What are membrane potentials (electrochemical gradients)?

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Membrane potentials reflect stored energy created by unequal ion distributions across a membrane. When ions like sodium, potassium, and others are not evenly spread on each side, and the membrane is selectively permeable, a charge difference builds up across the membrane. That electrical potential, combined with the ion concentration differences, forms the electrochemical gradient, which is the energy that cells can tap for processes like transport and electrical signaling. This energy isn't about bonds in molecules or about energy released from metabolic steps; it's the voltage created by a separation of charge across the membrane, maintained by ion pumps and channels. For example, neurons keep a resting potential around -70 mV to enable rapid responses when channels open.