Professor Dave here, let’s learn about prokaryotic cells.
As we just discussed, we may not know exactly how it happened, but we do have a pretty reasonable
grasp of how the self-assembly of the first primitive cells may have happened, and these
first cells that existed billions of years ago were much simpler than the cells that
are present in all the plants and animals that are alive today.
These very first living things, the first earthlings in effect, were made of just one
cell, and we can categorize these cells as being prokaryotic.
Let’s go through the features of a prokaryotic cell now.
First, any cell must have some kind of plasma membrane that separates what’s inside the
cell from what’s outside of the cell.
This is the barrier between the organism and its environment, made of a phospholipid bilayer
and the many different proteins floating around within it.
The membrane lets some things through and prevents others, so that the organism can
take in food and energy and expel waste.
Beyond this membrane, most prokaryotes also have a rigid structure called the cell wall,
made of peptidoglycan, a polymer of sugars and amino acids that creates a mesh-like layer
around the cell.
The interior of the cell is generally called the cytoplasm, but the actual fluid present
within the membrane is more specifically called the cytosol.
This is a jelly-like substance where all the cellular components float around.
These components include a nucleoid, which is the region where the genetic information
for a prokaryotic cell can be found.
This genetic information is a chromosome or set of chromosomes, which are huge DNA molecules.
We learned all about DNA and its functions in the biochemistry course, but if you’re
new to DNA, it’s a nucleic acid made of millions of atoms, and certain portions of
this molecule are called genes.
These are the parts that code for proteins.
During transcription and translation, molecules read these genes and deliver the instructions
to ribosomes, complex macromolecules made of RNA and proteins, which also sit in the
cell.
These ribosomes take the instructions and build all the different proteins that the
cell needs to survive.
These include enzymes that copy DNA, as well as ones that float around in the cytosol and
perform metabolic functions, producing the energy needed for the cell to be able to move
around and reproduce.
Since these enzymatic functions require that certain substances be transferred into and
out of the cell, and only a certain amount of these materials can pass through the plasma
membrane at any given time, this gives us a clue as to why cells are the size that they
are.
To best supply the cell with the necessary tools for survival, it ought to have a high
surface area to volume ratio, to maximize exchange with the surroundings while minimizing
the volume inside that has to be serviced.
This ratio shrinks as cells get bigger, so nature seems to have found a good anchor point
for cell size, making them somewhere around a hundredth of a millimeter in diameter, depending
on the type of cell we are looking at.
When we later examine larger organisms, these will not contain bigger cells, but simply
more cells, since the rules that govern ideal cell size apply to all life on earth.
Speaking of larger organisms, the cells of a human are not prokaryotic.
Any prokaryotic cell will be a unicellular organism, meaning it is made of just one cell,
like bacteria and archaea, and although these are so tiny, they are incredibly numerous.
They are literally everywhere, including inside your body.
But we humans are multicellular, meaning one human is made of many cells, trillions in
fact, and any multicellular organism is made of eukaryotic cells.
What are those and how do they differ?
Let’s take a look at them next.