Question 6 (1 point) Similar to the firing of a neuron in the brain channelrhodopsin triggers an action potential. What specifically causes an action potential? A. An action potential is triggered when a voltage difference is established across the plasma membrane. B. An action potential is triggered when positively charged ions move into the cell. C. An action potential is triggered when light is absorbed by a pigment molecule. D. An action potential is triggered when a G-protein binds to a receptor molecule.

The correct answer is **B. An action potential is triggered when positively charged ions move into the cell.**<br /><br />Here's why:<br /><br />* **A. An action potential is triggered when a voltage difference is established across the plasma membrane.** While a voltage difference (membrane potential) exists across the membrane and is essential for neuronal function, the *change* in this voltage difference, specifically depolarization caused by ion movement, triggers the action potential. The mere presence of a voltage difference doesn't trigger it.<br /><br />* **C. An action potential is triggered when light is absorbed by a pigment molecule.** This describes the activation of channelrhodopsin itself. Channelrhodopsin acts as a light-gated ion channel. The absorption of light opens the channel, *allowing* the influx of positive ions, which *then* triggers the action potential. Light absorption is the initial step, not the direct trigger of the action potential.<br /><br />* **D. An action potential is triggered when a G-protein binds to a receptor molecule.** G-protein coupled receptors are involved in various signaling pathways, some of which can *indirectly* influence membrane potential. However, the direct trigger of an action potential is the influx of positive ions, not G-protein binding.<br /><br />**In summary:** The influx of positive ions (primarily sodium ions) depolarizes the membrane, reaching a threshold that triggers the rapid opening of voltage-gated sodium channels. This rapid influx of sodium ions is the defining event of an action potential.<br />