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Universal features of cells. Characteristics of prokaryotic cells. Surface area-to-volume ratio.
Introduction
Take a moment and look at yourself. How many organisms do you see? Your first thought might be that there's just one: yourself. However, if you were to look closer, at the surface of your skin or inside your digestive tract, you would see that there are actually many organisms living there. That’s right - you are home to around 100 trillion bacterial cells!
This means that your body is actually an ecosystem. It also means that you—for some definition of the word you—actually consist of both of the major types of cells: prokaryotic and eukaryotic.
All cells fall into one of these two broad categories. Only the single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes—pro means before and kary means nucleus. Animals, plants, fungi, and protists are all eukaryotes—eu means true—and are made up of eukaryotic cells. Often, though—as in the case of we humans—there are some prokaryotic friends hanging around.
That depends who you ask! In general, prokaryotes are single-celled organisms. However, there's increasing evidence that some groups of prokaryotic cells can organize to form structures that resemble multicellular organisms. Whether this counts as "real" multicellularity is a question hotly debated by researchers today.
For instance, certain types of cyanobacteria form long, filament-like chains, see image below. In these chains, cells remain connected to each other after division and acquire unique cellular identities and functions. Some cells in the chain are specialized to carry out photosynthesis, the production of sugars using energy from the sun. In the image below, these are the smaller, darker cells that make up most of the chain. Others are specialized to fix nitrogen, converting atmospheric
Components of prokaryotic cells
There are some key ingredients that a cell needs in order to be a cell, regardless of whether it is prokaryotic or eukaryotic. All cells share four key components:
The plasma membrane is an outer covering that separates the cell’s interior from its surrounding environment.
Cytoplasm consists of the jelly-like cytosol inside the cell, plus the cellular structures suspended in it. In eukaryotes, cytoplasm specifically means the region outside the nucleus but inside the plasma membrane.
DNA is the genetic material of the cell.
Ribosomes are molecular machines that synthesize proteins.
Despite these similarities, prokaryotes and eukaryotes differ in a number of important ways. A prokaryote is a simple, single-celled organism that lacks a nucleus and membrane-bound organelles. We’ll talk more about the nucleus and organelles in the next article on eukaryotic cells, but the main thing to keep in mind for now is that prokaryotic cells are not divided up on the inside by membrane walls, but consist instead of a single open space.
The majority of prokaryotic
Bacteria are very diverse in form, so not every type of bacterium will have all of the features shown in the diagram.
Most bacteria are, however, surrounded by a rigid cell wall made out of peptidoglycan, a polymer composed of linked carbohydrates and small proteins. The cell wall provides an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. Many bacteria also have an outermost layer of carbohydrates called the capsule. The capsule is sticky and helps the cell attach to surfaces in its environment.
Some bacteria also have specialized structures found on the cell surface, which may help them move, stick to surfaces, or even exchange genetic material with other bacteria. For instance, flagella are whip-like structures that act as rotary motors to help bacteria move.
Fimbriae are numerous, hair-like structures that are used for attachment to host cells and other surfaces. Bacteria may also have rod-like structures known as pili, which come in different varieties. For instance, some types of pili allow a bacterium to transfer
You may have heard fimbriae referred to as a type of pili—for instance, some sources call them common pili or attachment pili
Most typically, the term fimbriae is used to describe a class of cell surface protrusions that are numerous, relatively short, and involved in attachment to surfaces, while the term pili is used for less numerous, longer, and more specialized cell surface protrusions. These are the meanings we use for these two terms in this article.
Archaea may also have most of these cell surface features, but their versions of a particular feature are typically different from those of bacteria. For instance, although archaea also have a cell wall, it's not made out of peptidoglycan—although it does contain carbohydrates and proteins.
Cell size
Typical prokaryotic cells range from 0.1 to 5.0 micrometers (μm) in diameter and are significantly smaller than eukaryotic cells, which usually have diameters ranging from 10 to 100 μm.
The figure below shows the sizes of prokaryotic, bacterial, and eukaryotic, plant and animal, cells as well as other molecules and organisms on a logarithmic scale. Each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured, so these are big size differences we’re talking about!
With a few cool exceptions—check out the single-celled seaweed Caulerpa—cells must remain fairly small, regardless of whether they’re prokaryotic or eukaryotic. Why should this be the case? The basic answer is that as cells become larger, it gets harder for them to exchange enough nutrients and wastes with their environment. To see how this works, let’s look at a cell’s surface-area-to-volume ratio.
Suppose, for the sake of keeping things simple, that we have a cell that’s shaped like a cube. Some plant cells are, in fact, cube-shaped. If the length of one of the cube’s sides is
Thus, as a cell gets bigger, its surface-area-to-volume ratio drops. For example, the cube-shaped cell on the left has a volume of 1 mm
Surface-area-to-volume ratio is important because the plasma membrane is the cell’s interface with the environment. If the cell needs to take up nutrients, it must do so across the membrane, and if it needs to eliminate wastes, the membrane is again its only route.
Each patch of membrane can exchange only so much of a given substance in a given period of time – for instance, because it contains a limited number of channels. If the cell grows too large, its membrane will not have enough exchange capacity (surface area, square function) to support the rate of exchange required for its increased metabolic activity (volume, cube function).
The surface-area-to-volume problem is just one of a related set of difficulties posed by large cell size. As cells get larger, it also takes longer to transport materials inside of them. These considerations place a general upper limit on cell size, with eukaryotic cells being able to exceed prokaryotic cells thanks to their structural and metabolic features—which we’ll explore in the next section.
Some cells also use geometric tricks to get around the surface-area-to-volume problem. For instance, some cells are long and thin or have many protrusions from their surface, features that increase surface area relative to volume
Attribution
This article is a modified derivative of “Prokaryotic cells” by OpenStax College, Biology, CC BY 3.0. Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:17/Biology.
The modified article is licensed under a CC BY-NC-SA 4.0 license.
Works cited
Telford, John L., Michèle A. Barocchi, Immaculada Margarit, Rino Rappuoli, and Guido Grandi. "Pili in Gram-positive Pathogens." Nature Reviews Microbiology Nat Rev Micro 4, no. 7 (2006): 509-19. http://dx.doi.org/10.1038/nrmicro1443. Retrieved from https://med.uth.edu/mmg/files/2014/12/Ton-That_Telford_012215.pdf.
Reece, J. B., L. A. Urry, M. L. Cain, S. A. Wasserman, P. V. Minorsky, and R. B. Jackson, "A tour of the cell," in Campbell biology, 10th ed. (San Francisco, CA: Pearson, 2011), 98.
Additional references
"Bacteria - pili." Cronodon Museum. http://cronodon.com/BioTech/Bacteria_pili.html.
"Caulerpa." Wikipedia. December 1, 2014. Accessed August 9, 2015. https://en.wikipedia.org/wiki/Caulerpa.
Esko, J. D., T. L. Doering, and C. R. H. Raetz. "Eubacteria and Archaea." In Essentials of Glycobiology, edited by A. Varki, R. D. Cummings, J. D. Esko, H. H. Freeze, P. Stanley, C. R. Bertozzi, G. W. Hart, and M. E. Etzler. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.
"Eukaryotic cells." Scitable. 2014. http://www.nature.com/scitable/topicpage/eukaryotic-cells-14023963.
Jarrell, K. F., D. J. VanDyke, and J. Wu. "Archael flagella and pili." In Pili and flagella: Current research and future trends, edited by K. F. Jarrell, 215-234. Norfolk, UK: Caister Academic Press, 2009.
Kaiser, G. E. "Cellular organization: prokaryotic and eukaryotic cells." Dr. Kaiser’s microbiology course (BIOL 230). April, 2014. http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit1/proeu/proeu.html.
"Pilus." Wikipedia. March 7, 2015. Accessed August 9, 2015. https://en.wikipedia.org/wiki/Pilus.
Raven, P. H., G. B. Johnson, K. A. Mason, J. B. Losos, and S. R. Singer. "Cell structure." In Biology, 59-87. 10th ed. AP ed. New York, NY: McGraw-Hill, 2014.
Reece, J. B., L. A. Urry, M. L. Cain, S. A. Wasserman, P. V. Minorsky, and R. B. Jackson. "A tour of the cell." In Campbell biology , 92-123. 10th ed. San Francisco, CA: Pearson, 2011.
Reece, J. B., L. A. Urry, M. L. Cain, S. A. Wasserman, P. V. Minorsky, and R. B. Jackson. "Structural and functional adaptations contribute to prokaryotic success, 92-123. 10th ed. San Francisco, CA: Pearson, 2011.
"Surface-area-to-volume ratio." Wikipedia, August 1, 2015. Accesssed August 9, 2015. https://en.wikipedia.org/wiki/Surface-area-to-volume_ratio.
Todar, K. "Structure and function of bacterial cells." Todar's online textbook of bacteriology. 2012. http://textbookofbacteriology.net/structure_3.html.
Log in wist626 7 years agoPosted 7 years ago. Direct link to wist626's post “Are red blood cells consi...” Are red blood cells considered to be prokaryotic since they don't have a nucleus? • (39 votes) VaeSapiens 7 years agoPosted 7 years ago. Direct link to VaeSapiens's post “They are considered Eukar...” They are considered Eukaryots. They loose the nucleus when they mature and many non-vertebrates have erythrocytes with a nucleus. (43 votes) sadaf2005 6 years agoPosted 6 years ago. Direct link to sadaf2005's post “How do prokaryotes get ri...” How do prokaryotes get rid of waste? • (26 votes) Johnny a year agoPosted a year ago. Direct link to Johnny's post “Prokaryotes use the proce...” Prokaryotes use the process of diffusion, to either take in (endo) or release (exo), materials through their semi-permeable plasma membrane. Now you might say, what about their cell wall, the cell wall must be stopping these materials. Well actually, the cell wall (made of peptidogylcan in bacteria) is fully permeable, as it's role is not to stop materials from going in or out. It's function is to give support to the cell, which is why it is rigid but fully permeable, thereby allowing exchange of materials. Hope this helps! (11 votes) Vinnette Mcmillan 7 years agoPosted 7 years ago. Direct link to Vinnette Mcmillan's post “Do humans have prokaryoti...” Do humans have prokaryotic cells in their bodies • (12 votes) evangundy 7 years agoPosted 7 years ago. Direct link to evangundy's post “Humans do have bacteria i...” Humans do have bacteria in their saliva and stomach that we use to break down things we need, but we can't make that ourselves. So technically there is some in our bodies, we just didn't make it. (14 votes) asd0105as 8 years agoPosted 8 years ago. Direct link to asd0105as's post “From paragraph 10 to 16, ...” From paragraph 10 to 16, I still don't get idea of surface-area-to-volume. • (9 votes) Liu, Andrew 8 years agoPosted 8 years ago. Direct link to Liu, Andrew's post “If you're still confused,...” If you're still confused, you can watch the video "cell size" (13 votes) Nesta 4 years agoPosted 4 years ago. Direct link to Nesta's post “What makes Trichodina dif...” What makes Trichodina different from all prokaryotes? • (8 votes) Ivana - Science trainee 4 years agoPosted 4 years ago. Direct link to Ivana - Science trainee's post “Trichodina is considered ...” Trichodina is considered Eukaryotic protozoa -meaning it has a nucleus. That sets its apart from all Prokaryotes. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3750665/ (7 votes) FREE PALESTINE 10 months agoPosted 10 months ago. Direct link to FREE PALESTINE's post “Do I have to memorize all...” Do I have to memorize all of this or just stick up with the -Prokaryotes and eukaryotes review- • (4 votes) 💜PURPLE4VICTORY💜 10 months agoPosted 10 months ago. Direct link to 💜PURPLE4VICTORY💜's post “*You can do whatever you ...” You can do whatever you want to do when it comes to your learning. If you want to study the major highlights of the unit, then go look at the Prokaryotes and Eukaryotes Review. If you want to have a better understanding of prokaryotes in a more detailed sense, then I guess "memorize all of this" in this article. Or if you want to understand prokaryotes a little bit more and then look at an overall review, then do it! But please don't make it about energy points. Even though the perks are nice (or was nice--I still miss backgrounds) and it's soo satisfying to see that blue check mark after you read a long article or watched a long video, that's a secondary priority. Your first priority on this website (I hope it's your first priority) is to learn. I want you to know that no one is stopping you from doing what you need to do when learning. After all, learning all of this information on Khan Academy is for your benefit, not someone elses's. So learn however you want to, as long as you feel like you're learning! I wish you all the best in your learning journey. :) (15 votes) Brittany 8 years agoPosted 8 years ago. Direct link to Brittany's post “Why is the flagellum only...” Why is the flagellum only in the prokaryotic cells and not in the eukaryotic cells? • (7 votes) alex00hf 5 years agoPosted 5 years ago. Direct link to alex00hf's post “Eukaryotic cells they do ...” Eukaryotic cells they do have flagella (only one: Sperm cells) but they have a different composition (6 votes) Nikhil Chowdhury 7 years agoPosted 7 years ago. Direct link to Nikhil Chowdhury's post “are there are any multi c...” are there are any multi celluler prokaryotic • (5 votes) Ivana - Science trainee 5 years agoPosted 5 years ago. Direct link to Ivana - Science trainee's post “No, there are not known m...” No, there are not known multicellular Prokaryotes. Distinguishing characteristic among Eukaryote and Prokaryote Domains is being unicellular or multicellular. (6 votes) tamiadockery 4 years agoPosted 4 years ago. Direct link to tamiadockery's post “what cell is used by prok...” what cell is used by prokaryotic cells and some other single celled organisms for movement. • (4 votes) Ellie Castillo 4 years agoPosted 4 years ago. Direct link to Ellie Castillo's post “Flagella: are whip-like ...” Flagella: (6 votes) Shelly Woyicki 4 years agoPosted 4 years ago. Direct link to Shelly Woyicki's post “How does the eukaryotic c...” How does the eukaryotic cell size impact the cell's ability to tranport materials into and out of the cell? • (4 votes) Ivana - Science trainee 4 years agoPosted 4 years ago. Direct link to Ivana - Science trainee's post “Cell size does not have m...” Cell size does not have much impact on the transport - what really has impact is proximity to the next cell, the junctions between cells in the epithelial tissue and the shape of the cell (the more circular cell the greater surface vs volume ratio). (6 votes)Want to join the conversation?
https://elps.eastlongmeadowma.gov/files/9214/3525/7686/MCAS_review_packet_1.pdf
are whip-like structures that act as rotary motors to help bacteria move.
Pili:
Rod like structures
Fimbriae :
are numerous, hair-like structures that are used for attachment to host cells and other surfaces.
