Origin of Fast Signaling System of Life

Although genes, proteins, and various chemical reactions can help the life forms reproduce and evolve their offspring, they require a faster signaling system to respond to the external environment. Fast signaling system is indispensable for survival, especially for animals that constantly seek food and avoid danger, unlike plants that do not move and live on the spot. As mentioned in the previous post, this fast signal system that animals have is called the nervous system. So how did this signal system start in the beginning? Its origin goes back a long time ago.

Prokaryotes, the most ancient organisms were very simple in structure. They do not have nuclear membranes and a variety of intracellular organelle. They have a lipid membrane surrounding the cell, which allows the vital macromolecules to be engulfed into the cells and retained. However, simply by having a diffusion barrier that small molecules can not pass through, it is impossible to access the required molecule, pass it through the cell, and release the waste generated through the metabolism of the cell. Some problems could be solved by retaining macromolecules and passing small molecules through cell membrane pores selectively responding to size, but it was necessary to be able to exchange the inside and outside of cells more selectively. The first living organisms capable of this function were primitive eukaryotic cells that appeared 1.5 billion years ago.

As the Earth becomes richer with oxygen, new eukaryotic cells have a new way of producing energy using oxygen. Using these energies, they can actively exchange materials inside and outside the cell, regardless of the electrochemical gradient . They also refined the structure of the cell membrane to make it less permeable than prokaryotic cells. Another problem was to create a means of communication between the outside world and the cell. To do this, they had to create a kind of messenger that could react to external stimuli and trigger specific biological cellular responses.

The Birth of Intracelluar Messenger

In order to become an intracellular messenger, it had to meet some requirements. First, its intracellular concentration was very low and could be precisely controlled. Thus, the brief transmembrane fluxes of messenger should be able to change its intracellular concentration and to change the cell. In addition, it had to be able to bind selectively with the substrate.

It was Ca²⁺ ions that could be easily obtained in the global environment while satisfying these conditions and capable of binding as a biological molecule. To use these as messengers, it was necessary to transport them outside the cell to reduce the intracellular concentration of Ca²⁺ ions, which was increased by certain circumstances. The cell membranes were less permeable to the ions to prevent passive influx due to the concentration gradient, so they had to create transportation methods of Ca²⁺ ions into and out of the cell. To do this, the calcium ion transport mechanism had to be able to regulate through certain substances attached to the cell membrane, using energy created from oxygen metabolism. For the first time, the cell had an ion channel that is able to regulate calcium ion concentrations.

Voltage-Gated Ion Channels

Unicellular eukaryotes such as Euglena were able to move cells through flagella composed of microtubules and cilia surrounding the entire cell. They could obtain information on external environmental changes through voltage-gated, hyperpolarizing potassium, and depolarizing calcium ion channels. The calcium and potassium channels were directly involved in the forward and backward movement of cells.

In addition, many of these protozoans could respond to chemical, tactile, temperature, and visual stimuli. Until now, however, there was no sodium (Na⁺) ion channel, indispensable ion channels of nervous system in higher evolutionary stages, found in these organisms.

Anyway, the interior of these cells has already accumulated a certain amount of membrane-impermeable macromolecules for cellular metabolism. Their migration has led to the resting membrane potential within the cell. To date, most eukaryotic cells have been known to have a resting membrane potential between -20 and -90 mV.

In order to change this resting membrane potential, the ion channels had to be actively opened and closed. Voltage-gated ion channels were responsible for this role. These are a type of transmembrane proteins, ion channels that are activated in response to changes in electrical membrane potential near the channel. Changes in the membrane potential can change the shape of the protein in the channel and open and close gates that ions can pass through. Through this process, various signals can be delivered to the cells. The voltage-gated ion-channels found so far are those for sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl-) ions.

The open conformation of the ion channel allows for the translocation of ions across the cell membrane, while the closed conformation does not from Wikipedia.org


Voltage-gated ion channel by Wikipedia.org


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