1.) Fungi are a kingdom consisting entirely of absorptive heterotrophs. In contrast to certain bacteria and autotrophic plants, they need pre-formed organic compounds as energy sources, and as carbon sources for cellular synthesis. Fungi characteristically live embedded in some form of food substrate where they absorb simple, soluble nutrients through the wall and cell membrane. In many cases, these simple nutrients may be released from more complex polymers by depolymerases that are secreted into the external medium. It is therefore unlikely that there is a substrate anywhere in the world that a fungus cannot utilize or benefit from. It should be noted the cell wall of the fungi prevents food being engulfed by phagocytosis.
2.) Fungi usually are filamentous, with the single filaments being termed hyphae. Fungal hyphae grow and branch to produce a network of filaments which constitutes the mycelium. The mycelium enlarges by extension of single hyphae which show polar growth, meaning they grow only at their extreme tips. This apical growth is in contrast to the intercalary growth of most other filamentous organisms. Expansion of the mycelium is continuous if the hyphae can keep on extending on the medium they are residing in. However, changes do take place as the mycelium ages and as that part of the food source on which it is growing is no longer able to provide sufficient nutrients. It should be noted here that though many fungi are hyphae in character, with an indeterminate mycelium capable of producing the largest of organisms, there are actually five major body forms in the kingdom.
3.) Fungi can reproduce by both asexual and sexual means. Reproduction is invariably connected to the production of spores, produced at specialized structures and fully equipped to start a new colony independent of the parent mycelium, and usually some distance from it. Fungal spores vary enormously in shape, size and other special properties, linked to their numerous roles in dispersal or survival.
4.) Fungi are all eukaryotic. This means they have a membrane-bound nucleus containing several chromosomes (unlike a circular strand of DNA found in prokaryotes), and a number of membrane-bound organelles including mitochondria and vacuoles. Eukaryotes also contain DNA that includes non-coding regions entitled introns, and ribosomes of the 80S type, contrary to the 70S type found in prokaryotes.
In summary, fungi are a kingdom of heterotrophic absorptive eukaryotes which probably arose from a choanoflagellate like protozoan by the origin of beta-glucan/chitin walls, with the simultaneous loss of phagotrophy. Multiple losses and origins of complex characters would have occurred since then, including major changes in wall chemistry, sometimes totally losing the whole vegetative wall. Fungi are ordinarily aerobic, having mitochondria with flat cristae and peroxisomes, the latter giving yeasts some of their chemical virtuosity. The 'true' fungi consist of the phyla Chytridiomycota, Zygomycota, Ascomycota and Basidiomycota, each sharing important morphological and biochemical features, such as walls typically containing chitin. Fungi are restricted to a monophyletic lineage, the closest relatives to these true fungi being the Choanoflagellates, a group ancestral to multicellular animals too. The true fungi display evident evolutionary trends with respect to their colony structure, ecological relationships, cell form, life cycle and sexuality.(1)
(1) World of Fungus