A single-celled world: all about prokaryotes

While the term ‘prokaryote’ may not be all that familiar, everyone will have heard of the most common examples – bacteria. Often referred to as microbes as a nod to their microscopic size, these creatures, while tiny, pack an outsized punch. And they’re everywhere.

The name prokaryote means ‘without a nucleus’, meaning they lack the distinguishing feature of the more complex cells that make up organisms like you and me (‘eukaryotes’). Despite their comparative simplicity, they are in fact much more ancient than eukaryotes and have played a key role in the history of life on Earth in the almost 4 billion years they’ve been here.

Here I’ll introduce some key features of prokaryotes, explaining how their cells work, different types, and why they’re important for all other organisms and ecosystems across the planet.

Firstly, let’s get clear on what prokaryotes are and are not.

Many people confuse prokaryotes with other, similar, tiny organisms, which is understandable – they all have odd names and are too small to see, so can be difficult to get your head round at first.

Evidence from RNA (a molecule in all living cells very like the more famous DNA) taken from different types of microbes shows that what scientists used to refer to as one big group are actually very diverse, and fall into two main groups. Prokaryotes are now known to belong to either Bacteria (also called Eubacteria) or Archaea, which each have slightly different cell structure. Archaea are often associated with extreme environments – very hot or very acidic, like hydrothermal vents in the deep sea for instance – but in reality they, like bacteria, exist everywhere.

More on those in a later lesson. For now: prokaryotes include bacteria and another group of organisms like them. What are not prokaryotes are viruses, another common point of confusion. They are something altogether different, and best left to another time!

As well as having no nucleus, prokaryotic cells differ from other cell types in that they lack the other cellular sub-compartments you’ll be familiar with in animal and plant cells. They have no chloroplasts or mitochondria, or any other organelles like these that are bound by a membrane. They do have some features in common with eukaryotic cells: a cell surface (or plasma) membrane, cytoplasm, ribosomes, and genetic material, either in the form of DNA, or RNA (mentioned above).

Prokaryotes have a number of additional features that mark them out. These include a cell wall that protects and supports the cell, enclosing it around its cell surface membrane. This is similar to the cell wall of plant cells, though are made of different materials. Where plant cell walls are made of cellulose, those of bacteria for example are made of a substance called peptidoglycan.

They also keep their genetic material in an area of the cell called the nucleoid. Many prokaryotes have their DNA in the form of a circle, rather than linear chromosomes. They may also have other small rings of DNA called plasmids, which they’re able to transfer between each other. While eukaryotes have complicated packaging around their DNA (more in another lesson), prokaryotes do not.

While it differs between prokaryotic species, some may have additional structures on their external surface, including pili or flagella, which can be involved in helping the cell communicate or move around.

As we’ve hinted at, there are many different types of organism falling under the heading ‘prokaryote’. This diversity can also be seen in that they can be found in almost every environment on Earth, and often live closely with other species. This is becoming better known today as we hear more about the human gut microbiome, which refers to the many billions of prokaryotic cells that live in our guts. Though it might seem counterintuitive, we need them to stay healthy.

Living across diverse environments means prokaryotes have adapted to make use of a range of resources to sustain themselves. We call this metabolic diversity, meaning they have a variety of ways of metabolising resources. Scientists are using this ability nowadays to try to engineer bacteria to digest novel resources, like plastic, that we need help getting rid of.

But naturally occurring bacteria are just as interesting. Some are photosynthetic, like plants, able to produce their own carbohydrates using sunlight. Others can live in places with very low oxygen, like water-logged soils, where they play vital roles in local ecosystems just as they do everywhere else. We’ll learn about the contributions of prokaryotes to various ecological cycles in a later topic, but for now know that they are important in everything from breaking down matter from dead organisms and allowing it to return to the food chain, to ensuring nitrogen in the atmosphere gets into the soil in a form that plants can use.

So, despite their simplicity, prokaryotes play a disproportionate role across our planet. More and more attention is deservedly being paid to these tiny creatures in the scientific world and outside it. You’ll find they crop up all over the A Level syllabus, most notably in the infectious disease topic, but also later when we learn about biotechnology and genetic engineering, and at many other points along the way.