. When oil immersion lenses are used for the study of small objects, magnification is usually increased to 1,000 times. In order to gain a better understanding of cellular structure and function, scientists typically use electron microscopes.

Part a: This light microscope has binocular lenses and four objective lenses. The sample stage is directly beneath the objective lens. The light microscope sits on a tabletop and can be easily carried. Part b: The electron microscope shown here sits in a museum. It is about the size of a desk, and a person can sit in front of it to operate it. A column taller than a person rises from the center of the scope.
(a) Most light microscopes used in a college biology lab can magnify cells up to approximately 400 times and have a resolution of about 200 nanometers. (b) Electron microscopes provide a much higher magnification, 100,000x, and a have a resolution of 50 picometers. (credit a: modification of work by "GcG"/Wikimedia Commons; credit b: modification of work by Evan Bench)

Electron microscopes

In contrast to light microscopes, electron microscopes ( [link] b ) use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail ( [link] ), it also provides higher resolving power. The method used to prepare the specimen for viewing with an electron microscope kills the specimen. Electrons have short wavelengths (shorter than photons) that move best in a vacuum, so living cells cannot be viewed with an electron microscope.

In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes.

Part a: Salmonella through a light microscope appear as tiny purple dots. Part b: In this scanning electron micrograph, bacteria appear as three-dimensional ovals. The human cells are much larger with a complex, folded appearance. Some of the bacteria lie on the surface of the human cells, and some are squeezed between them.
(a) These Salmonella bacteria appear as tiny purple dots when viewed with a light microscope. (b) This scanning electron microscope micrograph shows Salmonella bacteria (in red) invading human cells (yellow). Even though subfigure (b) shows a different Salmonella specimen than subfigure (a), you can still observe the comparative increase in magnification and detail. (credit a: modification of work by CDC/Armed Forces Institute of Pathology, Charles N. Farmer, Rocky Mountain Laboratories; credit b: modification of work by NIAID, NIH; scale-bar data from Matt Russell)

For another perspective on cell size, try the HowBig interactive at this site .

Cell theory

The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of protista (a type of single-celled organism) and sperm, which he collectively termed “animalcules.”

In a 1665 publication called Micrographia , experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory    , which states that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory.

Career connection

Cytotechnologist

Have you ever heard of a medical test called a Pap smear ( [link] )? In this test, a doctor takes a small sample of cells from the uterine cervix of a patient and sends it to a medical lab where a cytotechnologist stains the cells and examines them for any changes that could indicate cervical cancer or a microbial infection.

Cytotechnologists (cyto- = “cell”) are professionals who study cells via microscopic examinations and other laboratory tests. They are trained to determine which cellular changes are within normal limits and which are abnormal. Their focus is not limited to cervical cells; they study cellular specimens that come from all organs. When they notice abnormalities, they consult a pathologist, who is a medical doctor who can make a clinical diagnosis.

Cytotechnologists play a vital role in saving people’s lives. When abnormalities are discovered early, a patient’s treatment can begin sooner, which usually increases the chances of a successful outcome.

Both normal cells and cells infected with HPV have an irregular, round shape and a well-defined nucleus. Infected cells, however, are two to three times as large as uninfected cells, and some have two nuclei.
These uterine cervix cells, viewed through a light microscope, were obtained from a Pap smear. Normal cells are on the left. The cells on the right are infected with human papillomavirus (HPV). Notice that the infected cells are larger; also, two of these cells each have two nuclei instead of one, the normal number. (credit: modification of work by Ed Uthman, MD; scale-bar data from Matt Russell)

Section summary

A cell is the smallest unit of life. Most cells are so tiny that they cannot be seen with the naked eye. Therefore, scientists use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution, and more detail than light microscopes. The unified cell theory states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.

Questions & Answers

Is there any other type of a eukaryotic cell.
Grace Reply
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Rachaelda Reply
state the role of mitochondria
Rachaelda
mitochondria ia power House of the cell. it provides energy and as ATP. Cells energy currency.
Haider
The scientific method of giving short names on the basis of genius and species.
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it is introduce by carlous Lennieus
Haider
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Structure of water molecule and it's biological significance. .....help guys
Ashly
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Justo Reply
Why mitochondria is called the power house of the congo the bahamas cell
Farrukh Reply
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mascuud Reply
what's microscope?
Mathias Reply
A device used to study a very small specimen thst cannt br seen with the naked eyes for example cells, or microorganisms.
Danisha
a medical device used to study cells bacteria viruses and parasites e.g electron microscope for studying cells.
Grace
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Randa
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Amar Reply
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Nyabuoy
test
Nyuongatdet
which party of an internal leaf which represent organ and tissue
fernando
3 trilleons cells on the human
Jyoti
name the groups of bacteria, what they cause and explain the components of bacterial cell
Emmanuel
what are the three level of relationship that exist between organism?
Chinedu
trillions of cells
Grace
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Ayasso Reply
trillions of cells
Grace
what causes coloring of skin variation
Prince Reply
what is your answer
Jonathan Reply
which qn
Randa
what is chemosynthesis
Irene
who many cell are in the human body
Ayasso
there are billion cells in human body
fazeela
what are three stages of mitosis
jerry
they're alot cells in our body
jerry
what are the stages of mitosis
jerry
they are prophez methaphez anaphez. thelophez
fazeela
anyone to explain each of the following,, prophase, metaphase, anaphase and telophase
jerry
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Mbah Reply
what is the difference between chlorophyll and photosynthesis
Rahman Reply
Chloe is the green pigment found in green plants while photosynthesis is the process by which plant produce their own food
mary
photosynthesis is the production of food by plant while chlorophyll is the green pigment that is found in chloroplast..
jerry
chrolophyll (green colouring matter in leaves) while photosynthesis (process by which green plants make their own food)
Nakhombi
What isaac life
Farrukh
chlorophyll is the pigment that gives a plant its green color while photosynthesis is when a plant makes it's own food.
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Bami Reply
base to provide support
Ian
only base what about the other
Bami
has only one function
Mark
Mirror ... used to reflect light
Irene

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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