Cells use different forms of energy depending on the function

The electrical difference generated in a system bounded by a phospholipid membrane selective for potassium ions, and that contains a negative core with a high affinity for this ion, has a limited ability to accumulate potential energy. In fact, the mechanisms that lead to charge separation are essentially passive with low efficiency from a thermodynamic point of view.

Moreover, the accumulated potential is exhausted in a short time, since the membrane is not a perfect insulator due to its fluidity. Therefore, slowly but surely, the differences in ion concentration are dissipated, bringing the system back to static equilibrium. However, the little energy accumulated in a passive way may have been sufficient, in the course of evolution, to perform a minimum amount of work.

Although small, this amount of energy may have allowed biological systems to build elements capable of generating even greater potential energy accumulation. The possibility of accessing greater amounts of energy has been instrumental to the construction of increasingly complex biological organisms. The different types of cells that make up a biological organism can be grouped, from a functional point of view in terms of use of energy, into two distinct categories.

Figure 3.9. An absorptive epithelium, such as the intestine, must continuously transport sodium (red dots) and potassium (blue dots) with an active system (purple oval) that consumes ATP

There are cells that carry out acute, high-intensity work, exploiting the energy accumulated by the system in a short or very short time, such as the cells of skeletal muscles (Figure 3.8). There are also cells that carry out chronic work of low intensity, prolonged in time or even continuous, such as the cells of an absorbent epithelium (Figure 3.9). The two types of cells have evolved by developing the ability to increase the availability of potential energy, but with different timescales and ways of using it.

If a cell has an intense activity, working in an acute mode, it needs to use a large part of the available energy in a short time, dissipating it rapidly. For this reason, the systems that work in acute mode, in addition to using chemical energy to restore potential energy, have developed mechanisms to partially recover the energy used, to reduce consumption, for example, by alternating motor units in the muscle (Figure 3.8).

For chronic work, the continuous use of stored potential energy is supported by continuous production and consumption of chemical energy, which always keeps the system in a dynamic equilibrium with high potential energy and the ability to perform work. In both cases, the cell ensures that there is always an excess of potential energy compared to basic needs, either to be able to perform work even in the presence of a temporary block of chemical energy production, or to be able to cope with work of higher intensity if required.

In order to perform the work required for complex biological functions, cells must be able to increase their potential energy stores. This has been accomplished by generating molecular structures able to maintain the dynamic equilibrium and at the same time increase the potential energy of the biological system.

Over time and the course of evolution, billions of random events have occurred, during which increasingly complex molecular structures have been selected. The close link between structure and function may have allowed the selection of the primordial cell shown in Figure 3.7, which could be a reasonable model. It is also plausible that the initial low potential energy passively accumulated by the primordial cell was used to form chemical bonds and to synthesize increasingly complex and specialized molecules.

Such molecules could have been able to increase the potential energy accumulated across the membrane, in turn allowing increasingly complex molecules to be built by means of a positive feedback mechanism (Figure 3.10).

Figure 3.10. Simple molecules (1) can give rise to more complex ones (2) which increase the potential energy (3) with which increasingly complex and specialised molecules are built (4) which in turn increase the potential energy (5) with which increasingly complex and specialised molecules are built (6)

An effective way of increasing the potential energy of the cell, and thus the possibility of performing more and more complex work, is to increase the differences in ion concentration across the membrane. This can be achieved using specialized molecules which, using chemical energy, can move ions across the membrane against their concentration gradient.

The event that enabled the cell to make a quantum leap in energy production was probably the simultaneous selection of complex molecules for the production of chemical energy in the form of ATP, stored inside the cell, and molecules stored in the lipid membrane that can use this energy to perform work. In addition, in order to ensure continuous production of energy in the form of ATP, the selection of intracellular structures capable of performing this task with high efficiency was once again favorable.

According to the most widely accepted current hypothesis, these structures are nothing more than protobacteria that have been incorporated into cells and become symbionts, evolving into mitochondria.

It has already been discussed that among the ions available in primordial seawater (table 3.1), those most suitable for creating a difference between two environments divided by a phospholipid membrane are sodium and potassium (paragraph 3.2.2).

In particular, potassium was the most suitable to be the main element in the internal environment. Cell dimensions are in a range of tens of micrometers (10^-6 meter) for many reasons. As an example, a restricted space allows a higher probability of interactions between molecules. A limited volume also makes it easier to control ionic concentrations. In any case, to increase the potassium in the internal compartment, there must be energy-dependent ion transport because the movement is against the natural gradient.

In this way, the concentration difference across the membrane can be increased and the ionic gradient maintained, compensating for passive second- gradient exchanges across the membrane or those due to imperfect membrane selectivity. It is therefore necessary for the mitochondria to be able to continuously produce sufficient amount of ATP.

To do this, they need a constant and consistent supply of chemical energy-rich molecules that can be used to produce ATP, which must necessarily enter the cell from the external environment. From the millions of different molecules that have formed randomly over the course of evolution, sugars have been selected as the most suitable because they are extremely rich in high energy chemical bonds, relatively simple and small in size, and therefore, despite being hydrophilic, are easy to transport across the hydrophobic phospholipid membrane.

There are now all of the conditions to build a primordial cell: a selective membrane that is mainly permeable to potassium ions, bordering a cytoplasm-containing protein cluster and being rich in mitochondria capable of synthesizing small high-energy molecules such as ATP. In order for the mitochondria to produce energy, sugars must be transported into the cell. Specific membrane transporters for sugars are needed to carry out the transport.

The initial low potential energy stored passively at the sides of the membrane and the asymmetric distribution of the sodium, potassium and chloride ions ensure a concentration and charge gradient for sodium ions, which is certainly sufficient to supply the cytoplasm with sugars, albeit in limited quantities.

It has long been known that glucose is transported within the cell by a sodium-dependent mechanism, but how the transporter works, from a molecular point of view, is still a matter of hypothesis. What is possible is to construct a plausible model from an energetic point of view.