🔬General Biology I Unit 24 – Fungi

What Are Fungi?

• Fungi are eukaryotic organisms that belong to their own kingdom separate from plants and animals
• Consist of a diverse group of species ranging from single-celled yeasts to multicellular mushrooms and molds
• Obtain nutrients through absorption by secreting enzymes to break down organic matter externally and then absorbing the nutrients
• Possess cell walls made of chitin, a tough polysaccharide that provides structure and support
• Lack chlorophyll and are therefore heterotrophic, relying on other organisms for their energy and carbon sources
• Play crucial roles in ecosystems as decomposers, symbionts, and pathogens
• Estimated to have over 1.5 million species, with only about 5% currently described by scientists

Fungal Cell Structure

• Fungal cells are eukaryotic, containing membrane-bound organelles such as a nucleus, mitochondria, and endoplasmic reticulum
• Cell walls are composed primarily of chitin, a polysaccharide that provides structural integrity and protection
  • Chitin is also found in the exoskeletons of arthropods (insects and crustaceans)
• Plasma membrane is similar to other eukaryotic cells, regulating the flow of substances in and out of the cell
• Cytoplasm contains various organelles, including ribosomes for protein synthesis and Golgi apparatus for processing and packaging proteins
• Some fungi have specialized structures called septa, which are cross-walls that divide the hyphae into individual cells
  • Septa may be complete or incomplete, allowing for the flow of cytoplasm and organelles between cells
• Many fungi are multinucleate, meaning they have multiple nuclei within a single cell, which is an adaptation for rapid growth and efficient nutrient distribution

Types of Fungi

• Fungi are classified into four main phyla: Ascomycota, Basidiomycota, Zygomycota, and Chytridiomycota
• Ascomycota, also known as sac fungi, is the largest phylum and includes yeasts, molds, and cup fungi
  • Produce sexual spores called ascospores within sac-like structures called asci
  • Examples include baker's yeast (Saccharomyces cerevisiae) and Penicillium, which is used to produce antibiotics
• Basidiomycota, or club fungi, includes mushrooms, puffballs, and shelf fungi
  • Produce sexual spores called basidiospores on club-shaped structures called basidia
  • Many edible mushrooms (shiitake and portobello) belong to this phylum
• Zygomycota, or conjugation fungi, includes bread molds and other fast-growing species
  • Reproduce sexually through the formation of zygospores, which are thick-walled spores that can withstand harsh conditions
• Chytridiomycota, or chytrids, are primarily aquatic fungi that produce flagellated spores called zoospores
  • Some species are parasitic and have been implicated in the decline of amphibian populations worldwide

Fungal Reproduction

• Fungi can reproduce both asexually and sexually, depending on the species and environmental conditions
• Asexual reproduction occurs through fragmentation, budding, or the production of asexual spores
  • Fragmentation involves the breaking off of hyphae, which can then grow into new individuals
  • Budding is common in yeasts, where a small outgrowth (bud) forms on the parent cell and eventually separates to form a new individual
• Sexual reproduction involves the fusion of two compatible haploid cells to form a diploid zygote
  • Meiosis then occurs, producing haploid spores that can develop into new individuals
• Many fungi have a complex life cycle that alternates between haploid and diploid stages
  • Haploid stage is often the dominant stage, with the diploid stage being short-lived and primarily for sexual reproduction
• Some fungi are heterothallic, requiring two genetically distinct individuals for sexual reproduction, while others are homothallic, able to self-fertilize

Ecological Roles of Fungi

• Fungi play critical roles in ecosystems as decomposers, symbionts, and pathogens
• As decomposers, fungi break down dead organic matter, releasing nutrients back into the environment for other organisms to use
  • Essential for nutrient cycling and maintaining soil fertility
  • Wood-decaying fungi (white rot and brown rot) are the primary decomposers of lignin, a complex polymer found in wood
• Mycorrhizal fungi form symbiotic relationships with the roots of plants, providing the plant with increased water and nutrient uptake in exchange for carbohydrates
  • Estimated that 90% of land plants engage in mycorrhizal associations
  • Two main types: endomycorrhizae (penetrate root cells) and ectomycorrhizae (form a sheath around roots)
• Some fungi are pathogens, causing diseases in plants, animals, and humans
  • Plant pathogens (rusts and smuts) can devastate crops, leading to significant economic losses
  • Animal pathogens (Batrachochytrium dendrobatidis) have been linked to the decline of amphibian populations
  • Human pathogens (Candida albicans) can cause infections, particularly in immunocompromised individuals

Fungi in Human Life

• Fungi have been used by humans for thousands of years in food production, medicine, and biotechnology
• Yeasts, particularly Saccharomyces cerevisiae, are used in the production of bread, beer, and wine
  • Fermentation by yeasts converts sugars into ethanol and carbon dioxide, giving these products their distinctive flavors and textures
• Many mushrooms are edible and are cultivated or collected for their nutritional and medicinal properties
  • Shiitake (Lentinula edodes) and oyster mushrooms (Pleurotus ostreatus) are popular in culinary applications
  • Reishi (Ganoderma lucidum) and lion's mane (Hericium erinaceus) are used in traditional medicine for their potential health benefits
• Fungi are the source of numerous antibiotics, including penicillin, which was discovered from the fungus Penicillium notatum in 1928 by Alexander Fleming
  • Antibiotics produced by fungi have revolutionized medicine, saving countless lives and enabling the treatment of previously deadly bacterial infections
• In biotechnology, fungi are used to produce enzymes, organic acids, and other valuable compounds
  • Citric acid, used in food and beverage production, is produced by the fungus Aspergillus niger
  • Fungi are also used in bioremediation to break down pollutants and toxins in the environment

Key Concepts to Remember

• Fungi are eukaryotic organisms that belong to their own kingdom and play crucial roles in ecosystems as decomposers, symbionts, and pathogens
• Fungal cell walls are composed of chitin, and many fungi are multinucleate, adaptations for their lifestyle
• The four main phyla of fungi are Ascomycota, Basidiomycota, Zygomycota, and Chytridiomycota, each with distinct characteristics and life cycles
• Fungi reproduce both asexually (fragmentation, budding, and asexual spores) and sexually (fusion of haploid cells and meiosis)
• As decomposers, fungi are essential for nutrient cycling and maintaining soil fertility
• Mycorrhizal fungi form symbiotic relationships with plants, improving water and nutrient uptake
• Fungi are used by humans in food production (bread, beer, and mushrooms), medicine (antibiotics), and biotechnology (enzymes and organic acids)

Cool Fungal Facts

• The largest living organism on Earth is a honey fungus (Armillaria ostoyae) in Oregon, covering an area of 2,385 acres (965 hectares) and estimated to be between 2,400 and 8,650 years old
• Fungi are more closely related to animals than plants, sharing a common ancestor that diverged from plants around 1.1 billion years ago
• Some fungi, called endophytes, live inside plant tissues without causing harm and can even protect the plant from herbivores and pathogens
• Zombie ants are created by the fungus Ophiocordyceps unilateralis, which infects and manipulates the behavior of ants to spread its spores
• The fungus Aspergillus tubingensis can break down plastics like polyester polyurethane, offering a potential solution for plastic waste management
• Fungi have been used in traditional medicine for centuries, with the Chinese using the reishi mushroom (Ganoderma lucidum) for over 2,000 years to promote health and longevity
• The fungus Penicillium roqueforti is responsible for the blue veins and distinct flavor of blue cheese, such as Roquefort and Stilton
• Some fungi, like the shaggy mane (Coprinus comatus), can degrade heavy metals and other pollutants in soil, making them useful for bioremediation efforts