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You are watching: Which of the following would diffuse through the lipid bilayer of a plasma membrane most rapidly?

Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. Fourth edition. Brand-new York: Garland Science; 2002.


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We begin this section by relenten the permeability nature of protein-free, man-made lipid bilayers. We then introduce some of the terms offered to explain the various develops of membrane transport and also some techniques for characterizing the proteins and also processes involved.


Protein-free Lipid Bilayers Are very Impermeable to Ions

Given sufficient time, virtually any molecule will certainly diffuse across a protein-free lipid bilayer under its concentration gradient. The price at which the does so, however, varies enormously, depending partly on the size of the molecule, however mostly on its loved one solubility in oil. In general, the smaller sized the molecule and also the an ext soluble that is in oil (the much more hydrophobic, or nonpolar, it is), the much more rapidly it will certainly diffuse throughout a lipid bilayer. Small nonpolar molecules, such as O2 and also CO2, conveniently dissolve in lipid bilayers and also therefore diffuse rapidly across them. Tiny uncharged polar molecules, such as water or urea, additionally diffuse throughout a bilayer, albeit much an ext slowly (Figure 11-1). By contrast, lipid bilayers are extremely impermeable to fee molecules (ions), no matter just how small: the charge and high degree of hydration of such molecules stays clear of them native entering the hydrocarbon step of the bilayer. Thus, fabricated bilayers room 109 times more permeable to water 보다 to also such small ions together Na+ or K+ (Figure 11-2).


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Figure 11-1

The loved one permeability of a artificial lipid bilayer to different classes of molecules. The smaller sized the molecule and, more importantly, the much less strongly the associates v water, the much more rapidly the molecule diffuses across the bilayer.


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Figure 11-2

Permeability coefficients for the i of miscellaneous molecules through synthetic lipid bilayers. The rate of flow of a solute throughout the bilayer is straight proportional to the distinction in that is concentration on the two sides the the membrane. Multiply (more...)


There space Two key Classes that Membrane deliver Proteins: Carriers and Channels

Like artificial lipid bilayers, cabinet membranes permit water and also nonpolar molecule to permeate by an easy diffusion. Cell membranes, however, also have to permit the i of miscellaneous polar molecules, such as ions, sugars, amino acids, nucleotides, and also many cabinet metabolites the cross fabricated lipid bilayers only really slowly. One-of-a-kind membrane carry proteins space responsible for delivering such solutes across cell membranes. This proteins happen in numerous forms and in all varieties of biological membranes. Every protein transports a details class the molecule (such as ions, sugars, or amino acids) and often only particular molecular varieties of the class. The specificity that membrane deliver proteins was first indicated in the mid-1950s by research studies in which single gene mutations were discovered to abolish the ability of bacteria come transport details sugars across their plasma membrane. Comparable mutations have now been uncovered in humans suffering native a selection of inherited illness that impact the move of a certain solute in the kidney, intestine, or numerous other cabinet types. Individuals with the inherited condition cystinuria, for example, space unable come transport details amino acids (including cystine, the disulfide-linked dimer the cysteine) from one of two people the urine or the intestine right into the blood; the resulting buildup of cystine in the urine leads to the development of cystine stones in the kidneys.

All membrane move proteins that have been learned in detail have been found to be multipass transmembrane proteins-that is, their polypeptide chains traverse the lipid bilayer multiple times. By creating a constant protein pathway across the membrane, this proteins enable particular hydrophilic solutes to overcome 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 significant classes that membrane transport proteins. Transport proteins (also dubbed carriers, permeases, or transporters) tie the certain solute to it is in transported and also undergo a collection of conformational changes to transport the bound solute throughout the membrane (Figure 11-3). Channel proteins, in contrast, connect with the solute to it is in transported much much more weakly. They type aqueous pores that extend across the lipid bilayer; once these pores space open, lock allow particular solutes (usually inorganic ions of appropriate size and charge) to pass through them and thereby overcome the membrane (see number 11-3). No surprisingly, transport with channel proteins wake up at a much quicker rate than transport mediated by carrier proteins.


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Figure 11-3

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


Active transport Is Mediated by carrier Proteins combination to an power Source

All channel proteins and also many transport proteins allow solutes to cross the membrane just passively (“downhill”), a process called passive transport, or promoted diffusion. In the case of carry of a single uncharged molecule, it is merely the difference in the concentration on the two sides of the membrane—its concentration gradient—that drives passive transport and determines that is direction (Figure 11-4A).


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Figure 11-4

Passive and energetic transport compared. (A) Passive deliver down an electrochemical gradient occurs spontaneously, either by simple diffusion with the lipid bilayer or by promoted diffusion through channels and also passive carriers. By contrast, energetic (more...)


If the solute tote a net charge, however, both its concentration gradient and the electrical potential difference throughout the membrane, the membrane potential,influence that transport. The concentration gradient and also the electric gradient have the right to be an unified to calculation a net driving force, the electrochemical gradient, because that each charged solute (Figure 11-4B). We discuss this in an ext detail in thing 14. In fact, practically all plasma membranes have actually an electrical potential difference (voltage gradient) throughout them, with the inside usually negative with respect to the outside. This potential distinction favors the entry of positively fee ions right into the cell but opposes the entry of negatively charged ions.

Cells additionally require move proteins that will proactively pump specific solutes throughout the membrane versus their electrochemical gradient (“uphill”); this process, recognized as energetic transport, is mediated by carriers, which are additionally called pumps. In active transport, the pumping task of the transport protein is directional because it is tightly combination to a resource of metabolic energy, such as ATP hydrolysis or one ion gradient, as debated later. Thus, transport by carriers can be either energetic or passive, whereas move by channel protein is constantly passive.


Ionophores can Be supplied as tools to increase the Permeability of membrane to certain Ions

Ionophores are small hydrophobic molecules the dissolve in lipid bilayers and also increase your permeability to particular inorganic ions. Most are synthesized by microorganisms (presumably as organic weapons against competitors or prey). They are widely supplied by cabinet biologists as tools to rise the ion permeability of membranes in researches on fabricated bilayers, cells, or cell organelles. There space two class of ionophores—mobile ion carriers and channel formers (Figure 11-5). Both types operate by shielding the charge of the transported ion so the it can penetrate the hydrophobic inner of the lipid bilayer. Since ionophores space not combination to energy sources, they allow the net movement of ions just down your electrochemical gradients.


Figure 11-5

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


Valinomycin is an instance of a cell phone ion carrier. The is a ring-shaped polymer that transports K+ under its electrochemical gradient by picking up K+ on one side of the membrane, diffusing across the bilayer, and also releasing K+ top top the various other side. Similarly, FCCP, a mobile ion transport that renders membranes selectively leaky come H+, is regularly used come dissipate the H+ electrochemical gradient throughout the mitochondrial inside membrane, in order to blocking mitochondrial ATP production. A23187 is yet another example the a mobile ion carrier, only it transports divalent cations such as Ca2+ and also Mg2+. Once cells space exposed to A23187, Ca2+ beginning the cytosol indigenous the extracellular liquid down a steep electrochemical gradient. Accordingly, this ionophore is widely used to increase the concentration of totally free Ca2+ in the cytosol, thereby mimicking certain cell-signaling instrument (discussed in thing 15).

Gramicidin A is an example of a channel-forming ionophore. That is a dimeric compound of two linear peptides (of 15 hydrophobic amino mountain each), i m sorry wind approximately each other to kind a twin helix. 2 gramicidin dimers space thought to come together end to end throughout the lipid bilayer to type what is most likely the easiest of all transmembrane channels, i m sorry selectively permits monovalent cations to circulation down their electrochemical gradients. Gramicidin is made by details bacteria, perhaps to kill other microorganisms by collapsing the H+, Na+, and K+ gradients that are crucial for their survival, and it has actually been useful as an antibiotic.


Summary

Lipid bilayers are very impermeable to most polar molecules. To transport small water-soluble molecules right into or the end of cells or intracellular membrane-enclosed compartments, cell membranes contain assorted membrane carry proteins, every of i beg your pardon is responsible for delivering a specific solute or class of solutes throughout the membrane. There room two great of membrane move proteins—carriers and channels. Both form constant protein pathways across the lipid bilayer. Whereas move by carriers can be either energetic or passive, solute circulation through channel proteins is always passive. Ionophores, which are small hydrophobic molecule made by microorganisms, can be used as tools to increase the permeability of cabinet membranes to details inorganic ions.

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