Chapter 31 Fungi Mighty Mushrooms It is estimated that there are actually as many as 1 5 million species of fungi Some fungi are exclusively single celled though most have complex multicellular bodies They break down organic material and recycle nutrients allowing other organisms to assimilate essential chemical Chapter 31 Fungi 1 elements Concept 31 3 Fungi are heterotrophs that feed by absorption Nutrition and Ecology Fungi are heterotrophs they absorb nutrients from the environment outside of its body They release hydrolytic enzymes into the surrounding that break down complex molecules into smaller organic compounds so that the fungi can absorb it into their bodies Other fungi use enzymes to penetrate the walls of cells enabling the fungi to absorb nutrients from the cells Decomposer fungi break down and absorb nutrients from nonliving organic material Parasitic fungi absorb nutrients from the cells of living hosts some are pathogenic including species that cause diseases Mutualistic fungi also absorb nutrients from a host organism but they reciprocate with actions that benefit the host in plants Body Structure The most common fungal body structures are multicellular filaments and single cells yeasts Many species can grow as both filaments and yeasts but even more grow only as filaments relatively few species grow only as yeast The bodies of these fungi typically form a network of tiny filaments called hyphae consisting of tubular cell walls surrounding the plasma membrane and cytoplasm of the cells Fungal cell walls are strengthened by chitin a strong but flexible nitrogen containing polysaccharide Fungal hyphae form an interwoven mass called a mycelium that infiltrates the material on which the fungus feeds The structure maximizes its surface to volume ratio making feeding very efficient In most fungi the hyphae are divided into cells by cross walls or septa Septa generally have pores large enough to allow ribosomes mitochondria and even nuclei to flow from cell Fungi that lack septa are known as coenocytic fungi these organisms consist of a continuous cytoplasmic mass having to cell hundreds or thousands of nuclei Coenocytic condition results from repeated division of nuclei without cytokinesis Specialized Hyphae in Mycorrhizal Fungi Some fungi have specialized hyphae that allow them to feed on living animals Haustoria are specialized hyphae in which the fungi use to extract nutrients from or exchange nutrients with their plant host Mycorrhizae are a mutually beneficial relationship between fungi and plant roots They can improve delivery of Phosphate ions and other minerals to plants from the soil and the plant provides the fungi with organic nutrients such as carbohydrates Chapter 31 Fungi 2 Ectomycorrhizal fungi form sheaths of hyphae over the surface of a root and typically grow into the extracellular space of the root cortex invagination of root cell plasma membrane Arbuscular mycorrhizal fungi extend branching hyphae through the root cell wall and into tubes formed by Almost all vascular plants have mycorrhizae and rely on their fungal partners for essential nutrients In the absence of human intervention mycorrhizal fungi colonize soils by dispersing haploid cells called spores that form a new mycelia after germinating Concept 31 1 Summary Fungi are heterotrophs that feed by absorption All fungi including decomposers and symbionts are heterotrophs that acquire nutrients by absorption Many fungi secrete enzymes that break down complex molecules to smaller molecules that can be absorbed Most fungi grow as thin multicellular filaments called hyphae relatively few species grow only as single celled yeasts In their multicellular form fungi consist of mycelia networks of branched hyphae adapted for absorption Mycorrhizal fungi have specialized hyphae that enable them to form a mutually beneficial relationships with plants Concept 31 2 Fungi produce spores through sexual and asexual life cycles Sexual Reproduction The nuclei of fungal hyphae and the spores of most fungal species are haploid although many fungi have transient diploid stages that form during sexual cycles Sexual reproduction often begins when hyphae from two mycelia release sexual signaling molecules called The union of the cytoplasms of two parent mycelia is known as plasmogamy In most fungi the haploid nuclei contributed by each parent do not fuse right away Parts of the fused mycelium contain coexisting genetically different nuclei such a mycelium is said to be pheromones heterokaryon In other species the haploid nuclei pair off two to a cell one from each parent dikaryotic Such a dikaryotic mycelium grows the two nuclei in each cell divide in tandem without fusing The next stage may take hours or days karyogamy the haploid nuclie contributed by the two parents fuse Meiosis then restores the haploid conditions leading to the formation of spores that enable the fungus to producing diploid cells disperse Asexual Reproduction Many fungi reproduce asexually by growing as filamentous fungi that produce haploid spores by mitosis such species are known as molds if they form visible mycelia Molds typically grow rapidly and produce many spores asexually enabling the fungi to colonize new sources of food their species of a different mating type Many species that produce such spores can also reproduce sexually if they happen to contact a member of Other fungi reproduce asexually by growing as single celled yeasts Instead of producing spores asexual reproduction in yeasts occurs by ordinary cell division or by the pinching of small bud cells off a parent cell Mycologists have traditionally lumped all fungi lacking sexual reproduction into a group called deuteromycetes Concept 31 2 Summary Fungi produce spores through sexual or asexual life cycles The sexual cycle involves cytoplasmic fusion plasmogamy and nuclear fusion karyogamy with an intervening heterokaryotic stage in which cells have haploid nuclei from two parents The diploid cells resulting from karyogamy are short lived and undergo meiosis producing haploid spores Many fungi can reproduce asexually as filamentous fungi or yeasts DNA sequence data now allow mycologists to classify Chapter 31 Fungi 3 all fungi even those lacking a known sexual cycle Concept 31 3 The ancestor of fungi was an aquatic single celled flagellated protist The Origin of Fungi Phylogenetic systematics suggests that fungi evolved from a flagellated ancestor Some of the earliest