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===Facilitated diffusion and transport易化扩散和转运 ===

===Facilitated diffusion and transport易化扩散和转运 ===

[[File:Scheme facilitated diffusion in cell membrane-en.svg|thumb|300px|right|Facilitated diffusion in cell membranes, showing ion channels and [[carrier proteins]]|链接=Special:FilePath/Scheme_facilitated_diffusion_in_cell_membrane-en.svg]]

[[File:Scheme facilitated diffusion in cell membrane-en.svg|thumb|300px|right|细胞膜上的易化扩散，显示了离子通道和载体蛋白（carrier protein）|链接=Special:FilePath/Scheme_facilitated_diffusion_in_cell_membrane-en.svg]]

The resistance of a pure lipid bilayer to the passage of ions across it is very high, but structures embedded in the membrane can greatly enhance ion movement, either [[active transport|actively]] or [[passive transport|passively]], via mechanisms called [[facilitated transport]] and [[facilitated diffusion]]. The two types of structure that play the largest roles are ion channels and [[ion transporter|ion pump]]s, both usually formed from assemblages of protein molecules. Ion channels provide passageways through which ions can move. In most cases, an ion channel is permeable only to specific types of ions (for example, sodium and potassium but not chloride or calcium), and sometimes the permeability varies depending on the direction of ion movement. Ion pumps, also known as ion transporters or carrier proteins, actively transport specific types of ions from one side of the membrane to the other, sometimes using energy derived from metabolic processes to do so.

The resistance of a pure lipid bilayer to the passage of ions across it is very high, but structures embedded in the membrane can greatly enhance ion movement, either [[active transport|actively]] or [[passive transport|passively]], via mechanisms called [[facilitated transport]] and [[facilitated diffusion]]. The two types of structure that play the largest roles are ion channels and [[ion transporter|ion pump]]s, both usually formed from assemblages of protein molecules. Ion channels provide passageways through which ions can move. In most cases, an ion channel is permeable only to specific types of ions (for example, sodium and potassium but not chloride or calcium), and sometimes the permeability varies depending on the direction of ion movement. Ion pumps, also known as ion transporters or carrier proteins, actively transport specific types of ions from one side of the membrane to the other, sometimes using energy derived from metabolic processes to do so.

===Ion pumps===

===Ion pumps===

[[File:Scheme sodium-potassium pump-en.svg|thumb|right|350px|The sodium-potassium pump uses energy derived from ATP to exchange sodium for potassium ions across the membrane.钠-钾泵利用 ATP 产生的能量，通过膜将钠离子与钾离子交换。|链接=Special:FilePath/Scheme_sodium-potassium_pump-en.svg]][[Ion transporter|Ion pumps]] are [[integral membrane protein]]s that carry out [[active transport]], i.e., use cellular energy (ATP) to "pump" the ions against their concentration gradient.<ref name="hodgkin_1955">{{cite journal | author = [[Alan Lloyd Hodgkin|Hodgkin AL]], [[Richard Keynes|Keynes RD]] | year = 1955 | title = Active transport of cations in giant axons from ''Sepia'' and ''Loligo'' | journal = J. Physiol. | volume = 128 | pages = 28–60 | pmid = 14368574 | issue = 1 | pmc = 1365754 | doi=10.1113/jphysiol.1955.sp005290}}</ref> Such ion pumps take in ions from one side of the membrane (decreasing its concentration there) and release them on the other side (increasing its concentration there).

[[File:Scheme sodium-potassium pump-en.svg|thumb|right|350px|钠-钾泵利用 ATP 产生的能量，将钠离子与钾离子跨膜交换。|链接=Special:FilePath/Scheme_sodium-potassium_pump-en.svg]][[Ion transporter|Ion pumps]] are [[integral membrane protein]]s that carry out [[active transport]], i.e., use cellular energy (ATP) to "pump" the ions against their concentration gradient.<ref name="hodgkin_1955">{{cite journal | author = [[Alan Lloyd Hodgkin|Hodgkin AL]], [[Richard Keynes|Keynes RD]] | year = 1955 | title = Active transport of cations in giant axons from ''Sepia'' and ''Loligo'' | journal = J. Physiol. | volume = 128 | pages = 28–60 | pmid = 14368574 | issue = 1 | pmc = 1365754 | doi=10.1113/jphysiol.1955.sp005290}}</ref> Such ion pumps take in ions from one side of the membrane (decreasing its concentration there) and release them on the other side (increasing its concentration there).

离子泵是内在膜蛋白，进行主动运输，也就是说，使用细胞能量（ATP）“泵”离子逆浓度梯度.<ref name="hodgkin_1955" />。这种离子泵从膜的一边吸收离子（顺浓度梯度），然后从另一侧释放离子（增加那边的浓度）。

离子泵是内在膜蛋白，进行主动运输，也就是说，使用细胞能量（ATP）“泵”离子逆浓度梯度.<ref name="hodgkin_1955" />。这种离子泵从膜的一边吸收离子（顺浓度梯度），然后从另一侧释放离子（增加那边的浓度）。

泄漏通道是最简单的离子通道类型，因为它们的渗透率几乎是恒定的。在神经元中，钾离子通道和氯离子通道是泄漏通道中最重要的类型。即使它们的性质也不是完全恒定的: 首先，它们中的大多数是电压依赖性的，因为它们在一个方向上比在另一个方向上导电更好(换句话说，它们是整流器) ; 其次，它们中的一些能够被化学配体关闭，即使它们不需要配体来操作。

泄漏通道是最简单的离子通道类型，因为它们的渗透率几乎是恒定的。在神经元中，钾离子通道和氯离子通道是泄漏通道中最重要的类型。即使它们的性质也不是完全恒定的: 首先，它们中的大多数是电压依赖性的，因为它们在一个方向上比在另一个方向上导电更好(换句话说，它们是整流器) ; 其次，它们中的一些能够被化学配体关闭，即使它们不需要配体来操作。

====Ligand-gated channels====

====Ligand-gated channels 配体门控离子通道====

[[File:LGIC.png|thumb|right|300px|Ligand-gated calcium channel in closed and open states配体门控钙离子通道闭合态和开放态|链接=Special:FilePath/LGIC.png]]

[[File:LGIC.png|thumb|right|300px|闭合态和开放态的配体门控钙离子通道|链接=Special:FilePath/LGIC.png]]

[[Ligand-gated ion channel]]s are channels whose permeability is greatly increased when some type of chemical ligand binds to the protein structure. Animal cells contain hundreds, if not thousands, of types of these. A large subset function as [[neurotransmitter receptor]]s—they occur at [[postsynaptic]] sites, and the chemical ligand that gates them is released by the presynaptic [[axon terminal]]. One example of this type is the [[AMPA receptor]], a receptor for the neurotransmitter [[glutamic acid|glutamate]] that when activated allows passage of sodium and potassium ions.  Another example is the [[GABAA receptor|GABA_A receptor]], a receptor for the neurotransmitter [[GABA]] that when activated allows passage of chloride ions.

[[Ligand-gated ion channel]]s are channels whose permeability is greatly increased when some type of chemical ligand binds to the protein structure. Animal cells contain hundreds, if not thousands, of types of these. A large subset function as [[neurotransmitter receptor]]s—they occur at [[postsynaptic]] sites, and the chemical ligand that gates them is released by the presynaptic [[axon terminal]]. One example of this type is the [[AMPA receptor]], a receptor for the neurotransmitter [[glutamic acid|glutamate]] that when activated allows passage of sodium and potassium ions.  Another example is the [[GABAA receptor|GABA_A receptor]], a receptor for the neurotransmitter [[GABA]] that when activated allows passage of chloride ions.

配体门控离子通道是当某种类型的化学配体与蛋白质结构结合时，其通透性大大增加的通道。动物细胞包含成百上千种这样的细胞。神经递质受体的一个很大的子集功能ーー它们发生在突触后位点，而与它们相关的化学配体是由突触前轴突末端释放的。这种类型的一个例子是 AMPA 受体，一种神经递质谷氨酸的受体，当激活时允许钠离子和钾离子通过。另一个例子是 GABA < sub > a  受体，一种神经递质 GABA 的受体，当被激活时允许氯离子通过。

配体门控离子通道是当某种类型的化学配体与蛋白结构结合时，其通透性大大增加的通道。动物细胞含有成百上千种这样的通道。很大一部分是作为神经递质受体的一个很大的子集功能ーー它们发生在突触后位点，而与它们相关的化学配体是由突触前轴突末端释放的。这种类型的一个例子是 AMPA 受体，一种神经递质谷氨酸的受体，当激活时允许钠离子和钾离子通过。另一个例子是 GABA < sub > a  受体，一种神经递质 GABA 的受体，当被激活时允许氯离子通过。

Neurotransmitter receptors are activated by ligands that appear in the extracellular area, but there are other types of ligand-gated channels that are controlled by interactions on the intracellular side.

Neurotransmitter receptors are activated by ligands that appear in the extracellular area, but there are other types of ligand-gated channels that are controlled by interactions on the intracellular side.

对于固定的离子浓度和固定的离子通道电导值，等效电路可以进一步缩小，使用下面描述的戈德曼方程电路，变成一个包含电容和电池电导并联的电路。在电学术语中，这是一种 RC 电路(阻容电路) ，其电学特性非常简单。从任何初始状态开始，流过电导或电容的电流以 EXPTIME 衰变，其时间常数为，这里是膜片的电容，这里是网电阻。在现实情况下，时间常数一般在1ー100毫秒的范围内。在大多数情况下，离子通道电导的变化发生在一个更快的时间尺度上，所以 RC 电路不是一个好的近似值; 然而，用于模拟膜片的微分方程通常是 RC 电路方程的修正版。

对于固定的离子浓度和固定的离子通道电导值，等效电路可以进一步缩小，使用下面描述的戈德曼方程电路，变成一个包含电容和电池电导并联的电路。在电学术语中，这是一种 RC 电路(阻容电路) ，其电学特性非常简单。从任何初始状态开始，流过电导或电容的电流以 EXPTIME 衰变，其时间常数为，这里是膜片的电容，这里是网电阻。在现实情况下，时间常数一般在1ー100毫秒的范围内。在大多数情况下，离子通道电导的变化发生在一个更快的时间尺度上，所以 RC 电路不是一个好的近似值; 然而，用于模拟膜片的微分方程通常是 RC 电路方程的修正版。

==Resting potential==

==Resting potential静息电位==

When the membrane potential of a cell goes for a long period of time without changing significantly, it is referred to as a [[resting potential]] or resting voltage.  This term is used for the membrane potential of non-excitable cells, but also for the membrane potential of excitable cells in the absence of excitation.  In excitable cells, the other possible states are graded membrane potentials (of variable amplitude), and action potentials, which are large, all-or-nothing rises in membrane potential that usually follow a fixed time course.  Excitable cells include [[neuron]]s, muscle cells, and some secretory cells in [[gland]]s.  Even in other types of cells, however, the membrane voltage can undergo changes in response to environmental or intracellular stimuli. For example, depolarization of the plasma membrane appears to be an important step in [[apoptosis|programmed cell death]].<ref name=":17">{{cite journal |vauthors=Franco R, Bortner CD, Cidlowski JA |title=Potential roles of electrogenic ion transport and plasma membrane depolarization in apoptosis |journal=J. Membr. Biol. |volume=209 |issue=1 |pages=43–58 |date=January 2006 |pmid=16685600 |doi=10.1007/s00232-005-0837-5|s2cid=849895 |url=https://zenodo.org/record/1232645 }}</ref>

When the membrane potential of a cell goes for a long period of time without changing significantly, it is referred to as a [[resting potential]] or resting voltage.  This term is used for the membrane potential of non-excitable cells, but also for the membrane potential of excitable cells in the absence of excitation.  In excitable cells, the other possible states are graded membrane potentials (of variable amplitude), and action potentials, which are large, all-or-nothing rises in membrane potential that usually follow a fixed time course.  Excitable cells include [[neuron]]s, muscle cells, and some secretory cells in [[gland]]s.  Even in other types of cells, however, the membrane voltage can undergo changes in response to environmental or intracellular stimuli. For example, depolarization of the plasma membrane appears to be an important step in [[apoptosis|programmed cell death]].<ref name=":17">{{cite journal |vauthors=Franco R, Bortner CD, Cidlowski JA |title=Potential roles of electrogenic ion transport and plasma membrane depolarization in apoptosis |journal=J. Membr. Biol. |volume=209 |issue=1 |pages=43–58 |date=January 2006 |pmid=16685600 |doi=10.1007/s00232-005-0837-5|s2cid=849895 |url=https://zenodo.org/record/1232645 }}</ref>

= = 静息电位 = = 当一个细胞的膜电位长时间不发生明显变化时，它被称为静息电位电压或静息电压。这个术语用于描述不可兴奋细胞的膜电位，也用于描述缺乏兴奋时可兴奋细胞的膜电位。在可兴奋细胞中，其他可能的状态是分级膜电位(可变振幅)和动作电位，这些电位通常在一个固定的时间过程中，在膜电位一分钟内上升很大，要么全有要么全无。可兴奋细胞包括神经元、肌细胞和腺体中的一些分泌细胞。然而，即使在其他类型的细胞中，膜电位也会因为环境或细胞内的刺激而发生变化。例如，去极化的质膜似乎是一个重要的步骤，在细胞程序性死亡.<ref name=":17" />。

= = = = 当一个细胞的膜电位长时间不发生明显变化时，它被称为静息电位电压或静息电压。这个术语用于描述不可兴奋细胞的膜电位，也用于描述缺乏兴奋时可兴奋细胞的膜电位。在可兴奋细胞中，其他可能的状态是分级膜电位(可变振幅)和动作电位，这些电位通常在一个固定的时间过程中，在膜电位一分钟内上升很大，要么全有要么全无。可兴奋细胞包括神经元、肌细胞和腺体中的一些分泌细胞。然而，即使在其他类型的细胞中，膜电位也会因为环境或细胞内的刺激而发生变化。例如，去极化的质膜似乎是一个重要的步骤，在细胞程序性死亡.<ref name=":17" />。

The interactions that generate the resting potential are modeled by the [[Goldman equation]].<ref name="Goldman">Purves ''et al.'', pp. 32&ndash;33; [[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, pp. 138&ndash;140; Schmidt-Nielsen, pp. 480; Junge, pp. 35&ndash;37</ref>  This is similar in form to the Nernst equation shown above, in that it is based on the charges of the ions in question, as well as the difference between their inside and outside concentrations. However, it also takes into consideration the relative permeability of the plasma membrane to each ion in question.

The interactions that generate the resting potential are modeled by the [[Goldman equation]].<ref name="Goldman">Purves ''et al.'', pp. 32&ndash;33; [[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, pp. 138&ndash;140; Schmidt-Nielsen, pp. 480; Junge, pp. 35&ndash;37</ref>  This is similar in form to the Nernst equation shown above, in that it is based on the charges of the ions in question, as well as the difference between their inside and outside concentrations. However, it also takes into consideration the relative permeability of the plasma membrane to each ion in question.

<nowiki>E_{m} = \frac{RT}{F} \ln{ \left( \frac{ P_{\mathrm{K}}[\mathrm{K}^{+}]_\mathrm{out} + P_{\mathrm{Na}}[\mathrm{Na}^{+}]_\mathrm{out} + P_{\mathrm{Cl}}[\mathrm{Cl}^{-}]_\mathrm{in}}{ P_{\mathrm{K}}[\mathrm{K}^{+}]_\mathrm{in} + P_{\mathrm{Na}}[\mathrm{Na}^{+}]_\mathrm{in} + P_{\mathrm{Cl}}[\mathrm{Cl}^{-}]_\mathrm{out}} \right) }</nowiki>

<nowiki>E_{m} = \frac{RT}{F} \ln{ \left( \frac{ P_{\mathrm{K}}[\mathrm{K}^{+}]_\mathrm{out} + P_{\mathrm{Na}}[\mathrm{Na}^{+}]_\mathrm{out} + P_{\mathrm{Cl}}[\mathrm{Cl}^{-}]_\mathrm{in}}{ P_{\mathrm{K}}[\mathrm{K}^{+}]_\mathrm{in} + P_{\mathrm{Na}}[\mathrm{Na}^{+}]_\mathrm{in} + P_{\mathrm{Cl}}[\mathrm{Cl}^{-}]_\mathrm{out}} \right) }</nowiki>

The three ions that appear in this equation are potassium (K⁺), sodium (Na⁺), and chloride (Cl^&minus;). Calcium is omitted, but can be added to deal with situations in which it plays a significant role.<ref name="goldman_calcium">{{cite journal | author = Spangler SG | year = 1972 | title = Expansion of the constant field equation to include both divalent and monovalent ions | journal = Alabama Journal of Medical Sciences | volume = 9 | pages = 218–23|pmid=5045041 | issue = 2 }}</ref>  Being an anion, the chloride terms are treated differently from the cation terms; the intracellular concentration is in the numerator, and the extracellular concentration in the denominator, which is reversed from the cation terms. ''P''_i stands for the relative permeability of the ion type i.

The three ions that appear in this equation are potassium (K⁺), sodium (Na⁺), and chloride (Cl^&minus;). Calcium is omitted, but can be added to deal with situations in which it plays a significant role.<ref name="goldman_calcium">{{cite journal | author = Spangler SG | year = 1972 | title = Expansion of the constant field equation to include both divalent and monovalent ions | journal = Alabama Journal of Medical Sciences | volume = 9 | pages = 218–23|pmid=5045041 | issue = 2 }}</ref>  Being an anion, the chloride terms are treated differently from the cation terms; the intracellular concentration is in the numerator, and the extracellular concentration in the denominator, which is reversed from the cation terms. ''P''_i stands for the relative permeability of the ion type i.

在这个方程式中出现的三个离子是钾(k +)、钠(Na +)和氯(Cl -)。钙是省略的，但可以添加到处理的情况下，它发挥了重要的作用le.<ref name="goldman_calcium" />  。作为一个阴离子，氯离子项处理不同于阳离子项; 细胞内浓度是在分子，胞外浓度在分母，这是反向阳离子项。Pi 代表离子类型 i 的相对渗透率。

在这个方程式中出现的三个离子是钾（K⁺）（Na⁺）和（Cl^&minus;）。钙是省略的，但可以添加到处理的情况下，它发挥了重要的作用le.<ref name="goldman_calcium" />  。作为一个阴离子，氯离子项处理不同于阳离子项; 细胞内浓度是在分子，胞外浓度在分母，这是反向阳离子项。Pi 代表离子类型 i 的相对渗透率。

In essence, the Goldman formula expresses the membrane potential as a weighted average of the reversal potentials for the individual ion types, weighted by permeability. (Although the membrane potential changes about 100 mV during an action potential, the concentrations of ions inside and outside the cell do not change significantly. They remain close to their respective concentrations when then membrane is at resting potential.) In most animal cells, the permeability to potassium is much higher in the resting state than the permeability to sodium.  As a consequence, the resting potential is usually close to the potassium reversal potential.<ref name="resting_potential">Purves ''et al.'', p. 34; [[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, p. 134; [[Knut Schmidt-Nielsen|Schmidt-Nielsen]], pp. 478&ndash;480.</ref><ref name=":18">Purves ''et al.'', pp. 33&ndash;36; [[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, p. 131.</ref>  The permeability to chloride can be high enough to be significant, but, unlike the other ions, chloride is not actively pumped, and therefore equilibrates at a reversal potential very close to the resting potential determined by the other ions.

In essence, the Goldman formula expresses the membrane potential as a weighted average of the reversal potentials for the individual ion types, weighted by permeability. (Although the membrane potential changes about 100 mV during an action potential, the concentrations of ions inside and outside the cell do not change significantly. They remain close to their respective concentrations when then membrane is at resting potential.) In most animal cells, the permeability to potassium is much higher in the resting state than the permeability to sodium.  As a consequence, the resting potential is usually close to the potassium reversal potential.<ref name="resting_potential">Purves ''et al.'', p. 34; [[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, p. 134; [[Knut Schmidt-Nielsen|Schmidt-Nielsen]], pp. 478&ndash;480.</ref><ref name=":18">Purves ''et al.'', pp. 33&ndash;36; [[Theodore Holmes Bullock|Bullock]], Orkand, and Grinnell, p. 131.</ref>  The permeability to chloride can be high enough to be significant, but, unlike the other ions, chloride is not actively pumped, and therefore equilibrates at a reversal potential very close to the resting potential determined by the other ions.

Maintenance of the resting potential can be metabolically costly for a cell because of its requirement for active pumping of ions to counteract losses due to leakage channels. The cost is highest when the cell function requires an especially depolarized value of membrane voltage. For example, the resting potential in daylight-adapted [[Calliphoridae|blowfly]] (''Calliphora vicina'') [[Simple eyes in invertebrates|photoreceptor]]s can be as high as -30 mV.<ref name="Juusola M et al., 1994">{{cite journal | doi = 10.1085/jgp.104.3.593 |vauthors=Juusola M, Kouvalainen E, Järvilehto M, Weckström M | date =  Sep 1994| title = Contrast gain, signal-to-noise ratio, and linearity in light-adapted blowfly photoreceptors| journal = J Gen Physiol| volume = 104| issue = 3| pages = 593–621|pmid = 7807062 | pmc = 2229225}}</ref> This elevated membrane potential allows the cells to respond very rapidly to visual inputs; the cost is that maintenance of the resting potential may consume more than 20% of overall cellular [[Adenosine triphosphate|ATP]].<ref name="Laughlin SB et al., 2008">{{cite journal |vauthors=Laughlin SB, de Ruyter van Steveninck RR, Anderson JC | date = May 1998| title = The metabolic cost of neural information| journal =  Nat. Neurosci.| volume = 1| issue = 1| pages = 36–41|pmid = 10195106 | doi = 10.1038/236| s2cid = 204995437}}</ref>

Maintenance of the resting potential can be metabolically costly for a cell because of its requirement for active pumping of ions to counteract losses due to leakage channels. The cost is highest when the cell function requires an especially depolarized value of membrane voltage. For example, the resting potential in daylight-adapted [[Calliphoridae|blowfly]] (''Calliphora vicina'') [[Simple eyes in invertebrates|photoreceptor]]s can be as high as -30 mV.<ref name="Juusola M et al., 1994">{{cite journal | doi = 10.1085/jgp.104.3.593 |vauthors=Juusola M, Kouvalainen E, Järvilehto M, Weckström M | date =  Sep 1994| title = Contrast gain, signal-to-noise ratio, and linearity in light-adapted blowfly photoreceptors| journal = J Gen Physiol| volume = 104| issue = 3| pages = 593–621|pmid = 7807062 | pmc = 2229225}}</ref> This elevated membrane potential allows the cells to respond very rapidly to visual inputs; the cost is that maintenance of the resting potential may consume more than 20% of overall cellular [[Adenosine triphosphate|ATP]].<ref name="Laughlin SB et al., 2008">{{cite journal |vauthors=Laughlin SB, de Ruyter van Steveninck RR, Anderson JC | date = May 1998| title = The metabolic cost of neural information| journal =  Nat. Neurosci.| volume = 1| issue = 1| pages = 36–41|pmid = 10195106 | doi = 10.1038/236| s2cid = 204995437}}</ref>

对于电池来说，维护静息电位的代谢成本可能很高，因为它需要主动泵入离子来抵消泄漏通道造成的损失。当细胞功能需要特别去极化值膜电位时，成本最高。例如，静息电位在日光适应的绿头丽蝇(红头丽蝇)光感受器可高达 -30mv.<ref name="Juusola M et al., 1994" /> 。这种升高的膜电位可以使细胞对视觉输入作出非常迅速的反应; 维持静息电位可能消耗超过20% 的细胞总 ATP.<ref name="Laughlin SB et al., 2008" />。

对于细胞来说，维护静息电位的代谢成本可能很高，因为它需要主动泵入离子来抵消泄漏通道造成的损失。当细胞功能需要特别去极化值膜电位时，成本最高。例如，静息电位在日光适应的绿头丽蝇(红头丽蝇)光感受器可高达 -30mv.<ref name="Juusola M et al., 1994" /> 。这种升高的膜电位可以使细胞对视觉输入作出非常迅速的反应；代价就是维持静息电位可能消耗超过 20% 的细胞总 ATP。<ref name="Laughlin SB et al., 2008" />

On the other hand, the high resting potential in undifferentiated cells does not necessarily incur a high metabolic cost. This apparent paradox is resolved by examination of the origin of that resting potential. Little-differentiated cells are characterized by extremely high input resistance,<ref name="Magnuson DS et al., 1995" /> which implies that few leakage channels are present at this stage of cell life. As an apparent result, potassium permeability becomes similar to that for sodium ions, which places resting potential in-between the reversal potentials for sodium and potassium as discussed above. The reduced leakage currents also mean there is little need for active pumping in order to compensate, therefore low metabolic cost.

On the other hand, the high resting potential in undifferentiated cells does not necessarily incur a high metabolic cost. This apparent paradox is resolved by examination of the origin of that resting potential. Little-differentiated cells are characterized by extremely high input resistance,<ref name="Magnuson DS et al., 1995" /> which implies that few leakage channels are present at this stage of cell life. As an apparent result, potassium permeability becomes similar to that for sodium ions, which places resting potential in-between the reversal potentials for sodium and potassium as discussed above. The reduced leakage currents also mean there is little need for active pumping in order to compensate, therefore low metabolic cost.

另一方面，未分化细胞中的高静息电位并不一定导致高代谢成本。这个明显的悖论通过研究静息电位的起源得到了解决。小分化细胞具有极高的拥有属性输入电阻,<ref name="Magnuson DS et al., 1995" /> ，这意味着在细胞生命的这个阶段很少有泄漏通道存在。作为一个明显的结果，钾离子的渗透性变得类似于钠离子的渗透性，正如上面讨论的，钠离子和钾离子的反转电位之间有静息电位。泄漏电流的减少也意味着不需要主动抽水来补偿，因此代谢成本低。

另一方面，未分化的细胞的高静息电位并不一定导致高代谢成本。这个明显的悖论通过研究静息电位的起源得到了解决。较少分化的细胞具有极高的的输入电阻,<ref name="Magnuson DS et al., 1995" /> ，这意味着在细胞生命的这个阶段很少存在泄漏通道。明显的结果就是，钾离子的渗透性变得类似于钠离子的渗透性，正如上面讨论的，使静息电位位于钠离子和钾离子的反转电位之间。泄漏电流的减少也意味着不需要活跃的钠钾泵活动来补偿，因此代谢成本低。

==Graded potentials==

==Graded potentials级量电位==

As explained above, the potential at any point in a cell's membrane is determined by the ion concentration differences between the intracellular and extracellular areas, and by the permeability of the membrane to each type of ion. The ion concentrations do not normally change very quickly (with the exception of Ca²⁺, where the baseline intracellular concentration is so low that even a small influx may increase it by orders of magnitude), but the permeabilities of the ions can change in a fraction of a millisecond, as a result of activation of ligand-gated ion channels. The change in membrane potential can be either large or small, depending on how many ion channels are activated and what type they are, and can be either long or short, depending on the lengths of time that the channels remain open. Changes of this type are referred to as '''graded potentials''', in contrast to action potentials, which have a fixed amplitude and time course.

As explained above, the potential at any point in a cell's membrane is determined by the ion concentration differences between the intracellular and extracellular areas, and by the permeability of the membrane to each type of ion. The ion concentrations do not normally change very quickly (with the exception of Ca²⁺, where the baseline intracellular concentration is so low that even a small influx may increase it by orders of magnitude), but the permeabilities of the ions can change in a fraction of a millisecond, as a result of activation of ligand-gated ion channels. The change in membrane potential can be either large or small, depending on how many ion channels are activated and what type they are, and can be either long or short, depending on the lengths of time that the channels remain open. Changes of this type are referred to as '''graded potentials''', in contrast to action potentials, which have a fixed amplitude and time course.

= = 分级电位 = = 如上所述，细胞膜上任何一点的电位取决于细胞内和细胞外区域离子浓度的差异，以及细胞膜对每种离子的通透性。离子浓度通常不会很快改变(除了 Ca2 + ，其细胞内基线浓度非常低，即使是很小的内流量也可能增加1毫秒数量级) ，但是由于配体门控离子通道的激活，离子的通透性可以在几分之一毫秒内改变。膜电位的变化可大可小，取决于激活的离子通道的数量和类型，也可长可短，取决于通道保持开放的时间长短。这种类型的变化被称为分级电位，与动作电位相反，动作电位有固定的振幅和时间进程。

如上所述，细胞膜上任何一点的电位取决于细胞内和细胞外区域离子浓度的差异，以及细胞膜对每种离子的通透性。离子浓度通常不会很快改变(除了 Ca2 + ，其细胞内基线浓度非常低，即使是很小的内流量也可能增加1毫秒数量级) ，但是由于配体门控离子通道的激活，离子的通透性可以在几分之一毫秒内改变。膜电位的变化可大可小，取决于激活的离子通道的数量和类型，也可长可短，取决于通道保持开放的时间长短。这种类型的变化被称为分级电位，与动作电位相反，动作电位有固定的振幅和时间进程。

As can be derived from the [[Goldman equation]] shown above, the effect of increasing the permeability of a membrane to a particular type of ion shifts the membrane potential toward the reversal potential for that ion. Thus, opening Na⁺ channels shifts the membrane potential toward the Na⁺ reversal potential, which is usually around +100 mV. Likewise, opening K⁺ channels shifts the membrane potential toward about –90 mV, and opening Cl⁻ channels shifts it toward about –70 mV (resting potential of most membranes). Thus, Na⁺ channels shift the membrane potential in a positive direction, K⁺ channels shift it in a negative direction (except when the membrane is hyperpolarized to a value more negative than the K⁺ reversal potential), and Cl⁻ channels tend to shift it towards the resting potential.

As can be derived from the [[Goldman equation]] shown above, the effect of increasing the permeability of a membrane to a particular type of ion shifts the membrane potential toward the reversal potential for that ion. Thus, opening Na⁺ channels shifts the membrane potential toward the Na⁺ reversal potential, which is usually around +100 mV. Likewise, opening K⁺ channels shifts the membrane potential toward about –90 mV, and opening Cl⁻ channels shifts it toward about –70 mV (resting potential of most membranes). Thus, Na⁺ channels shift the membrane potential in a positive direction, K⁺ channels shift it in a negative direction (except when the membrane is hyperpolarized to a value more negative than the K⁺ reversal potential), and Cl⁻ channels tend to shift it towards the resting potential.