6.5: Introduction to the Structure of Prokaryotes - Biology

What you’ll learn to do: Describe the structure of prokaryotic cells

There are many differences between prokaryotic and eukaryotic cells. Prokaryotes come in various shapes, but many fall into three categories: cocci (spherical), bacilli (rod-shaped), and spirilli (spiral-shaped) (Figure 1).


Certain prokaryotes can live in extreme environments such as the Morning Glory pool, a hot spring in Yellowstone National Park. The spring’s vivid blue color is from the prokaryotes that thrive in its very hot waters. (credit: modification of work by Jon Sullivan)

In the recent past, scientists grouped living things into five kingdoms—animals, plants, fungi, protists, and prokaryotes—based on several criteria, such as the absence or presence of a nucleus and other membrane-bound organelles, the absence or presence of cell walls, multicellularity, and so on. In the late 20 th century, the pioneering work of Carl Woese and others compared sequences of small-subunit ribosomal RNA (SSU rRNA), which resulted in a more fundamental way to group organisms on Earth. Based on differences in the structure of cell membranes and in rRNA, Woese and his colleagues proposed that all life on Earth evolved along three lineages, called domains. The domain Bacteria comprises all organisms in the kingdom Bacteria, the domain Archaea comprises the rest of the prokaryotes, and the domain Eukarya comprises all eukaryotes—including organisms in the kingdoms Animalia, Plantae, Fungi, and Protista.

Two of the three domains—Bacteria and Archaea—are prokaryotic. Prokaryotes were the first inhabitants on Earth, appearing 3.5 to 3.8 billion years ago. These organisms are abundant and ubiquitous that is, they are present everywhere. In addition to inhabiting moderate environments, they are found in extreme conditions: from boiling springs to permanently frozen environments in Antarctica from salty environments like the Dead Sea to environments under tremendous pressure, such as the depths of the ocean and from areas without oxygen, such as a waste management plant, to radioactively contaminated regions, such as Chernobyl. Prokaryotes reside in the human digestive system and on the skin, are responsible for certain illnesses, and serve an important role in the preparation of many foods.

If the DNA from all 46 chromosomes in a human cell nucleus was laid out end to end, it would measure approximately two meters. However, the diameter would be only 2 nm. Considering that the size of a typical human cell is about 10 µm (100,000 cells lined up to equal one meter), DNA must be tightly packaged to fit in the cell&rsquos nucleus. At the same time, it must also be readily accessible for the genes to be expressed. During some stages of the cell cycle, the long strands of DNA are condensed into compact chromosomes. There are a number of ways that chromosomes are compacted to fit in the cell&rsquos nucleus and be accessible for gene expression.

In the first level of compaction, short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the entire length of the chromosome. The DNA-histone complex is called chromatin. The beadlike, histone DNA complex is called a nucleosome. DNA connecting the nucleosomes is called linker DNA. A DNA molecule in this form is about seven times shorter than the double helix without the histones. The beads are about 10 nm in diameter, in contrast with the 2-nm diameter of a DNA double helix. The next level of compaction occurs as the nucleosomes and the linker DNA between them are coiled into a 30-nm chromatin fiber. This coiling further shortens the chromosome so that it is now about 50 times shorter than the extended form. In the third level of packing, a variety of fibrous proteins is used to pack the chromatin. These fibrous proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that does not overlap with that of any other chromosome.

Figure (PageIndex<1>): Levels of DNA Compaction: Double-stranded DNA wraps around histone proteins to form nucleosomes that have the appearance of &ldquobeads on a string.&rdquo The nucleosomes are coiled into a 30-nm chromatin fiber. When a cell undergoes mitosis, the chromosomes condense even further.

Prokaryote cell structure & function

This activity includes a full set of resources for teaching about prokaryotes, a student activity sheet, with examiner's hints, a short screencast explaining how to draw a prokaryote cell,flashcards to help students' learn the prokaryote labels,a quick test of the labels & IB style questions, with answers and as an extension activity an arcade game to further consolidate learning.

Prokarote cells outnumber human cells ten to one in the human body. Ref: American Society for Microbiology. "Humans Have Ten Times More Bacteria Than Human Cells: How Do Microbial Communities Affect Human Health?." ScienceDaily, 3 Jun. 2008. Web. 4 Jul. 2012

Lesson Description

Guiding Question

How can a Biologist illustrate the structures of prokaryote cells which are so small in electron microscope images that they are barely visible, even to the trained eye of the microbiologist?

Activity 1: Screencast

Play this screencast and complete the Prokaryote diagram student worksheet

Screencast using the Diagram Activity Sheet & Nice microscope image of a real prokaryote cell

Activity 2: Learn the labels

Study the diagram label flashcards and then try one of the online activities with the quizlet cards below.

Activity 3: IB Style questions

Quick test - questions to test knowledge of the labels

Test understanding with these IB style questions

Teachers' notes

This page is part of a series of lesson resources for the diagrams of the IB syllabus.

There are many ways to use this page. Students can work independently or the resources can be projected in the classroom. Worksheets allow you to print as much or as little as you wish. The online activities can be guided or let the students choose. The choice is yours.

Answers to the questions can be found in the resources section. Teachers can makes these separate pages available to students once the questions are completed.

A Suggested Lesson Plan.

Starter: 10 minutes

  • Students complete the Diagram Activity worksheet while watching the Screencast as a whole class.

Main: 40 minutes

  • Students study printed (or online) Flashcards to help memorizing of the labels
  • Students take a Quick test - to see how well they understand the labels
  • Students review the Screencast and attempt the diagram (with labels) from memory.

End: 10 minutes

  • Plenary - students play the arcade game to further consolidate learning
  • Differentiation - students who need additional time for main can continue independently.

Complete the IB style questions.

Interesting extension

Following a comment from Anthony Rose I have recently adjusted the description of prokaryote flagella to include name of the protein flagellin and to remove any mention of microtubules.

I have learned that E.coli produces approximately four tubular filaments at one pole of the cell. These flagella filaments can change shape and can rotate independently in both clockwise and anticlockwise directions thanks to an ingenious microscopic motor made from an assemblage of a few dozen proteins. E.coli can thus change direction as is moves.

It is thought that the Eukaryote and the Prokaryote flagella are an example of convergent evolution (useful in topic 5) and that there may be a third type of flagella in the Archea (the other Domain in classification of organisms!)

There is even a theory that flagella could be an example of endosymbiosis and there is an ongoing debate about the 'irreducible complexity' of flagella. The idea that without just one protein, a flagella stops working, so it could be evidence that evolution by natural selection couldn't have happened. However, evolutionary scientists refute these claims citing several mechanisms which are possible.

Watch the video: Prokaryotic Cells - Introduction and Structure - Post 16 Biology A Level, Pre-U, IB, AP Bio (January 2022).