Membrane fluidity

In biology, membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane or a synthetic lipid membrane. Lipid packing can influence the fluidity of the membrane. Viscosity of the membrane can affect the rotation and diffusion of proteins and other bio-molecules within the membrane, there-by affecting the functions of these things.^[1]

Membrane fluidity is affected by fatty acids. More specifically, whether the fatty acids are saturated or unsaturated has an effect on membrane fluidity. Saturated fatty acids have no double bonds in the hydrocarbon chain, and the maximum amount of hydrogen. The absence of double bonds increases fluidity. Unsaturated fatty acids have at least one double bond, creating a "kink" in the chain. The double bond decreases fluidity. While the addition of one double bond raises the melting temperature, research conducted by Xiaoguang Yang et. al. supports that four or more double bonds has a direct correlation to membrane fluidity. Membrane fluidity is also affected by cholesterol.^[2] Cholesterol can make the cell membrane fluid as well as rigid.

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Why doesn't our cell membrane fall apart when it's too hot, or why doesn't our cell membrane freeze when it gets too cold? Well, our cell membrane actually has a very unique property called membrane fluidity. Now, a lot of different factors can affect membrane fluidity. But the three most important ones that we're going to focus on today are, number one, temperature, number two, cholesterol, and number three, which is whether we have unsaturated or saturated fatty acids. Now, just to quickly to remind us, the building blocks of a cell membrane are what we call phospholipids. And it looks like this. There's a phosphate head group that's represented by a circle and two fatty acid chains, kind of like strings hanging below. And in this video, we're actually really going to focus in on the impact of phospholipids in our cell membrane. So the first thing we're going to start off with is temperature. We have low temperature, and obviously we have high temperature. So let's pretend that our cell membrane is only made up of phospholipids. What do you think our cell membrane's going to look like at low temperatures? Since the temperature is low, our phospholipids are actually going to start clustering together really closely, kind of like that. And the reason why is because these phospholipids are at low temperature, which means they don't have a lot of energy to move around a lot. So they're going to huddle really close together. At extremely low temperatures, we actually call this a crystallized state. And since they're huddled so close together and they don't have a lot of energy to move around, the fluidity is actually pretty low. So as the temperature decreases, the fluidity of the cell membrane also decreases. What happens at high temperatures? Well, at high temperatures, our phospholipids have a little more energy. So they're going to move around a little bit more and cause themselves to have more of a distance between each other, kind of like that. So you'll notice that the distance between phospholipids is now much greater than what it was over here, at low temperatures, which is very, very small. So this increased distance allows our fluidity to increase, because there's much more room for the cell membrane to move around. So as the temperature increases, our membrane fluidity also increases. What happens when we add cholesterol? Well, at low temperatures, our phospholipids still tend to cluster pretty closely together. But occasionally, something really interesting happens, which is when cholesterol actually inserts itself between the phospholipids, like this. And it doesn't do this for every single phospholipid, but it'll occasionally insert itself into the membrane. The same goes for the phospholipids that are underneath. And you'll notice that the membrane doesn't always have to line up in the sense that the phospholipids can actually be in the same place as the ones above or in a slightly different place. In some membranes there's more cholesterol, and in others there is less. But the presence of cholesterol itself does something really unique. And what that is, is it actually increases the distance between some of the phospholipids. And like we've talked about for the high temperatures, as the distance between the phospholipids increases, the fluidity can also increase. What happens at high temperatures with cholesterol? At high temperatures, our phospholipids are already pretty far apart, just like the above picture. But just like before, the cholesterol will insert itself into the membrane at random places. And what this will actually do is it will cause the phospholipids to pull themselves closer together, because they kind of want to attach to that cholesterol. So now there's more stuff inserted throughout the membrane, and so the molecules in the membrane are now closer together. So the fluidity actually decreases. So cholesterol is actually really interesting, because at low temperatures, the fluidity will increase. And at high temperatures, the fluidity will decrease. You can kind of think about cholesterol like a buffer, kind of like in chemistry. It allows our cell membrane to remain at a fairly stable and normal level of fluidity. When the temperature gets too low, the fluidity will increase a little. And when the temperature gets too high, the fluidity will decrease. So moving on to our third one, which is the presence of saturated versus unsaturated fats, we're going to go ahead and make a new canvas to give ourselves a little bit of room. So in number three, we're comparing the presence of saturated versus unsaturated fats. And when we're talking about saturated versus unsaturated, we're talking about the fatty acid chains that are hanging below our phosphate head group. So just to remind us from chemistry, a saturated fatty acid can be represented like this, where every angle or pointy end is a carbon. In the case of an unsaturated fatty acid, it can look pretty different, because an unsaturated fatty acid means that we have some double bonds. So let's say we have two double bonds like that. By themselves, it doesn't seem to be anything special. Granted, they look different. But how will these interact with multiple fatty acids next to them? So in the case of a saturated one and in the case of an unsaturated one, our molecule will still have some double bonds. And what's really unique is you'll notice that in the saturated fatty acid, these two fatty acid chains stack together really neatly, kind of like Legos. But in our unsaturated fatty acid, these two don't really stack together that neatly. How will this affect our membrane fluidity? Well, for the sake of this particular explanation, we're going to draw the saturated fatty acid chains as straight lines, like this and just because we're trying to represent the fact that these straight lines stack together really well. So what's going to happen is they'll stack pretty closely together, and so will the ones underneath. And since the distances between the molecules is pretty small, our fluidity is actually pretty low. So what do you think will happen with our unsaturated fatty acids? Well, you'll notice that there's a little bit of a bend now in these fatty acid chains. So I'm actually going to represent the phospholipid with a little bend in it. And these might occur at different places, or they might have both of them being bent. But you'll notice that I'm unable to draw these phospholipids as closely together. There becomes more distance between these phospholipids because of this unsaturated bend in our phospholipids. So since there's more distance between our phospholipids, the fluidity increases. Just to quickly sum up, today we learned the three factors that can affect membrane fluidity, the first being temperature. As temperature increases, fluidity also increases. The second is cholesterol. And cholesterol acts as a buffer, increasing fluidity at low temperatures and decreasing fluidity at high temperatures. And the last are unsaturated fatty acids in our phospholipid. When we increase the amount of unsaturated fatty acids in our cell membrane, the fluidity also increases.

Factors determining membrane fluidity

Membrane fluidity can be affected by a number of factors.^[1] The main factors affecting membrane fluidity are environmental (ie. temperature), and compositionally.^[3] One way to increase membrane fluidity is to heat up the membrane. Lipids acquire thermal energy when they are heated up; energetic lipids move around more, arranging and rearranging randomly, making the membrane more fluid. At low temperatures, the lipids are laterally ordered and organized in the membrane, and the lipid chains are mostly in the all-trans configuration and pack well together.

The melting temperature $T_{m}$ of a membrane is defined as the temperature across which the membrane transitions from a crystal-like to a fluid-like organization, or vice versa. This phase transition is not an actual state transition, but the two levels of organizations are very similar to a solid and liquid state.

$T<T_{m}$ : The membrane is in the crystalline phase, the level of order in the bi-layer is high and the fluidity is low.
$T>T_{m}$ : The membrane is in the liquid-crystal phase, the membrane is less ordered and more fluid. At 37 °C, this is the state of the membrane: the presence of cholesterol, though, allows for the membrane stabilization and a more compact organization.

The composition of a membrane can also affect its fluidity. The membrane phospholipids incorporate fatty acyl chains of varying length and saturation. Lipids with shorter chains are less stiff and less viscous because they are more susceptible to changes in kinetic energy due to their smaller molecular size and they have less surface area to undergo stabilizing London forces with neighboring hydrophobic chains. Molecules with carbon-carbon double bonds (unsaturated) are more rigid than those that are saturated with hydrogens, as double bonds cannot freely turn. As a result, the presence of fatty acyl chains with unsaturated double bonds makes it harder for the lipids to pack together by putting kinks into the otherwise straightened hydrocarbon chain. While unsaturated lipids may have more rigid individual bonds, membranes made with such lipids are more fluid because the individual lipids cannot pack as tightly as saturated lipids and thus have lower melting points: less thermal energy is required to achieve the same level of fluidity as membranes made with lipids with saturated hydrocarbon chains.^[1] Incorporation of particular lipids, such as sphingomyelin, into synthetic lipid membranes is known to stiffen a membrane. Such membranes can be described as "a glass state, i.e., rigid but without crystalline order".^[4]

Cholesterol acts as a bidirectional regulator of membrane fluidity because at high temperatures, it stabilizes the membrane and raises its melting point, whereas at low temperatures it intercalates between the phospholipids and prevents them from clustering together and stiffening. Some drugs, e.g. Losartan, are also known to alter membrane viscosity.^[4] Another way to change membrane fluidity is to change the pressure.^[1] In the laboratory, supported lipid bilayers and monolayers can be made artificially. In such cases, one can still speak of membrane fluidity. These membranes are supported by a flat surface, e.g. the bottom of a box. The fluidity of these membranes can be controlled by the lateral pressure applied, e.g. by the side walls of a box.

Heterogeneity in membrane physical property

Discrete lipid domains with differing composition, and thus membrane fluidity, can coexist in model lipid membranes; this can be observed using fluorescence microscopy.^[4] The biological analogue, 'lipid raft', is hypothesized to exist in cell membranes and perform biological functions.^[5] Also, a narrow annular lipid shell of membrane lipids in contact with integral membrane proteins have low fluidity compared to bulk lipids in biological membranes, as these lipid molecules stay stuck to surface of the protein macromolecules.

Measurement methods

Membrane fluidity can be measured with electron spin resonance, fluorescence, atomic force microscopy-based force spectroscopy, or deuterium nuclear magnetic resonance spectroscopy. Electron spin resonance measurements involve observing spin probe behaviour in the membrane. Fluorescence experiments involve observing fluorescent probes incorporated into the membrane. Atomic force microscopy experiments can measure fluidity on synthetic^[6] or isolated patches of native membranes.^[7] Solid state deuterium nuclear magnetic resonance spectroscopy involves observing deuterated lipids.^[1] The techniques are complementary in that they operate on different timescales.

Membrane fluidity can be described by two different types of motion: rotational and lateral. In electron spin resonance, rotational correlation time of spin probes is used to characterize how much restriction is imposed on the probe by the membrane. In fluorescence, steady-state anisotropy of the probe can be used, in addition to the rotation correlation time of the fluorescent probe.^[1] Fluorescent probes show varying degree of preference for being in an environment of restricted motion. In heterogeneous membranes, some probes will only be found in regions of higher membrane fluidity, while others are only found in regions of lower membrane fluidity.^[8] Partitioning preference of probes can also be a gauge of membrane fluidity. In deuterium nuclear magnetic resonance spectroscopy, the average carbon-deuterium bond orientation of the deuterated lipid gives rise to specific spectroscopic features. All three of techniques can give some measure of the time-averaged orientation of the relevant (probe) molecule, which is indicative of the rotational dynamics of the molecule.^[1]

Lateral motion of molecules within the membrane can be measured by a number of fluorescence techniques: fluorescence recovery after photobleaching involves photobleaching a uniformly labelled membrane with an intense laser beam and measuring how long it takes for fluorescent probes to diffuse back into the photobleached spot.^[1] Fluorescence correlation spectroscopy monitors the fluctuations in fluorescence intensity measured from a small number of probes in a small space. These fluctuations are affected by the mode of lateral diffusion of the probe. Single particle tracking involves following the trajectory of fluorescent molecules or gold particles attached to a biomolecule and applying statistical analysis to extract information about the lateral diffusion of the tracked particle.^[9]

Phospholipid-deficient bio-membranes

A study of central linewidths of electron spin resonance spectra of thylakoid membranes and aqueous dispersions of their total extracted lipids, labeled with stearic acid spin label (having spin or doxyl moiety at 5,7,9,12,13,14 and 16th carbons, with reference to carbonyl group), reveals a fluidity gradient. Decreasing linewidth from 5th to 16th carbons represents increasing degree of motional freedom (fluidity gradient) from headgroup-side to methyl terminal in both native membranes and their aqueous lipid extract (a multilamellar liposomal structure, typical of lipid bilayer organization). This pattern points at similarity of lipid bilayer organization in both native membranes and liposomes. This observation is critical, as thylakoid membranes comprising largely galactolipids, contain only 10% phospholipid, unlike other biological membranes consisting largely of phospholipids. Proteins in chloroplast thylakoid membranes, apparently, restrict lipid fatty acyl chain segmental mobility from 9th to 16th carbons vis a vis their liposomal counterparts. Surprisingly, liposomal fatty acyl chains are more restricted at 5th and 7th carbon positions as compared at these positions in thylakoid membranes. This is explainable as due to motional restricting effect at these positions, because of steric hindrance by large chlorophyll headgroups, specially so, in liposomes. However, in native thylakoid membranes, chlorophylls are mainly complexed with proteins as light-harvesting complexes and may not largely be free to restrain lipid fluidity, as such.^[10]

Diffusion coefficients

Diffusion coefficients of fluorescent lipid analogues are about 10⁻⁸cm²/s in fluid lipid membranes. In gel lipid membranes and natural biomembranes, the diffusion coefficients are about 10⁻¹¹cm²/s to 10⁻⁹cm²/s.^[1]

Charged lipid membranes

The melting of charged lipid membranes, such as 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol, can take place over a wide range of temperature. Within this range of temperatures, these membranes become very viscous.^[4]

Biological relevance

Microorganisms subjected to thermal stress are known to alter the lipid composition of their cell membrane (see homeoviscous adaptation). This is one way they can adjust the fluidity of their membrane in response to their environment.^[1] Membrane fluidity is known to affect the function of biomolecules residing within or associated with the membrane structure. For example, the binding of some peripheral proteins is dependent on membrane fluidity.^[11] Lateral diffusion (within the membrane matrix) of membrane-related enzymes can affect reaction rates.^[1] Consequently, membrane-dependent functions, such as phagocytosis and cell signalling, can be regulated by the fluidity of the cell-membrane.^[12]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k Gennis, R. B. (1989) Biomembranes: Molecular Structure and Function. Springer, ISBN 0387967605.
^ Yang, Xiaoguang; Sheng, Wenwen; Sun, Grace Y.; Lee, James C-M. (February 2011). "Effects of fatty acid unsaturation numbers on membrane fluidity and α-secretase-dependent amyloid precursor protein processing". Neurochemistry International. 58 (3): 321–329. doi:10.1016/j.neuint.2010.12.004. ISSN 0197-0186. PMC 3040984. PMID 21184792.
^ Los, Dmitry A.; Murata, Norio (2004-11-03). "Membrane fluidity and its roles in the perception of environmental signals". Biochimica et Biophysica Acta (BBA) - Biomembranes. Lipid-Protein Interactions. 1666 (1): 142–157. doi:10.1016/j.bbamem.2004.08.002. ISSN 0005-2736.
^ ^a ^b ^c ^d Heimburg, T. (2007) Thermal Biophysics of Membranes. Wiley-VCH, ISBN 3527404716.
^ Simons K, Vaz WL (2004). "Model systems, lipid rafts, and cell membranes" (PDF). Annual Review of Biophysics and Biomolecular Structure. 33: 269–95. doi:10.1146/annurev.biophys.32.110601.141803. hdl:10316/11254. PMID 15139814.
^ Chiantia, Salvatore (2006). "Combined AFM and Two‐Focus SFCS Study of Raft‐Exhibiting Model Membranes". ChemPhysChem. 7 (11): 2409–2418. doi:10.1002/cphc.200600464. PMID 17051578.
^ Galvanetto, Nicola (2018). "Single-cell unroofing: probing topology and nanomechanics of native membranes". Biochimica et Biophysica Acta (BBA) - Biomembranes. 1860 (12): 2532–2538. arXiv:1810.01643. doi:10.1016/j.bbamem.2018.09.019. PMID 30273580. S2CID 52897823.
^ Baumgart, Tobias; Hunt, Geoff; Farkas, Elaine R.; Webb, Watt W.; Feigenson, Gerald W. (2007). "Fluorescence probe partitioning between L_o/L_d phases in lipid membranes". Biochimica et Biophysica Acta (BBA) - Biomembranes. 1768 (9): 2182–94. doi:10.1016/j.bbamem.2007.05.012. PMC 2702987. PMID 17588529.
^ Almeida, P. and Vaz, W. (1995). "Lateral diffusion in membranes", Ch. 6, pp. 305–357 in: Lipowsky, R. and Sackmann, E. (eds.) Handbook of biological physics. Elsevier Science B.V. doi:10.1016/S1383-8121(06)80023-0, ISBN 978-0-444-81975-8
^ YashRoy R C (1990) Magnetic resonance studies of dynamic organisation of lipids in chloroplast membranes. Journal of Biosciences, vol. 15(4), pp. 281-288.https://www.researchgate.net/publication/225688482_Magnetic_resonance_studies_of_dynamic_organisation_of_lipids_in_chloroplast_membranes?ev=prf_pub
^ Heimburg, Thomas & Marsh, Derek (1996). "Thermodynamics of the Interaction of Proteins with Lipid Membranes". In Kenneth M. Merz Jr. & Benoît Roux (eds.). Biological Membranes. Boston: Birkhäuser. pp. 405–462. doi:10.1007/978-1-4684-8580-6_13. ISBN 978-1-4684-8580-6.
^ Helmreich EJ (2003). "Environmental influences on signal transduction through membranes: A retrospective mini-review". Biophysical Chemistry. 100 (1–3): 519–34. doi:10.1016/S0301-4622(02)00303-4. PMID 12646388.

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