Most Ions Can Travel Directly From the Cytoplasm Across the Plasma Membrane
Due to their polarity, most ions cannot cross directly from the cytoplasm across the plasma membrane. To facilitate cell diffusion – an attempt at crossing hydrophobic plasma membrane with help from a protein mediator – most ions require help.
Facilitated cell diffusion is the selective passive movement of molecules along a concentration gradient that exists between the cytoplasm and its environment. This motion is primarily controlled by both molecular concentrations and their electrical potentials.
Furthermore, ions require the presence of a specific channel in order to pass across the lipid bilayer. Fortunately, most cells already possess these channels built-into their plasma membranes.
Ion channels in the plasma membrane are a dynamic system that opens and shuts ion channels according to chemical changes within the cell as well as signals from external molecules. These channels are activated by chemical changes within the cell and external signals to ensure correct ion flow across the membrane.
They open and close in response to several factors, such as ionic concentrations inside and outside the cell, organic molecules within and outside, and ATP concentration in the cell (see Table 12.1). When these channels open or close, an electrochemical gradient across their plasma membrane is created which can be controlled using active transport pumps using ATP.
These pumps can move Na+ and K+ ions from the cytoplasm into extracellular fluid, neutralizing their negative charge. This pumping activity accounts for much of the energy used by nerve and muscle cells to carry out their intricate tasks.
In most cases, ion channels in the plasma membrane open within a fraction of a second after receiving a signal from another cell. While this period is brief, it still represents significant energy storage. This energy helps sustain action potentials in neurons and muscles by keeping ionic concentrations constant.
The process of opening ion channels is highly dependent on the ionic composition of the cytoplasm and the osmotic pressure of the surrounding liquid. Generally, the concentration of ions within the cytoplasm is much lower than that found outside, creating an electrochemical gradient consisting both of chemical charges and charge gradients.
This dual gradient can be disrupted by a variety of agents, from simple chemicals to large-scale processes. Disruption to this pathway is one of the major causes of cell death and has been used in both natural and artificial biocides.
Ions can also be produced non-chemically, such as by high voltage or temperature sources. These ions are employed in many scientific instruments and devices like mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.