Energy storage in biological systems

The mechanism most frequently used by cells to respond to stimuli exploits previously accumulated potential energy. The potential difference across the membrane is characterized by the possibility of rapid transfer into work with timescales in the order of hundreds of microseconds. The system has a great capacity to accumulate electrical charges across the

plasma membrane, in a range from 10 to 100-120 mV, which corresponds to a massive electric field of about 10 x 10⁶ V/m. The aim is to maintain the widest possible charge separation using a mechanism that is a highly efficient combination of selectively permeable barriers and ATP- consuming ion transporters. In this case, the energy stored is in the form of an electric potential difference, the use of which occurs when any given stimulus succeeds in removing the constraints responsible for maintaining the dynamic equilibrium.

Their removal causes an immediate lowering of the membrane's electrical resistance, creating a short circuit that leads to the dissipation of a certain amount of potential energy. The physical phenomenon of the passage of electric current is practically immediate, since it is a movement of charges comparable to that in metallic conductors, and therefore at a speed close to that of light (3 x 10⁵ km/s).

It should be noted that the electric current, induced by a difference in electrical potential and produced by the flow of ions through the membrane, takes advantage of the passage of charges from one environment to another without the ion necessarily having to physically cross the membrane (Data Sheet 2.1); an approximate estimate of the time it takes an electric charge to cross the plasma membrane is about 0.3 x 10^-15 seconds.

A mechanical model that can describe how and how fast a charge passes through a plasma membrane is the Newton Collision Ball Device. The energy, in the form of momentum, of the incident ball (1 of Figure 3.1) is transferred instantaneously and entirely, except for energy losses in the form of heat, from one ball to another (2 of Figure 3.1) until the last ball (3 of Figure 3.1), which moves upwards against the force of gravity.

Figure 3.1. In the Newton Collision Ball Device, the momentum of the incident sphere (1), which falls by gravity from position a to position b and hits the central spheres (2), is transferred to the last sphere of the series (3) which passes from position c to position d, without the central spheres moving

In ion channels, the charge of a single ion is transferred according to a similar energetic process: each charge, which randomly bounces against the plasma membrane due to thermal agitation, is a candidate to go through the ion channel. If the ion channel opens and the thermodynamic conditions favor its passage, the ion induces the release of an identical charge on the opposite membrane side.

The purpose of this phenomenon is to generate a local and momentary variation in the electrical potential across the membrane, which is used as a primary signal for the cell itself and for different cells.

A second cellular mechanism is used when the accumulation of energy occurs through the sequestration of particular ionic species. The storage of calcium ions in intracellular reservoirs is the most common example. By an endoplasmic reticulum membrane transport mechanism that uses ATP, calcium ions are actively stored in intracellular stores (section 3.3).

Potential energy is stored inside the stores as a chemical gradient with respect to the outside, where the concentration of calcium ions can reach a concentration of the order of 10^-3 M/L, several orders of magnitude higher than in the cytoplasm, which is of the order of 10^-9 M/L.

The constraint that keeps the system in dynamic equilibrium is again represented by the membrane's resistance to the passage of ions. Once the membrane, following a stimulus, becomes permeable to calcium ions, they are free to diffuse into the intracellular environment. In this case, calcium ions physically cross the membrane through gaps (Figure 3.2).

Figure 3.2. The permeation of a plasma membrane by a molecule with or without an electric charge. A) A single particle bounces randomly in the channel’s external aperture (black dot). In active transport and/or in electrochemically favorable conditions, the physical passage of one particle to the other side of the membrane (B, black dot) takes a time period that is a few orders of magnitude higher than that for charge permeability

This necessarily entails longer timescales than the passage of electrical charges. In fact, it can be roughly calculated that a generic particle, whether neutral or charged, takes about 50 x 10^-9 seconds (50 ns) to cross the phospholipid bilayer, whereas an electric charge generating a current through the membrane takes about 10^-6 ns.

Calcium ions are essential as intracellular messengers to initiate many cytoplasmic functions. Given the high reactivity of calcium ions, due to the presence of two highly reactive free positive charges, the release time and cytoplasmic concentration must be finely regulated to prevent the cell from undergoing serious degenerative phenomena.

This type of potential energy, which affects a very large and varied group of intracellular molecules, has, precisely because of its heterogeneous characteristics, action times ranging from a few tens of milliseconds to a few seconds. The two forms of energy storage, an electrical gradient and a chemical gradient, are characterized by different mechanisms of accumulation and use, and above all, by very different timelines by which accumulation and use take place.

It should be kept in mind that from a physiological point of view, the overproduction and accumulation, both in the cytoplasm and in the nucleus, of messenger RNA as well as various proteins, can also be considered accumulations of potential energy. However, this is a form of potential energy that has response times to stimuli with a different order of magnitude compared to electrical potential and the chemical gradient of calcium.

This type of accumulation will not be examined further. However, the common feature of these three different ways of storing energy is that they maintain the state of the cells in a dynamic equilibrium with high potential energy levels, always ready for a more-or-less rapid reaction in response to a stimulus generated by changes in the surrounding environment.