Fungi can absorb only small molecules such as sugars or peptides less than size amino acids. The reproductive organs of the fungi are called fruiting bodies or sporangia, which are sacs or other tissues that contain the fungi spores. Fungi are uncommon in aquatic environments. On land, the amount of hyphae in the soil is measured in hundreds or thousands of meters of length per gram of soil. Fungi secrete enzymes that can break down cellulose into glucose, one of the few kinds of organisms able to do this.
Fungi are the only known organisms that degrade lignin completely.
Cellulose and lignin are structural materials in plants that are difficult to degrade. The fungi do not use the breakdown products of lignin, but instead they use hydrogen peroxide to oxidize lignin in place. The breakdown products diffuse away, exposing the cellulose to enzymatic attack. In oceans and lakes, the small 2-10 m m long flagellates are the most important predators on bacteria. In some termites, anaerobic protozoans in the gut degrade cellulose.