The "Fire" type is unique in its members' ability to create or control fire. Apparently, their cells are protected with a flame-retardant cocktail, while sulfur, phosphorus, and carbon are stored in special organs to produce the flame. These Pokemon tend to be colored with "warm" tones, and since they are virtually fire-proof, can live in incredibly hot conditions (e.g. the inside of a volcano). Fire Pokemon tend to devour metals and minerals straight from the rock, since their cells are unable to properly digest normal food.
"Water"-type Pokemon are generally aquatic or partially so. There is not much of a cellular difference between Water types and Normal types, but they do possess the ability to absorb and release vast quantities of water. Some such Pokemon have the ability to change the weather, generate a wall of concentrated water, and even blast a never-seen plasma form of water.
The "Grass" type is similar in ways to the Bug type, but instead of a primitive mite living in the bloodstream, algae (Xenoalgus supernaturum) shares a symbiotic relationship with a Grass-type Pokemon's cells. In exchange for protection and some carbon dioxide, the algae grant the Pokemon with oxygen, sugars, and special abilities. In some cases, the algae appear as a visible vegetable organ or apparatus (e.g. leaves, flowers, vines, etc.), and in some such Pokemon, there is more plant than animal material controlling the organism, resulting in them commonly being mistaken as a form of sentient plant.