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Cell membrane, History of Development of the Fluid Mosaic Model, Practice Problems and FAQs

Cell membrane, History of Development of the Fluid Mosaic Model, Practice Problems and FAQs

The universe has a huge variety of living creatures which are diverse with respect to their size, morphology, appearance etc. You must be aware of the existence of bacteria which are unicellular organisms. Have you ever wondered if the bacterial cell and the cells comprising your body are the same or not? They are not the same. There are many differences between a bacterial cell and the animal cell as bacterial cells are prokaryotic whereas animal cells are eukaryotic in nature. But they have some similarities too. One of the major similarities between the prokaryotic and eukaryotic cells is that they have a similar structure of cell membrane, except for prokaryotic archaebacteria in which the cell membrane is modified to withstand extreme conditions of heat or pressure.



                                               Fig: Prokaryotic and eukaryotic cells

How did we know that the cell membranes of prokaryotic and eukaryotic cells are similar? We could not have simply predicted that they are the same. Many eminent scientists have researched over the years to understand the structure of the cell membrane. Initial research only laid down some basic ideas related to the structure of the cell membrane but along with the growth in the technologies, the research also developed. As a result, by 1972 two scientists named Singer and Nicolson proposed the most accepted structure of the cell membrane. Aren’t you intrigued to know the history behind the study and understanding of the cell membrane structure? In this article we will discuss more about the structure of the cell membrane and the story of how we gradually arrived at the current and most widely accepted model of the cell membrane structure.

Table of contents:

Cell membrane 

In prokaryotes, the innermost layer of the cell envelope is called the cell membrane. The structure of the cell membrane is the same in both prokaryotes and eukaryotes. The outer covering made up of complex molecules that encloses the cytoplasm is known as plasma membrane, cell membrane or plasmalemma. The name plasma membrane was given by Nageli and Cramer and plasmalemma was given by Plowe.

The cell membrane is usually not visible under the light microscope, hence, its structure can only be studied by using an electron microscope. It is selectively permeable in nature. This means that the cell membrane allows the passage of only selected molecules across it. It is a living and dynamic structure. 



                                                                     Fig: Plant cell

There are intracellular membranes in eukaryotic cells. It includes the membranes that surround the vacuole and certain cell organelles such as the mitochondria, plastids, nucleus, endoplasmic reticulum, etc. The membranes of the nucleus, endoplasmic reticulum, golgi complex, vacuoles and lysosomes are connected and form the endomembrane system. The plasma membrane and the subcellular or intracellular membranes of the cell are grouped into biological membranes or biomembranes.



                                                       Fig: Animal cell

Structure of cell membrane

Biomembranes are typically made up of phospholipids, proteins and a few polysaccharides attached to their surface. The surface carbohydrates serve as surface markers. Cell membrane is about 75Å thick. They have a trilaminar structure (three layered). The three layers are as follows:

  • Outer electron dense layer (20 Å)
  • Middle pale coloured layer (35 Å)
  • Inner electron dense layer (20 Å)

The outer layer and inner layer is made up of extended ꞵ proteins and the middle layer is made up of two layers of phospholipids.

Components of cell membrane

The major components of a cell membrane are as follows:

  • Phospholipids which form the major part of the membrane.
  • Peripheral proteins that are present on the outer or inner surface of the membrane.
  • Integral proteins that are found inside the membrane layers. 
  • Transmembrane proteins that are found partially inside and outside the cell membrane.
  • Cholesterol which is folded between the membrane.
  • Carbohydrates that are attached to the lipids (glycolipids) or proteins (glycoproteins) on the extracellular side of the membrane. 



                         Fig: Structure of Cell membrane

History of development of fluid mosaic model

Many scientists suggested many models for the structure of plasma membranes. The most accepted models are Robertson’s unit membrane model and the fluid mosaic model. The currently accepted fluid mosaic model of plasma membrane by Singer and Nicolson was discovered after many attempts by different scientists.

Groter and Grendel’s membrane theory 

The first lipid bilayer concept was discovered by Evert Gorter and Francois Grendel. Their hypothesis was, if the cell membrane is bilayered, then the surface area of the single lipid layer would be double the surface area of the cell membrane. They believed that in the bilayer, the hydrophilic heads of the lipids face outside towards the aqueous environment whereas the hydrophobic tails face inwards away from the aqueous environment.

Although their conclusion of the lipid bilayer arrangement was correct, some of their calculations were wrong regarding the area and pressure of the lipid monolayer. They also failed to explain the functions of the membrane. Another misconception of Gorter and Grendel was that the cell membrane is composed mostly of lipids.

Davson and Danielli’s pauci-molecular model

By taking the drawbacks of Groter and Grendel’s membrane theory, Davson and Daniel proposed another model for cell membranes. According to their model, the membranes of the cell have a trilaminar structure and are composed of lipoproteins. It consists of a phospholipid bilayer lying between two layers of globular proteins. They have mentioned this model as a sandwich arrangement of protein-lipid-protein in the cell membrane. They explained the role of proteins in biological membranes. 

But there were some shortcomings for this model. Davson and Danielli believed that all living membranes had the same structure but their model was not able to explain how different functions were performed by different types of membranes. According to this theory, proteins form the outer layers of the trilaminar membrane structure. But, the proteins are amphipathic and mostly hydrophobic. Hence the existence of outer proteins was an issue, since they have to interact with the water molecules.

Robertson’s unit membrane theory 

When the cell biologists verified the plasma membrane through the electron microscope, Robertson discovered and described the trilaminar structure of the cell membrane and the biomembranes inside the cells. Robertson’s model is known as the unit membrane model as he proposed the same model for all living biomembranes. In this model the lipid bilayer is sandwiched between the two protein layers. The outer proteins are thin sheets of mucoproteins and inner proteins are non mucoid proteins. 

Unit membrane model agreed with the pauci-molecular model. But it also had the same drawbacks as that of the Davson–Danielli model. Both the models explained the proteins as layers on both sides of the lipid bilayer which cannot be supported by the fact that proteins are mostly hydrophobic in nature. Davson and Danielli explained the outer proteins as globular proteins which did not fit into the unit membrane model which explained the outer proteins as thin sheets.

Singer and Nicolson’s fluid mosaic model 

The most important and highly advanced model of the structure of the cell membrane was explained by Seymour Jonathan Singer and Garth L. Nicolson in early 1975. They actually expanded the Davson–Danielli model, by including some extra components like transmembrane proteins and modifying the protein sheet structure on either side of the lipid bilayer. 



                                                       Fig: Singer and Nicolson 

According to the model, biomembranes are not solid structures but are viscous fluids in nature or can be a quasi-fluid structure. Fluid mosaic models are considered satisfactory to understand the molecular structure of plasma membranes. The phospholipid and all protein molecules are not arranged uniformly in this structure. But they are mosaic in nature. Hence this model is called the ‘fluid mosaic model’.



                GIF: Fluid mosaic model

They have explained that the biomembranes are primarily made up of two components and they are as follows: 

  • Lipids (Mostly phospholipids) 
  • Proteins



                                             Fig: Fluid mosaic model of cell membrane

Now we will see more about the components in the structure of cell membrane according to the fluid mosaic model.

Lipids

Mostly phospholipids are found in the cell membrane and form the structural framework of the membranes. Phospholipid molecules are ‘amphipathic’ in nature, which means they have both hydrophilic and hydrophobic regions. Phospholipids are arranged in two layers known as the ‘phospholipid bilayer’ and it has a thickness of 35Å. 

They have a polar or hydrophilic head towards the outside and nonpolar or hydrophobic tail towards the inner side of the bilayer. Hence they will only allow the lipid soluble substances to pass through the membrane and it is impermeable to water soluble substances. The quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer, hence giving the cell membrane fluidity. Flip flop type of movement from one monolayer to another can be observed too. 



                                                             Fig: Lipid bilayer

Proteins 

They form a layer on both the sides of the lipid bilayer. Proteins in biomembranes are of three types:

  • Extrinsic or peripheral proteins
  • Intrinsic or integral proteins
  • Transmembrane proteins 



                                                     Fig: Types of membrane proteins

Extrinsic or peripheral proteins

The proteins that are present on the outer surface of the lipid bilayer are called extrinsic or peripheral proteins. They are also easily removable from the surface of the cell membrane, since they are loosely bound to the intrinsic proteins or to the polar ends of lipid molecules. They bind through weak electrostatic or ionic bonds. 

Intrinsic or integral proteins

The proteins that are embedded partially or completely in the lipid bilayer are called extrinsic or peripheral proteins. They cannot be easily removed from the membrane. These proteins are tightly held by strong hydrophilic or hydrophobic interactions. There are different types of integral proteins according to their form and function. 

Transmembrane proteins

The large globular protein molecules that are projected beyond the lipid bilayer on both sides are called transmembrane proteins or channel proteins. They have hydrophobic and hydrophilic regions. The hydrophobic region passes through the lipid bilayer and hydrophilic regions extend out of the membrane. There are single pass proteins and multipass proteins in transmembrane proteins. Single pass transmembrane proteins are those whose polypeptide chains cross only once through the membrane. If the polypeptide passes through the membrane multiple times, then it is called a multipass protein. Channel proteins provide passage for water soluble substances. These proteins are responsible for the selective permeability of the cell membrane.

Apart from the proteins and lipids, the cell membrane also consists of oligosaccharides, cholesterol, glycolipids and glycoproteins. The carbohydrates help in the cell-to-cell recognition or cell signalling in eukaryotes. 



                                     Fig: Glycoproteins and glycolipids on the cell membrane

Functions of cell membrane

The lipid bilayer and quasi fluid structure of the cell membrane allows it to perform various functions. Some of them are as follows:

  • Helps in the transitional movements of intrinsic proteins and lipid molecules.
  • Allows the fat soluble substances to pass through.
  • Permits the movement of cells.
  • Helps to change the shape of the cells.
  • Cell to cell recognition is executed.
  • Selective transport of the molecules.

Practice Problems

Q 1. Which of the following is/are correct about the eukaryotic cell membrane?

1. Cell membrane allows the passage of only selected molecules across it.
2. They have a trilaminar structure with an outer electron dense layer, middle pale coloured layer and inner electron dense layer.
3. According to Davson and Daniel, the cell membrane model was a sandwich arrangement of protein-lipid-protein.
4. According to the unit membrane theory, biomembranes are not solid structures but are viscous fluids in nature or can be a quasi-fluid structure. 

a. A, B, C, D
b. A, B, C
c. A and C
d. Only D

Answer: The outer covering of complex molecules enclosing cytoplasm is known as plasma membrane, cell membrane or plasmalemma. It is selectively permeable in nature. This means that the cell membrane allows the passage of only selected molecules across it. Cell membranes are typically made up of lipoproteins, and a few polysaccharides attached to their surface. They have a trilaminar structure with an outer electron dense layer, middle pale coloured layer and inner electron dense layer. According to the model of Davson and Daniel, the membranes of the cell have a lipid bilayer which is trilaminar and composed of lipoproteins. This lipid bilayer is lying between two layers of globular proteins. They have mentioned this model as a sandwich arrangement of protein-lipid-protein in the cell membrane. The most important and highly advanced model was explained by Seymour Jonathan Singer and Garth L. Nicolson in early 1975. According to the model, biomembranes are not solid structures but are viscous fluids in nature or can be a quasi-fluid structure. Hence the correct option is b.

Q 2. Which of the following drawbacks of Robertson’s unit membrane theory lead to its failure?

a. They failed to explain the functions of the membrane 
b. They found that the cell membrane is composed mostly of only lipids
c. They were not able to explain the different functions of different types of membranes
d. They explained the proteins as globular proteins and they cannot be fixed in the structure they have explained as protein sheets

Answer: Robertson discovered and described the trilaminar structure of the cell membrane through the electron microscope. He also described the biomembranes inside the cells. The name of the structure of cell membrane by Robertson is unit membrane model. In this model the lipid bilayer is sandwiched between the two protein layers. The outer proteins are mucoproteins and inner proteins are non mucoid proteins. Unit membrane model agreed with the pauci-molecular model. But it also had the same drawbacks as that of the Davson–Danielli model. Both the models explained the proteins as layers on both sides of the lipid bilayer. They explained the proteins as globular proteins and they cannot be fixed in the structure they have explained as protein sheets. Hence the correct option is d. 

Q 3. What are the types of proteins present in a cell membrane?

Answer: Proteins form a layer on both the sides of the lipid bilayer. Proteins in biomembranes are of three types as follows:

  • Extrinsic or peripheral proteins
  • Intrinsic or integral proteins
  • Transmembrane proteins 

The proteins that present on the outer surface of the lipid bilayer are called extrinsic or peripheral proteins. The proteins that are embedded partially or completely in the lipid bilayer are called extrinsic or peripheral proteins. The large globular protein molecules that are projected beyond the lipid bilayer on both sides are called transmembrane proteins or channel proteins. 

Q 4. Explain the structure of lipid bilayer in cell membrane?

Answer: Phospholipids are arranged in two layers known as the ‘phospholipid bilayer’ and it has a thickness of 35Å. They have a polar or hydrophilic head towards the outside and nonpolar or hydrophobic tail towards the inner side of the bilayer. Hence they will only allow the lipid soluble substances to pass through the membrane and it is impermeable to water soluble substances. The quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer, hence giving the cell membrane fluidity. Flip flop type of movement from one monolayer to another can be observed too. 

FAQs

Q 1. What are desmosomes?
Answer: Desmosomes or maculae are disc shaped thickenings between the plasma membranes of two adjacent cells. They act as spot welds and keep the cells firmly together. They are hence called spot desmosomes. 

Q 2. What is an axolemma?
Answer: The action potential is generated by a specialised plasma membrane found on the axons of nerve cells called axolemma. It is made up of a granular and tightly packed lipid bilayer. This interacts with spectrin and actin which are the two components of the cytoskeleton. The transmembrane proteins of axolemma can bind to and interact with these cytoskeleton components.

Q 3. What are flippases?
Answer: The transmembrane proteins which help in movement of phospholipids between the two layers of the cell membrane are called flippases. They belong to the ABC transporter or P-4 type ATPase families. They are responsible for the movement of phospholipid molecules from the extracellular side to the cytosolic side. It is known as transverse diffusion or flip-flop transition. 

Q 4. In which step of the cell division the nuclear membrane is formed?
Answer: The formation of cell membrane happens during the telophase stage of cell division. The chromosomes will reach their poles during the telophase stage, and the mitotic spindle will separate. The two sets of chromosomes will now be in close contact with the vesicles that hold the original fragment of the nuclear membrane. Thus, the nuclear membrane will develop around each set of chromosomes to divide the nuclear DNA and the cytoplasm. 

YOUTUBE LINK: https://www.youtube.com/watch?v=-NOY_k8iN9A&t=2525s

Related Topics

Cell Theory: Introduction, History, Modern cell theory, Practice problems and FAQs 

An Overview of Cell: classification, structure, functions, Practice problems and FAQs 

Eukaryotic Cell, Practice problems and FAQs 

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