Do all cells have a cell membrane? – (ANSWERED)

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Do all prokaryotic cells have a cell membrane?

Yes, all prokaryotic cells have a cell membrane. `The cell membrane is also known as the plasma membrane.

In prokaryotic cells, just like in the cell of bacteria, etc. there’s not just the cell membrane. They have an overall cell-protective layer known as the cell envelope.

The cell envelope of a prokaryotic cell consists of a tightly bound three-layered structure i.e. the outermost glycocalyx followed by the cell wall and then the cell membrane.

Although each layer of the envelope performs a distinct function, they act together as a protective unit.

The plasma membrane (also called cell membrane) in prokaryotes is composed of a semi-fluid lipid bilayer model. It is a thin lipid bilayer (6 to 8 nanometers) that completely surrounds the cell and separates the inside from the outside.

It’s non-permeable to ions, proteins, and other molecules, while permeable to other molecules that may move through the membrane.

The cell membrane of the prokaryotes is not so hard and protective as those of the eukaryotes. And as the prokaryotes live in some of the most harsh habitats therefore the protection of their cell is a must.

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So, the cell membrane of prokaryotes is supported by other two protective layers above it which are the glycocalyx layer and the cell wall layer.

The glycocalyx exists in bacteria either as a capsule or as a slime layer. In a capsule, polysaccharides are firmly attached to the cell wall, while in a slime layer, the glycoproteins are loosely attached to the cell wall.

The cell wall is another protective layer that surrounds the cells and gives them the shape and rigidity. It is located outside the cell membrane and prevents osmotic lysis (bursting due to increasing volume).

Cell membrane and its structure
Cell Membrane and its structure
(Derivative work: Mariana Ruiz Villarreal / CC BY-SA)

Do all eukaryotic cells have a cell membrane?

Yes, all eukaryotic cells have a cell membrane. Among the eukaryotes, some protists, and all fungi and plants, have cell walls with the cell membrane.

In the eukaryotic plant cell there’s a cell wall, as well as a cell membrane. In plants, the cell wall surrounds the cell membrane.

In case of the eukaryotic animal cell there’s only a cell membrane and no cell wall is present.

So in very simple words, in the animals cells, the Plasma Membrane is the only membrane of the cell. And, in the plant cells, the cell wall is also there for extra protection.

The cell membrane in eukaryotes is semi-permeable in nature and interacts with the outside world. This membrane is similar in structure with that of the prokaryotes.

There in the membrane you will see special extensions in the form of vesicles, tubules, and lamellae extending inwards into the cell from the plasma membrane. These are also called mesosome.

The plasma membrane is made up of a phospholipid bilayer with embedded proteins that separate the internal contents of the cell from its surrounding environment.

Only relatively small, nonpolar materials can easily move through the lipid bilayer of the plasma membrane.

In plant cells, the cell wall surrounds the plasma membrane of plant cells and provides tensile strength and protection against mechanical and osmotic stress.

The cell wall along with the plasma membrane also allows the plant cells to develop turgor pressure, which is the pressure of the cell contents against the cell wall.

Why do all cells have a cell membrane?

The first and foremost benefit of having a cell membrane is the protection of the cell. The cell membrane bounders the protoplasm and keeps the structure of the cell intact.

The cell membrane not only gives structural support but also provides protection to the inner cell organelles.

It also provides a fixed environment inside the cell for carrying on the different cellular metabolic activities, and also that cell membrane has several different functions.

The cell membrane is the one and the only route to transport nutrients into the cell and also to transport toxic substances out of the cell.

The food that you eat is broken down into simple nutrients and is then absorbed into the cell only through the cell membranes. The toxic substances from cell organelles like peroxisomes and lysosomes are removed out of the cell only through the cell membrane when the need arises.

Cell signaling and cell signaling pathways are dependent on the receptors present in the cell membrane just like the G-protein coupled receptor.

Cells have proteins called receptors in their cell membrane that bind to signaling molecules and initiate a physiological response. And as because, membrane receptors interact with both extracellular signals and molecules within the cell, they permit signaling molecules to affect cell function without actually entering the cell.

The cell membrane can control the rate at which certain molecules can enter and exit the cell as it is semi-permeable in nature.

That’s why water, oxygen, and carbon dioxide can easily travel through the membrane by simple diffusion, whereas ions (e.g. sodium, potassium) and polar molecules cannot pass through the membrane without active transportation by porin proteins.

The cell membrane plays an important role in both Endocytosis and Exocytosis.

Endocytosis means the taking in of matter by a living cell by invagination of its membrane to form a vacuole. Whereas, Exocytosis is a process by which the contents of a cell vacuole are released to the exterior through the fusion of the vacuole membrane with the cell membrane.

What is a cell membrane made of?

The cell membrane is made up of phospholipid molecules, cholesterol, glycoproteins, various cell surface receptors, and various transport proteins.

More importantly, the four parts of the cell membrane are phospholipids, proteins, carbohydrates, and cholesterol.

The cell membrane is made up of many kinds of phospholipid (consisting of phosphorus and lipid) molecules, with the different sized head (water-loving) and tail (water-hating) molecules.

Along with the phospholipids, proteins named porins are also present between the lipids that make up the membrane and allow hydrophilic molecules to pass through by forming channels, pores, or gates.

Carbohydrates in the form of glycoproteins are also seen in the cell membrane that helps in cell adhesion between cells.

Cell membranes also have proteins called receptors that bind to signaling molecules and initiate a physiological response. Example: Ephrin receptors, Olfactory receptors, etc.

Cholesterol in the cell membrane can dedicatedly regulate the activity of certain integral membrane proteins by helping the cell to restrict the passage of molecules by increasing the packing of phospholipids.

It has been seen that the Cholesterol can fit into spaces between phospholipids and prevent water-soluble molecules from diffusing across the membrane.

According to the Fluid Mosaic Model, it is said that the phospholipids arrange themselves into a double layer pattern with hydrophilic (water-loving) heads on the outside, and hydrophobic (water-hating) tails on the inside.

In simple words, Phospholipids arranged in a bilayer make up the basic structural fabric of the plasma membrane. They are well-suited for this role because they are amphipathic, meaning that they have both hydrophilic and hydrophobic regions.

The hydrophilic end is made up of a negatively charged phosphate group with an additional small organic R group. On the other hand, the hydrophobic end is made up of a long nonpolar fatty acid tail.

This amphipathic nature gives the plasma membrane selectively permeable capability. So, that’s why hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot.

What does the cell membrane look like?

The cell membrane looks like a bilayer structure as seen under the microscope with at least a magnification of about 1000x.

According to the Fluid Mosaic Model (Singer & Nicolson 1972), the plasma membrane is composed and looks like a semi-fluid bilayer model. This bilayer is interspersed with protein in a mosaic pattern.

The Fluid Mosaic Model is the model of the cell membrane that describes the composition of the plasma membrane and how phospholipids, proteins, and carbohydrates freely move within it.

As if you visualize the phospholipids then you’ll find that it is the bilayer (one upper layer and one down layer) structure of the fabric of the cell membrane that is a lipid made along with glycerol, two fatty acid tails, and a phosphate-linked head group.

The phosphate-linked head group is the upper layer that is made up of a phosphate group linked with a small organic R group.

The down layer i.e. the second layer of the bilayer structure is made up of fatty acid tails that are nonpolar in nature.

You will also see various Integral membrane proteins and Peripheral membrane proteins.

Integral membrane proteins (IMP) are membrane-bound proteins that will always remain permanently attached to the biological cell membrane.

Whereas, Peripheral membrane proteins are found on the outside and inside surfaces of membranes, attached either to integral proteins or to phospholipids.

What makes the cell membrane fluid? Is the cell membrane soft or hard?

The cell membrane is neither very soft not very hard. It’s like a flexible jelly type bilayer that is bordering, protecting, and giving the shape to the cell structure.

It’s not a hard solid structure because it is made of millions of smaller molecules like phospholipids and cholesterol that creates a flexible and porous container with sponginess in it.

It’s also not a hard solid structure because the cell membrane is a bit watery nature with fluids in it.

It has fluids because the individual phospholipid molecules and proteins can diffuse within their monolayer and thus move around with ease.

That is simply because the intermolecular attractions between the different biomolecules of the cell membrane aren’t so strong and rigid but like that of a jelly fluid.

The fluidity of the layer is affected by the length of the fatty acid chain of the phospholipid bilayer. Because the shorter the chain, the more fluid is the cell membrane.

The fluidity nature of the membrane also depends on the level of saturation of the fatty acids. As the more unsaturated fatty acids there are, the more fluid is the membrane.

Also, the presence of cholesterol molecules can’t be ignored. As more cholesterol there is, the more fluid is the cell membrane.

Also, it is very important to note that, the structure of the cell membrane is termed as the Fluid Mosaic model. But, why so?

It is termed as the fluid mosaic because it has many types of molecules that float along with the lipids and along with the many types of molecules that make up the cell membrane. The liquid part is the lipid bilayer which floats along with the lipids due to the many types of molecules that make up the cell.

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