Forms of Energy
The energy used by your computer to download this post, your screen to display it, and your brain to read it are all fundamentally the same. Energy can take many forms, and there is nothing particularly special about the kind that runs humans. Life is complex, but it always obeys chemical and physical laws. People used to believe in an "animus", a special form of energy that creates life but this idea has long since been debunked. For fun, let's take a look at what it really takes to give a person the ability to perform a seemingly mundane task, pressing a key on a computer keyboard.
This story started a very long time ago. Though the very earliest moments are poorly understood, it is generally accepted that our universe underwent a very rapid period of inflation about 14 billion years ago, transforming from an unimaginably dense, high-energy, homogeneous state to a plasma of quarks and gluons. Today quarks are invariably found locked up as the constituent components of hadrons like protons and neutrons so having a state of free quarks is very "strange". As quarks make up mass, gluons are carriers of energy. Specifically, they carry a fundamental force called the strong nuclear force which generally binds quarks together to form hadrons, in addition to gluing protons and neutrons together to form the nucleus of an atom. However, at this incredibly high level of energy both were free and continually colliding near relativistic speeds, creating and annihilating matter – antimatter pairs. At some point, for some reason, an important balance called the Baryon number was slightly upset and there ended up being more matter than antimatter, which eventually resulted in a universe made of matter.
The universe continued to expand and cool and eventually the other three fundamental forces, weak nuclear, electromagnetism, and gravity, separated and various other elementary particles began to take shape. Quarks began to settle in to their baryons, forming protons and neutrons, as well as their antimatter counterparts, but when temperatures dropped too low to create new matter – antimatter pairs, they mostly annihilated each other, leaving only the slightly offset balance of regular matter from before. A similar process then happened for electrons and positrons (anti-electrons), leaving the energy of the universe dominated by photons and neutrinos, which are the pure-energy products of these annihilations. This all happened in less than a second.
Some of the free protons and neutrons combined to form Helium, bound by the strong nuclear force, but most protons remained free and eventually combined with electrons to form the Hydrogen that makes up the vast majority of the atoms in the universe and our bodies today. As mass came to dominate the universe regions of slightly higher density eventually coalesced via the gravitational force, attracting ever more mass and increasing in density to form gas clouds, some of which reached high enough levels of density and heat to catalyze a nuclear fusion reaction. Hydrogen atoms normally repel each other due to the electrostatic force, but if the surrounding energy is high enough, a percentage can get close enough together for the strong nuclear force to cause their nuclei to fuse, since it is stronger than electrostatic forces at very close distances. This nuclear fusion releases the overcome electrostatic energy as a positron and neutrino, and forms Deuterium, which may then fuse with another atom of Hydrogen, forming Helium-3 and releasing a Gamma Ray, a very high energy photon.
The Sun is a huge nearly perfect sphere of plasma consisting mostly of Hydrogen and Helium, reacting in the core in the aforementioned way. However, since fusion happens only within the core, and the sun is very dense, the gamma rays are continually absorbed and re-emitted by the surrounding mass, taking tens of thousands of years to eventually reach the surface. At this point, each gamma ray is split in to millions of photons of lower energy, visible light. About 8.5 minutes later, some of these photons strike the surface of the earth.
All life takes energy. Without the intake of energy from the environment, an ordered system such as an organism quickly succumbs to entropy and death. Some forms of life have evolved to use the energy produced by the sun in the form of photons. Chlorophyll, for example, appears green because it absorbs mostly blue light, which is shorter wavelength and thus higher energy. The energy from a photon strips an electron from a suitable molecule, such as water, via the photoelectic effect. This ionizes and separates the hydrogen from water. The oxygen is then discarded and the ionized, or charged hydrogen and free electron are transferred to a substance such as NADP+, which becomes NADPH and is later consumed to generate ATP, via the Calvin cycle.
Besides ATP, the chief currency of energy for all cells, the Calvin cycle produces something very important for humans: sugar. Let's take corn for example. Humans have brilliantly optimized the growth of corn by introducing extra nitrogen in to the soil. This means we can cheaply convert the sun's energy in to sugar, or glucose, and then consume it for energy ourselves. After a human consumes this glucose, the small intestine diffuses it in to the bloodstream and a complex mechanism triggers the release of insulin, a protein whose job it is to transport the molecules of glucose to the cells that require them. The Krebs cycle essentially does the opposite of the Calvin cycle, using the glucose to produce ATP, the same molecule used by the plant cells for energy.
The brain, which also uses glucose for energy, "fires" a neuron, changing the electric potential along a pathway travelling down the top of the spinal column, down the arm, to a group of skeletal muscle cells in the forearm called the "lumbrical muscle". As our nerves make for poor electrical conductors, the signal must activate sodium channels on the way to the muscle in order to propagate. Eventually the signal reaches the junction between nerve and muscle, and activates an influx of calcium ions, which in turn releases the neurotransmitter acetylcholine, opening a channel for sodium and potassium ions, forming yet another electric action potential. This potential spreads throughout the muscle, releasing more calcium which binds to muscle fiber regions called Troponin.
This reaction changes the Troponin, which was previously blocking binding sites on the muscle filaments for Myosin, which in turn acts as a binding site for ATP. The ATP binds the the Myosin, causing it to release Actin, a protein serving as a microfilament to stabilize it, and the Myosin extracts energy from the ATP molecule via hydrolysis. Hydrolysis releases chemical energy by breaking the relatively weak phosphate bonds in ATP. These bonds are easy to break, but contain high energy electrons, which is how they introduce extra energy in to a system. This process causes the muscle to contract about 10-12nm, and is repeated as long as the muscle is signaled and there is sufficient ATP and Calcium to drive the reaction. The average key press distance on a keyboard is 3.81mm so it takes about 381,000 iterations per muscle fiber to perform this action.
What triggered the brain to fire the neurons? Information is also form of energy.