Lecture 8.1 - Fungi

chitin

• a polysoccharide found in the cell wall of fungi

• this tough polysaccharide contains N-acetylglucosamine

fungal structure

varies:

• since cell yeasts

• multicellular molds

• macroscopic mushrooms

spore

a reproductive structure that can develop into an individual without fusing with another reproductive cell

fungal distribution

• global

• primarily terrestrial

• associate with plants and animals

• non-motile (not exhibiting/capable of movement)

hyphae + structure

tubular, threadlike structures found in fungi. they define fungi

structure:

• it’s like a tube and is segregated into different compartments. these are naturally individual cells

• tough cell wall

• plasma membrane

• the inner lumen contains the cytoplasm and organelles

• it has a large surface:volume ratio

• it also has efficient nutrient absorption

what do some hyphae have

• cross walls called septa (septate hyphae)

• these septa don't isolate into different cells, but they form compartments in the hyphae, allowing cellular content to flow into the different compartments

• septa can have single or multiple pores

• cytoplasm can flow through each compartment

• septet hyphae are not discrete cells

aseptate (in hyphae)

where there are no cross walls separating cells

the hyphen tip

• these are structurally and functionally unique

• they are organelle-dense

• essential for apical growth

nutrition in fungi

fungi are heterotrophic: either parasitic or saprophytic

• saprophyte - feeds on dead organic material by decomposing it, which releases nutrients which can be reused, e.g. bread mould

• parasite - obtain their food from a live host of a different species, usually causing it harm, e.g. athlete’s foot

• symbiotic - mutual benefit of two independent organisms

rhizopus stolonifer

• a saprophyte that colonises moist, carbohydrate-rich, starchy foods such as bread

• it loves bread as it’s a huge source of starch, hence glucose will be available

• consists of threadlike structures called hyphae, which are multinucleate (many nuclei)

• each nucleus is a haploid

structures of rhizopus and their roles

rhizoid:

• anchorage (holds in place)

• secretes digestive enzymes

• absorption of digested food

stolon:

• colonisation → allows rhizopus to spread out and find new areas of growth

sporangiophore:

• supports the sporangium

sporangium:

• produces and releases spores for asexual reproduction

• sporangia contain black sporangiospores

apophysis:

• swelling below sporangium

columella:

• wall pushing up into the sporangium

• separates the spores from sporangiophore

types of hyphae

• rhizoids - penetrates the the food source horizontally and absorb nutrients

• stolons - horizontal hyphae

• sporangiophores

• upright hyphae - can produce asexual sporangia

nutrition in rhizopus

1. rhizopus is a saprophyte

2. fungus secretes enzymes into the substrate

3. enzymes digest the starch

4. digested food is then absorbed

5. rhizoids are hyphae that provide extra surface area for absorption of the digested material

asexual reproduction in rhizopus (how they usually reproduce)

1. cells within the sporangium divide by mitosis to produce spores (haploid)

2. the sporangium dries out and opens, releasing many spores → sporulation

3. each spore will grow/germinate into a new hypha and mycelium if it lands on a suitable substrate

sexual reproduction in rhizopus

this is they reproduce when conditions are unfavourable/adverse, such as very dry conditions or unsuitable temperatures:

1. two hyphae of opposite strains (+ & -) grown beside each other

2. swellings develop opposite to each other

3. haploid nuclei move into the swellings, forming progametangia

4. cross walls develop and gametangia are formed

5. haploid nuclei fuse, forming many diploid zygotes/nuclei

6. a tough wall forms around these nuclei, forming a zygospore (a thick wall structure)

7. zygospore is resistant to harsh conditions and can remain dormant

8. when conditions are suitable, the zygospore germinates by meiosis to form a new mycelium

9. a hypha grows out of the zygospore and produces a sporangium at the tip

advantages of zygospore formation

1. dispersal

2. survival of adverse conditions

yeast

• single-celled fungi

• round or oval

• cell walls are thin and made of chitin, cytoplasm is dense and contains many food storage vacuoles

• respire anaerobically to produce ethanol and carbon dioxide

glucose → 2 ethanol + 2 carbon dioxide

asexual reproduction in yeast

Budding:

1. nucleus of the parent cell divides by mitosis

2. a bud develops on the side of the cell and one nucleus plus organelles and cytoplasm enter the bud

3. this bud can separate from the parent to become a new yeast cell

4. in some cases, the bud does not detach and forms a colony

*new cells are genetically identical to the parent cell

phylogeny of fungi

these are defined by their sexual spores

basidiomycetes

• the mushroom is a reproductive structure of aggregated hyphae

• cap gills bear numerous basidia, where meiosis occurs in each basidium

• resultant basidiospore are released