The cellular structure of a Kryptonian is similar to that of humans. However, a certain chemical makeup in the ribosomes (organelles responsible for protein synthesis) is capable of absorbing solar energy and storing it. Once the ribosomes achieve saturation levels of this radiation, they begin to re-radiate the energy in the form of a bioelectric forcefield. Each individual cell within the field's radius becomes reinforced in such a way that the cells are not only protected from external harm, but internal forces as well. It is this last which is key to Superman's great powers. Since the individual cells are wholly invulnerable, they are capable of operating at levels of efficiency that would destroy normal humanoid cells. These "super-cells" lend an enhancing effect to all bodily systems...muscular, metabolic, nervous, and so forth. The continual absorption of solar energy provides a constant power source for these enhanced systems. Thus, Superman is capable of incredible feats.
Naturally, the basic forcefield effect renders him invulnerable to large amounts of damage. Unlike a human, Superman's body has more than simple cohesion and adhesion holding his tissues together. The bioelectric forcefield has a set tolerance for how much stress occurs before it reacts energetically in opposition to the external force. The tolerance for how much stress occurs before Superman's tissues counteract it varies according to tissue type. His bones allow virtually no compressional or tensional strain (deformation in shape) and his fleshy tissues remain supple under soft pressure, but under stress react to restrict any further deformation in shape. Since the strength of Superman's cellular forcefields greatly exceeds the cohesive force of any substance, no ordinary object can puncture his skin.
Superman can withstand a 1-megaton nuclear explosion without ill effect, and has been able to tolerate the molten conditions in the Earth's core with only discomfort. A 40-Megaton nuclear explosion knocked him unconscious for over an hour. He could not, however, survive the conditions inside a star, but can safely approach to within several miles of Earth's Sun, probably due to a feedback loop of solar energy powering his bioelectric field. Superman can easily survive conditions in deep space, but he does need to breathe, eat, sleep, and excrete. Superman can hold his breath for a maximum of twelve hours, and requires two hours of sleep a night to function at peak mental efficiency, as well needing to dream at least half an hour a night, or experience the psychological effects of sleep deprivation as would any other person who missed a night of sleep. When his solar energy is depleted, his resistance to physical punishment drops to approximately one-quarter the normal amount. Withstanding large amounts of damage continuously over time will rapidly deplete his solar reserves.
Superman's cells provoke one another into releasing their respective energy when one begins to threaten the integrity of the other. Because of a certain degree of constant pressure, there is a continuously emitted radiance of the bioelectric field reaching slightly beyond his body, a sort of aura that protects items near his skin e.g. his costume. The aura also protects Superman's teeth, however his hair and nails are only partially protected due to certain cellular properties they possess. They are invulnerable to fire, however they can still be cut.
Due to the highly charged nature of his solar storage systems, his immune system deals with most normal poisons and diseases quickly and with no ill effect. With the exception of kryptonite emissions, radiation and similar harmful energies are similarly neutralized.
His physical strength is greatly magnified by the combination of his more-efficient muscular system and the enhancement of his skeletal system's structural integrity, but is primarily augmented by the sheer force of his solar forcefield. When Superman appears to be lifting a weight, he is actually holding and pushing it with the bioelectric field. His energy stores push not from himself, but from the shielding that surrounds him. So the shielding is controlled on the highest level and simulates strength. He can lift 100,000 tons with serious effort. The conjectured upper limit of his physical strength has been determined to be approximately 250,000 tons with great effort, and causing strain damage to himself equivalent to an average adult human male lifting 500 pounds. Continual exertion will deplete his solar energy, and greatly exceeding his normal lift will drain him more rapidly. At the level of complete solar depletion, Superman can lift/exert no more than 100 tons of weight or force.
Similarly, Superman's enhanced metabolic rate, muscular speed, and nervous system grant him incredible speed. His Kryptonian structure and the solar energy paths that course along his neurophysical structure allow him to react at many times normal human speed. He can read a complete encyclopedia set in seconds, wash a sink full of dishes in the blink of an eye, or run from New York City to Paris in a matter of minutes. He can move faster than the human eye can perceive if necessary, but this requires extra effort. His reflexes are quick enough to allow him to dodge incoming automatic laser fire.
Superman's perceptions are similarly rapid, allowing him to see individual bullets fired from high-speed automatic weaponry in flight and outrace them easily. Moving at extreme speeds for extended periods of time rapidly drains his solar reserves. In a full drained state, his reflexes are only sufficient enough to dodge semi-automatic projectile weapons and read the encyclopedia set in under 10 minutes.
The enhanced structure of Superman's sensory organs combined with the increased capacity of his nervous system provides for the greatly enhanced senses. His range of hearing extends from the subsonic through to the point where signal modulation in broadcast communications can be sensed and demodulated into "sounds" if he concentrates. He can distinctly hear soft sounds anywhere on Earth if he is listening for them. He has perfect pitch. He has, on occasion, sat and perceived every sound on Earth as a symphony of a living planet.
His visual reception range covers the entirety of the electromagnetic spectrum. The difference between human and Superman's vision is that Superman has the capability to see a much wider band of the electromagnetic spectrum; while humans only sense a very narrow band of the spectrum we call visible light. Superman can change the spectrum he perceives at any given time, by changing the chemical receptors in the rods and cones of his retinas. He can adjust his photovoltaic detection as an act of will to sense longer wavelengths down to the infrared, and higher energy light, up to X-ray and gamma radiation.
Kryptonian muscle tissue when super-charged with solar-energy, can generate a powerful gravitational field. These fields in combination with Superman's personal bioelectric field give him the ability to negate gravity, allowing him to fly. When in flight, he's fully suspended by this energy, almost completely repelling Earth's gravimetric influence over him. He becomes his own center of gravity, and no longer has to physically leverage against the gravity of whatever planetary body he's on, or any object. By instinctively utilizing a precise muscular control, Superman is able to control the pull and force of his personal gravitational field, allowing him to essentially fall forward in a direction and speed of his own choosing. Through sheer force of will, he is capable of Mach speeds in excess of Mach 10 in atmosphere. He can fly to the Moon in minutes, but faster than that outside of atmospheric interference. His control of his flight is perfect and he can perform aerobatic feats such as hovering, flying backwards and even lifting great weights while flying.
Superman can use the gravitational fields generated by the muscles in his eyes to produce a gravity lens, allowing him to focus far beyond the range of the human eye. Combined with Superman's sensitivity to light, he can magnify an image a virtually countless number of times. Superman can see through solids objects by using a combination of these telescopic and microscopic visual abilities to see through the atomic structure of an object and focus past it, as a camera focuses beyond the dust on a lens. Certain dense materials, notably lead, obstruct this ability.
Superman has the ability to expel stored solar and electromagnetic internal energy in the form of focused beams of radiation from his eyes. He emits light along various frequencies in high energy bursts that flash-melts materials in seconds, weaken structural integrity over a smaller area, such as melting the barrel of a gun but leaving the handle untouched. Superman can control his heat vision and direct it wherever he wants to. He directs it with his eyes. In actuality, Superman could not emit the beams directly from his eyes. The energy is actually built up on the surface of the bio-energetic field surrounding his body and emitted from there. This allows his body to be protected from the energy and absorbing stray electromagnetic energy back into his body. Superman's energy reclamation aura is so efficient that any radiation, even those emitted from him, once in the environment are able to be reabsorbed. The heat beams are psionically controlled and that control is reflexive, so he can shut it off without accidentally damaging anything that he does not want to. He can emit the radiation visibly or invisibly at lower settings. His energy has a high visibility profile if emitted at higher levels.
Superman has the power to release gale-force winds from his lungs. He can create ice blocks from his breath, as well as freeze objects and opponents. The psionic portion of his powers are made evident by these depictions. The only way for objects to freeze is if their molecular composition slows down in motion at an incredible rate. The application of intense winds from Superman's breath is actually not sufficient to freeze things. Since Superman can agitate molecules to increase in motion, he can do the reverse. He is using the same effect that amplifies converted solar energy in reverse. He is using it as a catalyst to access higher-order energies through an unknown physical mechanism. That same amplification mechanism is reversed to dampen or neutralize energies in the environment to produce freeze breath. Additionally, his vast lung capacity is not physically possible given the dimensions of the humanoid physique. However, his super-breath includes tesseract-type properties.
Both Superman's Kryptonian physiology and the fact that his engrams are stabilized by a solar-frequency hologram reinforcing his neural matrix allow him near-perfect recall of events he pays specific attention to or that have emotional significance or are particularly striking.
Superman is a talented mimic, and can reproduce most sounds that he can hear. He can modulate his voice to transmit on radio or microwave/radar frequencies, and can expel a minute amount of air in space at a warm enough temperature and speed to carry sub-vocalized sound directly to one or more targets.
Superman requires solar energy to function at peak capacity, or his powers fade to greatly reduced levels, some disappearing entirely if his solar reserves are empty. Superman cannot absorb energy from the lower frequency orange O-class stars and cannot process the higher-frequency White K or blue F classes. Blue B and yellow G type stars are ideal.
Superman's body stores and processes solar energy at a rapid rate and for a variety of effects, which are explained in his other powers. His storage capacity is sufficient to let him function at full capacity for up to 14 days depending on his physical condition and state of rest. While under a star of the right class (G-type dwarfs in the blue-white through yellow spectrum are ideal) and within a planetary atmosphere, he replenishes energy at a constant rate.
If forced to expend large amounts of energy over a relatively short time, or to maintain constant exertion over longer periods of time, he tires and his power begins to diminish. Constant exertion will revert him to normal humanoid levels after 14 days without rest. The power recovers at the rate of normal rest, and a full recharge can be accomplished with two days of no more than moderate, normal-level exertion. A process of solar recharge is also known, but requires massive amounts of fusion energy of solar/light-based superpowers.
The fatal flaw in Kryptonian cellular physiology is the unique radiation given off by kryptonite. In the most minute amounts, the radiation waveforms and high-energy particles emitted by kryptonite can react catastrophically with the very ribosomal chemical that grant a Kryptonian powers. The briefest contact can strip these powers, and further begin a fatal poisoning that totally shuts down cellular activity in the affected areas. As kryptonite radiation makes its way through the bloodstream, a Kryptonian becomes instantly weak and ill in its unshielded presence, and prolonged exposure will render him or her unconscious (after 30 minutes to an hour depending on the size of the sample, and the current state of his solar reserves), and will eventually (after two to four hours) reach all parts of his body and bring death, usually as the body's immune system attempts to expel the deadly toxin. However, the rate of recovery once removed from the emissions is as rapid as the rate of damage, and the Kryptonian will be back to normal in under an hour after exposure in optimal conditions. Kryptonite emissions can be blocked by lead and other super-dense materials.
Further, the radiation signature of the red sun of the Krypton system (and similar M-class stars) instantly causes the ribosomal structures to lose their stored energy, reducing a Kryptonian to normal humanoid levels.