Prokaryotic Cell

Prokaryotic Cell Definition

Prokaryotic cells are cells that do not have a true nucleus or most other cell organelles. Organisms that have prokaryotic cells are unicellular and called prokaryotes. Bacteria and archaea are prokaryotes. Prokaryotic cells can be contrasted with eukaryotic cells, which are more complex. Eukaryotic cells have a nucleus surrounded by a nuclear membrane and also have other organelles that perform specific functions in the cell.

Prokaryotic Cell Structure

Prokaryotic cells do not have a true nucleus that contains their genetic material as eukaryotic cells do. Instead, prokaryotic cells have a nucleoid region, which is an irregularly-shaped region that contains the cell’s DNA and is not surrounded by a nuclear envelope. Some other parts of prokaryotic cells are similar to those in eukaryotic cells, such as a cell wall surrounding the cell (which is also found in plant cells, although it has a different composition). Like eukaryotic cells, prokaryotic cells have cytoplasm, a gel-like substance that makes up the “filling” of the cell, and a cytoskeleton that holds components of the cell in place. Both prokaryotic cells and eukaryotic cells have ribosomes, which are organelles that produce proteins, and vacuoles, small spaces in cells that store nutrients and help eliminate waste. Some prokaryotic cells have flagella, which are tail-like structures that enable the organism to move around. They may also have pili, small hair-like structures that help bacteria adhere to surfaces and can allow DNA to be transferred between two prokaryotic cells in a process known as conjugation. Another part that is found in some bacteria is the capsule. The capsule is a sticky layer of carbohydrates that helps the bacterium adhere to surfaces in its surroundings.

Prokaryotic Cell Diagram

The following image is a diagram of a prokaryotic cell; in this case, a bacterium.
Prokaryote cell

Characteristics of Prokaryotic Cells

All prokaryotic cells have a nucleoid region, DNA and RNA as their genetic material, ribosomes that make proteins, and cytoplasm that contains a cytoskeleton, which organizes and supports the parts of the cell. Prokaryotic cells are simpler than eukaryotic cells, and an organism that is a prokaryote is unicellular; it is made up of only one prokaryotic cell.

Prokaryotic cells are usually between 0.1 to 5 micrometers in length (.00001 to .0005 cm). Eukaryotic cells are generally much larger, between 10 and 100 micrometers. Prokaryotic cells have a higher surface-area-to-volume ratio because they are smaller, which makes them able to obtain a larger amount of nutrients via their plasma membrane.

Examples of Prokaryotic Cells

Bacterial Cells

Bacteria are single-celled microorganisms that are found nearly everywhere on Earth, and they are very diverse in their shapes and structures. There are about 5×1030 bacteria living on Earth, including in our own bodies; in the human gut, bacteria outnumber human cells 10:1.

The cell walls of bacteria contain peptidoglycan, a molecule made of sugars and amino acids that gives the cell wall its structure and is thicker in some bacteria than others. Bacteria contain certain structures unique to them as previously mentioned, such as the capsule, flagella, and pili. Most bacteria have just one chromosome that is circular, which can range from about 160,000 base pairs (bp) to 12,200,000 bp. They also contain plasmids, which are small circular pieces of DNA that replicate independently of the chromosome. Some bacteria can form endospores. These are tough, dormant structures that the bacteria can reduce themselves to under starvation conditions when not enough nutrients are available. They do not need nutrients and are resistant to extreme temperatures, UV rays, and chemicals. When environmental conditions become favorable again, the endospore can reactivate.

Archaeal Cells

Archaea are similar in size and shape to bacteria, and they are also unicellular. Since bacteria and archaea are the two types of prokaryotes, this means that all prokaryotes are unicellular. Some archaea are found in extreme environments, such as hot springs, but they can be found in a variety of locations, such as soils, oceans, marshlands, and inside other organisms, including humans.

Like bacteria, archaea can have a cell wall and flagella. However, the structure of these organelles is different. For example, archaeal cell walls do not contain peptidoglycan. In addition, the flagella of archaea work the same way as those of bacteria, but they evolved from different structures. Membranes of archaea are very different than those of all other lifeforms; they contain different lipids, which have a different stereochemistry. Archaea usually have one circular chromosome, like bacteria do. The archaeal chromosome can range from less than 491,000 bp to about 5,700,000 bp. They can also contain plasmids. Less is known about archaea than bacteria; they were not classified as a separate group of prokaryotes until 1977.

Related Biology Terms

  • Eukaryotic cell – A cell that has a true nucleus surrounded by a nuclear envelope, and contains organelles such as mitochondria.
  • Prokaryote – An organism that has prokaryotic cells; this includes the domains Bacteria and Archaea.
  • Microorganism – A living thing that is microscopic.
  • Nucleoid – A region in prokaryotes that contains the cell’s DNA; it is not a true nucleus because it does not have a nuclear envelope.

Quiz

1. Which organism is made up of prokaryotic cells?
A. Human
B. Archaeon
C. Red squirrel
D. Seaweed

Answer to Question #1

2. What is the nucleoid?
A. An organelle that makes proteins
B. A region that contains loosely organized DNA
C. An organelle that contains DNA surrounded by a nuclear envelope
D. An organelle that produces energy

Answer to Question #2

3. Which of the following is characteristic of prokaryotic cells?
A. Small, between 0.1-5 µm in length
B. High surface-area-to-volume ratio
C. Lack of a true nucleus
D. All of the above

Answer to Question #3
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