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You are watching: Why do nonpolar molecules diffuse more rapidly through membranes

Alberts B, Johnson A, Lewis J, et al. Molecule Biology the the Cell. 4th edition. Brand-new York: Garland Science; 2002.


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We start this ar by describing the permeability nature of protein-free, artificial lipid bilayers. We then introduce some that the terms supplied to describe the various forms of membrane transport and also some strategies for characterizing the proteins and processes involved.


Protein-free Lipid Bilayers Are very Impermeable come Ions

Given enough time, virtually any kind of molecule will diffuse throughout a protein-free lipid bilayer under its concentration gradient. The rate at which it does so, however, varies enormously, depending partly on the dimension of the molecule, but mostly on its relative solubility in oil. In general, the smaller the molecule and the more soluble that is in oil (the more hydrophobic, or nonpolar, that is), the much more rapidly it will diffuse throughout a lipid bilayer. Little nonpolar molecules, such together O2 and also CO2, conveniently dissolve in lipid bilayers and also therefore diffuse rapidly throughout them. Small uncharged polar molecules, such together water or urea, also diffuse throughout a bilayer, albeit much much more slowly (Figure 11-1). By contrast, lipid bilayers are highly impermeable to charged molecules (ions), no matter just how small: the charge and also high level of hydration of together molecules avoids them indigenous entering the hydrocarbon step of the bilayer. Thus, fabricated bilayers room 109 times much more permeable come water 보다 to also such little ions together Na+ or K+ (Figure 11-2).


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Figure 11-1

The loved one permeability that a fabricated lipid double layer to various classes the molecules. The smaller sized the molecule and, more importantly, the much less strongly that associates with water, the more rapidly the molecule diffuses across the bilayer.


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Figure 11-2

Permeability coefficients because that the passage of assorted molecules through artificial lipid bilayers. The rate of circulation of a solute throughout the bilayer is straight proportional come the distinction in the concentration ~ above the 2 sides that the membrane. Multiplying (more...)


There room Two main Classes that Membrane move Proteins: Carriers and also Channels

Like synthetic lipid bilayers, cell membranes allow water and nonpolar molecule to pass through by an easy diffusion. Cell membranes, however, also have to permit the i of various polar molecules, such as ions, sugars, amino acids, nucleotides, and also many cabinet metabolites that cross man-made lipid bilayers only very slowly. Distinct membrane deliver proteins room responsible for moving such solutes across cell membranes. These proteins take place in countless forms and in all types of biological membranes. Every protein transports a specific class of molecule (such as ions, sugars, or amino acids) and also often only specific molecular varieties of the class. The specificity the membrane move proteins was very first indicated in the mid-1950s by studies in which solitary gene mutations were found to abolish the capability of bacteria come transport details sugars across their plasma membrane. Similar mutations have now been uncovered in people suffering from a variety of inherited conditions that influence the carry of a certain solute in the kidney, intestine, or countless other cabinet types. People with the inherited disease cystinuria, because that example, space unable come transport details amino mountain (including cystine, the disulfide-linked dimer that cysteine) from either the urine or the intestine right into the blood; the resulting accumulation of cystine in the urine leads to the development of cystine stones in the kidneys.

All membrane transfer proteins that have actually been studied in detail have been discovered to it is in multipass transmembrane proteins-that is, your polypeptide chain traverse the lipid double layer multiple times. By developing a constant protein pathway across the membrane, these proteins enable certain hydrophilic solutes to cross the membrane without coming right into direct call with the hydrophobic inner of the lipid bilayer.

Carrier proteins and also channel proteins space the two major classes the membrane transfer proteins. Carrier proteins (also dubbed carriers, permeases, or transporters) bind the certain solute to be transported and also undergo a collection of conformational alters to transport the bound solute throughout the membrane (Figure 11-3). Channel proteins, in contrast, interact with the solute to be transported much an ext weakly. They kind aqueous pores the extend throughout the lipid bilayer; once these pores room open, lock allow specific solutes (usually not natural ions of suitable size and charge) come pass v them and thereby overcome the membrane (see figure 11-3). No surprisingly, transport v channel proteins wake up at a much much faster rate than move mediated by transport proteins.


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Figure 11-3

Carrier proteins and channel proteins. (A) A carrier protein alternates in between two conformations, so that the solute-binding site is sequentially obtainable on one side of the bilayer and then ~ above the other. (B) In contrast, a channel protein forms a (more...)


Active carry Is Mediated by transport Proteins coupled to an power Source

All channel proteins and many transport proteins enable solutes to overcome the membrane only passively (“downhill”), a process called passive transport, or facilitated diffusion. In the instance of move of a solitary uncharged molecule, it is just the difference in its concentration top top the two sides the the membrane—its concentration gradient—that drives passive transport and also determines that direction (Figure 11-4A).


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Figure 11-4

Passive and energetic transport compared. (A) Passive deliver down an electrochemical gradient wake up spontaneously, one of two people by basic diffusion v the lipid double layer or by assisted in diffusion through channels and passive carriers. By contrast, active (more...)


If the solute dead a net charge, however, both that is concentration gradient and the electric potential difference throughout the membrane, the membrane potential,influence its transport. The concentration gradient and also the electrical gradient can be combined to calculate a network driving force, the electrochemical gradient, for each charged solute (Figure 11-4B). We talk about this in much more detail in chapter 14. In fact, virtually all plasma membranes have an electric potential distinction (voltage gradient) across them, v the within usually negative with respect come the outside. This potential difference favors the entry of positively fee ions right into the cell but opposes the entry of negatively fee ions.

Cells additionally require transport proteins that will actively pump certain solutes across the membrane against their electrochemical gradient (“uphill”); this process, well-known as active transport, is mediated through carriers, i beg your pardon are additionally called pumps. In active transport, the pumping task of the carrier protein is directional since it is tightly coupled to a resource of metabolic energy, such together ATP hydrolysis or one ion gradient, as disputed later. Thus, carry by carriers can be either active or passive, whereas deliver by channel proteins is always passive.


Ionophores have the right to Be provided as tools to boost the Permeability of membrane to certain Ions

Ionophores are small hydrophobic molecules the dissolve in lipid bilayers and increase your permeability to particular inorganic ions. Many are synthesized by microbe (presumably as biological weapons against competitors or prey). They room widely provided by cell biologists as devices to rise the ion permeability of membranes in research studies on man-made bilayers, cells, or cabinet organelles. There space two classes of ionophores—mobile ion carrier and channel formers (Figure 11-5). Both species operate by shielding the charge of the transported ion so that it have the right to penetrate the hydrophobic inner of the lipid bilayer. Due to the fact that ionophores room not coupled to power sources, they permit the net movement of ions only down your electrochemical gradients.


Figure 11-5

Ionophores: a channel-former and a mobile ion carrier. In both cases, network ion circulation occurs only down an electrochemical gradient.


Valinomycin is an example of a mobile ion carrier. That is a ring-shaped polymer the transports K+ down its electrochemical gradient by choose up K+ ~ above one next of the membrane, diffusing throughout the bilayer, and releasing K+ on the various other side. Similarly, FCCP, a mobile ion transport that makes membranes selectively leaky to H+, is frequently used to dissipate the H+ electrochemical gradient throughout the mitochondrial within membrane, in order to blocking mitochondrial ATP production. A23187 is yet another example that a mobile ion carrier, only it transports divalent cations such together Ca2+ and also Mg2+. When cells room exposed come A23187, Ca2+ beginning the cytosol indigenous the extracellular fluid down a steep electrochemical gradient. Accordingly, this ionophore is widely used to rise the concentration of free Ca2+ in the cytosol, thereby mimicking certain cell-signaling instrument (discussed in chapter 15).

Gramicidin A is an instance of a channel-forming ionophore. It is a dimeric link of two direct peptides (of 15 hydrophobic amino acids each), which wind around each various other to type a double helix. 2 gramicidin dimers space thought to come together finish to end throughout the lipid double layer to kind what is probably the simplest of all transmembrane channels, which selectively permits monovalent cations to circulation down their electrochemical gradients. Gramicidin is made by specific bacteria, probably to kill various other microorganisms through collapsing the H+, Na+, and also K+ gradients the are vital for your survival, and also it has been valuable as an antibiotic.


Summary

Lipid bilayers are very impermeable to many polar molecules. To transport little water-soluble molecules right into or the end of cell or intracellular membrane-enclosed compartments, cell membranes contain miscellaneous membrane transport proteins, each of i beg your pardon is responsible for delivering a certain solute or course of solutes throughout the membrane. There room two classes of membrane transport proteins—carriers and also channels. Both form constant protein pathways throughout the lipid bilayer. Whereas transfer by carriers have the right to be either active or passive, solute flow through channel protein is always passive. Ionophores, which are little hydrophobic molecules made by microorganisms, have the right to be provided as tools to boost the permeability of cell membranes to specific inorganic ions.

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