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Cells

Cells

Prokaryotic Cell Structure and Function

The vast majority of cells on Earth are actually prokaryotic, so we are in the minority. Do you feel outnumbered?

If it weren't so ugly, it would be kind of cute:



There are two major kinds of prokaryotes:
  • Bacteria
     
  • Archaea (single-celled organisms) 
As you may have read earlier in this unit, biologists now estimate that each human being carries nearly 20 times more bacterial, or prokaryotic, cells in his or her body than human, or eukaryotic, cells. If that statistic overwhelms you, rest assured that most of these bacteria are trying to help, and not hurt, you. Numerically, at minimum, there are 20 times more prokaryotic cells on Earth than there are eukaryotic cells. This is only a minimum estimate because there are trillions of trillions of bacterial cells that are not associated with eukaryotic organisms. In addition, all Archaea are also prokaryotic. As is the case for bacteria, it is unknown how many Archaean cells are on Earth, but the number is sure to be astronomical. In all, eukaryotic cells make up only a very small fraction of the total number of cells on Earth. So who runs this place, anyway?

There are four main structures shared by all prokaryotic cells, bacterial or Archaean:
  1. The plasma membrane
     
  2. Cytoplasm
     
  3. Ribosomes
     
  4. Genetic material (DNA and RNA) 
Some prokaryotic cells also have other structures like the cell wall, pili (singular pillus), and flagella (singular flagellum). Each of these structures and cellular components plays a critical role in the growth, survival, and reproduction of prokaryotic cells.

Prokaryotic Plasma Membrane

Prokaryotic cells can have multiple plasma membranes. Prokaryotes known as "gram-negative bacteria," for example, often have two plasma membranes with a space between them known as the periplasm. As in all cells, the plasma membrane in prokaryotic cells is responsible for controlling what gets into and out of the cell. A series of proteins stuck in the membrane (poor fellas) also aid prokaryotic cells in communicating with the surrounding environment. Among other things, this communication can include sending and receiving chemical signals from other bacteria and interacting with the cells of eukaryotic organisms during the process of infection. Infection is the kind of thing that you don't want prokaryotes doing to you. Keep in mind that the plasma membrane is universal to all cells, prokaryotic and eukaryotic. Because this cellular component is so important and so common, it is addressed in great detail in its own In Depth subsection.

Prokaryotic Cytoplasm

The cytoplasm in prokaryotic cells is a gel-like, yet fluid, substance in which all of the other cellular components are suspended. Jello for cells. It is very similar to the eukaryotic cytoplasm, except that it does not contain organelles. Recently, biologists have discovered that prokaryotic cells have a complex and functional cytoskeleton similar to that seen in eukaryotic cells.2 The cytoskeleton helps prokaryotic cells divide and helps the cell maintain its plump, round shape. As is the case in eukaryotic cells, the cytoskeleton is the framework along which particles in the cell, including proteins, ribosomes, and small rings of DNA called plasmids, move around. The "highway system" suspended in Jello.

Prokaryotic Ribosomes

Prokaryotic ribosomes are smaller and have a slightly different shape and composition than those found in eukaryotic cells. Bacterial ribosomes, for instance, have about half of the amount of ribosomal RNA (rRNA) and one third fewer ribosomal proteins (53 vs. ~83) than eukaryotic ribosomes have.3 Despite these differences, the function of the prokaryotic ribosome is virtually identical to the eukaryotic version. Just like in eukaryotic cells, prokaryotic ribosomes build proteins by translating messages sent from DNA.

Prokaryotic Genetic Material

All prokaryotic cells contain large quantities of genetic material in the form of DNA and RNA. Because prokaryotic cells, by definition, do not have a nucleus, the single large circular strand of DNA containing most of the genes needed for cell growth, survival, and reproduction is found in the cytoplasm.

The DNA tends to look like a mess of string in the middle of the cell:



Transmission electron micrograph image source

Usually, the DNA is spread throughout the entire cell, where it is readily accessible to be transcribed into messenger RNA (mRNA) that is immediately translated by ribosomes into protein. Sometimes, when biologists prepare prokaryotic cells for viewing under a microscope, the DNA will condense in one part of the cell producing a darkened area called a nucleoid.

As in eukaryotic cells, the prokaryotic chromosome is intimately associated with special proteins involved in maintaining the chromosomal structure and regulating gene expression. In addition to a single large piece of chromosomal DNA, many prokaryotic cells also contain small pieces of DNA called plasmids. These circular rings of DNA are replicated independently of the chromosome and can be transferred from one prokaryotic cell to another through pili, which are small projections of the cell membrane that can form physical channels with the pili of adjacent cells.

The transfer of plasmids between one cell and another is often referred to as "bacterial sex." Sounds dirty. The genes for antibiotic resistance, or the gradual ineffectiveness of antibiotics in populations, are often carried on plasmids. If these plasmids get transferred from resistant cells to nonresistant cells, bacterial infection in populations can become much harder to control. For example, it was recently learned that the superbug MRSA, or multidrug-resistant Staphylococcus aureus, received some of its drug-resistance genes on plasmids.4

Prokaryotic cells are often viewed as "simpler" or "less complex" than eukaryotic cells. In some ways, this is true: prokaryotic cells usually have fewer visible structures, and the structures they do have are smaller than those seen in eukaryotic cells. Don’t be fooled, however, into thinking that just because prokaryotic cells seem "simple" that they are somehow inferior to or lower than eukaryotic cells and organisms. Making this assumption can get you into some serious trouble.

Biologists are now learning that bacteria are able to communicate and collaborate with one another on a level of complexity that rivals any communication system ever developed by humans.5 Prokaryotes showed you, Facebook and Twitter. In addition, some Archaean cells are able to thrive in environments so hostile that no eukaryotic cell or organism would survive for more than a few seconds.6 Try living in a hot spring, salt lake, deep Earth, or a volcano!

Prokaryotic cells are also able to pull off stuff that eukaryotic cells could only dream of, in part because of their increased simplicity. Being bigger and more complex is not always better. These cells and organisms are just as adapted to their local conditions as any eukaryote, and in that sense, are just as “evolved” as any other living organism on Earth.

Brain Snack

One kind of bacterial communication, also known as quorum sensing, is where small chemical signals are used to count how many bacteria there are. Hear more about it here.

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