MODELS OF PLASMA MEMBRANE

(i) Lipid Monolayer Model Of Langmuir-

The first scientific attempt to know the membrane was made by someone who suggested that the membrane was composed of phospholipid of one molecule thick.
It was shown by an experiment in which the phospholipid was spread on water.
This forms a layer of one molecule thick on the water surface.
Phospholipid are known as amphiphilic molecules which contain both hydrophilic & hydrophobic regions
.Langmuir interpreted his model that hydrophilic or head groups of the lipid molecules remain attached to the water surface and the hydrophobic tails remain free towards the air.

(ii) Lipid Bilayer Model Of Gorter & Grendel-

Gorter & Grendel proposed a lipid bilayer model of membrane structure from the experiments of RBCs. When lipid extracted from RBC’s where spread on the water surface,
It was found that lipids were also spread as one layer on water.
But it covers twice the area on the water surface than that the area of the cell from which lipid is extracted.
The model of Gorter & Grendel gives a new impetus to membrane research as they first tried to describe the structure of membrane at the molecular level.

(iii) The Danielli-Davson Model-

They concluded that the biological membrane could not be of lipid alone. Danielli & Davson proposed a molecular model of the membrane in which hydrophilic groups of the lipid molecules are covered on both sides by a protein layer .
The proteins are attached to the hydrophilic head groups by lipid bilayer by ionic bonds .
But in this model , the distance between ends of the fatty acid chains (hydrophobic tails) is not specified .

(iv) Robertson’s Model Or Unit Membrane Hypothesis:–

The presence of common structure in all biological membranes led to the postulate Unit membrane hypothesis .
For detailed study of the membrane structure & its molecular organization , Robertson selected myelin as its experimental sample .
He selected myelin rather than the typical membrane because in case of myelin , multiple layers of membrane are present which forms quasi-crystalline structure .
He carried out investigations on electron microscopes using different stains for lipids & proteins .
He found that both lipid & proteins are present in the membrane .
Lipid are present in two layers covered by proteins with lipid head groups projecting outward towards both membrane surfaces .
Robertson’s observation corroborates the structure proposed by Danielli & Davson .
The electron microscopic observations & X-rays diffraction data confirmed the Danieli & Davson model of membrane structure . .

(v) Fluid Mosaic Model–

In this model the main component is the lipid bilayer with hydrophilic groups oriented towards the outside & the hydrophilic groups towards the inside of the layer .
The basic requirement for the basic requirements of the molecular organization of the membrane is free energy.
The term fluid is given because the lipid layer is present in the fluid state.
The transition of the fluid layer from non fluid (gel) conditions to a liquid crystalline (fluid) state depends on the temperature of the cell.
According to this model, proposed by SJ Singer & John Nicholson, the principle of membrane organization is as follows:

1. Lipids are present in two layers.

2. Proteins are arranged in two ways:

a) Some are embedded in lipid layer, called integral proteins &

b) Some are present on the surface of the lipid bilayer, called the peripheral proteins.

3. The lipid layer is usually in the liquid crystal line, i.e., fluid state. .

FUNCTION

It forms protective covering over cytoplasmic organelles.
It is selectively permeable in nature which allows only selectable molecules to pass through it.
Substances that pass through it by simple diffusion,facilitated diffusion and by active transport method.
Simple diffusion drives the net movement of dissolved solutes as well as water molecules and the process is termed as osmosis.
Facilitated diffusion refers to the assisted movement of a substance down its electrochemical gradient.
Active transport is carried out by membrane transport proteins.

MCQ’s

1-10: Lipid Monolayer & Bilayer Models

Who was the first to suggest that membranes are composed of phospholipids? (CSIR-NET 2018)
a) Overton
b) Langmuir
c) Gorter & Grendel
d) Danielli & Davson
Ans: (b) Langmuir
What is the key feature of phospholipids in a membrane? (NEET 2021)
a) Hydrophobic heads & hydrophilic tails
b) Hydrophilic heads & hydrophobic tails
c) Fully hydrophilic
d) Fully hydrophobic
Ans: (b) Hydrophilic heads & hydrophobic tails
Gorter & Grendel’s model was based on experiments with (CSIR-NET 2022)
a) Myelin
b) Mitochondria
c) Red Blood Cells (RBCs)
d) Nerve cells
Ans: (c) Red Blood Cells (RBCs)
Which property of lipids supports bilayer formation? (NEET 2019)
a) Hydrophobic tails repel water
b) Hydrophilic heads interact with water
c) Both a & b
d) Neither a nor b
Ans: (c) Both a & b
What did Gorter and Grendel’s experiment conclude? (GATE 2020)
a) Membranes are monolayers
b) Membranes are bilayers
c) Membranes contain no proteins
d) Membranes are randomly organized
Ans: (b) Membranes are bilayers
Which molecules make up most of the plasma membrane? (CSIR-NET 2017)
a) Carbohydrates
b) Proteins
c) Lipids
d) DNA
Ans: (c) Lipids
Which part of a phospholipid is hydrophobic? (NEET 2018)
a) Phosphate head
b) Glycerol backbone
c) Fatty acid tails
d) None of the above
Ans: (c) Fatty acid tails
Which of the following statements about the plasma membrane is false? (GATE 2019)
a) It is selectively permeable
b) It is made up of a single lipid layer
c) It contains embedded proteins
d) It allows communication between cells
Ans: (b) It is made up of a single lipid layer
Gorter and Grendel’s model lacked which component? (CSIR-NET 2016)
a) Proteins
b) Phospholipids
c) Cholesterol
d) Both a & c
Ans: (d) Both a & c
Which property of lipid bilayers allows for membrane fluidity? (NEET 2020)
a) Hydrophobic interactions
b) Covalent bonding
c) Ionic interactions
d) Presence of carbohydrates
Ans: (a) Hydrophobic interactions

11-20: Danielli-Davson & Robertson Models

Who proposed the ‘sandwich’ model of the plasma membrane? (CSIR-NET 2015)
a) Singer & Nicolson
b) Danielli & Davson
c) Gorter & Grendel
d) Robertson
Ans: (b) Danielli & Davson
What is a major feature of the Danielli-Davson model? (GATE 2017)
a) Phospholipid monolayer
b) Protein-lipid-protein arrangement
c) Cholesterol-free bilayer
d) Non-fluid structure
Ans: (b) Protein-lipid-protein arrangement
Why was the Danielli-Davson model rejected? (NEET 2021)
a) Lack of cholesterol
b) Could not explain membrane fluidity
c) Absence of proteins
d) Incorrect lipid composition
Ans: (b) Could not explain membrane fluidity
Who proposed the Unit Membrane Hypothesis? (CSIR-NET 2018)
a) Gorter & Grendel
b) Danielli & Davson
c) Robertson
d) Singer & Nicolson
Ans: (c) Robertson
Which experimental sample did Robertson use to study membranes? (GATE 2020)
a) Myelin
b) RBC membrane
c) Bacterial cell wall
d) Chloroplast membrane
Ans: (a) Myelin
Robertson’s model suggested that all membranes share a similar (NEET 2018)
a) Basic structure
b) Composition
c) Transport mechanism
d) Asymmetry
Ans: (a) Basic structure
Which microscopy technique helped confirm membrane structure? (CSIR-NET 2017)
a) Fluorescence microscopy
b) Scanning electron microscopy
c) Transmission electron microscopy
d) Confocal microscopy
Ans: (c) Transmission electron microscopy
Which key feature did Robertson’s model confirm? (NEET 2016)
a) Protein-free membrane
b) Unit membrane structure
c) Monolayer organization
d) No lipid movement
Ans: (b) Unit membrane structure
Which problem led to Robertson’s model being questioned? (GATE 2015)
a) Lack of transport proteins
b) Over-simplification
c) No evidence of asymmetry
d) Lack of integral proteins
Ans: (c) No evidence of asymmetry
What was missing in Robertson’s model? (CSIR-NET 2019)
a) Phospholipids
b) Transmembrane proteins
c) Carbohydrates
d) ATP synthase
Ans: (b) Transmembrane proteins

21-30: Fluid Mosaic Model

Who proposed the Fluid Mosaic Model? (NEET 2019)
a) Gorter & Grendel
b) Singer & Nicolson
c) Danielli & Davson
d) Overton
Ans: (b) Singer & Nicolson
Why is the Fluid Mosaic Model called “fluid”? (CSIR-NET 2020)
a) Water is present inside the membrane
b) Phospholipids move freely
c) Proteins are fixed in the bilayer
d) Membrane does not change
Ans: (b) Phospholipids move freely
Which proteins are embedded in the membrane? (NEET 2021)
a) Peripheral proteins
b) Cytoskeletal proteins
c) Integral proteins
d) None of the above
Ans: (c) Integral proteins
What is the role of peripheral proteins in membranes? (GATE 2018)
a) Transport of ions
b) Cell signaling
c) Provide structural support
d) Both b & c
Ans: (d) Both b & c
Which of the following describes the Fluid Mosaic Model? (CSIR-NET 2022)
a) Lipids and proteins are static
b) Phospholipids form a rigid structure
c) Lipids and proteins move within the bilayer
d) Carbohydrates are absent
Ans: (c) Lipids and proteins move within the bilayer
Which component of the membrane is responsible for fluidity? (NEET 2018)
a) Cholesterol
b) Integral proteins
c) Carbohydrates
d) Peripheral proteins
Ans: (a) Cholesterol
Which type of proteins span across the lipid bilayer? (GATE 2019)
a) Integral proteins
b) Peripheral proteins
c) Cytoskeletal proteins
d) Carrier proteins
Ans: (a) Integral proteins
Which transport process requires energy? (CSIR-NET 2021)
a) Diffusion
b) Osmosis
c) Facilitated diffusion
d) Active transport
Ans: (d) Active transport
Which transport method does not require ATP? (NEET 2020)
a) Active transport
b) Endocytosis
c) Diffusion
d) Exocytosis
Ans: (c) Diffusion
What is the main function of membrane carbohydrates? (CSIR-NET 2019)
a) Cell recognition
b) Transport
c) Providing energy
d) Protein synthesis
Ans: (a) Cell recognition