What are Micelles?

Micelles are the type of lipid molecules that organize themselves in spherical shape in aqueous solutions. The production of the micelle is a reply to the amphipathic behavior of fatty acids. It means that they consist of both hydrophobic regions and hydrophilic regions. In micelles, polar head groups (hydrophilic groups) produce an outer layer associated with water whereas nonpolar tails (hydrophobic groups) are organized in the interior.

Formation of micelles

The phase of the micelle is formed by the packing nature of single-tail lipids in the bilayer. The struggling filling whole volumes of the inner shell of bilayer whereas housing the region per head group imposed on the molecule through the lipid head group hydration lead to the production of the micelle. This kind of micelle is called a normal-phase micelle. In the case of micelles, where the head groups present in the center with the tails stretching out are called inverse micelles.

The micelle’s shape is spherical. Other shapes like ellipsoids, bilayers, and cylinders are also known. The micelle’s shape & size is an outcome of the geometry of the surfactant and the conditions of the solution such as temperature, pH, concentration & the ionic strength of the surfactant. The production of micelles is named micellization.

The micelles are produced only if the temperature is higher than the critical micelle temperature (CMC) that is named a kraft temperature & the surfactant’s concentration is higher than CMC. CMC is the essential characteristic of surfactants. It could be defined as the surfactant’s concentration beyond which micelles are produced. Due to the balance between entropy & enthalpy, micelles could be produced spontaneously.


A single surfactant molecule that is present in the system but is not a region of a micelle is named monomers. Micelles indicate the assembly of molecules where the individual parts are thermodynamically in equilibrium with the monomers of the equivalent species in the encircled system. In the case of water, the hydrophilic heads of the surfactant molecule are consistently associated with solvent, nevertheless if the surfactant exists as a region of a micelle or a monomer.

But the lipophilic tails of surfactant molecules are poorly associated with water if they are the section of a micelle. This is an important energetic drive for the micelle’s production. In micelles, the hydrophobic tails of various surfactant molecules aggregate into an oil-like core, the highly stable arrangement that is not associated with water. By contrast, the water that surrounds the surfactant molecules forms a solvation shell or cage joined through hydrogen bonds.

Micelles consist of ionic surfactants which have an electrostatic attraction to the ion which encircled them in the solution. It is called counterions.

Packing parameter of micelle

The molecular self-gathering in the solutions of surfactant is calculated by using the micelle packing parameter. It is given by,

ʋo / αeτo


Surfactant tail volume is represented by ʋo.

Tail length is represented by τo.

The equilibrium region for a molecule or the aggregate surface is represented by αe.

What are biological membranes?

Biological membranes are also known as cell membranes. It is a permeable membrane that isolates cells from the outer environment or produces intracellular sections. Biological membranes look like a eukaryotic cell membrane and it is made up of phospholipid bilayer with an implant, peripheral and integral proteins which are helpful in communication and the transportation of ions and chemicals. The flow of messages within the cells by sending, processing, and receiving information in the shape of the electrical and chemical signals is controlled by biological membranes.

Compositions of biological membranes

The lipid bilayer is made up of two layers- an inner leaflet & an outer leaflet. The constituent of the bilayers is irregularly diffused between the surfaces to form asymmetry between the outer & the inner surfaces. Such asymmetric organization is important for cell functions like cell signaling. The biological membrane’s asymmetry shows the various functions of the two leaflets of the membrane. Like in the case of the fluid membrane model of a phospholipid bilayer, the outer & the inner leaflet of membranes are uneven in their arrangement.

Formation of biological membranes

The production of phospholipid bilayer is because of the gathering of membrane lipids in the aqueous solutions. The hydrophobic effect will cause aggregation where the hydrophobic ends are joined with each other and are arranged far from the water. Due to this arrangement by hydrophobic effect, the hydrogen bonding between the water & hydrophilic heads is maximized whereas the unfavorable connection between the water and hydrophobic tails is minimized. This hydrophobic effect is also necessary for vesicle formation, the introduction of a membrane protein into the nonpolar lipid environment, protein folding, and association of protein-small molecule. The entropy of the system is enhanced because of the rise in the available hydrogen bonding, thus it will lead to a spontaneous process.

The fluid membrane model of the phospholipid bilayer is shown in the figure.
CC BY 4.0 | Image Credits: https://commons.wikimedia.org | OpenStax

Functions of biological membrane

Biological molecules are hydrophilic & hydrophobic, i.e are simultaneously amphipathic or amphiphilic. A charged hydrophilic head group is located in the phospholipid bilayer that reacts with polar water. The hydrophobic tails are also present in this layer that are in contact with the hydrophobic tails of a complementary layer. The fatty acids work as hydrophobic tails that vary in length. The physical properties of the lipids like fluidity could be found by the hydrophobic tails. Membranes in cells usually define enclosed compartments or spaces where the cells could maintain a biochemical or chemical environment that varies from the outside.

Selective permeability of biological membrane

A selectively permeable shape is the most essential feature of the biomembrane which means the charge, size & other chemical properties of the molecules and atoms will decide whether they achieve success by doing this. For the effective separation of an organelle or a cell from its surroundings, selective permeability is very important. These membranes also have particular elastic and mechanical properties which permit them to modify shape and move as required. The small hydrophobic molecules could easily cross the bilayer of phospholipid through simple diffusion.

Context and Applications

This topic is significant for all undergraduates and postgraduates, especially for

  • Bachelors and Masters in Chemistry
  • Bachelors and Masters in Biochemistry

Practice Problems

Question 1: The transformation of ions to micelle is __________.

  1. Irreversible
  2. Reversible
  3. None of the above

Answer: Option 2 is correct.

Explanation: It is a reversible process. Because in the micelle’s production, the water-soluble heads are located on the surface which is in touch with water whereas the water-insoluble tails are placed towards the center.

Question 2: Micelles act as colloids only when ______.

  1. The concentration is higher than CMC
  2. The concentration is smaller than CMC
  3. The concentration is equal to CMC
  4. Always acts as colloids

Answer: Option 1 is correct.

Explanation: If the concentration is higher than CMC, then micelles act as colloids. Before attaining the critical micellar concentration, the surface tension alters strongly with the surfactant’s concentration. After attaining the critical micellar concentration, the surface tension remains constant or alters with a lower slope.

Question 3: What is CMC?

  1. The minimum quantity of concentration which is needed for the production of micelle
  2. The maximum quantity of concentration which is needed for the production of micelle
  3. The minimum quantity of concentration which is not needed for the production of micelle
  4. The maximum quantity of concentration which is not needed for the production of micelle.

Answer: Option  1 is correct.

Explanation: The minimum quantity of concentration that is needed for the production of the micelle is named critical micelle concentration. It is also defined as the surfactant’s concentration above which micelles are produced.

Question 4: The movement of the integral proteins could be determined by the physical state of _________.

  1. External phospholipids
  2. Membrane phospholipids
  3. Amino acids
  4. Membrane appendages

Answer: Option 2 is correct.

Explanation: The movement of integral proteins could be determined by the physical state of membrane phospholipids because the integral proteins are implanted in the phospholipid bilayer of the membrane.

Question 5: Among the following diseases, which disease is produced because of the mutations in the membrane protein?

  1. Anemia
  2. Alzheimer’s disease
  3. Hemolytic anemia
  4. Parkinson’s disease

Answer: Option 3 is correct.

Explanation: Hemolytic anemia is produced by the irregular arrangement of erythrocytes which is traced to the mutations in spectrin or ankyrin that are plasma membrane proteins established in erythrocytes.

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