## 19.7 Bioelectricity Bioelectricity is the study of the generation and action of electric currents or voltages in biological processes. This electrical activity is a cellular phenomenon that relies on the properties of the cell membrane. ### The Cell Membrane as a Capacitor The cell membrane separates the intracellular and extracellular environments. It acts like a capacitor by storing energy in the form of electrically charged ions on its opposite sides. ### Semipermeable Nature The cell membrane is semipermeable, meaning it allows certain ions to pass through while blocking others. This selective permeability is crucial for establishing electrical potential. ### Ion Channels * These are proteins embedded in the cell membrane that form hydrophilic tunnels. * They allow specific inorganic ions, such as $\mathrm{Na}^{+}$, $\mathrm{K}^{+}$, $\mathrm{Ca}^{2+}$, or $\mathrm{Cl}^{-}$, to diffuse rapidly across the membrane down their electrochemical gradients. * Ion channels are highly selective, typically allowing only one type of ion (or a few closely related ones) to pass through. * See Figure 19.9 for a visual representation. ### Generation of Electric Potential An electric potential difference is created when solutions with different ion concentrations are separated by a semipermeable membrane. In nerve and muscle cells, the membrane is slightly permeable to positive potassium ions ($\mathrm{K}^{+}$). These ions diffuse outward, leaving a net negative charge inside the cell. The Sodium-Potassium pump actively maintains these concentration gradients. ### Resting and Action Potentials * **Resting Potential:** The stable bioelectric potential across a non-stimulated cell membrane. In many neurons, this is typically about $-70\mathrm{mV}$ (though values can vary, such as $50\mathrm{mV}$ in specific contexts). During this state, the outer surface is more positive relative to the inside. * **Action Potential:** When a cell is stimulated, ion channels open, allowing positive charges (like $\mathrm{Na}^{+}$) to enter the cell. This influx of positive charge makes the inside of the cell more positive (reaching approximately $+40\mathrm{mV}$), triggering an electrical pulse known as an action potential. These pulses are fundamental for nerve signaling, thoughts, and muscle contraction. ### Electrogenic and Electroreceptive Animals * **Electrogenic Animals:** Species that can naturally generate external electric shocks. Examples include electric eels (*Electrophorus electricus*), some bacteria, yeast, and fish. Electric eels can deliver shocks up to 860 volts. * **Electroreceptive Animals:** Species that can detect electricity. Examples include platypuses, bees, sharks, rays, and dolphins. * Most electrogenic animals are also electroreceptive, but many electroreceptive animals are not electrogenic. ### Medical Applications In clinical medicine, the electrical activity of the heart and brain is monitored using specialized equipment to diagnose health conditions. * **ECG (Electrocardiogram):** Monitors the electrical impulses of the heart. * **EEG (Electroencephalogram):** Monitors the electrical activity of the brain.