The Development of Cell Theoryscientistcindy.com /the-development-of-cell-theory.html

Before Cell Theory...

The Theory of Spontaneous Generation

The Greek philosopher Aristotle (384–322 BC) published writings that theorized how some organisms come intoexistence through non-living materials. This theory is known as the theory of spontaneous generation. Aristotlesupposed that if a non-living material has a spiritual essence or "vital heat" (or pneuma), it could give rise to livingorganisms! The theory persisted for thousands of year, because without microscopes, it did APPEAR as though animals orinsects were spontaneously appearing out of non-living things! For example, fish would seemingly appear in apuddle water, maggots would appear in rotting meats, fleas would appear out of dust, mice would appear out of pilesof grain, frogs appeared along river banks, etc.

First Evidence Against Spontaneous Generation - 1668

The first evidence showing that the theory of spontaneous generation is incorrect, was presented byItalian physician Francesco Redi in 1668. Redi designed an experiment that disproved that maggots wereable to spontaneously generate on rotting meat. His experiments also demonstrated that flies must havedirect contact with the rotting meat in order for maggots to appear.

Francesco Redi's 1668 Experiment

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Redi performed an experiment using six containers containing meat. Two were covered with gauze to prevent fliesrom coming into direct contact with the meat, two were left open to the air allowing flies to have direct contact withthe meat, and two were covered preventing flies from coming into direct contact with the meat. Redi's experimentsshowed that maggots could only appear when flies had direct contact with meat. Therefore, maggots were formedas offspring of flies and not from spontaneous generation by meat.

John Needham (1713–1781)

Redi's conclusions were met with disbelief by many scholars and scientists. In 1745, John Needham sought todisprove Redi's findings and performed the following experiment. Needham briefly boiled broth containing plant andanimal matter, in an effort to kill any existing microorganisms. The broth was then kept sealed and left to incubate forthree days. After three days, the broth was visibly cloudy and microbes were observed in the broth using amicroscope. Needham concluded that these microscopic organisms had spontaneously generated from the broth.His conclusion supported the Theory of Spontaneous Generation and refuted Redi's claims. Later, it was found thatJohn Needham's experiment was flawed, because he had not allowed the broth to boil long enough to kill all of thepre-existing microbes.

Lazzaro Spallanzani (1729–1799)

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LazzaroSpallanzani(1729–1799)

Lazzaro Spallanzani (1729–1799) performed a number of experiments that elicited results thatcontradicted those of John Needham. Spallanzani boiled broth containing plant and animal matter.He then poured some of the heated broth into sealed containers and poured the remaining brothinto unsealed containers. The broth that was kept sealed, remained microbe-free, whereas thebroth exposed to air were found to carry microorganisms. Spallanzani’s results contradicted thenotion of spontaneous generation and suggested that the microbes found in the broth samples thatwere exposed to air, came from microbes that were present in the air.

Sealed containers containing brothdid not contain microbes after 3days.

The broth that was kept sealed,remained microbe-free, whereas thebroth exposed to air were found tocarry microorganisms.

Broth containing plant and animal matter was boiledthen placed in a sealed container for 3 days.After 3days, the broth contained no microorganisms.

Broth containing plant and animal matter was boiledthen placed in an unsealed container for 3 days.After3 days, the broth was cloudy and contained severaltypes of microorganisms.

John Needham held fast to his belief in spontaneous generation, despiteSpallanzani's experimental evidence. Needham explained away Spallanzani'sexperimental results by postulating that the "life force" needed for spontaneousgeneration was destroyed when the broth was boiled, and that sealing theboiled broth prevented any new "life force" from forming. Spallanzanihypothesized that spontaneous generation could not occur in the broth withoutthe broth having an intact "life force".

The legitimacy of the theory of spontaneous generation continued tobe the source of heated debate until the mid 1800's.

Louis Pasteur - 1858

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(The End of Spontaneous Generation?)

LouisPasteur

Louis Pasteur performed a series of experiments that presented overwhelming evidence againstthe theory of spontaneous generation.

Louis Pasteur was a pioneer in microbial fermentation and he invented the process ofpasteurization which bears his name.

FUN FACT! There is also a laboratory instrument called the Pasteur pipette which also bears his name.

A Plastic Disposable Pasteur Pipette

Pasteur pipettes, are named after the scientist LouisPasteur, who invented them for his own laboratory use.Pasteur pipettes allow for sterile conditions, which is of theutmost importance when studying microorganisms. ThePasteur pipette can be used to transfer liquids from onecontainer to another WITHOUT the liquid being exposed tothe open air. These pipettes can be made of glass ordisposable plastic and are a staple in biology laboratoriesstill to this day.

In 1858, Pasteur successfully transferred microbes that had attached themselves onto a piece of gun-cotton from the air, to broth. He was able to perform this transfer without exposing the broth to the air. Afterseveral days of incubation, the broth contained many microorganisms. Pasteur concluded thatmicroorganisms were coming from the air directly. This experiment showed evidence against the idea thatthe microorganisms appear in broth due exposure to the air which gave the broth a "life force".

Louis Pasteur and his famous swan-neck flasks experiment.

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Pasteur designed special flasks containing swan neck tubes thatprotruded from the top. The design allowed for air to pass throughto the broth, but prevented any microbes from passing through theair into the broth.

GIF If the theory of spontaneous generation was correct, microbes should appear in the broth in the swan-neckflasks, because the swan-neck flasks would have allowed any "life force" to pass from the air to the broth.

If Pasteur's hypothesis that microbes come from the air is correct, the broth in the swan-neck flask would notcontain any microbes. The swan-neck flasks allowed air passage, but prevented airborne microbes from coming intocontact with the broth. This is because any microbes would get stuck or trapped in the twists and turns of the swan-neck.

If Pasteur's hypothesis that microbes come from the air is correct, broth in a flask exposed to airborne microbeswould contain microorganisms. When the swan-neck was removed from the flask, both air and airborne microbeshad access to the broth.GIF COURTESYOF upload.wikimedia.org/wikipedia/commons/c/cd/Pasteur%27s_experiment_testing_spontaneous_generation_and_biogenesis.gif

Microbes were observed in the broth exposed to air AND airborne microbes, whereas broth exposed toair only did not contain microbes after the incubation period. This gave conclusive evidence against thetheory of spontaneous generation.

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THE ORIGINS OF

CELL THEORY

THE DISCOVERY OF CELLS AND CELL THEORY

Life comes in a huge variety of shapes, sizes, composition and physiology. Some of the largest organisms to everroam the Earth, like the blue whale, exceed 150 tons, whereas the smallest organisms are only visible usingmicroscopes. Our current understanding of "life" is centered on The Cell Theory which was developed overhundreds of years, due to the contributions of countless scientists and numerous inventions, experiments andobservations. These discoveries were made possible due to the invention of high-powered microscopes in the 17thcentury.

The Cell Theory:

1) The fundamental unit of life is the cell.2) All organisms contain one or more cells.3) All cells come from other cells.

All living beings are made up of cells. Some of them are made up of only one cell and others have manycells.The adult human body is made up of about 37 trillion cells. That is 37,000,000,000,000 cells! WOW!!!

Robert Hooke (1635 - 1703)

Cells Were NamedAfter the Cells of aHoneycomb (shownhere) by RobertHooke in 1665

The Cell Theory began with Robert Hooke's discovery and observation ofcells. As awesome as this discovery was, the true significance of thisdiscovery was not fully realized until centuries later! It was only then that thestructures Hooke coined as "cells" would be found to be the fundamentalunit of life!

MicroscopicViewAnimalCells

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Hooke's

Microscope

Robert Hooke was the first person to view tissuesunder a microscopic. He identified the distinctstructures that made up the tissue as "cells". Hepublished a book called Micrographia in 1665 inwhich he described and illustrated the microscopicview of a variety of samples and specimens. In addition to his contributions to microbiology,Hooke made contributions to physics (Hooke's Lawof Elasticity), astronomy, philosophy, and evenarchitecture.

Hooke's observations began with viewing sectionsof cork under the microscope. Hooke was observingplant cells which made up the cork.

Hooke's book, Micrographia, included spectacularcopperplate engravings of microscopic views ofdifferent organisms. Some of these, like theillustration of the louse, was a fold-out plate fourtimes the size of the page. Interestingly,Micrographia also describes distant planetarybodies, the wave theory of light, evolution, etc.

"In the collection of most of which I made use ofmicroscopes and some other glasses andinstruments that improve the senses... only topromote the use of mechanical helps for the Senses,both in the surveying the already visible World, andfor the discovery of many others hitherto unknown"

- Micrographia, by Robert Hooke (1665)

https://commons.wikimedia.org/w/index.php?curid=7667243

Diagram of a louse from Hooke'sMicrographia

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Illustration of CorkUnder theMicroscope byRobert Hooke

Robert Hooke identified the first cells by viewing a sample of cork.He described cells as structures resembling a

1. Honey Comb2. Small Boxes3. Bladders of Air4. Cavern5. Bubble

Hooke decided to call these structures cells.. The word "cell" or Cell” is distinct from theothers (in Latin, “cell” literally means “small room”). They likely appeared to Hooke to be filledwith air because the cork cells were dead, with only the rigid cell walls providing the structure.

What is a cell?

Cells are the fundamental unit of life!

Matthias Schleiden (1804–1881)

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Microscopic View of Plant Cells

A botanist, MatthiasSchleiden, took inventory of themicroscopic view of hundreds ofplant tissues. Plant cells are veryrecognizable due to theirpronounced cell walls, whichgives them their characteristicbrick-like shape. Interestingly,Schleiden believed the cell wallswere due to some sort ofcrystallization process. We nowknow that the cell walls arecreated in the cell divisionprocess, not crystallization.

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Theodor Schwann (1810–1882) Theodore Schwann, a German physiologist, surveyed a multitude of animal tissuesamples under the microscope and made several significant contributions to the BiologicalSciences. Schwann and Schleiden realized the stark similarities between cells of plants andcells of animals.

Theodore Schwann and Matthias Jakob Schleiden were both instrumental in creating CellTheory.

In 1837, Schleiden stated that new plant cells were formed from old plant cells.

Many of these observations were published in Schwann's book, M icroscopicalResearches into the Accordance in the Structure and Growth of Animals and Plants,. In it hedeclared, "All living things are composed of cells..." This statement is the heart of CellTheory

Robert Remak - 1852 Robert Remak performed experiments that presented convincing evidence that, indeed, newcells are formed from other cells through a process of cellular division. Remak's statements weremet with disbelief by many of the prominent scientists of his time. The ideas of Cell Theory did notbegin to be popularized until a publication by Rudolf Virchow, "Cellular Pathology". was publishedin 1857.

Despite Remak's contribution, most texts attributed the founding of Cell Theory to RudolfVirchow. In fact, some historians go so far as to declare that Virchow plagiarized Remak's ideas bystating that all cells come from pre-existing cells.

Rudolf Virchow - 1857 The Cell Theory was then expand by Rudolf Virchowin 1857, to include the statement, "every cell arisesfrom another cell".

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Rudolf Virchow - Archiv für Pathologische Anatomieund Physiologie (now known as “Virchow’s Archives”),1847, first issue

Cell Theory

The Cell Theory was fueled by the invention of progressively more powerfulmicroscopes. Viewing the microscopic world allowed scientists to directly observecellular structure and cellular reproduction.

Cell Theory was developed through the observations and conclusions of severalscientists in the mid-1800's. Cell Theory is now universally accepted in the sciences.The cell theory is made up of 3 statements:

1. All living organisms are composed of one or more cells. 2. The cell is the basic unit of structure and organization in organisms.3. Cells arise from pre-existing cells

Prokaryotic and Eukaryotic Cells

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