A physical space in which there is a different concentration of a single substance is said to have a concentration gradient. Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the interior of the membrane is not identical to the exterior of the membrane. Integral proteins that act as channels or pumps work in one direction.
Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane. These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes. Recall that plasma membranes have hydrophilic and hydrophobic regions. This characteristic helps the movement of certain materials through the membrane and hinders the movement of others.
Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion. Polar substances, with the exception of water, present problems for the membrane.
While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes.
Simple sugars and amino acids also need help with transport across plasma membranes. Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space.
You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of perfume in a room filled with people. The perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the bottle, and gradually, more and more people will smell the perfume as it spreads.
Diffusion expends no energy. Rather the different concentrations of materials in different areas are a form of potential energy, and diffusion is the dissipation of that potential energy as materials move down their concentration gradients, from high to low. Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials. Additionally, each substance will diffuse according to that gradient.
For an animation of the diffusion process in action, view this short video on cell membrane transport. In facilitated transport, also called facilitated diffusion, material moves across the plasma membrane with the assistance of transmembrane proteins down a concentration gradient from high to low concentration without the expenditure of cellular energy. However, the substances that undergo facilitated transport would otherwise not diffuse easily or quickly across the plasma membrane.
The solution to moving polar substances and other substances across the plasma membrane rests in the proteins that span its surface. The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane.
This allows the material that is needed by the cell to be removed from the extracellular fluid. Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it.
After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another.
This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:. A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly.
This occurs in the kidney where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules. The rate of diffusion in this instance is almost totally dependent on pressure. Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
Facilitated transport is a type of passive transport. Unlike simple diffusion where materials pass through a membrane without the help of proteins, in facilitated transport, also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are ions or polar molecules that are repelled by the hydrophobic parts of the cell membrane.
Facilitated transport proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell. The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane. This allows the material that is needed by the cell to be removed from the extracellular fluid. The substances are then passed to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of beta-pleated sheets that form a channel through the phospholipid bilayer.
Others are carrier proteins which bind with the substance and aid its diffusion through the membrane. The integral proteins involved in facilitated transport are collectively referred to as transport proteins; they function as either channels for the material or carriers. In both cases, they are transmembrane proteins. Channels are specific for the substance that is being transported. Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids; they additionally have a hydrophilic channel through their core that provides a hydrated opening through the membrane layers.
Passage through the channel allows polar compounds to avoid the nonpolar central layer of the plasma membrane that would otherwise slow or prevent their entry into the cell. Aquaporins are channel proteins that allow water to pass through the membrane at a very high rate. Channel Proteins in Facilitated Transport : Facilitated transport moves substances down their concentration gradients. They may cross the plasma membrane with the aid of channel proteins.
The attachment of a particular ion to the channel protein may control the opening or other mechanisms or substances may be involved.
In some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues, a gate must be opened to allow passage. An example of this occurs in the kidney, where both forms of channels are found in different parts of the renal tubules.
Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes. Opening and closing of these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting in the facilitation of electrical transmission along membranes in the case of nerve cells or in muscle contraction in the case of muscle cells. Another type of protein embedded in the plasma membrane is a carrier protein.
This protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior; depending on the gradient, the material may move in the opposite direction.
Carrier proteins are typically specific for a single substance. This adds to the overall selectivity of the plasma membrane. The exact mechanism for the change of shape is poorly understood.
Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough of the material for the cell to function properly.
Carrier Proteins : Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane. An example of this process occurs in the kidney. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney.
This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported; it is excreted from the body in the urine. Channel and carrier proteins transport material at different rates.
Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.
Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration. Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes.
Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion. While diffusion transports materials across membranes and within cells, osmosis transports only water across a membrane. The semipermeable membrane limits the diffusion of solutes in the water. Not surprisingly, the aquaporin proteins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.
Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all?
Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane otherwise the concentrations on each side would be balanced by the solute crossing the membrane.
If the volume of the solution on both sides of the membrane is the same but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane.
If there is more solute in one area, then there is less water; if there is less solute in one area, then there must be more water. To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar.
If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it. Osmosis : In osmosis, water always moves from an area of higher water concentration to one of lower concentration.
In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can. Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of passing through the membrane will diffuse through it.
In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system.
Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. In the beaker example, this means that the level of fluid in the side with a higher solute concentration will go up.
Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.
In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water.
In Fig. This result is in agreement with the molecular dynamics simulation of DPPC lipid 23 and the previously reported 27 values for a similar lipid, 1,2-dilauroyl- sn -glycerophosphocholine DLPC , measured by IXS. This distance also corresponds to the first peak in the static structure factor S Q,0 measured during the experiment Fig.
These areas per molecule are in agreement with the ones obtained by coarse grained molecular dynamic simulation 45 and more precise measurements 46 , Not unexpectedly, similar longitudinal phonon dispersions have been reported for deuterated 1,2-dimyristoyl- sn -glycerophosphocholine DMPC using INS However, our results reveal that the minimum in the longitudinal dispersion relation is deeper in the fluid phase, which is supportive of previous IXS measurements 27 but in contradiction with INS ones The inconsistency of the results measured by INS can possibly be attributed to the uncontemplated effects due to the selective deuteration of hydrocarbon tails used for the neutron scattering experiment.
Sound propagates well in lipid membranes in the form of the longitudinal phonons described above. What is not obvious, though, is whether transverse phonons also occur in disordered systems such as lipid membranes. Because each phonon mode is responsible for the sound propagation and heat transfer, it is important to know how many modes occur in a lipid membrane of interest.
In contrast to longitudinal phonons routinely observed in lipids, high-frequency transverse modes have not been experimentally observed in such systems so far. Recently, weakly dispersive transverse phonon modes were observed in DMPC membrane in the presence of ethanol.
In contrast, in this work we clearly observe high-frequency in-plane propagating transverse phonon modes in DPPC lipid membrane, similar to that observed in water and other liquids 48 , The good agreement between the longitudinal and the transverse excitations obtained by MD simulation 50 and in our work suggests that previous oversimplified interpretation of the IXS spectra may not be appropriate.
The phonon gap is indicated by the double arrow in Fig. This phenomenon can be explained in the framework of the recently developed phonon theory of liquids 36 , 37 , 38 , which predicts that transverse phononic gaps appear in disordered materials with increasing temperature.
The first experimental evidence for such phononic gaps using IXS along with the theoretical explanation supported by MD simulations was shown for liquid Ar As can be seen further, the appearance of the phononic band gaps leads to important implications for membrane dynamics and passive membrane transport.
As shown before 36 , 37 , 38 , 48 , the emergence of the phononic gaps is associated with diffusion and relaxation processes occurring in the lipid membrane. This point is of particular relevance, since the emergence of low-frequency phononic gaps see Fig.
In turn, the loss of long-range pair correlations activates diffusion processes at higher rates. This reveals two intrinsically different mechanisms of sound propagation and heat transfer in lipid membranes.
The first mechanism is characterized by the short and intermediate length scales and is governed primarily by phononic excitations. The other mechanism is originated from highly diffusive dynamics due to heavily damped phonons at long-length scales. Such picosecond clustering was previously observed in liquid Sn and Ga 51 , 52 and ascribed to transverse phonon localization on nanometre-length scales.
Specifically, the cluster size was estimated 0. The lipids, participating in the formation of these nanometre-sized clusters are schematically indicated by dark purple colour.
The rarified regions around the dark purple DPPC clusters indicate the formation of transient pores on longer length scales. To gain a deeper insight into the origin of the acoustic modes propagating throughout the membrane, in Fig. In the former regime, the density fluctuation is so strongly damped that it dies-off before experiencing a single oscillation and thus the mode essentially acquires a diffusive, rather than oscillatory, character. On a more formal ground, one may recognize that in the strongly overdamped regime R «1 the mode can be well described by a real-valued merely dissipative eigenvalue, while in the underdamped regime an imaginary oscillatory part emerges.
The oscillatory character of the transverse perturbation is provided by shear restoring forces that in a liquid are weaker and much shorter lasting than in solids. Generally, R is a measure of the number of oscillations exhibited by the vibrational mode before it becomes damped off. A damped acoustic wave is described by a function, which has the general form:.
In this respect, the comparison shown in Fig. In particular, one readily notices that, at lower temperature, the transverse modes become increasingly solid-like at intermediate Q range where R reaches a clear maximum. At the same time, the longitudinal modes become more damped, reflecting the increasing difficulty of the system in supporting the high wavevector acoustic sound wave.
In this Q -range, shear restoring forces cause the transverse waves to experience, within their lifetime, a number of oscillations even larger than the longitudinal modes. Our observation of the phononic gaps sheds light on the mechanism of the transmembrane solute diffusion. There has been much experimental evidence that the increased lipid packing reduces the diffusion across the membrane.
For example, reducing the temperature across the lipid main transition or adding cholesterol leads to increased surface density and decreases the solute partitioning into the membrane Different explanations of the solute diffusion were proposed based on MD simulations.
Another suggestion was that the partition coefficient strongly depends on the local chain ordering within the bilayer, which leads to the solute exclusion within the region of highly ordered chain packing 53 , Such local chain packing is short-living on the picosecond time scale and is localized on the scale of d gap , where the concentration of a solute can be potentially reduced.
On the other hand, long-range disorder, where the transverse phonon propagation is impaired, can be a signature of short-lived volume voids notice the rarefied lipid regions surrounding the DPPC clusters in Fig. The phononic landscape of biomembranes appears to be very rich 50 and is still largely unexplored experimentally.
The precise experimental determination of each phonon branch be it acoustic or optical using IXS remains very challenging for various reasons. The development of new IXS spectrometers with higher energy and Q resolutions with sharper resolution line shape 57 therefore, offers great opportunities to improve our understanding of the collective dynamics and the role it plays in the passive transport and other biological functions of biomembranes.
Here, we reveal for the first time, the existence of propagating transverse phonon modes in DPPC lipid membrane. Remarkably, we observe phononic gaps in the transverse phonon dispersion and evidence gap opening with increasing temperature.
This result confirms the theoretical prediction 38 that transverse phononic gaps appear in disordered materials over heat production due to the phonons interaction.
Such band gaps are related to the diffusion and relaxation processes occurring in the lipid membrane. According to the phonon theory of liquids 36 , 37 , 38 , the band gap emerges when the transverse phonon propagation is no longer supported due to the increasing lipid chain disorder on long-length scales.
For short and intermediate length scales, the band gap is an evidence of short-lived on the picosecond time scale local lipid clustering in lipid membranes. Such clustering local chain ordering supports the hypothesis that the local chain ordering may play an important role in solute diffusion by mediating the entropic expulsion exclusion of the solute from the inter-chain regions of the lipid membrane On the other hand, lipid chain disorder on long-length scale can potentially be an evidence for short-lived volume voids.
Thermally triggered void formation can account for the abnormally fast diffusion of small solutes via hopping between voids The universal phonon-triggered mechanism may possibly help shed light on other permeation properties, such as the anomalously high proton permeability of lipid bilayers, which cannot be explained just in the framework of solubility-diffusion mechanism The formation of water wires, or fingers inside the short-lived voids or transient defects that are mediated by phonons and the heat transfer in the lipid bilayer can account for proton translocation along such hydrogen-bonded water chains.
Single crystal synthetic diamond substrate 4. The rotational frequency had been chosen such that no tossing of lipid solution from the substrate was observed As a result, well oriented DPPC multilayer containing up to a thousand bilayers was formed. It should be noted, that a lipid sample prepared by the spin coating may exhibit imperfections, which in turn may affect the phonon spectra of disordered materials, like lipid membranes.
Nevertheless, this can be easily observed by analyzing the phonon branches. Therefore, such defects cannot be seen in our experiment and they have no effect on the results presented in this work.
The spectrometer was operated at Further details of the experimental setup can be found elsewhere The DPPC multilayer sample was placed inside a humidity chamber where the sample temperature and the relative humidity can be precisely controlled. The scattering geometry used during the experiment is shown in Fig. The scattering intensity is collected in the plane, parallel to the sample surface. Under these conditions, the scattering vector Q remains essentially within the DPPC lipid multilayer plane and, thus, only in-plane dynamics is probed.
In this experiment a very wide Q range from 2. The total data acquisition time for a given temperature shown in Fig. During the experiment, in order to mitigate the beam damage the sample was continuously scanned across the beam. Possible beam damage was examined both visually by looking at the sample under a microscope and by comparing the IXS scans for the same crystal analyser at the beginning and at the end of the experiment. Neither significant deterioration of the sample nor the degradation of the IXS spectra quality was observed.
The data that support the findings of this study are available from the corresponding author upon request. How to cite this article: Zhernenkov, M. Revealing the mechanism of passive transport in lipid bilayers via phonon-mediated nanometer-scale density fluctuations.
This Article was originally published with an incorrect publication date. This paper was due to be published on 26 May together with other content on a similar topic, but due to an error was published earlier on the 12 May Alberts, B. Molecular Biology of the Cell 4th edn Garland Science Cooper, G. The Cell: A Molecular Approach. Missner, A. Dioxide transport through membranes. Kleinzeller, A. Chen, I. RNA catalysis in model protocell vesicles. Mansy, S. Template-directed synthesis of a genetic polymer in a model protocell.
Nature , — Membrane transport in primitive cells. Cold Spring Harb. Article Google Scholar. Nagle, J. Structure of lipid bilayers. Acta , — Lingwood, D. Lipid rafts as a membrane-organizing principle. Science , 46—50 Edidin, M. The state of lipid rafts: from model membranes to cells. Akabori, K.
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