Prokaryotes VS Eukaryotes

** Similarities, Differences & Identification 


Defined as pre-nucleus, prokaryotes are cells that contain no membrane-based organelles, including a nucleus.

Mostly recognized as bacteria, two prokaryotic kingdoms exist: Monera or Bacteria and Archaea.

Seemingly simple in structure and markedly different from eukaryote and protist organisms, many scientists believe prokaryotic cells were amongst the first on the Earth and very well may out-survive all other organisms.




Cell Theory

Prokaryotes are, with few exceptions, unicellular organisms; many bacteria live in colonies, making them appear larger at first glance, but individual cells are visible under a microscope.

These cells do not possess membrane-based organelles, but the fundamentals of cell theory still apply.


Initially proposed by German scientists Matthias Schleiden and Theodor Schwann, and later amended by Rudolf Virchow, four basic rules apply to eukaryotic, prokaryotic and protist cells:


  • All forms of life contain one or more cells
  • All cells come from pre-existing cells

  • All functions that make life possible occur within the boundaries of cells
  • All cells possess genetic material required to regulate cell functions and replicate, passing this genetic information to new cells


Scientists have expanded and refined cell theory with the advent of advanced microscopy instruments, but these basic rules still apply to all cells.


Prokaryotes vs. Eukaryotes – Similarities?


prokaryotic Cell Structure


Prokaryotic cells, like those in eukaryotic uni- and multi- cellular organisms contain ribosomes and DNA – genetic matter that control all cell functions, including replication.

All cells require energy to survive and undergo chemical processes to sustain life.


Biochemical processes often involve the use of carbohydrates, proteins, lipids and nucleic acids for cell functions such as:


  • Obtaining food
  • Turning food into usable energy
  • Cell growth
  • Cell replication


Elemental processes essential to the life – acquisition of energy, reproduction – are similar in prokaryotic and eukaryotic cells, regardless of the difference in structure and means.


Prokaryotes vs. Eukaryotes – Differences


Eukaryotes as well as protists, a miscellany group made up of eukaryote-like plant, animal and fungi missing one or more characteristics to be defined as purely eukaryotic, differ greatly from prokaryotes in structure.

Most significant is the lack of a nucleus in prokaryotic cells as well as membrane-based organelles found in all eukaryotic cells.

Both have DNA, but eukaryote-DNA contain histones and chromosomes in a linear structure.


Mitochondria (or the plant equivalent chloroplasts) is one of the many membrane-bound organelles present in eukaryotes, along with:



In addition, while eukaryotes can be uni- or multi- cellular, heterotroph or autotroph, prokaryotes are heterotrophic single-celled organisms.


The Prokaryotic Cell


Ranging from two to five micrometers (um), typical prokaryotic cell structure includes:


  • Cell Wall – lends to the shape of the cell; two types are gram positive and gram negative

  • Pili – hair-like projections surrounding the outer layer of the cell; enables bacteria to stick-on surfaces or latch-on other cells
  • Capsule – thick covering of the cell wall that can provide protection from phagocytosis, chemicals and dehydration; the sticky nature allows it to adhere to other cells; found in gram-positive bacteria and blue-green algae
  • Flagellum – attached to the cell wall, usually described as “whip-like;” most prokaryotes are in constant motion and only able to move forward and backward
  • Plasma Membrane – a thin, flexible asymmetrical “sac” that essentially contains the cell; serves as a passageway for anything that enters or leaves the cell such as nutrients and gases; also holds the cytoplasm
  • Cytoplasm – can be compared, in terms of purpose, as carrying out similar functions to membrane-bound organelles; contains enzymes for and carries out metabolism; important to note that nothing within the cytoplasm is separated via membrane or well-defined sections, items are suspended in the semi-fluid gel
  • Plasmids – tiny rings of DNA that can be transferred to other cells; anti-biotic resistance is a prime illustration of the way prokaryotes share information with other prokaryote cells, enabling cells to make adjustments that ensure survival
  • Ribosomes – combination of RNA and protein, the function of prokaryotic ribosomes widely depends on the bacteria


Prokaryotes consist of two Kingdoms: Monera or Bacteria (sometimes called eubacteria), which includes cyanobacteria noted for the ability to carry out photosynthesis, and Archaea (archaebacteria).

With the greatest ability to withstand the most severe environmental conditions, science believes the latter contains the oldest cells/organisms on the planet, and sometimes referring to it as “ancient bacteria.”




Microbiology


The study of prokaryotic cells involves the study of bacteria – single cells that can be as tiny as two microns and look like dots under a compound microscope.


Bacteria are fascinating subjects for many reasons:


·  Range of purpose – without the existences of many “good” bacteria, many species could not exist, including humans


·  Are found everywhere – bacteria are not limited to causing disease, for example, they:


  • Line human intestines to help with digestion
  • Aid in the digestive process of rudiment animals
  • Help decompose waste material


· Adaptability – some bacteria create endospores, enabling them to survive if placed in a different or “harsh” environment


· Can survive conditions that would kill other cells – for example, some live in exceptionally hot or cold temperatures, gaseous environments or in places with intense high or low Ph


· Function – even though the cell structure seems simple, especially when compared to eukaryotes, unicellular bacteria and archaea organisms not only perform complex functions to survive, they are the oldest known cells


· Colonization – many bacteria grow in colonies, yet each cell maintains its autonomy


· Reproduction – cells pass along genetic information via a process called binary fission; cells create duplicate DNA and divide



Identifying Bacteria


Prokaryotes are classified through characteristics such as shape, behavior, size, growth, and stains. 

Bacteria are separated into three classes based on shape: cocci, bacilli, and spirilla. Although defined by morphology, they might not fall into the same classification – the only commonality might be shape. 

Also important to note, due to the size of bacteria, shapes are the only aspects visible under a light microscopes, such as electron microscopes, which offer more powerful magnification, in order to see the internal structures of the cell.



Shapes include:


  • Cocci are described as round, flat spheres and can be observed as lone cells, pairs, chains, tetrads (4 cells), clusters or cubes (8 cells); streptococcus is a chain of cocci-shaped bacteria cells responsible for the common sore throat infection
  • Rods, sometimes described as cylindrical and called as bacilli exist as singles, pairs and chains; unlike the simpler cocci, the length of chains has no bearing on identification
  • Spirilla, known for their spiral shape can appear as one curve, like elbow-macaroni, twists or genuine spirals


The way bacteria behaves is also important in identification; attributes include:


  • Whether a culture grows in a warm or cold environment
  • Whether cells colonize
  • DNA tests
  • Cell behavior when exposed to a variety of filters, chemicals, elements, gases or states (i.e., dehydration, change in Ph)
  • Growth (i.e., observing sample in a Petri-dish over time, possibly changing the external conditions)


In addition, bacteria are separated into gram positive and gram negative, easily discerned from one another by the use of a stain.


Summary


Prokaryote cells are especially intriguing to individuals interested in microbiology and microscopy.

Although shapes can be discerned under a compound microscope, powerful electron microscopes are required to observe internal details of the cell. Separated into Kingdoms Monera and Archaea, prokaryotes consist primarily of bacteria cells.

Ranging from 2-5 um, these impressive one-celled organisms survived millions of years; notorious for the result of diseases, most eukaryotic organisms could not exist without “good” bacteria – including humans.

Using microscopy, researchers try to understand their ability to adapt and survive, the way they help the human body and how to use them to improve the earth.


See Differences between Cytosol and Cytoplasm here.

See Differences between Microbiology and Biochemistry

Learn about Translation in Eukaryotes and Prokaryotes 

See Also:   Eubacteria main page - Helicobacter Pylori, Campylobacter

To Read further about Eukaryotes, Cell Division and Cell Differentiation and more about Unicellular Organisms - discussing further about Bacteria, Fungi, Algae and Archaea.

And learn about Autotrophs

More specifically to learn about Bacteria Size, Shape and Arrangement

Protists as well as Microorganisms in Pond Water

Return from Prokaryotes to Cell Theory

Return to Bacteriology main page

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