Written By Manisha Minni
Last Modified 25-01-2023

Cell Membrane: Definition, Structure, Models, and Functions

How do osmosis and diffusion take place? The diffusion or osmosis process of a cell is controlled by the cell membrane. Cell membranes are thin membranes that enclose every living cell and separate it from its surroundings. Proteins and lipids make up most of the cell membrane’s structure. The cell membrane performs a variety of important activities for the cell.

The cytoplasm is held in place by cell walls, which also hold the cell’s form. It’s a semipermeable, selective membrane that permits the cell to maintain homeostasis by controlling what it allows. Both active and passive transport is used by cells to absorb nutrients and eliminate waste through the cell membrane. Let us scroll down to know more about the Cell membrane Structure and Functions.

Cell Membrane: Definition

Cell membrane term was used originally by Nageli and Cramer (1855) for the membrane covering of protoplast. The cell membrane, also known as the plasma membrane (PM), cytoplasmic membrane, or plasmalemma, is a biological membrane that separates the interior of all cells from the external environment (extracellular space) and protects the cell from its surroundings. Schwann discovered the plasma membrane in the year 1838.

Structure of Cell Membrane

The detailed structure of the cell membrane was investigated in 1950 due to the introduction of the electron microscope. The cell membrane of human RBCs is made up of a lipid bilayer with protein molecules embedded in it, according to research. Protein and carbohydrates were later shown to be present in cell membranes.

1. Lipid

The molecules of lipids are amphipathic.
Phospholipids or phosphoglycerides are the most common lipid molecules.
Both polar hydrophilic and nonpolar hydrophobic endings are present. The hydrophilic part is shaped like a head, whereas the hydrophobic part has two fatty acid tails.
The hydrophobic heads of phospholipids will orient themselves to be on the inside.
The hydrophilic heads, on the other hand, will be on the exterior, in contact with the water.
As a result, a double layer of phospholipids forms, with the hydrophobic heads clustering in the centre and the hydrophilic tails forming the structure’s exterior.
Only specific compounds can diffuse through the membrane because the lipid bilayer is semipermeable.

Fig: Structure of Lipid

Cholesterol is a lipid that is found in animal cell membranes.
Cholesterol molecules are distributed between membrane phospholipids preferentially.
By preventing phospholipids from being too closely packed together, it helps to protect cell membranes from becoming stiff.

2. Protein

Proteins may be fibrous or globular, structural, carrier, receptor, or enzymatic.
There are two types of associated proteins in the cell membrane.
Peripheral membrane proteins are found on the outside of the membrane.
Most integral membrane proteins pass through the membrane after being introduced into it.
On both sides of the membrane, portions of these transmembrane proteins are exposed.
Both polar and nonpolar side chains can be found in protein molecules.
The lipid matrix is supported by the elasticity and mechanical support of the protein monolayer.

3. Carbohydrates

Carbohydrates in the membrane are oligosaccharides, which can be branched or unbranched.
To form glycolipids and glycoproteins, these are attached to phospholipids or peripheral proteins, respectively.

Fig: Structure of the Cell Membrane

Different Models of Cell Membrane

Several models have been proposed to describe the structure of a cell membrane. The following are the numerous cell membrane models:

1. Lipid and Lipid Bilayer Model: Overton discovered in 1902 that substances soluble in lipid might flow through membranes selectively. On this basis, he claimed that the plasma membrane is made up of a thin layer of lipid. Gorter and Grendel demonstrated in 1925 that the lipid taken from the red cell ghost spread across twice the area of a simple molecular film based on studies of erythrocyte cell membranes. As a result, it was assumed that the membrane was made up of two layers of lipid molecules. These Gorter and Grendel models did not explain the suitable structure of the plasma membrane, but they provided the basis for future membrane structure models.

Fig: Lipid and Lipid Bilayer Model

2. Dannelli and Davson Model or Sandwich Models: The sandwich or trilamellar model of cell membrane structure was proposed by James Danielli and Hugh Davson in 1935. The Danielli and Davson model was the first to attempt to characterise membrane structure in terms of molecules and relate it to biological and chemical properties. According to Danielli and Davson’s model, the plasma membrane is a sheath-like structure made up of two layers of phospholipid molecules arranged so that the hydrophilic heads of the phospholipid molecules face outside, and the hydrophobic nonpolar lipid chains are associated in the leaflet’s inner region. The polar ends of lipid molecules are also linked to the monomolecular layer of globular proteins, according to the hypothesis. A double layer of phospholipid molecules would be sandwiched between two essentially continuous layers of protein, forming the plasma membrane.

Until Singer and Nicolson introduced the fluid mosaic model in 1972, the Davson–Danielli model prevailed. The fluid mosaic model added transmembrane proteins to the Davson–Danielli model and eliminated the previously proposed flanking protein layers, which were not strongly supported by experimental evidence.

Fig: Sandwich or Davson–Danielli Model of Plasma Membrane

3. Unit Membrane Model or Protein-Lipid Bilayer-Protein: J.David Robertson, in 1959, revised Danielli and Davson’s concept by introducing the unit membrane hypothesis, which claims that all cellular membranes have the same structure. The unit membrane, according to this hypothesis, is made up of bimolecular lipid leaflets packed between the outer and inner layers of protein. The cell membrane is shown as a trilaminar structure with two dark osmiophilic layers separated by a light osmiophilic layer in the unit membrane model. The term “unit membrane” comes from the physical look of this trilaminar type.

A trilaminar appearance is implied by the unit membrane idea, with a bimolecular lipid layer placed between two protein layers. It is around 75 Å thick, with a 35 Å thick central phospholipid layer sandwiched between two 20 Å thick protein layers. The plasma membrane that surrounds the cell is thicker on the cell’s surfaces than it is in contact with other cells. Protein layers in the unit membrane model are asymmetrical. Mucoprotein is found on the outer surface, while non-mucoid protein is found on the inner surface.

Fig: Unit Membrane Model

4. Fluid Mosaic Model: The Fluid Mosaic Model of Plasma Membrane was proposed and widely accepted by S.J. Singer and Garth L. Nicolson in 1972 to describe the structure of the plasma membrane. The fluid mosaic model defines the plasma membrane’s composition as a mosaic of components, including phospholipids, cholesterol, proteins, and carbohydrates, that give it a fluid appearance. Both lipids and proteins are spread in a mosaic pattern according to this hypothesis. All biological membranes are quasi structures that allow lipids and proteins to move around. Plasma membranes have a thickness of 5 to 10 nm. Proteins, lipids, and carbohydrates make up different amounts in the plasma membrane depending on the cell type.

Each phospholipid molecule has a head that is attached to water and repels the tail. The hydrophilic heads of both layers of the plasma membrane point outward, while the hydrophobic tails comprise the bilayer’s interior.

Proteins and chemicals like cholesterol embedded themselves in the bilayer, giving it a mosaic appearance. The fluidity of the lipid portion is determined by the capacity of proteins to move within the membrane.

Fig: Fluid Mosaic Model

Functions of Cell Membrane

The functions of the cell membrane are as follows:

The role of the plasma membrane is to protect the cell from its surroundings. It regulates the movement of substances in and out of cells by being selectively permeable to ions and organic molecules.
In some animals, the plasma membrane serves as a base of attachment for the cytoskeleton, while in others, it serves as a base of attachment for cell walls.
It is in charge of keeping the cell’s structure and form.
Extracellular signals are received and transferred within the cell by the cell membrane.
The permeability and retentivity of the plasma membrane, as well as the permeability and retentivity of other biomembranes, regulate cell metabolism.
Enzymes are found on the surface of cell membranes and are used to carry out certain reactions.
Antigen specificity is determined by substances adhering to the cell membrane.
The cell membrane plays a role in the transmission of impulses in nerve cells.
It has active transport carrier proteins.
By forming undulations or pseudopodia, it helps in the movement of certain cells.

Different Methods of Transport Across Cell Membrane

Passive transport, active transport, and bulk transport are the three methods used to transport substances across cell membranes.

Passive Transport

The movement of material along a concentration gradient is referred to as passive transport. The cell expends no energy during this transport. It happens in both directions. Diffusion, osmosis, facilitated diffusion, and ion channels are the four main types of passive transport.

Active Transport

The movement of materials against a concentration gradient is referred to as active transport. It necessitates the use of energy by the cells. It only happens in one direction. Active transportation is classified into two types: primary (direct) active transport and secondary (indirect) active transport.

Bulk Transport

It occurs through two mechanisms: pinocytosis and phagocytosis. They involve the enclosing of material in transit in the vesicles of the membrane.

Summary

The cell membrane is the cell’s outermost layer. It’s a delicate, thin membrane. Plasma membrane or plasmalemma are other names for it. Lipids (phospholipids and cholesterol), proteins, and carbohydrates are the main components of the plasma membrane. Several models have been proposed to describe the structure of a cell membrane.

The Fluid Mosaic Model of Plasma Membrane was proposed and widely accepted. S.J. Singer and Garth L. Nicolson proposed the fluid mosaic model in 1972 to describe the structure of the plasma membrane. The role of the plasma membrane is to protect the cell from its surroundings. It regulates the movement of substances in and out of cells by being selectively permeable to ions and organic molecules.

FAQs

Q.1. What do you mean by cell membrane?
Ans: Cell membrane is a biological membrane that separates the interior of all cells from the external environment (extracellular space) and protects the cell from its surroundings.

Q.2. What are the main components of the cell membrane?
Ans: Lipids (phospholipids and cholesterol), proteins, and carbohydrate groups attached to some of the lipids and proteins are the main components of the plasma membrane.

Q.3. Why does the cell membrane’s phospholipid bilayer require a hydrophobic and hydrophilic end?
Ans: Phospholipid’s dual nature allows membranes to develop without the use of a catalyst or energy. Membrane formation would be problematic if phospholipids were different. The hydrophobic core also acts as a barrier to molecule movement, allowing the cell to regulate.

Q.4. What causes lipids in the plasma membrane to be quasi-fluid?
Ans: The quasi-fluid property of lipid allows proteins to move sideways within the lipid bilayer.

Q.5. What are the three functions of the cell membrane?
Ans: The three functions of the cell membrane are:
1. The role of the plasma membrane is to protect the cell from its surroundings. It regulates the movement of substances in and out of cells by being selectively permeable to ions and organic molecules.
2. The cell membrane plays a role in the transmission of impulses in nerve cells.
3. It has active transport carrier proteins.

Learn About Cell Organelles Here

We hope this detailed article on Cell Membrane helps you. If you have any queries, feel to ask in the comment section below and we will get back to you at the earliest.