Classification and structure of prokaryotic cells: MCAT

Medistudents Team
May 3, 2023

The classification and structure of prokaryotic cells in a MCAT topic included in the Biological and Biochemical Foundations of Living Systems section of the exam. To achieve a good MCAT score, it’s essential that you prepare for all key topics listed in the MCAT syllabus.

To help you to revise the classification and structure of prokaryotic cells topic, this comprehensive overview covers everything you need to know, including:

  • Prokaryotic domains
  • Major classifications of bacteria by shape
  • Lack of nuclear membrane and mitotic apparatus
  • Lack of typical eukaryotic organelles
  • Presence of cell wall in bacteria
  • Flagellar propulsion

For more information about the growth and physiology of prokaryotic cells for the MCAT, visit our blog.

Prokaryotic domains

Prokaryotes (a kingdom) have two domains – Archaea and Bacteria.


Bacteria are known to be the simplest form of life as they are single-celled organisms devoid of any membrane-bound organelles and are typically around 1-2 μm in diameter. Some species of bacteria have incredibly short generation times (<30 minutes) and can therefore lead to rapid progression of bacterial diseases. However, not all bacteria are pathogenic (harmful), some have a symbiotic (beneficial) relationship with other species such as plants or even humans!


Archaea are single-celled organisms that have a structure similar to bacteria but are able to survive in extreme environments, such as extreme temperature, pH or levels of radiation for example. Ribosomal RNA sequencing has shown that archaea are evolutionarily and genetically distinct from bacteria and are more closely related to Eukarya. Research has shown that almost all archaea are not harmful to humans however studies are ongoing to look into the opportunistic pathogenesis of archaea.

Classification of bacteria by shape

The MCAT syllabus states that you must understand the major classifications of bacteria by shape, which include:

  • Bacilli (rod-shaped)
  • Spirilli (spiral-shaped)
  • Cocci (spherical)

Bacilli (rod-shaped)

Bacilli are bacteria or archaea that are rod-shaped and can only divide into one plane. Some well known examples of bacilli include Escherichia coli and Aspergillus nidulens.

Most bacilli are a single rod but you can also have a pair of rods (known as diplobacilli), an elongated chain of rods (known as streptobacilli) or a single rod that appears rounder than a normal rod (known as coccibacilli).

MCAT: A photomicrographic view of Moraxella lacunata, a diplobacillus.
Figure 1: A photomicrographic view of Moraxella lacunata, a diplobacillus.

Spirilli (spiral-shaped)

Spirilli are bacteria/archaea that have a curved shape (from gently curved to tightly corkscrewed). A common example of spirilli is Helicobacter pylori.

Vibrio are spirilli with less than one complete twist – creating a comma-shaped cell. Spirillum is a cell that has a rigid helical structure and spirochete is a cell with a flexible helical structure.

MCAT: A false coloured electron micrograph of Vibrio vulnificus
Figure 2: A false coloured electron micrograph of Vibrio vulnificus.

Cocci (spherical)

Cocci are bacteria or archaea that are spherical-shaped and can divide in more than one plane, creating a variety of arrangements. Some common examples of cocci bacteria include Staphylococcus and Streptococcus.

There are two classes of cocci: truly round coccus cells that usually divide in alternating perpendicular planes leading to the cells forming tetrads or three-dimensional cuboidal shapes and; organisms whose cells are elongated to form ellipsoid cells and then divide into successive parallel planes creating diplococci (two cocci together) or small chains of coccus cells. These various arrangements are shown in Figure 3 below.

MCAT: Cocci division – (a) represents a chain of cocci, (b) represents diplococci, (c) represents tetrads of cocci and (d) represents the cubodial arrangement of cocci.
Figure 3: Cocci division – (a) represents a chain of cocci, (b) represents diplococci, (c) represents tetrads of cocci and (d) represents the cubodial arrangement of cocci.

Lack of nuclear membrane and mitotic apparatus

Prokaryotes lack a nucleus and therefore their genetic material is not surrounded by a nuclear envelope. Instead, most prokaryotes carry their genetic material in a single molecule/chromosome of circular DNA in the central region of the cell known as a nucleoid. They also can carry plasmids in their cell (small amounts of circular DNA distinct from the DNA carried in the nucleoid).

Eukaryotes use specific apparatus (like spindles) in mitosis in order to separate the replicated eukaryotic chromosomes. In prokaryotes (devoid of chromosomes) they attach each copy of their genetic material to different parts of the cell membrane and then as the cell pulls apart so does the genetic material.

Lack of typical eukaryotic organelles

Eukaryotic cells have membrane-bound organelles that allow biochemical reactions to be compartmentalized within the cell. Prokaryotic cells do not usually have these types of organelles and therefore most chemical reactions happen in the same environment – the cytosol.

However, membrane-bound organelles can be found in specific groups of prokaryotes. For example, some types of aquatic cyanobacteria have gas vacuoles which allow them to control their position in a water column.

Presence of cell wall

The cell wall forms a rigid structure around the cell for protection whilst also allowing substances to pass through it. Cell walls also allow the cell to maintain its shape when changes in osmotic pressure occur.

Bacteria can be divided into Gram-negative and Gram-positive and each can be identified by examining the cell wall structure. In Gram-positive bacteria, the cell wall is thick and largely composed of peptidoglycan. It also lacks an outer lipopolysaccharide membrane making the cell wall more absorbent and less resistant to antibiotics. In Gram-negative bacteria, the cell wall has a much thinner layer of peptidoglycan but a lipopolysaccharide outer membrane as well; therefore making Gram-negative bacteria more resistant to antibiotics.

It must be noted that having peptidoglycan in a cell wall is a characteristic of prokaryotes only. Plant cells, for example, have a cell wall made of cellulose.

Flagellar propulsion

Most prokaryotes are motile usually due to the presence of specific structures like flagella but could also be due to the presence of gas vesicles (which allow some aquatic species to move up or down in a water column).

Flagella are long, thin appendages made up of flagellin. They have a helical shape and are attached to a structure similar to a ‘motor’. To move forward, flagella must rotate counter-clockwise (clockwise rotation to move backwards). There are two main types of flagella movement either running, where the cell moves forward in a straight line, or tumbling, where the flagella of the cell spread out and rotate clockwise, creating a tumbling motion with no true direction.

Prokaryotes perform chemotaxis, a process where a cell will move towards an attractant by responding to concentration gradients. In chemotaxis, the number of ‘runs’ outweighs the number of ‘tumbles’. When there is no attractant the number of ‘runs’ and ‘tumbles’ are equal and the cell will move in a random direction.

Hopefully you now have a better understanding of the classification and structure of prokaryotic cells for the MCAT. For more revision materials, visit our MCAT page, which has blogs covering a wide range of MCAT topics, including the metabolism of fatty acids and proteins and the principles of metabolic regulation.