Why a bilayer is formed in a watery medium

It sticks together, and floats on the surface, until it all just forms one big slick. If you shake the mixture vigorously however, you see thousands of tiny little globules floating on the surface which will eventually return to one big glob. This is due to forces between the water molecules and the oil molecules and is known as the 'hydrophobic effect'.

In the human body, water is present in almost every space you can imagine. When exposed to the water, the phospholipid bilayer spontaneously self-assembles.

If you can picture it, the hydrophilic heads begin to arrange themselves in such a manner that they are directly exposed to the water molecules, and the exposed tails attempt to isolate themselves from the water; leading to the creation of a sphere with the heads exposed, and the tails safe from water on the inside. As more spheres are created, they interact with each other and combine to create a larger, continuous phospholipid bilayer.

How do phospholipids arrange themselves into a bilayer? Nov 14, Lipids self-aggregate in water due to the hydrophobic interaction of the nonpolar chains forming closed particles such as liposomes lower part. In addition, different methodologies have been designed, afterwards, in order to obtain suspension of homogeneous size distribution of different magnitudes [ 10 , 11 , 12 ]. With this wide range of possibilities, it was immediate to infer that liposomes and its different versions of covered or uncovered unilamellar vesicles would be the ideal tools to trap, vehiculate specific compounds to drive them to specific targets and deliver drugs to organs and tissues, specifically for human beings pathologies Figure 3.

Liposomes are one of the most attractive biomimetic systems because its preparation is done with lipids extracted from cells.

In addition, other biomimetic nanoparticles can include lipids in its matrix. However, there are a number of difficulties for the direct use of these preparations that are mostly derived by limited knowledge of the physicochemical properties of the lipid bilayers. These include the presence of water as a major component in the membrane matrix; the thermodynamic properties derived from it in relation to the response from physicochemical stimuli and the interphase properties.

The purpose of this chapter is to analyze these points in order to propose new strategies for designing biomimetic lipid particles more efficiently. When dry phosphatidylcholines PCs of chain length above 12 hydrocarbon atoms are dispersed in water above their transition temperature as described in Figure 2 , they form lamellar onion-like structures in which bilayers are separated by aqueous spaces that are available to trap the compounds of interest to vehiculize and deliver Figure 4.

A Electronic microscopic traditional image of multilamellar liposomes; B the diffraction pattern illustrates the separation between bilayers; C water solution trapped in between bilayers is schematically represented. Different thermal profiles were obtained according to the lipid features and head group structure was found [ 16 , 17 , 18 ].

Lipids stabilize differently according to its geometry. Phosphatidylcholines, that have similar areas in the head group region and the acyl chain, form bilayers by stacking molecules in cylindrical shape. In contrast, phosphatidylethanolamine PE forms hexagonal phases due to the conical shape of molecules [ 19 , 20 ]. The process of lipid hydration that derives the formation of liposomes consists of different stages as described in Figure 5.

Hydration of lipids and swelling of liposomes. After this stage, area and thickness remain constant and the swelling of liposomes starts by the increase of water in the interlamellar space. This description of lipid swelling illustrates about several structural and physicochemical properties of the bilayers. The first observation is that there is a defined number of water molecules per lipid that determines the area per lipid and the bilayer thickness.

This number is around 7—8 below the phase transition temperature and 22—24 above as derived by DSC [ 17 , 18 , 21 , 22 ]. Thus, water is a component of the structure of the lipid bilayer, determining its thermodynamic stability. At least four features deserve discussion.

At equilibrium in fully hydrated state, the membrane thickness is composed by the excluded volume of the lipid molecules and the excluded volume of the water organized by them the hydration number denoted above. Thus, the barrier properties do not only merge with the head group and the acyl chain region per se but also of the packing and arrangement of water molecules in the hydration shell of the phospholipids. This means that for any solute, to overcome the bilayers, that is, releasing the trapped solute or incorporating some of them must permeate or alter the hydration shell.

This can, in principle, be done by some of these mechanisms: insertion in the water network removing or replacing water molecules in the hydration shell or changing the area per lipid by expansion or compression. The second feature of lipid bilayers is derived from the first one. The interbilayer space consists of water not bound to the membrane, that is, it can be displaced by changing the osmotic gradient between the inner spaces and the outer media of the liposome.

Water can permeate the lipid bilayer with certain facility depending on the phase state of the lipids, the presence of double bonds or ramifications in the acyl chains [ 23 , 24 ]. In contrast, membrane is completely or partially impermeable to some solutes, such as sugars, ions, depending on its size and molecular structure.

The differences in permeation rates between water and any of these solutes means that at least in the beginning of the process, a gradient of water chemical potential can be built with a difference in solute concentrations between the two sides of the bilayer.

Let us consider the interbilayer space described in Figure 5. If solute is more concentrated between the bilayers, water will be driven to enter due to a difference in osmotic pressure and them the spacing and hence the trapped volume will be larger. An opposite effect can be caused, if the solute is concentrated in the outer media of the liposomes. In this case, liposomes shrink and the interbilayer space decreases. Thus, any change in the number of these water molecules will affect thickness and area with concomitant effect on permeability.

Finally, the fourth feature is defined by the limit of the volume decrease. This is given by the steric repulsion of the groups in the surface of the bilayer, in which water plays a significant role.

Water associated with the lipids is oriented at the bilayer surface constituting an electrical potential that hinder bilayers approach.

This repulsive force is named as dipole potential or hydration forces [ 24 , 25 , 26 , 27 ] Figure 6. A The limit of approach of lipid bilayers, B water organized at the interphase determining the repulsion forces, C dipole potential at the lipid interphase.

This potential makes the bilayer interior positive and has important consequences in the binding and penetration of charged peptides and proteins. It is immediate to derive that the presence of these forces hinders the adhesion or fusion of membranes with different kinds of surfaces inorganic and organic materials, other membranes, proteins, tissues, etc. On the other way round, those processes will be feasible if water of hydration is totally or partially removed.

This point is essential to understand the role of water in terms of membrane response to biologically relevant effectors. Entering details of the dynamic properties of membranes in relation to water, we may again inspect Figure 5.

So, the question is which perturbations can trigger changes in hydration that can be dominated in order to promote controlled changes in permeation. In this direction, let us focus on the mechanical and chemical forces at constant temperature. Park, R. Boston: Little Brown, Sadava, D. Cell Biology, Organelle Structure and Function. Boston: Jones and Bartlett, Stoeckenius, W. Structure of the plasma membrane: An electron-microscope study.

Circulation 26 , — Cell Membranes. Microtubules and Filaments. Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes. Plant Cells, Chloroplasts, and Cell Walls.

Cytokinesis Mechanisms in Yeast. How Viruses Hijack Endocytic Machinery. Discovering the Lipid Bilayer. Discovery of the Giant Mimivirus. Endosomes in Plants. Mitochondria and the Immune Response. Plant Vacuoles and the Regulation of Stomatal Opening. The Discovery of Lysosomes and Autophagy.

The Origin of Plastids. The Origins of Viruses. Volvox, Chlamydomonas, and the Evolution of Multicellularity. Cephalopod Camouflage: Cells and Organs of the Skin.

Citation: Adams, M. Nature Education 3 9 We are taught that plasma membranes are a typical lipid bilayer, but how do we know this, and who figured it out? Aa Aa Aa. The Membrane Concept. Discovery of the Lipid Bilayer. Figure 2: Langmuir trough. Figure Detail.

Experimental Follow-Up with Microscopy. References and Recommended Reading Edidin, M. Tanford, C. Ben Franklin Stilled the Waves. Article History Close. When you go to the dentist to get a tooth pulled, you really do not want to feel any pain. The dentist injects an anesthetic into your gum and it eventually becomes numb.

One theory as to why anesthetics work deals with the movement of ions across the cell membrane. The anesthetic gets into the membrane structure and causes shifts in how ions move across the membrane. If ion movement is disrupted, nerve impulses will not be transmitted and you will not sense pain - at least not until the anesthetic wears off.

A phospholipid is a lipid that contains a phosphate group and is a major component of cell membranes.