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, which outnumber your own human cells by about 10 to onestart superscript, 1, end superscript.
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 N, start subscript, 2, end subscript into more biologically useful forms. In the image below, only one nitrogen-fixing cell is shown, and it appears rounder and lighter-colored than its neighbors.
Image of cyanobacterial cells forming a filament composed of dissimilar cell types. Most of the cells are small, but one is round and different in morphology. This larger cell is a nitrogen-fixing cell.
Image credit: modified from "Anabaena circinalis" by BdCarl, CC BY-SA 3.0. The modified image is licensed under a CC BY-SA 3.0 license.

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:
  1. The plasma membrane is an outer covering that separates the cell’s interior from its surrounding environment.
  2. 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.
  3. DNA is the genetic material of the cell.
  4. 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 D, N, A is found in a central region of the cell called the nucleoid, and it typically consists of a single large loop called a circular chromosome. The nucleoid and some other frequently seen features of prokaryotes are shown in the diagram below of a cut-away of a rod-shaped bacterium.
Image of a typical prokaryotic cell, with different portions of the cell labeled.
Image credit: modified from "Prokaryotic cells: Figure 1" by OpenStax College, Biology, CC BY 3.0
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 a 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 numeous, 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 D, N, A molecules to other bacteria, while others are involved in bacterial locomotion—helping the bacterium move.
You may have heard fimbriae referred to as a type of pili—for instance, some sources call them common pili or attachment pilistart superscript, 2, end superscript. These multiple names reflect a long history in which two different terms, fimbriae and pili, have both been used by scientists to describe the surface protrusions of bacterial cells.
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!
Graph showing the relative sizes of items from, in order, atoms to proteins to viruses to bacteria to animal cells to chicken eggs to humans.
Image credit: "Prokaryotic cells: FIgure 2" by OpenStax College, Biology, CC BY 3.0
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 l, the surface area of the cube will be 6, l, start superscript, 2, end superscript, and the volume of the cube will be l, start superscript, 3, end superscript. This means that as l gets bigger, the surface area will increase quickly since it changes with the square of l. The volume, however, will increase even faster since it changes with the cube of l.
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 mmstart superscript, 3, end superscript and a surface area of 6 mmstart superscript, 2, end superscript with a surface-area-to-volume ratio of six to one, whereas the cube-shaped cell on the right has a volume of 8 mmstart superscript, 3, end superscript and a surface area of 24 mmstart superscript, 2, end superscript with a surface area-to-volume ratio of three to one.
Image of two cubes of different sizes. The cube on the left has 1 mm sides, while the cube on the right has 2 mm sides.
Image credit: modified from "Prokaryotic cells: FIgure 3" by OpenStax College, Biology, CC BY 3.0
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 volumestart superscript, 3, end superscript.

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

  1. Brody, J. E. "We Are Our Bacteria." New York Times. July 14, 2014. http://well.blogs.nytimes.com/2014/07/14/we-are-our-bacteria/?_r=0.
  2. 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.
  3. 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.
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.
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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.