lecture 6 - fungi Flashcards
What are fungi?
“Unicellular, multicellular or coenocytic, heterotrophic, eukaryotic microorganisms which do not contain chlorophyll and which characteristically form a rigid cell wall containing chitin and/or cellulose”
Where are fungi on the tree of life?
Found in the Domain of Eukarya, Kingdom Fungi
More closely related to plants and animals than bacteria and archaea
What are the different phyla of fungi?
There is a lot of debate regarding the classification for fungi.
Originally classified as plants
Taxonomy of the Fungi is changing at the moment – no on can agree!
Genetic comparisons often overturn the assumptions of the older systems of classification
Phylogeny determined by rRNA analysis
What are hyphal growth forms?
Each hypha is a tube, containing cytoplasm surrounded by a cell wall
Hyphae may be aseptate (coenocytic)
-Several nuclei are found in one
“super cell”
Basically there are two forms of fungal growth – hyphal and unicellular.
Hyphae are 2-10 um in diameter and can be cm long.
Hyphae exhibit Apical growth (i.e. they elongate at their tips)
Hyphae branch to form a Mycelium
Mycelium is found in two forms
Aerial mycelium - involved in reproduction Vegetative mycelium - penetrates food source. Mycelial mats can be relatively small e.g. what you see growing on bread to massive “mats” which are effectively one organism that colonises whole forests.
What are macroscale fungal structures?
hyphae and fruiting bodies
How do unicellular growth forms (yeasts) replicate?
budding and binary fission
Difference between budding and binary fission is that parent cell can repeatedly bud up to a point when they are covered in bud scars then they die. Used as a model for ageing.
What are dimorphic forms?
show both hyphal and unicellular forms
What are fungal spores?
Fungi produce spores for
Dispersal
Survival
Sexual reproduction
These spores can be either sexual spores or asexual (collectively called Conidia)
How does fungal nutrition work?
Heterotrophic -require one or more carbon compounds as a carbon source.
Chemo-organotrophs obtain their energy from organic compounds.
Assimilate a wide range of organic compounds
Produce exoenzymes break down C sources external to cell and uptake products.
Produce a wide range of secondary metabolites
Responsible for decay and recycling of organic material – saprophytes
Includes recalcitrant macromolecules like cellulose, keratin and lignin.
How does lignin degradation take place?
Fungi use extracellular peroxidases to break bonds within the lignin to produce smaller molecules that can be absorbed and used as either a C or energy source.
Lot of interest in this for using this type of material as a feed stock for the production of organic chemicals and biofuels.
How does fungal reproduction take place?
Reproduce by means of the spores mentioned earlier, and most can produce asexual or sexual types.
Sexual reproduction:
Fungal nuclei can exist in two forms: either,
Haploid containing one of each chromosome (think of sperm/eggs in humans) ,
or
Diploid containing two of each chromosome (think normal animal cells)
Describe fungal sexual reproduction
In most Eukaryotes when the gametes fuse, cytoplasmic and nuclear fusion occur within a short period of time (think sperm/egg).
In fungi, cytoplasmic fusion can occur well before nuclear fusion.
Fungal reproduction cycles typically include haploid, dikaryotic and diploid phases
Some fungi need to mate with another individual of the opposing mating type (Heterothallic)
Some fungi can have hyphae that contain both “male” and “female” types in the same aggregate and can fertilise themselves (Homothallic)
Describe yeast reproduction
Cells of S. cerevisiae can exist in the vegetative or haploid stage indefinitely (16 Chromosomes)
Sometimes two yeast of different mating types fuse to form a diploid form (32 Chromosomes).
Cells have to be opposite mating types
To go back to haploid form ascospores are formed
Describe fungal disease in humans
Cause disease via
Hypersensitivity – allergic reaction
Mycotoxins – e.g. Ergot poisoning & St. Anthony’s Fire caused by Claviceps purpurea which produces toxic alkaloids.
How can fungal disease be controlled in humans?
Control is difficult.
Antibiotics exploit differences between eukaryotic and prokaryotic cells
e.g. Cell wall synthesis in Gram positives.
Any antifungal agent is likely to have side effects on the human host.
Fungi develop resistance to antifungal agents
Antifungal agents target membrane functions, nucleic acid and cell wall synthesis, microtubule formation
