# 物理的英文題目~星期四前要!謝謝~~

from a wire. It's a fluid-filled tube that's surrounded by another fluid.

Both fluids contain ions of sodium,potassium,and chlorine. Each

sodium or potassium ion has one elementary unit of poaitive charge

while each chlorine ion has elementary unit of negative charge. The

wall of the nerve cell separates the two fluids and normally prevents

these ions from passing through it.

(1).When the nerve cell is in its resting state, the fluid outside it has

slightly more sodium plus potassium ions than chlorine ions. The

fluid inside it has slightly more chlorine ions than sodium plus

potassium ions. What is the net electric charge of each of these two

fluids?

(2).How do you know that the resting nerve cell has electrostatic potential

energy?

(3).To produce its electrostatic potential energy, the nerve cell pumps sodium ions out ofthe cell. Show that the nerve cell must do work on the

sodium ions during this transfer.

(4).which part of the nerve cell has a positive voltage? which part has a

negative voltage?

(5).when the nerve cell "fires",it abruply allows sodium ions to pass

through its walls. Which way do they move and what happens to the

voltage of the various parts of the cell?

Rating

它問的只是靜電學的概念問題，不要被細胞生物外表( 細胞膜 @.@" ) 嚇到啊。

(1) 首先它已經告訴你

細胞外： [K ] [Na ] > [Cl-]

細胞內： [K ] [Na ] < [Cl-]

那細胞外 / 內分別帶何種電荷不是很清楚了嗎 ? ( 正or負 )

(2) 內外帶電荷不同，電位也不同 ( 事實上細胞內外約差了-70mV )，有電位差就是有能量囉。(想像儲能的電池兩端也是有電位差)

(3) 細胞為了維持這個電位差，需不停的把[Na ]離子從細胞把出細胞外，這是要耗能的。

你回想一下，把一個正電荷從低電位移到高電位 ( 或正電荷從平行電板的負極推到正極 ) 是不是要外力推 ? 是不是要做功 ?

(4) 同問題(1)(2)意思

(5) 這裡說細胞 "fire" 的現像正式叫action potential ( 相對於上面電壓差維持時的resting potential )，當細胞膜的[Na ]離子通導突然打開時，因[Na+]本身帶正電，所以自然會想跑到電位比較低的細胞內去，降低細胞內外電壓差。

ps. 上述過程事實上還關係到各種離子的濃度，所以離子除了會想平衡電壓差外，也會希望平衡濃度差 ( 高濃度想跑到低濃度去 )，這兩種力一起作用決定了resting potential (-70mV) 及action potential的反應，詳細請看：

http://en.wikipedia.org/wiki/Action_potential