Do prokaryotic cells have a plasma membrane?
Yes, prokaryotic cells do have a plasma membrane which is also called cell membrane. This plasma membrane is one of the three protective layers of the prokaryotic cell.
In a broader term if we say then it is to be mentioned that prokaryotic cells have a cell envelope that is made up of three protective layer: the glycocalyx (outer layer), the cell wall (middle layer), and the plasma membrane (inner layer).
The primary function of Glycocalyx layer is protection, of cell wall is structure and rigidity, of the Plasma membrane is to regulate the movement of substances in and out of the cells.
The structure of plasma membrane in both prokaryotic and eukaryotic cells is almost same with ignorable to a little differences between the various prokaryotic species and strains.
Actually, as commonly seen, both eukaryotic and prokaryotic cells have a semi-fluid phospholipid bilayer cell membrane.
In both types of cells, the cell membrane seems to be non-permeable to ions, proteins, and other molecules, while being permeable to other molecules that may move through the membrane.
This plasma membrane is the only membrane that has direct contact with the cytoplasm. The other two membranes need to communicate directly with the plasma membrane in order to consider with anything inside the cell.
So, this plasma membrane being the inner layer of protection, and cellular entry-exit route, surrounds the cytoplasm of a prokaryotic cell.
So, we can conclude that this membrane physically separates the cytoplasm from the outside environment.
To learn more precisely and clearly about this topic, you can also follow this article about “Do all cells have a cell membrane?” Or, just keep reading. There’s more to learn here in this post.
What is the plasma membrane in prokaryotes?
The plasma membrane in prokaryotes is one of the three layers of the cell envelope which is the inner most layer of the cell membrane. This is a lipid bilayer membrane that consists of phospholipids, proteins, carbohydrates, and very less amounts to no cholesterol at all.
We can term this plasma membrane as the cell membrane or lipid bilayer membrane or phospholipid membrane as well.
This plasma membrane is called cell membrane because it is one of the inner layer of cell envelope.
While the plasma membrane is called phospholipid membrane because this membrane consists of phosphorus and lipid molecules, with different sized head containing water-loving molecules and different sized tail containing water-hating molecules.
This plasma membrane together is made up of phospholipids, along with proteins, carbohydrates, and little to no cholesterol molecules as well.
Also, note that the word ‘plasma’ is derived from the word ‘plasm’ which means all those material forming cells meaning a living substance.
So, the plasma membrane is the membrane surrounding the plasma. So, that’s why the cell membrane is also termed the plasma membrane or plasmalemma.
The plasma membrane holds all the material inside the cell and the cytoplasm is the fluid that is present inside the plasma membrane.
In some books of biology, many authors have referred to all contents inside of the cell membrane as the cell’s plasma. Here, this plasma is the “filling” of the cell that holds all of the cell’s organelles.
So, the outermost membrane of the cell is sometimes called the cell membrane and sometimes called the plasma membrane, because that is what it is in contact with.
And we cannot imagine a cell in this world of biology without plasma membrane. Meaning that all cells, let it be prokaryotic or eukaryotic cells, have plasma membrane in common.
How many cell membranes do prokaryotes have?
A majority of the prokaryotes have a single cell membrane surrounding the cytoplasm of the cell.
It is a double layer membrane made up of phospholipid molecules that is why it is also said bilayer lipid membrane.
While a few of the prokaryotes that falls under the Gram-negative bacterial category such as Salmonella typhi, Escherichia coli, Haemophilus influenzae, etc. have two cellular membranes.
The two cellular membranes viz. the plasma membrane and an outer membrane, are separated by a periplasm with having a periplasmic space between 13 to 25 nm.
Both of these cellular membranes are lipid bilayer in nature with the periplasm in between having a multipurpose function that allows more efficient and diverse mechanisms of protein oxidation, folding, and quality control for the cell.
So, the structure of the cell membrane is the same. The structure can be considered as a two-dimensional liquid in which lipid, cholesterol molecules, and a large quantity of proteins molecules diffuse more or less easily.
Let the organism have a single cell membrane or two cell membranes, the point is that these membranes are usually responsible for the various cells’ biological functions like cell adhesion, communication, endocytosis, and exocytosis.
Just after the cell membrane, you will find the cytoskeleton in all cells which is actually a dynamic network of various interlinking protein filaments extending from the cell nucleus to the cell membrane.
This cytoskeleton helps cells maintain their shape and internal organization by supporting and keeping the structure of the plasma membrane in place and also aiding in cell division and cell movement.
How the plasma membrane differ between prokaryotes and eukaryotes?
The prokaryotic plasma membrane is different from the eukaryotic plasma membrane as it posses essential infolding called mesosomes. Mesosomes are formed by the extension of the plasma membrane into the cell.
Also that, the plasma membrane of prokaryotic cells is smaller than those of the eukaryotic ones. It’s because the diameter of a prokaryotic cell ranges between 0.1–5.0 µm in diameter, which is very significantly smaller than eukaryotic cells which have diameters ranging from 10–100 µm.
The plasma membrane of prokaryotes can have very little to no sterols. Meaning that the sterols like cholesterol are usually absent or present in minute amounts in the membrane.
While in the other case, eukaryotic plasma membranes contain a significant amount of carbohydrates and sterols, such as cholesterol.
If eukaryotes possess cholesterol sterol in significant amounts, then prokaryotes contain hapanoid which is another derivative of sterol in very significant amounts.
As prokaryotes lack sterols, it has been seen that the prokaryotic plasma membranes are significantly less rigid and stable than eukaryotic plasma membranes.
It is also to be noted that Prokaryotic plasma membranes have only a few types of phospholipid molecules. While in the case of eukaryotic plasma membranes there are more than 7 different types of phospholipid molecules.
In prokaryotes, the plasma membrane work in association with the glycocalyx layer and the cell wall layer to form the cell envelope.
While in the case of eukaryotic plant cell, the plasma membrane remain in association with the cell wall to form the cell boundary. And in the case of eukaryotic animal cell, the plasma membrane is the only membrane forming the cell boundary alone.
Do all cells have plasma membrane? What cells do not have a plasma membrane?
Yes, all cells (eukaryotic and prokaryotic) have plasma membrane. As the plasma membrane is a very essential cellular feature and a cell boundary marking, we can’t imagine any cell without having the cell membrane.
Prokaryotic cells have no membrane bound cell organelles but the presence of a plasma membrane is there as no cell is actually a cell with any cellular boundary.
While eukaryotic cells have both membrane bound cell organelles along with the cell’s plasma membrane that bounders the cell very well.
All cells have plasma membrane as without this semi-permeable membrane almost any chemical would be allowed to enter the cell leading to cell rapture in most of the cases.
So, the plasma membrane due to its selectively permeable nature only allows selected materials to move in and out of the cell as required either actively or passively, along with giving the cell a rigid protection against any biological disaster.
So, it’s due to the plasma membrane that forms a direct connection with the cytoplasm that the cells can stay alive.
It does so by maintaining a fixed environment inside the cell, by importing nutrients inside the cell, and also by exporting toxic substances out of the cell.