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Prokaryotic Cell Structure: A Visual Guide

Updated on February 24, 2017
The gneralised structure of the Prokaryotes
The gneralised structure of the Prokaryotes | Source

What Are Prokaryotes?

Prokaryotes are some of the oldest lifeforms on our planet. They have no nucleus and show huge variation. Many people know them better as 'bacteria' but, although all bacteria are prokaryotes, not all prokaryotes are bacteria.

Eukaryotes have diversified into forms that have taken to the air, seas and earth; they have evolved into forms that can reform the Earth itself. However, they are still outnumbered, outcompeted and outdiversified by Prokaryotes. The prokaryotes comprise the most successful division of life on our planet.

Quite different from the membrane-bound organelles of the Eukaryotes, the Prokaryotes are a stunning example of how there are many ways to build a cell, many ways to survive, and many ways to thrive.

Prokaryote Cell Growth

Why are Bacteria so Successful?

It is not the largest or the most intelligent of species, but those most adaptable to change who will survive long term - just ask the dinosaurs. It is in this respect that prokaryotes excel.

Prokaryotes divide rapidly. The doubling time across the group varies massively; some divide in a matter of minutes (E. coli - 20mins under optimum conditions; C. difficile - 7mins at optimum) others in a matter of hours (S. aureus - around an hour) and some double their number over days (T. pallidum - around 33hours). Even the longest of these doubling times is still hugely faster than the reproduction rates of eukaryotes.

As natural selection works on the generational time scale, the more generations that pass, the more 'time' natural selection has to select for or against the clay of evolution - the genes. As a batch of E. coli can double (with perfect conditions) 80 times in a 24 hour period, this provides huge opportunity for advantageous mutations to arise, be selected for, and spread throughout the population. This is, in essence, how antibiotic resistance develops.

This huge capacity for change is the secret of prokaryote's success.

Structure of Prokaryotic Cells

Prokaryotic cells are much older than Eukaryotes. Prokaryotes lack any membrane-bound organelles; that means no nucleus, no mitochondria or chloroplasts. Prokaryotes often have a slimy capsule and flagella for movement.
Prokaryotic cells are much older than Eukaryotes. Prokaryotes lack any membrane-bound organelles; that means no nucleus, no mitochondria or chloroplasts. Prokaryotes often have a slimy capsule and flagella for movement. | Source

Cell Structure

Structure
Prokaryotes
Eukaryotes
Nucleus
No
Yes
Mitochondria
No
Yes
Chloroplasts
No
Plants only
Ribosomes
Yes
Yes
Cytoplasm
Yes
Yes
Cell Membrane
Yes
Yes
Capsule
Sometimes
No
Golgi Apparatus
No
Yes
Endoplasmic Reticulum
No
Yes
Flagellum
Sometimes
Sometimes in animals
Cell Wall
Yes (not cellulose)
Plants and Fungi only
Eukaryotes and Prokaryotes share many features, but there are many key differences. Prokaryotes have no membrane-bound organelles, and Eukaryotes have no capsule, and their cell walls are structured differently

Prokaryotic Cell Micrograph

A fake colour micrograph of dividing E. coli
A fake colour micrograph of dividing E. coli | Source

Cytoplasm

The cytoplasm plays, if possible, an even more important role in prokaryotes than it does in eukaryotes. It is the site of all chemical reactions and processes that take place in the prokaryotic cell.

Another deviation from the eukaryotic cell is the presence of small, circular, extrachromosomal DNA known as plasmid. These replicate independently of the cell, and can be passed on to other bacterial cells. This occurs in two ways. The first is obvious - when the bacterial cell divides via a process called binary fission - plasmids are often passed on to the daughter cell because the cytoplasm is divided equally between the cells.

The second method of transmission is through bacterial conjugation (bacterial sex) where a modified pilus will be used for transfer of genetic material between two bacterial cells. This can result in a single mutation spreading through an entire bacterial population. This is why it is so important to finish any course of antibiotics prescribed. A single surivor can spread its advantageous genes to existing bacteria in your body, and any progeny of the cell will share its antibiotic resistance.

Plasmids can encode genes for virulence, antibiotic resistance, heavy metal resistance. These have been hijacked by humanity for genetic engineering

The DNA is in one long strand kept in a special area of the cytoplasm called the Nucleoid. It may look dark on a micrograph, but don't make the mistake of calling it a Nucleus!
The DNA is in one long strand kept in a special area of the cytoplasm called the Nucleoid. It may look dark on a micrograph, but don't make the mistake of calling it a Nucleus! | Source

Nucleoid

Prokaryotes are named for their lack of nucleus (pro= before; karyon=kernal or compartment). Instead, Prokaryotes have a single continuous strand of DNA. This DNA is found naked in the cytoplasm. The region of the cytoplasm where this DNA is found is called the 'Nucleoid'. Unlike eukaryotes, prokaryotes do not have several chromosomes...although one or two species do have more than one nucleoid.

The Nucleoid is not the only region where genetic material can be found, however. Many bacteria has circular loops of DNA called 'plasmids' that can be found throughout the cytoplasm.

The DNA is also organised differently in Prokaryotes and Eukaryotes.

Eukaryotes wrap their DNA carefully around proteins called 'histones'. Think of how cotton wool is wrapped around its spindle. These are laid on top of each other in rows to give the appearance of 'beads on a string'. This helps condense the enormous length of DNA into something small enough to fit into a cell!

Prokaryotes do not package their DNA in this way. Instead, prokaryotic DNA twists and twines around itself. Imagine twisting a couple of bracelets around one another.

Eukaryotes vs Prokaryotes

Ribosomes

Any difference between Eukaryotic and Prokaryotic cells has been exploited in the ongoing war with pathogenic bacteria, and the ribosomes are no exception. At its most simple, the ribosomes of bacteria are smaller, made of different subunits than those of eukaryotic cells. As such, antibiotics can be designed to target prokaryotic ribosomes whilst leaving the eukaryotic cells (e.g. our cells or the cells of animals) unharmed. With no functioning ribosomes, the cell cannot complete protein synthesis. Why is this important? Proteins (usually enzymes) are involved in almost all cellular functions; if proteins cannot be synthesised, the cell cannot survive.

Unlike in eukaryotic cells, ribosomes in prokaryotes are never found bound to other organelles

Low-temperature electron micrograph of a cluster of E. coli bacteria, magnified 10,000 times
Low-temperature electron micrograph of a cluster of E. coli bacteria, magnified 10,000 times | Source

The Prokaryotic Envelope

There are many common structures inside a prokaryotic cell, but it is the outside where we can see most of the differences. Each prokaryote is surrounded by an envelope. The structure of this varies between prokaryotes, and serves as a key identifier for many prokaryotic cell types.

The cell envelope is made up of:

  • A Cell Wall (made of peptidoglycan)
  • Flagella and Pili
  • A capsule (sometimes)

Prokaryotes

Did you know Bacteria were part of the Prokaryotes?

See results
Coloured Electron Micrograph of Pseudomonas fluorescens. The capsule provides protection for the cell and is seen in orange. Flagella are also seen (whiplike strands)
Coloured Electron Micrograph of Pseudomonas fluorescens. The capsule provides protection for the cell and is seen in orange. Flagella are also seen (whiplike strands) | Source

Capsule

The capsule is a protective layer possessed by some Bacteria that enhances their pathogenicity. This surface layer is made up of long strings of polysaccharides (long chains of sugar). Depending on how well this layer is stuck to the membrane it is called either a capsule or, if not well adhered, a slime layer. This layer enhances pathogenicity by acting as an invisibility cloak - it hides the cell surface antigens that white blood cells recognise.

So important is this capsule to the virulence of certain bacteria, that those strands without a capsule do not cause disease - they are avirulent. Examples of such bacteria are E. coli and S. pneumoniae

Bacterial cell walls are categorised according to whether they take up Gram Stain. Therefore they are named Gram positive and Gram Negative
Bacterial cell walls are categorised according to whether they take up Gram Stain. Therefore they are named Gram positive and Gram Negative | Source

Prokaryotic Cell Wall

The Prokaryotic Cell Wall is made of a substance called peptidoglycan - a sugar-protein molecule. The precise make up of this varies hugely from species to species, and forms the basis of prokaryotic species identification.

This organelle provides structural support, protection from phagocytosis and dessication and comes in two categories: Gram Positive and Gram Negative.

Gram Positive cells retain the purple gram stain because their cell wall structure is thick and complex enough to trap the stain. Gram Negative cells lose this stain because the wall if much more thin. A diagramatic representation of each type of cell wall is given opposite.

Flagellum Types

Pili

Bacterial Conjugation. Here we can see a plasmid being transferred along this pilus to another cell. This is how antibiotic resistance can be passed on to other pathogens
Bacterial Conjugation. Here we can see a plasmid being transferred along this pilus to another cell. This is how antibiotic resistance can be passed on to other pathogens | Source

Flagella and Pili

All living things react to their environment, and bacteria are no different. Many bacteria use flagella to move the cell towards or away from stimuli such as light, food or poisons (such as antibiotics). These motors are marvels of evolution - far more efficient than anything humanity has created. Contrary to common belief, these structures can be found all over the surface of a bacterium, not just at the end.

The video looks at some of the different organisations of flagella (sound quality is slightly fuzzy).

Pili are smaller, hairlike projections that sprout over the surface of most bacteria. These often act as anchors, securing the bacterium to a rock, intestinal tract, tooth or skin. Without such structures, the cell loses virulence (its' ability to infect) as it cannot hold on to the host structures.

Pili can also be used to transfer DNA between different prokaryotes of the same species. This 'bacterial sex' is known as conjugation, and allows for more genetic variation to develop.

How Small are Prokaryotes?

Prokaryotes are smaller than animal and plant cells, but much larger than viruses.
Prokaryotes are smaller than animal and plant cells, but much larger than viruses. | Source

How do Antibiotics work?

Unlike cancer therapy, the treatment of pathogens is usually well targetted. Antibiotics attack proteins or structures (such as the capsule or pili) that have no eukaryotic counterpart. Due to this, the antibiotic can kill prokaryotes whilst leaving the eukaryotic cells of the animal or human intact.

There are several classes of antibiotics, classified according to how they work:

  1. Cephalosporins: first discovered in 1948 - they prevent proper production of a bacterial cell wall.
  2. Penicillins: the first class of antibiotic discovered in 1896 then rediscovered by Flemming in 1928. Florey and Chain isolated the active ingredient from the penicillium mould in the 1940s. Prevent proper production of bacterial cell walls
  3. Tetracyclins: interfere with bacterial ribosomes, preventing protein synthesis. Due to more pronounced side-effects, this is not often used with common bacterial infections. Discovered in the 1940s
  4. Macrolides: another protein synthesis inhibitor. Erythromycin, the first of its class, was discovered in the 1950s
  5. Glycopeptides: prevent polymerisation of the cell wall
  6. Quinolones: interefere with important enzymes involved with DNA replication in prokaryotes. Due to this they have very few side-effects
  7. Aminoglycosides: Streptomycin, which was also developed in the 1940s, was the first to be discovered in this class. They bind to the smaller bacterial ribosome subunit, thus preventing protein synthesis. These do not work well against anaerobic bacteria.

Video Review of Prokaryotic Cells

© 2011 Rhys Baker

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    • TFScientist profile image
      Author

      Rhys Baker 3 years ago from Peterborough, UK

      Not weird-prudent. They are, after all, the dominant life form on our planet!

      Thanks for stopping by and welcoming me back. Hopefully you will check this out again when it is finished? You will be able to test your knowledge in the mighty bacteria quiz :)

    • Londonlady profile image

      Laura Writes 3 years ago

      Is it weird to say I love bacteria? Maybe a little, but now that I get to do research on them I find them even more fascinating little things