1.1 The Chaotic Life of an Aqueous/Gaseous Molecule
You have probably been introduced to the notion of chemical gradients before. If two salt solutions with different concentrations are separated by a permeable membrane, then molecules travel from high to low concentration such that after a set amount of time, both solutions are of equal concentration. In other words, molecules tend to diffuse down their concentration gradient. While this notion serves as a foundational principle in the natural sciences, there are often considerable misconceptions surrounding its ontology.
The notion of a concentration gradient is predictive. It accurately describes the net movement of molecules and thus allows us to build models for much more complicated topics. These topics range from ocean currents and chemical reactions to the hardware that generates human behavior, intelligence, and dare I say, consciousness. One important caveat is that chemical gradients describe NET movement of molecules, but in reality, molecules are constantly being exchanged in a bidirectional manner across any semi-permeable membrane. This continues until equal concentrations are achieved by a balanced yet continuous exchange of molecules.
As the title suggests, aqueous and gaseous molecules live a chaotic life. Any one molecule, in an uncharged environment, does not know whether it is in a solution of high or low concentration. It simply moves randomly, purposelessly, changing direction only when it bumps into other molecules. This random behavior of particles is termed Brownian motion, and its considered to be one of the strongest supportive evidence for atomic theory. From the perspective of the atom, there is no such thing as a chemical gradient. There is no force which drives molecules down their concentration gradient like the force of gravity drives our feet to the ground. Diffusion gradients are not an innate property of nature in the way that electric forces and the existence of quarks are. Instead, the phenomenon of diffusion is simply an accidental consequence, an epiphenomena, of the random and chaotic motion of particles.
In an uncharged environment, molecules move randomly, full stop. How is it then, that a population of molecules have net movement down their concentration gradient? Why has this principle been so predictive and foundational for understanding the natural world. The answer is best described using illustration.
To the left, we have an image of a molecule. To simplify the conceptual process, let us assume that the molecule can only move in four directions. Whether or not it moves in any one direction is random, with a 25% probability per path.
Now, if these were four particles, then we would have, on average, one moving in each of the four directions. As can be seen in the figure below, if one solution has more particles in it, then it will simply have more molecules moving in all directions. This includes the direction that leads to the semi-permeable membrane, resulting in a net exchange of molecules until equal concentrations are achieved.
In short, movement down a chemical gradient is an extension of the second law of thermodynamics; universal entropy always increases and molecules move randomly over time. Increased chaos is the default state of the universe. Order, in the form of maintaining different concentrations across a membrane, would require energy to be put into the system. Indeed, neurons purposefully maintain a concentration difference across a membrane, but they can only maintain such order using ATP as well as the hydrophobic interactions which make a membrane not permeable to all ions. In fact, ⅓ of all ATP metabolized in humans everyday is used purely to maintain stable concentration gradients. We will come back to this point later in the chapter, when discussing action potentials, sodium/potassium pumps, and neuronal electrical propagation.