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Study Guide for test 4 BIOS 205 Fungi ‐‐Earth time scale: ‐‐Earth originated about 4.6 billion years ago, condensing from a large cloud of dust and gas along with the Sun and the rest of the solar system ‐‐Hadean era: bombardment by many large objects, no solid surface or permanent atmosphere. Ended about 3.8 billion years ago. ‐‐Archaean Era: Beginning of life on Earth, all prokaryotes, about 3.8 billion years ago. Ended about 2.5 billion years ago, when cyanobacteria developed a form of photosynthesis that generates oxygen, which poisoned the atmosphere but also allowed for much more efficient metabolism. ‐‐Proterozoic Era: protists (single celled eukaryotes) dominated. Efficient aerobic metabolism allowed them to be larger and more complex than prokaryotes. Ended 600 million years ago with the Cambrian Explosion, the sudden appearance of hard parts: shells, teeth, claws, in the fossil record. ‐‐Paleozoic Era: at the beginning of this era, all life is in the sea, but during it plants, fungi, and animals invade the land. Amphibians and insects are dominant animals; coal swamps with primitive plants like club mosses and giant horsetails and tree ferns. Ends 250 million years ago with the Permian Mass Extinction, killing up to 95% of all species. Cause is unknown. ‐‐Mesozoic Era: dominated by reptiles including the dinosaurs. Cycads, a type of gymnosperm, dominated the plants. By the end of the Mesozoic, flowering plants (angiosperms) became prominent. Ends 65 million years ago with the Cretaceous Mass Extinction, caused by a large object from space hitting the Earth, which generated so much dust that photosynthesis stopped for several years. ‐‐Cenozoic Era: our current era, dominated by mammals and angiosperms (flowering plants). ‐‐Basic characteristics of fungi: ‐‐eukaryotes (and not prokaryotes) ‐‐heterotrophs that feed by absorption. That is, they eat organic matter, which they obtain by secreting digestive enzymes into the environment and then taking the digested food molecules in through the cell membrane. ‐‐have a cell wall composed of chitin ‐‐most don’t move under their own power ‐‐cells are usually haploid (1 copy of each chromosome) or dikaryon (each cell has two separate haploid nuclei in it). Fungi are diploid (2 copies of each chromosome in one nucleus) only briefly, when the two haploid nuclei in the dikaryon fuse to become a single diploid nucleus, which then undergoes meiosis immediately to produce new haploid offspring. ‐‐Fungal body forms: ‐‐Multicellular fungi are usually composed of thin filaments called hyphae. A mass of hyphae is called a mycellium, and we can also call a mycellium a mold. Mushrooms are the fruiting bodies (that is, the place where meiosis and sexual reproduction occurs) of some fungi; mushrooms are composed of hyphae that are organized into a specific shape. ‐‐unicellular fungi are called yeasts. ‐‐lichens are fungi that are in a symbiotic relationship with photosynthetic algae or cyanobacteria. ‐‐mycorrhizae are fungi that are in a symbiotic relationship with plant roots: the fungi provide mineral nutrients to the plants, and the plants provide sugar (from photosynthesis) to the fungi. ‐‐cell walls are rigid boxes on the outside of cells. They keep the cell from bursting due to osmotic pressure (water diffusing into the cell). Cell walls are found in fungi, plants, bacteria, and many protists. Cell walls are composed of polysaccharides (many sugar molecules linked together). Specifically, fungi cell walls are the polysaccharide chitin, plant use the polysaccharide cellulose, and bacterial use the polysaccharide peptidoglycan. ‐‐osmotic pressure inside the hyphae allows fungi to exert force, so they can penetrate plant cells to feed on them. ‐‐reproduction in fungi can be asexual (one parent produces spores that are genetically identical to itself) or sexual (two parents combine to form offspring that are genetically different from either parent). ‐‐The eukaryotic sexual cycle: a haploid organism produces gametes (sperm and eggs), which fuse during the process of fertilization to produce a diploid organism. Some cells in the diploid organism undergo the cell division process of meiosis, which produces new haploid offspring. ‐‐fungi spend most of their lives as haploids. When fungi of two different mating types (the equivalent of male and female sexes) meet, some cells fuse to form dikaryons, which contain two haploid nuclei, one from each parent. This process of fusing two haploid cells to form a dikaryon cell is called plasmogamy. At the moment of sexual reproduction, the two nuclei in a dikaryon to form a single diploid nucleus, a process called karyogamy. The diploid nucleus immediately undergoes meiosis to produce haploid offspring. ‐‐some fungi are not known to have any sexual reproduction. These are called imperfect fungi. DNA sequence analysis allows them to be classified into their proper phylogenetic groups. The imperfect fungi are not a monophyletic group. ‐‐fungi (and plants) defend themselves by producing secondary metabolites, which are chemical compounds that are not proteins, carbohydrates, lipids, or nucleic acids. ‐‐fungi are more closely related to animals than to plants. The nucleariids are the protists most closely related to fungi. ‐‐five groups of fungi: ‐‐chytrids: most basal group, live in the water, have zoospores with flagella. One species causes a frog‐killing disease: the fungus digests the keratin in their skin, which alters their internal ion balance. ‐‐zygomycetes: a fairly primitive group that produces large pigmented zygospores as part of sexual reproduction. Also reproduce asexually. ‐‐glomeromycetes: symbiotic with plant roots. They form arbuscular mycorrhizae, which means that part of the fungal hyphae are inside the plant root cell. Other fungal groups form ectomycorrhizae, which are just on the surface of the plant root cells, not inside them. ‐‐ascomycetes: a large group of fungi that has meiosis and the resulting sexual spores produced inside a sac. Truffles and morels are edible ascomycetes. Plant disease like Dutch elm disease, chestnut blight, and ergot are all caused by ascomycetes. Candida is a yeast that infects human mucus membranes. Most ascomycete reproduction is asexual, with spores produced on structures called conidia. ‐‐lichens are fungi (usually ascomycetes) living in symbiosis with photosynthetic organisms, either green algae or cyanobacteria. The two organisms remain separate from each other. The fungus supplies protection and it secretes acids that allow it to attach firmly to rocks. The photosynthetic organisms provides food. ‐‐the ascomycete yeast Saccharomyces cerevesiae is probably the most important fungus for humans. It is used to produce alcoholic beverages and to make bread rise. ‐‐basidomycetes are the group that contains most of the mushrooms. Mushrooms are the fruiting bodies of the fungus, which is mostly underground in the form of a mass of hyphae. Sexual reproduction produces spores that form on club‐shaped cells called basidia. Basidomycetes only reproduce sexually. ‐‐rusts are basidomycetes that cause plant diseases and have very complicated life cycles involving several different types of spore and more than one species of host plant. ___________________________________________________________________ Spore‐forming Plants ‐‐Linnaeus invented the classification system used today, based primarily on the characteristics of the reproductive system. Woese pioneered the use of DNA for classification, and determined that there were 3 main branches of life: bacteria, archaea, and eukaryotes. Plants, animals, and fungi are the three complex, multicellular clades of eukaryotes. In contrast, the protists are eukaryotes but they are not a monophyletic group. ‐‐the definition of “plant” is controversial: ‐‐the Archaeplastida are the protists derived from the primary endosymbiosis of a cyanobacteria by primitive eukaryote. The Archaeplastida consist of red algae, green algae, and plants ‐‐the Viridiplantae are the green algae plus the plants ‐‐the Embryophytes are plants that have a multicellular embryo growing within the mother’s body. This group contains all the land plants and excludes the algae; our current best definition of “land plant” is Embryophytes. This is the definition used in the textbook, and what will be used for the tests in this class. ‐‐green algae can be unicellular, colonial (a group of identical cells), or multicellular (a group of cells with different cell types that act as a single organism). ‐‐Green algae can be divided into the Chlorophytes and the Charophytes, ‐‐The charophytes are the group most closely related to plants; they live in the water but have complex multicellular bodies. Two characteristics shared by charophytes and plants but no other groups are sporopollenin, a tough polymer that protects spores and seeds, and the phragmoplast, a structure that dividing cells use as a scaffold to build their cell wall on. ‐‐the four main groups of plants: ‐‐bryophytes: mosses, liverworts and hornworts. The most primitive group, which is not monophyletic. No vascular tissue, so are very low to the ground. Gametophtye (haploid) stage is dominant, with the diploid sporophyte stage quite small and growing out of the gametophyte. The sperm have flagella and need to swim through water droplets to reach the eggs. Peat moss bogs cover a significant part of the Earth’s surface. ‐‐seedless vascular plants: lycophytes (club mosses) and pterophytes (ferns and horsetails). Have a vascular system. The diploid sporophyte is dominant, but there is a small, free‐living gametophyte stage as well. Sperm need to swim to reach the eggs, and the sporophyte grows directly out of the gametophyte. Although today lycophytes and pterophytes are small, during the Paleozoic era many were large trees, which fossilized to become the coal beds found in much of Illinois. ‐‐gymnosperms: conifers, cycads, and others we will discuss later. First plants to have seeds. “naked seed” ‐‐angiosperms: flowering plants, most plants today are angiosperms. ‐‐plant reproduction. The diploid stage is called the sporophyte. Sporophytes have a special structure called a sporangium, in which meiosis occurs to produce haploid spores. Spores disperse to new locations, then germinate to form the haploid stage, the gametophyte. Gametophytes have a structure called a gametangium, which produces the sperm and eggs (the gametes), which are haploid. The sperm and egg combine in the process of fertilization to produce a new diploid sporophyte. ‐‐the different plant groups have variations on this basic theme. However, both the haploid (gametophyte) and diploid (sporophyte) stages are multicellular in all plants (unlike animals or fungi). ‐‐in many plants, male and female spores are different from each other. In this case, “mega‐“ refers to female and “micro‐“ refers to male. For example, a megaspore (female spore) or a microgametophyte (male haploid stage). ‐‐photosynthesis occurs in the chloroplast. Plastid is another word for chloroplast. In the light reactions of photosynthesis, sunlight is used to remove electrons from water, producing oxygen. The high energy electrons from the light reactions are then used to convert carbon dioxide into sugar using the Calvin cycle, which is also called carbon fixation. ‐‐plant cell walls are composed of cellulose, which is made of glucose subunits. Cell walls are stiffened by a complex compound called lignin. ‐‐meristems are regions on the plant where cell division occurs. Once a new cell is formed in the meristems, it differentiates into its final cell type. Apical meristems are at the tip (apex) of the shoot and root. Lateral meristems, in the stem and root, produce lateral growth (increase in thickness). ‐‐vascular tissue conducts fluids between the roots and the leaves. Xylem cells are dead and hollowed out into tubes that pull water up from the roots by evaporating water out of the leaves (transpiration). Phloem cells are alive, and conduct the sugar produced by photosynthesis from the leaves to other parts of the plant. Xylem and phloem are both produced by meristems: xylem on the inside, phloem on the outside. Xylem cells get thickened and become wood. ‐‐leaves are the main site of photosynthesis. They are coated with a waxy layer called cuticle. Leaves have openings called stomata that open and close to let gases and water vapor in and out depending on conditions. Liverworts, the most primitive of the plants, don’t have stomata: they have opening that never close instead. ‐‐microphylls are leaves on primitive plants: they are thin and narrow and have just a single strand of vascular tissue. Most plants have megaphylls instead; megaphylls have branched leaf veins. _____________________________________________________ Seed Plants ‐‐a major life cycle change from the lower plants. Seed plants shorten the haploid stage (spore‐
gametophyte‐gamete) to just a few cells contained within the spore. The male haploid stage is the pollen grain and the female haploid stage is the ovule, which remains within the female body. ‐‐the male gamete (sperm) gets to the female gamete (egg) by growing a long pollen tube from the pollen grain to the ovule. The sperm nuclei migrate down this tube. ‐‐seeds are multi‐cellular diploid organisms that are dormant and able to withstand bad environments. The seed stage occurs within the life of the diploid sporophyte. In lower plants, the diploid sporophyte grows directly out of the body of the female gametophyte, with no dormant stage in between. Seeds contain a food source to live on before photosynthesis starts. The first leaves of the new plant, called cotyledons, are found in the seed. ‐‐wood is a composite of cellulose fibers and lignin matrix. Wood is found in all living gymnosperms and in many angiosperms. Wood develops from xylem cells whose cell walls have by thickened. Gymnosperms produce softwood, which is composed of tracheid cells only, and angiosperms produce hardwood, which contains vessel elements in addition to tracheids. ‐‐the two groups of seed plants are the gymnosperms and the angiosperms (flowering plants). Gymnosperms evolved before the angiosperms. Gymnosperm seeds develop on the outside of the plant, and angiosperm seeds develop inside an ovary. ‐‐there are 4 groups of gymnosperms, and all but the conifers are very small groups that were much more common in the past. ‐‐cycads were the dominant plant type during the Mesozoic (age of dinosaurs) ‐‐gingkos are represented today by just one species that is thought to have been kept alive by monks in China. ‐‐gnetophytes are the source of ephedrine, a common stimulant and decongestant drug ‐‐conifers are trees like pine and fir. Conifers are the dominant species in northern forests, because they are able to thrive in very cold climates. Most of our lumber and wood pulp (used to make paper) comes from conifer. ‐‐most gymnosperms are monoecious: they have both male and female parts on the same plant. Dioecious plants have separate male and female plants. ‐‐gymnosperm pollen is spread by the wind. When it lands on the female parts, a pollen tube grows to carry the sperm cell to the egg inside the ovule. The ovules are on the outside of the plant, attached to the base of the scales of pine cones. ‐‐angiosperms are defined as plants with flowers. They evolved much later than the gymnosperms. Angiosperms became dominant late in the Mesozoic era, and they remain the dominant form of plant life today. ‐‐flowers consist of 4 sets of organs (whorls) nested inside each other. The outermost parts are the sepals. In most plants the sepals are the green protective covering of the flower bud, but in some plants the sepals are brightly colored and look like petals. ‐‐the next set of organs are the petals. Petals are usually brightly colored and used to attract animal pollinators. ‐‐the third set of organs in a flower are the stamens, which are the male reproductive parts. The most important part of the stamens are the anthers, which produce the pollen. ‐‐the innermost set of flower organs are the carpels, which are the female reproductive structures. The ovules are located with the ovary, which is part of the carpel. ‐‐most flowers are perfect: they have both male and female parts in the same flower. In contrast, imperfect flowers have either male organs (stamens) or female organs (carpels) but not both. ‐‐dioecious plants, which have separate male and female plants, always have imperfect flowers. ‐‐some monoecious plants (one plant is both male and female) have imperfect flowers. A good example is corn: the tassel is the male flower and the ear is the female flower. ‐‐pollination is the process of getting the pollen to the female flower parts. ‐‐some monoecious plants allow self‐pollination: the same plant acts as both male and female parent. However, most plants develop mechanisms to avoid self‐fertilization, which reduces genetic diversity. ‐‐In gymnosperms and some angiosperms (especially the grasses), pollen is transported by the wind. ‐‐However, most angiosperms use animals to spread their pollen. The plants produce food in the form of nectar, a sugary liquid. The plants signal the presence of nectar with brightly colored flowers and scents. The pollen gets on the animals as they come to eat the nectar, and the animals spread it from flower to flower. ‐‐different plant species and their pollinating animals co‐evolve: they both develop adaptations that make it easier for the animals to find the flowers of the proper plant species and spread its pollen to other members of the species. The different colors, shapes, and scents of different kinds of flower are adaptations to attract specific types of pollinator. ‐‐fertilization is the union of the haploid sperm and egg to produce a new diploid organism. The pollen grain lands on the stigma and grows a long pollen tube to the ovule, which is contained within the ovary. Angiosperms have a unique process of double fertilization: two sperm nuclei move from the pollen grain down the pollen tube to the ovule. Inside the ovule, one sperm fertilizes the egg to create the zygote (which becomes the new plant), and the other sperm fertilizes 2 other cells in the ovule to produce endosperm, which is the nutritive tissue for the seed after it germinates. Endosperm is the equivalent of yolk in vertebrates. Endosperm is triploid: two sets of chromosomes from the mother plus one set from the father. ‐‐after fertilization, the embryo develops into a seed. The seed is contained within a fruit, which develops from the walls of the ovary (pericarp). The rest of the flower withers away. Some fruits are fleshy (like apples) while others are dry, like pea pods or maple seeds. Fruits are used to disperse the seeds, either because they are eaten by animals or because they use the wind to disperse. ‐‐there are 3 major groups of angiosperms. ‐‐the basal angiosperms are the most primitive types. Examples include water lilies and the magnoliids, which include magnolia, nutmeg, black pepper, and cinnamon. The basal angiosperms do not form a monophyletic group. ‐‐the monocots are a large group that contains the grasses, orchids, and lilies. Monocot seedlings have just one cotyledon leaf, and they have parallel leaf veins. ‐‐the eudicots are the largest plant group, including most of the common plants. Eudicot seedlings have 2 cotyledons, and the leaves have net‐like veins. ‐‐the mustard family of eudicots contains the genus Brassica. Many of our crops have been bred from brassica species: broccoli, cauliflower, cabbage, Brussels sprouts, turnip, canola, mustard. ‐‐the legume family is the source of most of the plant protein we eat. Legumes are unique in the plant world because they have root nodules that harbor nitrogen‐fixing bacteria. Nitrogen gas consists of two nitrogen atoms bound very tightly to each other: not usable by most organisms. To be useful, nitrogen must be fixed by converting it to ammonia or nitrate. Nitrogenase is the bacterial enzyme that fixes nitrogen. The process uses a lot of energy and is very sensitive to oxygen. The legume root nodules provide a good environment for this process to occur. ‐‐many plants produce secondary metabolites: chemical compounds that are not directly related to growth and reproduction. Secondary compounds are used as a defense against predators and pathogens, and to attract animals to help spread pollen and seeds. We use many secondary metabolites as medicines, food flavorings, and intoxicants. Two big classes of secondary metabolites: the alkaloids have rings of carbon and nitrogen atoms, are alkaline, and generally taste bitter. Glycosides are compounds with a sugar attached to the active component. They are harmless until the sugar has been removed by an animal’s digestive system. Cyanogenic glycosides release cyanide, and steroid glycosides release steroids, which are compounds with a particular 4‐ring structure that is found in many animal hormones. ‐‐grasses are the most important family of monocots. Grasses evolved during the Cenozoic era and have become one of the most important plant groups. The cereal grains such as corn, wheat, and rice are all grasses. ‐‐tropical grasses use a special form of photosynthesis called C4 photosynthesis, in which the leaf stomata are open at night to take in carbon dioxide and store it. During the day the stomata are closed to prevent water loss, but photosynthesis uses the stored carbon dioxide to make sugar. ‐‐the growing point (apical meristem) of grasses is below ground until the plant flowers. This makes it hard for grazing animals or fire to kill it. ‐‐grass flowers have the same basic structure as other flowers, but it is modified in many ways. The grain is the fruit of a grass plant. It is a dry fruit, with the ovary wall becoming the bran (outer coat) that surrounds the seed. Each grain contains a single seed. The seed consists of the embryo (which is called the germ) and the endosperm (nutritive tissue). The endosperm is mostly composed of starch, which we use as flour. ‐‐wheat is one of the most important cereal grains. Modern wheat is a hexaploid: it has 6 copies of each chromosome. Hexaploid wheat is the result of 2 different hybridizations between closely related species. This hexaploid hybrid produces much larger kernels than the original diploid species. A second important aspect of wheat domestication was selection for a non‐shattering head: the kernels don’t spontaneously fall off the plant, which allows easier harvesting. ‐‐barley is another important cereal grain. When allowed to germinate, barley kernels produce an enzyme that converts starch into sugar. The germinated and dried kernels are called malt. When mixed with yeast and other grains, the malt enzymes convert starch in the grain into sugar, and the yeast converts the sugar into ethanol (alcohol), which we consume as beer. The alcohol in beer killed harmful bacteria in water, which prevented much water‐borne disease in ancient civilizations.