Chapter 18, Problem 68RPP

BIO Electric discharge by eels In several aquatic animals such as the South American electric eel electric organs produce 600-V potential difference pulses to ward off predators as well as to stun prey Figure 18.29 illustrates the key component that produces this electric shock—an electrolyte. The interior of an inactive electrolyte (Figure 18.29a) has an excess of negatively charged ions. The exterior has an excess of positively charged sodium ions $\left({\text{Na}}^{\text{+}}\right)$ on the left side and positively charged potassium ions $\left({\text{K}}^{\text{+}}\right)$ on the right. There is a -90-mV electric potential difference from outside the cell membrane to inside the electrolyte cell membrane, but zero potential from one exterior side to the other exterior side. How then does an eel produce the 600-V potential difference necessary to stun an intruder?

The eel's long trunk and tail contain many electrolytes placed one after the other in columns (Figures 18.29b and c). Each electrolyte contains several types of ion channels, which when activated by a nerve impulse allow sodium ions to pass through channels on the loft flat side of each electrolyte from outside the cell to the inside. This causes the electric potential across that cell membrane to change from $\text{-90 mV to +50 mV}$. The electric potential from the left external side to the right external side of an electrolyte is now about 100 mV (Figure 18.29c). With about 6000 electrolytes placed one after the other (menses), the eel is able to produce an electric impulse of over 600 V (6000 electrolytes in series times 0.10 V per electrolyte) The discharge lasts about 2-3 ms.

Suppose that one cell of the electrolyte is regarded as a small capacitor with a 0.10-V potential difference across it How should we arrange 10 cells to get a 1.0-V potential difference across them?

a. in series, as in Figure P18.64a

b. in parallel, as in Figure P18.64b

c. randomly so that they do not cancel each other d not enough information