Read Aloud the Text Content

This audio was created by Woord's Text to Speech service by content creators from all around the world.

Text Content or SSML code:

Fungi are heterotrophs, meaning they depend on pre-existing organic molecules for both carbon and energy. Unlike animals, fungi do not have organs that enable them to ingest food and break it down in a digestive cavity. Instead, fungi absorb organic molecules directly through their cell walls. Simple molecules like amino acids and sugars are able to pass easily across the cell wall. Fungi secrete a diversity of enzymes that break down complex organic molecules like starch, cellulose, or lignin into simpler compounds that can be absorbed. Most fungi are multicellular, consisting of highly branched filaments called hyphae. Hyphae are slender, typically 10−50 times thinner than human hair. The numerous, long, thin hyphae provide fungi with a large surface area for absorbing nutrients. Fungi have no mode of locomotion so they must use the process of growth to find nourishment. Hyphae grow at their tips when resources are plentiful forming a network of branching hyphae called a mycelium. Mycelia can grow to be quite large—the largest known individual of the fungus Armillaria ostoyae covers over 2000 acres in the Blue Mountains of Oregon and weighs many hundreds of tons. When resources are low, hyphae growth is slow or may stop entirely. In fungi, cell walls are made of chitin, a modified polysaccharide that contains nitrogen, which is the same compound found in the exoskeleton of insects. Fungal cell walls are thinner than plant cell walls because of the chemical makeup of chitin. The cell wall prevents cells from rupturing when exposed to dilute solutions, such as in freshwater environments and in the soil. The cell wall also keeps the cells from expanding as water flows into the cytoplasm by osmosis. Fungi can transport food and signaling molecules across long distances in mycelia, driven by changes in turgor pressure. Molecules taken up actively from the environment drive water into the cell by osmosis and thus increase turgor pressure. At the same time, growth and respiration consume these molecules, decreasing turgor pressure. Such differences in turgor pressure along hyphae drives bulk flow in a manner similar to phloem transport in plants. Their ability to transport materials allows fungal hyphae to use materials in nutrient-rich locations to fuel hyphal elongation across nutrient-poor locations. It also allows fungi to build relatively large reproductive structures aboveground. Cytoplasmic continuity is essential for the long-distance movement of materials within mycelia. In early-diverging groups, the hyphae have many nuclei but no cell walls to separate them. In later-diverging groups, nuclear divisions are accompanied by the formation of septa, walls that partially divide the cytoplasm into separate cells. Each septum contains one or more pores that allow water and solutes to move freely between cells. Septa play an important role when hyphae are damaged. Injury activates sealing mechanisms that plug pores in the septa, preventing the loss of pressurized cytoplasm. Yeasts are single-celled fungi found in moist, nutrient-rich environments. Most yeasts divide by budding. A small outgrowth increases in size and eventually breaks off to form a new cell. The localized outgrowth that results in budding is similar to growth by elongation at hyphal tips. In fact, some yeasts can form hypha-like structures under certain conditions. Yeasts are common on the surface of plants, and to a lesser extent on the surfaces and in the gut of animals. Humans have long used yeast to ferment plant carbohydrates to produce leavened bread and alcoholic beverages. Most fungi use dead organic matter as their source of energy and raw materials. Bacteria and some protists can also subsist on this resource, but for the most part it is the fungi that convert dead organic matter back to carbon dioxide and water. Decomposition by fungi helps to keep the biological carbon cycle in balance and returns nutrients in leaves to the soil, where they will be available for new plant growth. Fungi can gain nutrients from dead animal, protozoan, or even bacterial cells, but the most abundant biomolecules on and within soils are cellulose and lignin, the principle components of plant cell walls. Cellulose is a rich source of carbon and energy. It is difficult to degrade because individual cellulose polymers bind tightly to one another. cellulose microfibrils are in intimate association with lignin, making the cellulose hard to get at. Lignin is even more difficult for enzymes to break apart because it lacks a regular chemical structure. Some bacteria are able to decompose lignin, but in nature, fungi account for most of the decomposition of wood. Fungal infections are rare in vertebrates. Severe infections are more frequent among fish and amphibians than in mammals, perhaps because fungi grow poorly at mammalian body temperatures. An apparent exception is the fungal infection that has caused dramatic declines in North American bat populations. The fungi infect the bats during their winter hibernation, when the bats have lowered body temperatures that conserve energy. In humans, most fungal infections are annoying rather than life threatening. Some examples include athlete’s foot and yeast infections. White-nose syndrome (WNS) is a poorly understood disease associated with the deaths of at least 5.7 million to 6.7 million North American bats Fungi are also well adapted to infect living tissues. Plants are vulnerable to a diverse array of fungal pathogens—rusts, smuts, and molds that cause huge losses in agricultural production. Successful pathogens must be able to get past a plant’s physical or chemical defenses. In many cases, fungi infect plants through wounds, which provide a route around a plant’s outer defenses. Some fungi enter through stomata. Others penetrate epidermal cells directly, degrading the wall with enzymes and then pushing their hyphae into the plant interior by turgor pressure. Aboveground, plant infections are usually transmitted by fungal spores, carried either by the wind or on the bodies of insects. Belowground, infection is typically transmitted by hyphae that penetrate the root. Insects and other invertebrates are also susceptible to fungal pathogens. Some fungal predators trap living animals by forming sticky traps with their hyphae; others lasso their prey. The hyphae of these latter fungi form rings that can inflate within seconds, trapping anything within the ring, such as nematodes, tiny worms common in soils. While some interactions between species benefit the fungus at the expense of its host, others benefit both partners. Mycorrhizal fungi supply plant roots with nutrients such as phosphorus from the soil and, in return, receive carbohydrates from their host. There are two main types of mycorrhizae. The hyphae of ectomycorrhizal fungi surround, but do not penetrate, root cells. The hyphae of endomycorrhizal fungi penetrate into root cells, where they produce highly branched structures that provide a large surface area for nutrient exchange. Other fungi, called endophytes, live within leaves. Endophytic hyphae grow within cell walls and in the spaces between cells. This relationship is thought to be beneficial, as the fungi may help the host plant by producing chemicals that deter pathogens and herbivorous insects. Mutually beneficial associations between fungi and animals are much less common, but a few examples are known. Most common are those between fungi and insects. The insects provide the fungi with shelter, food, and protection from predators and pathogens, while the fungi are used as a food source for the insect. Examples of this type of association include leaf-cutter ants in tropical forests, African termites, and in some wood-boring beetles. Lichens are familiar sights in many environments, often forming colorful growths on rocks or tree trunks. Lichens look, function, and even reproduce as single organisms, but they are actually stable associations between a fungus and a photosynthetic microorganism, usually green algae or a cyanobacterium. The dual nature of lichens was first proposed in 1867, by a Swiss botanist, Simon Schwendener, but it was not accepted until 1939 that their dual nature became widely accepted. In 1939, Eugene Thomas showed that lichens could be separated into their individual parts and then reassembled.