Talk:Lipid bilayer

Ye olde comments
What cellular structure is composed of a lipid bilayer?


 * All cell membrane's consist of a lipid bilayer.  It is not so explicit in the article, but it should be. --Lexor 01:55, 21 Sep 2003 (UTC)

Should this article be merged together with other similar structures to "lipid aggregates"? --Eleassar777 15:07, 24 May 2005 (UTC)
 * I think it should not Karol 15:30, May 24, 2005 (UTC)

Why not? Or lipid structures? --Eleassar777 16:26, 24 May 2005 (UTC)
 * Well, I just think that lipid bilayers are recognized as being so different than ex. micelles (maybe that's just my bias as a biophysicist) and monolayers. Also, large lipid vesicles for example consist of bilayers, so they are not bilayers, but a specific architecture for bilayers, which can be planar or closed surfaces. That said, I very much agree that there should be more information on lipid aggregate structures - as a section in the page on lipids or as a new article (although I think the first option is better). I would happily do this myself, but I'm working on a different topic just now. Karol 17:06, May 24, 2005 (UTC) P.S. look: the lipid page has something on aggregates already that could evolve into a section maybe

Model Over an aperture between two aqueous solutions, where it is called a black BLM.
 * Huh? This doesn't make sense. --mgambrell 20:32, 9 October 2006 (UTC)

This whole article is terrible, very poorly written. I've tried to address some of the problems, but it needs some serious work. Maybe it should be entirely re-written from scratch fraybentos 11:32, 14 Nov 2006 (UTC)

Bimolecular layer?
Is a lipid bilayer also called a bimolecular layer? -Velen117 08:11, 12 September 2006 (UTC)

Never heard of such a term. So, probably no. However it is the samething as a phospholipid bilayer--Biophysik (talk) 12:30, 15 January 2009 (UTC)

Fluidity
No one has mentioned the fluidity of the membrane, the fact that each phospholipid molecule swaps positions with its neighbour 10 million times a second!

Original Characterization methods section
Here is the original Characterization methods section before my recent edit, for use in subarticles or readds:



The physical nature of the lipid bilayer makes it a very difficult structure to study. An individual bilayer since it is only a few nanometers thick is invisible in traditional light microscopy. The bilayer is also a relatively fragile structure since it is held together entirely by non-covalent bonds and is irreversibly destroyed if removed from water. In spite of these limitations dozens of techniques have been developed over the last seventy years to allow investigations of the structure and function of bilayers.

Electrical measurements are the most straightforward way to characterize one of the more important functions of a bilayer, namely its ability to segregate and prevent the flow of ions in solution. Accordingly, electrical characterization was one of the first tools used to study the properties of model systems such as black membranes. By applying a bias across the bilayer and measuring the resulting current, it is possible to determine the resistance of the bilayer. This resistance is typically quite high, often exceeding 100 GO since the hydrophobic core is impermeable to charged species. Because this resistance is so large, the presence of even a few nanometer-scale holes results in a dramatic increase in current. The sensitivity of this system is such that even the activity of single ion channels can be resolved.



Electrical measurements are a useful tool for studying bilayer properties, but they do not provide an actual picture like imaging with a microscope can. Lipid bilayers cannot be seen in a traditional microscope because they are too thin and therefore do not interact sufficiently with light. In order to see bilayers, researchers often use fluorescence microscopy. In fluorescence microscopy a sample is excited with one color of light and observed in a different color of light, such that only fluorescent molecules with a matching excitation and emission profile will be seen. This specificity allows sensitive measurements with low background signal. Most materials, including natural lipid bilayers, are not fluorescent. Therefore, to use fluorescence microscopy to study bilayers, a fluorescent dye must be used. The resolution of fluorescence microscopy is typically limited to a few hundred nanometers, which is much smaller than a typical cell but much larger than the thickness of a lipid bilayer. More recently, advanced microscopy methods have allowed much greater resolution under certain circumstances, even down to sub-nm.

When researchers need higher resolution than optical methods can offer, they can use electron microscopy to study lipid bilayers. In an electron microscope, a beam of focused electrons interacts with the sample rather than a beam of light as in traditional microscopy. In 1960, when the structure of the bilayer was still debated, it was electron microscopy that offered the first direct visualization of the two apposing leaflets. In conjunction with rapid freezing techniques, electron microscopy has also been used to study the mechanisms of inter- and intracellular transport, for instance in demonstrating that exocytotic vesicles are the means of chemical release at synapses.

Another method that has been used in recent years to image and study lipid bilayers is Atomic force microscopy (AFM). Rather than using a beam of light or particles, AFM uses a small sharpened tip to scan the surface, much the same way a record player works. AFM is a promising technique because it has the potential to image with nanometer resolution at room temperature and even underwater, conditions necessary for natural bilayer behavior. Another advantage is that AFM does not require fluorescent or isotopic labeling of the lipids, since the probe tip interacts mechanically with the bilayer surface. Because of this, the same scan can image both lipids and associated proteins, sometimes even with single-molecule resolution. In addition to imaging, AFM can also probe the mechanical nature of lipid bilayers.

Question
This is a great article. One question about the lead. It says "Cell membranes are almost always made of bilayers in the fluid phase" - I hesitated to change this, since you're the expert, but I personally can't think of any counterexamples. What examples were you thinking of? Tim Vickers (talk) 17:25, 1 March 2009 (UTC)
 * Certain species of archaea use a lipid monolayer rather than a bilayer. This is an adaptation to their role as extremophiles. At very high temperatures a monolayer is more stable than a bilayer. There are also a few other unique several adaptations present, including the use of an ether headgroup linkage and isoprene sidechains on the tails. Thanks for the edits. I think they help the intro read more clearly. My remaining concerns are whether the intro is still too long and whether it adequately addresses each sub-section. What do you think? Also, what do you think about the overall length of the article? Is there anything else I need to address before FA submission? MDougM (talk) 21:22, 1 March 2009 (UTC)
 * Good point, we can still simplify and generalise this "Cell membranes are made lipids in the fluid phase, in part because fluid lipids can reseal small holes and tears." Tim Vickers (talk) 22:27, 1 March 2009 (UTC)
 * OK, that's probably as general as we can make it. Have you ever heard Einstein's aphorism about explaining science that is very applicable to writing Wikipedia articles - "Make things as simple as possible, but no simpler"! Tim Vickers (talk) 23:11, 1 March 2009 (UTC)

Commercial applications / removed Planar patch clamp diagrams
The diagram / photo referring to Planar patch clamp did not illustrate the section text, their presence thus confused the reader because section had no text at all referring to this highly specialized topic. Illustrations can be readily found in the PPC article itself, therefore I have removed them and linked to Planar patch clamp at the right text position where automated patch clamp systems are mentioned. Furthermore I have replaced the commercial nanion weblink with a link to the PPC article by Behrends and Fertig on the nanion site. Regards, --Burkhard (talk) 22:03, 28 February 2010 (UTC)

Aquaporins?
The section on diffusion should be edited to include new information on aquaporins and their role in osmosis. —Preceding unsigned comment added by Quadral (talk • contribs) 02:22, 16 September 2010 (UTC)

Use of Units
The "Structure and Organisation" section of the article seems to alternate between using angstroms and nanometres for measuring distance.

For clarity and ease of understanding, wouldn't it be better to stick with one unit type? Preferably nanometers since they are an SI defined unit, whilst angstroms are not?

Also, if angstroms are to be kept due to their common usage in biology, can the link on the Å symbol be changed to direct to the Angstrom article instead of the article for the actual Å character? It just confused me first time round!

Cybernetic cheese (talk) 20:14, 5 October 2010 (UTC)


 * Good point. I went ahead and changed everything to nm. MDougM (talk) 15:54, 17 March 2011 (UTC)

overly technical detail in Structure and Organization?
It seems like the paragraph led by "Phospholipid-deficient mixed lipid bilayers" is perhaps too technical for this general article. Unless someone has a good counter-argument, I'll remove it from the main article and post it here so that it could maybe be spun off into a subsidiary article. — Preceding unsigned comment added by MDougM (talk • contribs) 00:13, 7 March 2015 (UTC)
 * I agree that that paragraph is too technical as written. Perhaps, after some editing, it could be moved to the "Mixed Systems" section of Lipid bilayer phase behavior? Shanata (talk) 09:05, 7 March 2015 (UTC)
 * I went ahead and removed that section. Pasting here for posterity so that it can be moved to a more appropriate home at some point MDougM (talk) 20:46, 7 March 2015 (UTC)

Phospholipid-deficient mixed lipid bilayers are unique to plant thylakoid membranes. Earth's most abundant biological membrane system, plant thylakoid membranes, surprisingly, contain largely reverse-hexagonal cylindrical phase forming monogalactosyl diglyceride(MGDG) and only 10 percent phospholipids. However, the second most abundant lipid, digalactosyl diglyceride forms aqueous lamellar phase or lipid bilayer organisation. Nevertheless, total lipid extract of thylakoid membranes does form aqueous lipid bilayers and unilamellar liposomes, which find matching fluidity with native thylakoid membranes - thus interpreted to consist largely of lipid bilayer lamellae. It is interesting to study subtle changes in carbon-13 FT-NMR spectral linewidths and line shapes of lipid fatty-acyl, and headgroup carbon resonances  of lipids in unaggreated, reverse spherical micellar and lipid bilayer forms and note how segmental motions and fluidity gradient, characteristically change in different lipid dynamic organizations. Extent of resonance line broadening depends upon restriction of segmental motional freedom. For example, lipid headgroup (hg) is maximally broadened in reverse micellar (B) and least in un-aggregated (A) and in-between for bilayer (C) organisation of same lipids. Fatty-acyl carbonyl (C=O) resonance, being closest to lipid headgroups, follows almost the same line-broadening pattern. Terminal methyl (CH3) carbon resonances are only marginally broadened in lipid bilayer (C) and practically sharp in A and B. Mid-chain CH2 and HC=CH carbon resonances are also differentially broadened in the three states, depending on relatively more restriction of segmental motions in bilayer state compared to A and B. Overal examination of A, B and C states, educates about different flexibility or fluidity gradient in the three cases. Motional restriction due to 'peer pressure' on fatty acyl chains from bulk lipids in lipid bilayer organization, makes lipid bilayers show a 'characteristic', gradually-increasing fluidity-gradient conditions, from lipid head-groups to terminal methyl carbons; this becomes clear from line broadening pattern in their NMR spectra.

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Assessment comment
Substituted at 22:04, 29 April 2016 (UTC)

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Cytosis balancing
The article says about cytosis: ''The primary mechanism of this interdependence is the sheer volume of lipid material involved. In a typical cell, an area of bilayer equivalent to the entire plasma membrane will travel through the endocytosis/exocytosis cycle in about half an hour. If these two processes were not balancing each other, the cell would either balloon outward to an unmanageable size or completely deplete its plasma membrane within a matter of minutes.''

I would like to change sheer volume to large amounts or similar. I don't think there is a fault here. But the word volume is ambiguous and we want to avoid association with the other meaning, which is the primary meaning according to Wikipedia.

I would also like to change within a matter of minutes to in a short time or in half an hour or something similar. This is a matter of arithmetics. If exocytosis processes an area equivalent to the plasma membrane in half an hour, then it needs half an hour to completely deplete the plasma membrane. This is a mathematical necessity. --Ettrig (talk) 10:31, 18 February 2019 (UTC)

Repetition.
The Signalling section says again some things that were already, and in my view better, explained in the Assymetry section.

Compare the following for brevity and clarity, please.

Assymettry section: "when a cell undergoes apoptosis, the phosphatidylserine — normally localised to the cytoplasmic leaflet — is transferred to the outer surface: There, it is recognised by a macrophage that then actively scavenges the dying cell."

Signalling section: " A classic example of this is phosphatidylserine-triggered phagocytosis. Normally, phosphatidylserine is asymmetrically distributed in the cell membrane and is present only on the interior side. During programmed cell death a protein called a scramblase equilibrates this distribution, displaying phosphatidylserine on the extracellular bilayer face. The presence of phosphatidylserine then triggers phagocytosis to remove the dead or dying cell." Polar Apposite (talk) 20:54, 18 November 2022 (UTC)