Membranes of our cells consist of lipid bilayer (with some other components mixed in). Why wouldn't those lipid bilayers glue to each other and make the cells merge? If you suppose that it's due to additional components in the membrane that prevent them from gluing together, consider the case of liposomes, which don't contain additional components, but seemingly don't glue as well.

I'm primarily interested in quantitative physical chemistry aspect of the matter.

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    the membranes are not pure phospholipid double-layers, they contain proteins as well, those proteins have a plethora of functions, one being to prevent two membranes to stick together and merge. (Others being: gluing the membranes together at a distance, transporting chemicals through the membrane, being receptors for chemical signals, ...). But there is of course also an energy barrier due to the orientation of the molecules (with the polar ends sticking out in the water). – Sebastian Riese Jul 17 '15 at 22:59

Phospholipid bilayers found in cells are usually negatively charged. The phosphate groups repel each other by like charges and prevent two membranes from coming too close to each other. Membranes are also full of proteins and often coated with carbohydrates, which serve to keep membranes from interacting too strongly. See this diagram of a membrane bilayer: enter image description here

Even this diagram is probably cleaned up.

From a more physical chemistry perspective, the energy barrier for flipping a lipid in a bilayer is pretty high, since the polar head group doesn't like passing through the hydrophobic layer. Bringing two membranes together would require overcoming the charge-charge repulsion and flipping around the inner two layers of lipids, so you go from 4 layers to 2.

Interestingly, there are mechanisms for fusing membranes, such as using cationic lipids. These would turn charge repulsion into charge attraction. The lipid ion pairs tend to have shapes that don't support bilayer formation, and tend to prefer micelle or inverted micelle shapes. These have been utilized to break down vesicle membranes and allow large molecules to pass through, like DNA or proteins, and are used in genetic modification of cells.

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    Also, SNARE proteins facilitate membrane fusion. – James Jul 18 '15 at 15:06

well it's based on the tissue type for example take loose connective tissue which lies under epithelium they don't merge because of the profound matrix separating them while in the epithelium in cuboidal and columnar epithelium at side corner and upper portion of their membranes the two neighboring cells fused completely preventing any flow of material whatever may be the size it's called tight junction and is due to the physical change in lipid layer of both cell causing this adherence.

or if by merging you mean that the cells should remove their barriers (side membranes) to make their self one cell it is also depending on tissue like macro phages becoming giant cells by merging and forming one cell.

  • Yes, I mean why don't they make giant cells by merging? – Boris Burkov Jul 18 '15 at 12:38
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    well it's due to the enzymes synthesized by macrophages which other cells doesn't show this charesteristics and this fusion doesn't happens itself unless there is a stimulator like bacillus anthracis (causer of tuberculosis) when it enters the body the macrophages can swallow them but can't lyse them so they have to merge... so in that case the alterer of this phenomena is the bacteria mentioned by indirect stimulation of those gene which synthesis those enzymes which take part in fusion of macrophages...so the bottom line is not every cell can synthesis those fusion making enzymes. – user4147 Jul 18 '15 at 13:13

I think lipid membranes in mutual contact do in some cases merge, search liposomes. However, this does not always happen, because the membranes have some degree of cohesiveness and quasi-rigidity. This is said to be due to hydrophobic effect - in aqueous environment, the lipid molecules are repelled from water molecules as the latter attract each other much more strongly than lipid molecules do (water molecules are polar + they experience hydrogen bonding).

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