The science of Twilight vampires (i.e. vampires in the Twilight universe) is based around "vampire venom." This venom is not merely an organic chemical substance, it is actually a fluid-based lifeform. It also behaves like a virus or, more specifically, a retrovirus. It will attach itself to a host cell, sometimes shrinking to the size of a virus or most likely dividing itself into many virus sized molecular structures and attaching themselves to the host cell's membrane. The venom-virus then infects the host cell, making a DNA copy of the genome for use in making RNA. This RNA could be used as a template for making more viral genomes and as mRNA to produce viral proteins, which is known as reverse transcription. Venom cells grown in size and seem to attack white blood cells first. The host body triggers an autoimmune response to the infection, causing the release of white blood cells. Once the bloodstream of the subject has been flooded with white blood cells, the infected cells send out a signal to the other infected cells that are not producing the virus-like cells which in turn grow in size, causing swelling in tissues and extreme pain. In most cases, this results in convulsions and agonizing screams from victims. This most likely stems from disrupted neural pathways where the immense size of these predatory seeker cells turn on the host's white blood cells and devoures them.
Genetically different venom cells will adapt and mutate for the betterment of the host. There is another factor in mitosis of vampire cells that provides for extra "checks and balances" on the creation of new material. On one hand, the genome is thoroughly checked for anomalies in the DNA and can more likely spare the expense of ATP in these checks because of the plethora of energy in a "pure" cell. On the other hand, the DNA readily accepts foreign DNA if it mutates a cell for the betterment of its new host. The DNA of that cell is able to convert both unique venom, incorporating the DNA changes of both into the host cell.
Although the vampire venom lifeform acts like a retrovirus, it is also sentient to a degree. The venom being sentient is the causation of many functions in a vampire's physiology. The heart and many other major organs of a vampire no longer need to pump fluids throughout the body as the venom can move on its own. Also, since the venom acts quite aggressive in its spread, it pools in areas where it is most likely to be injected into another life form: the mouth and the sexual organs. That's why the sexual organs of a vampire continue to function when so many other organs do not. The venom specifically chooses a vampire's sexual organs as a transport mode to other organisms, allowing them to function normally.
The sentient venom along with the extremely rapid regeneration of cells are also the cause of the vampire ability to heal quickly and attach severed limbs. Each vampire has unique venom due to the DNA changes. The venom is able to recognize itself and send out signals over small distances. There is a symbiotic relationship between the sentient venom and the vampire host.
Vampire venom is highly flammable due to the extremely long chain of hydrocarbons, similar to petrol, which is a part of their cellular protein synthesis.
The vampire body temperature is low due to venom cells having an endothermic nature. Ammonium nitrate is a part of a vampire's cellular cycle and it has uses for the venom as well. The venom uses a thermodynamic chemical reaction that is endothermic: the ammonium nitrate combining with decreasing a vampire's body temperature by using the reactant heat of the cellular growth. That's why the vampire's body is able to undergo such complex chemical changes with an extremely short cellular half-life while appearing dead.
Vampire cells differ greatly from human cells. Vampire cell membranes consist of an organic based membrane "armored" with titanium molecules for strength. Titanium is recognized for its high strength-to-weight ratio, which means its use throughout the body of a vampires does not affect his or her weight drastically. It is a strong metal that has excellent malleability without shattering. It is also white in color, which explains the lightening of a vampire's skin color. The titanium not only resides in the cell membrane, but is also in all other portions of the cell including the nucleolus, mitochondria, ribosomes, and Golgi apparatus. Titanium found in the rough endoplasmic reticulum is linked to the extremely strong proteins synthesized by venom cells and is directly attributed to stronger muscles, faster protein synthesis due to conductivity within the cell and increased excitability in electron transport function, as well as energy released. The ribosomes in the endoplasmic reticulum produce the proteins. As these modified proteins and other molecular byproducts of the venom interact with the human cell in the smooth endoplasmic reticulum strong steroids and lipids are created.
Vampire ATP differs greatly from human ATP. Normally, mitochondria convert potential energy from sugars into ATP. The mitochondria of the venom are extremely dependent on finding new hosts and the average vampire cell has ten times the mitochondria as a normal human cell. This is what causes the higher amounts of energy production that a meta-physical being such as vampires are able to sustain, allowing superhuman feats. An average human cell produces about 32 molecules of ATP per cycle of respiration. Humans use oxygen to bind and trap waste products during this cycle, which creates carbon dioxide in anaerobic cycles, or fermentation, the cell creates a different sort of waste product (either lactic acid or ethanol) which only produces 2 ATP molecules. Vampire venom uses a process that binds nitrogen to create an unstable reactant, highly flammable, but with such a short half-life. The reactants are of an alcohol and an aldehyde. Ammonium nitrade is also part of the process.
Vampires do not age because of apoptosis. Venom and vampire DNA have programmed cell death, which ceases the aging process. The cells transmit a "suicide" signal terminating life with increased cell division when the genetic material has been damaged. Apoptosis is also the reason vampires have no hair or nail growth.
Some of a vampire's heightened senses are due to physiological changes that take place during transformation. The heightened speed and strength are due to the supercharged proteins synthesized by the venom cells and the titanium in the vampire cellular composition.
The vampire hearing range is greater than that of a human. The structure of a vampire's inner ear changes dramatically during transformation, increasing their hearing range to 20,000 – 20Hz. Once changed, they are able to hear over greater distances and communicate in infrasonic tones. A vampire's sight also physically transforms during the change. The amount of rods and cones in vampire eyes increases dramatically, increasing the photosensitivity of the eye. In addition the pigmented oils in the eyes increase, assisting vampires in seeing a greater spectrum of color. The fovea changes shape as well, increasing the number of receptors available. A vampire's heightened sense of smell is similar to a canine. Although the amount of a vampire's olfactory nerves does not change significantly, the amount of smell receptors increases by the millions. The visual cortex of a vampire's brain evolves into a visual and olfactory cortex and it is able to process both sight and sound simultaneously. That's why vampires see "red" tinting their vision when angered or in the throes of bloodlust.
While vampires do not require oxygen, breathing is still a necessity for them, and not only for scenting purposes. It is also important to their cellular structure. Their bodies need nitrogen and titanium to survive, both of which can be found within the air (as well as in the blood they sustain themselves with), and, although vampires can hold their breath for extended periods of time, cellular damage would eventually reach a critical level. Also, their gas exchange system is used to expel gaseous byproducts from their system, facilitating the need to breathe.
The physical appearance of a vampire's eyes change, to red, the color of blood. Although blood has nothing to do with the appearance. The eyes change color due to the effect of Rayleigh scattering (changes in directions of electromagnetic energy by particles whose diameters are 1/16 wavelength or less; this type of scattering is responsible for the sky being blue) which is caused by their unusual cell structure and the amount of bromine in their system, since bromine is a red color. Since their speed is increased, to many humans, it seems that their eye color changes when they feel bloodlust or sexual lust, but this is not the case. With the influx of adrenaline into a vampire's system, it triggers their feral instincts, causing their pupils to dilate rapidly and making their eyes appear to change from colored to black. Also, when vampires need to feed, the venom shuts down specific systems of their body until they are able to resupply it with energy, and replenishment of the replenishing of the colored oils in the cones appears to be the first function to wane, also causing their eyesight to decrease. The eyes of a vampire who only drinks animal blood will change to a yellow color. This is due to more alkalines in animal blood reacting to the bromine in a vampire's system.
The luminosity of a vampire is much different than the original host. This is because of the titanium in the cell membrane reacting to light; the coruscation, or glittery effect of a vampire's skin, is due to those metals reflecting the light. It is not noticeable to the human eye in low light.
A vampire's brain undergoes many changes during transformation, which causes partial memory loss. The brain structure changes dramatically, creating trillions more synapses than present in humans and making brain functions increase exponentially. The hardest hit section of the brain is the amygdala. The basolateral nuclei, the part of the brain that contains long-term memory storage, shrink dramatically during transformation. This is why so many vampires have very little memories of their human lives. Even though the basolateral nuclei shrink, the amount of synapses increases by the end of transformation, giving vampires an increased long-term memory. The best way to describe this phenomenon would be a computer memory-dump.
A vampire's baser instincts emerge after transformation due to brain physiology and chemistry. The bed nuclei of stria terminalis (BNST) and the temporal cortex hold most of the new synapses (along with the aforementioned amygdala). Since this is the part of the brain associated strongly with instincts and emotions, their feral instincts resurface. Hunting, mating, and aggression are all associated with these areas of our brain. In addition, the adrenal glands function in vampires at an increased rate, making their epinephrine (also known as adrenaline) production quadruple; this also increases their feral instincts.
Although it would seem that vampires do not sleep, they in fact do sleep, but only unihemispherically (using only one cerebral hemisphere of the brain at a time). This adaptation of the brain is similar to birds and some aquatic mammals. The slow wave sleep patterns of one brain hemisphere will last for less than one hour, because vampires need much less sleep than humans.
The vampire need to consume blood is due to the need for fibrinogen. Fibrin is abundant in blood plasma and it connects proteins together to form the vampire's complex chains of protein synthesis and, to produce them at such a rapid rate; they would need a large influx of fibrinogen. Besides the plasma, this is a simple way to receive all of the nutrients their cells need to reproduce. Adrenaline, water, amino acids, proteins, carbohydrates, and lipids are all available in blood, fueling their rapid cell growth quite successfully. Vampires do not need to defecate or urinate due to the efficiency of blood as an energy source. Their bodies efficiently use every compound contained in the blood for energy, leaving no solid waste products, only gases, eliminating their need to urinate and defecate. Their gas exchange system is advanced enough to eliminate all waste in a gaseous form (respiration).
Vampires in their first year of life as a vampire have heightened strength compared to older vampires. It is commonly believed amongst vampires that this is because they are fueled by the leftover human blood still in their system. The truth is a newborn vampire's strength is due to the amount of leftover fibrinogen in their cells. They actually have more fibrin in their system as the venom breaks down unneeded protein chains, enabling even faster cellular growth.
Vampire "gifts" are the result of a perceptual interaction with other spatial dimensions. How the human host perceived these dimensions explains how their gifts develop. These other dimensions hold the key to how humans can perceive the future and have other extra sensory experiences as vampires. Some humans have an extrasensory perception and are subconsciously aware of one or more of these dimensions. Since a vampire's body undergoes such great changes during the transformation including heightening all of their senses, vampires are able to perceive these extra dimensions in a new light. Because vampires are able to perceive them differently, their brain waves are able to send and receive signals from these other dimensions and the brains of some vampires are able to understand those signals and translate them into usable data, giving them "supernatural" gifts.