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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 of the Cell. 4th edition. New York: Garland Science; 2002.


By commitment with the publisher, this book is easily accessible by the find feature, however cannot it is in browsed.
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We begin this ar by relenten the permeability nature of protein-free, man-made lipid bilayers. Us then introduce some that the terms supplied to explain the various develops of membrane transport and some strategies for characterizing the proteins and processes involved.


Protein-free Lipid Bilayers Are very Impermeable come Ions

Given enough time, virtually any type of molecule will certainly diffuse throughout a protein-free lipid bilayer down its concentration gradient. The rate at which that does so, however, varies enormously, depending partly on the dimension of the molecule, yet mostly on its loved one solubility in oil. In general, the smaller the molecule and the an ext soluble it is in oil (the much more hydrophobic, or nonpolar, the is), the much more rapidly it will certainly diffuse across a lipid bilayer. Small nonpolar molecules, such as O2 and CO2, conveniently dissolve in lipid bilayers and also therefore diffuse rapidly throughout them. Small uncharged polar molecules, such as water or urea, likewise diffuse across a bilayer, albeit much much more slowly (Figure 11-1). By contrast, lipid bilayers are highly impermeable to charged molecules (ions), no matter exactly how small: the charge and also high degree of hydration of together molecules stays clear of them from entering the hydrocarbon step of the bilayer. Thus, man-made bilayers room 109 times much more permeable come water than to even such little ions as Na+ or K+ (Figure 11-2).


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Figure 11-1

The family member permeability the a synthetic lipid bilayer to different classes the molecules. The smaller the molecule and, much more importantly, the much less strongly that associates through water, the more rapidly the molecule diffuses throughout the bilayer.


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Figure 11-2

Permeability coefficients because that the passage of assorted molecules through man-made lipid bilayers. The rate of flow of a solute across the bilayer is directly proportional come the distinction in that concentration top top the 2 sides that the membrane. Multiply (more...)


There room Two key Classes the Membrane deliver Proteins: Carriers and also Channels

Like fabricated lipid bilayers, cell membranes allow water and nonpolar molecule to permeate by an easy diffusion. Cell membranes, however, additionally have to enable the passage of various polar molecules, such together ions, sugars, amino acids, nucleotides, and also many cell metabolites that cross fabricated lipid bilayers only very slowly. One-of-a-kind membrane move proteins are responsible for moving such solutes across cell membranes. This proteins take place in countless forms and in all species of biological membranes. Each protein transports a specific class that molecule (such together ions, sugars, or amino acids) and often only particular molecular varieties of the class. The specificity of membrane deliver proteins was very first indicated in the mid-1950s by research 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 actually now been uncovered in people suffering from a range of inherited conditions that impact the transport of a certain solute in the kidney, intestine, or countless other cell types. Individuals with the inherited disease cystinuria, because that example, are unable come transport details amino mountain (including cystine, the disulfide-linked dimer of cysteine) from either the urine or the intestine into the blood; the resulting build-up of cystine in the urine leads to the formation of cystine stones in the kidneys.

All membrane transport proteins that have been learned in detail have been found to be multipass transmembrane proteins-that is, your polypeptide chains traverse the lipid bilayer multiple times. By forming a constant protein pathway across the membrane, these proteins enable details hydrophilic solutes to overcome the membrane there is no coming into direct contact with the hydrophobic inner of the lipid bilayer.

Carrier proteins and channel proteins room the two major classes that membrane transport proteins. Transport proteins (also called carriers, permeases, or transporters) tie the details solute to it is in transported and undergo a collection of conformational alters to move the bound solute throughout the membrane (Figure 11-3). Channel proteins, in contrast, interact with the solute to it is in transported much more weakly. They form aqueous pores that extend throughout the lipid bilayer; as soon as these pores are open, castle allow particular solutes (usually inorganic ions of ideal size and also charge) come pass with them and also thereby overcome the membrane (see figure 11-3). Not surprisingly, transport v channel proteins wake up at a much faster rate than transfer mediated by transport proteins.


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Figure 11-3

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


Active deliver Is Mediated by transport Proteins coupled to an energy Source

All channel proteins and also many carrier proteins allow solutes to overcome the membrane only passively (“downhill”), a procedure called passive transport, or promoted diffusion. In the situation of move of a single uncharged molecule, the is merely the difference in that is concentration top top the 2 sides the the membrane—its concentration gradient—that cd driver passive transport and also determines that direction (Figure 11-4A).


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Figure 11-4

Passive and energetic transport compared. (A) Passive transport down one electrochemical gradient occurs spontaneously, either by simple diffusion v the lipid bilayer or by facilitated diffusion v channels and passive carriers. By contrast, active (more...)


If the solute dead a network charge, however, both that is concentration gradient and also the electric potential difference across the membrane, the membrane potential,influence its transport. The concentration gradient and also the electrical gradient can be linked to calculation a network driving force, the electrochemical gradient, because that each fee solute (Figure 11-4B). We discuss this in more detail in chapter 14. In fact, almost all plasma membranes have actually an electric potential difference (voltage gradient) across them, through the inside usually an adverse with respect come the outside. This potential difference favors the entrance of positively fee ions into the cell however opposes the entry of negatively fee ions.

Cells likewise require transfer proteins the will actively pump details solutes throughout the membrane against their electrochemical gradient (“uphill”); this process, recognized as energetic transport, is mediated by carriers, i beg your pardon are also called pumps. In active transport, the pumping activity of the carrier protein is directional due to the fact that it is tightly coupled to a source of metabolic energy, such together ATP hydrolysis or an ion gradient, as questioned later. Thus, transport by carriers deserve to be either active or passive, whereas carry by channel proteins is always passive.


Ionophores can Be offered as tools to boost the Permeability of membranes to specific Ions

Ionophores are tiny hydrophobic molecules the dissolve in lipid bilayers and also increase their permeability to certain inorganic ions. Many are synthesized by microbe (presumably as biological weapons versus competitors or prey). They are widely used by cell biologists as tools to boost the ion permeability of membrane in research studies on artificial bilayers, cells, or cell organelles. There space two great of ionophores—mobile ion carrier and channel formers (Figure 11-5). Both varieties operate through shielding the charge of the transported ion so the it deserve to penetrate the hydrophobic inner of the lipid bilayer. Since ionophores room not coupled to energy sources, they permit the net activity of ions only down your electrochemical gradients.


Figure 11-5

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


Valinomycin is an instance of a mobile ion carrier. The is a ring-shaped polymer the transports K+ under its electrochemical gradient by picking up K+ on one next of the membrane, diffusing across the bilayer, and releasing K+ on the other side. Similarly, FCCP, a mobile ion carrier that provides membranes selectively leaky come H+, is regularly used come dissipate the H+ electrochemical gradient across the mitochondrial inner membrane, in order to blocking mitochondrial ATP production. A23187 is yet another example of a cell phone ion carrier, only it transports divalent cations such together Ca2+ and also Mg2+. When cells room exposed to A23187, Ca2+ beginning the cytosol from the extracellular liquid down a steep electrochemical gradient. Accordingly, this ionophore is widely supplied to rise the concentration of cost-free Ca2+ in the cytosol, thereby mimicking details cell-signaling mechanisms (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 mountain each), i m sorry wind about each other to kind a twin helix. 2 gramicidin dimers are thought come come together finish to end throughout the lipid bilayer to kind what is probably the most basic of all transmembrane channels, i m sorry selectively allows monovalent cations to flow down their electrochemical gradients. Gramicidin is make by certain bacteria, possibly to kill other microorganisms by collapsing the H+, Na+, and K+ gradients the are important for their survival, and it has been helpful as one antibiotic.


Summary

Lipid bilayers are highly impermeable to many polar molecules. To transport little water-soluble molecules into or the end of cells or intracellular membrane-enclosed compartments, cabinet membranes contain various membrane transport proteins, every of i m sorry is responsible for transferring a details solute or class of solutes across the membrane. There space two class of membrane deliver proteins—carriers and channels. Both form consistent protein pathways throughout the lipid bilayer. Whereas carry by carriers deserve to be either energetic or passive, solute circulation through channel proteins is always passive. Ionophores, i m sorry are small hydrophobic molecule made by microorganisms, have the right to be supplied as devices to boost the permeability of cabinet membranes to details inorganic ions.

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