Fungi are heterotrophs, meaning that they fulfill their needs for carbon and energy by breaking down and assimilating organic matter. Fungi are capable of absorbing and metabolizing a wide variety of carbohydrates, including glucose, xylose and fructose. More difficult carbohydrates are also easily digested, such as cellulose and hemicellulose.
Fungi use enzymes to break down the materials on which they are growing. When absorbing the digested material, the food must be in the form of a solution in order to enter the hyphal walls of the fungi. Saprobic fungi and bacteria are largely responsible for the bulk of organic decomposition. In general, fungi require oxygen to survive, but they are also able to use fermentation when they lack oxygen.