What is an ion?

An ion is defined as an atom or a group of atoms that has gained or lost one or more electrons, resulting in an electric charge. This charge can be positive (cation) when electrons are lost or negative (anion) when electrons are gained. The nature of ions is crucial in various biological processes, such as maintaining cellular homeostasis, transmitting nerve impulses, and muscle contraction. Understanding ions and their charges is fundamental in biology, particularly in processes involving electrical gradients across membranes and the functioning of nerve cells and muscle fibers.

Other options, while related to different biological concepts, do not accurately define an ion. For instance, neurotransmission may involve ions but it specifically refers to the process by which signals are transmitted across synapses. Cellular structures within neurons would pertain to components like dendrites or axons, and hemoglobin is a protein in red blood cells that carries oxygen, which does not relate directly to the definition of an ion. Thus, the definition of an ion being about the electric charge of atoms is essential and correctly described in the selected answer.