Prokaryotes vs Eukaryotes
We know how to tell if something is alive or not, but if a bacterium and a dog are both living organisms, then what differentiates them? There are actually two distinct types of living beings, prokaryotes and eukaryotes, each made up of specialised prokaryotic and eukaryotic cells. In the three phylums of life—bacteria, archaea, and eukarya—prokaryotes cover the first two, and eukaryotes cover eukarya. You can probably already guess which groups a bacterium and a dog belong to, but let’s find out why.
All cells (i.e., both prokaryotes and eukaryotes) contain four common structures:
- A plasma membrane, which is a “barrier” that separates the cell from the outside world, like how your skin prevents your organs from falling out.
- The cytoplasm, which is the jelly-like substance that takes up the spaces inside the cell that aren’t already occupied by organelles.
- Nucleic acids, the genetic material, which tell the cell how to operate and reproduce.
- Ribosomes, where protein synthesis takes place according to the information contained in the genetic material. Proteins are organic compounds essential to living organisms, and they’ll be explored in more detail in a later article.
But there are also fundamental differences between living cells. Prokaryotic organisms as a whole are much smaller than eukaryotes, because they’re just made up of single cells, while eukaryotic organisms are made up of many specialised cells. The size of individual cells is different, too: prokaryotes are about 1-10 µm (micrometres) in diameter, while eukaryotes are 10-100 µm. If you want to get your head around the scale of things, go nuts with this interactive page.
Prokaryotes also lack a nuclear compartment and other membrane-bound organelles (which are like little organs within cells, each performing specific functions), so their genetic material and basic functioning processes happen out in the open, in the cytoplasm. This allows for less specialisation, so prokaryotes turn out to be pretty simple cells.
They reproduce asexually by binary fission, meaning that each cell splits in two to create a copy of itself. This gives rise to less diversity, but there is some scope for something called “horizontal gene exchange”: directly exchanging genetic information between the same generation, as opposed to passing genetic information onto the next generation. See illicit bacterial sex tape here.
Eukaryotes, on the other hand, have a range of organelles designed to perform specialised functions, such as the mitochondria, which creates the cell’s energy, the chloroplast, which converts light energy to chemical energy in plants, and the Golgi body, which modifies and processes proteins. This “compartmentalisation” allows for greater complexity—different compartments can have different functions even if they conflict, because they’re sealed off from each other.
Eukaryotes divide and reproduce by mitosis (the division of cells for tissue growth) and meiosis (the division of sex cells), and what results is two parents passing their genetic information onto the next generation. This creates the opportunity for more diversity, though it’s a longer process—some prokaryotes can divide and create a new organism in 20 minutes flat, while in humans it’s just a tad longer than that.
So what’s the difference between a bacterium and a dog? You can probably answer that yourself: bacteria are prokaryotic organisms and dogs are eukaryotic.
Further resources: Comparison table and Khan Academy video