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FUNGI


Fungi include 2 groups of "moulds" (yeasts are included in one), and also a group to which most of the mushrooms, toadstools, bracket fungi and puffballs belong. The best known evidence of fungi - their spore-bearing structures or fruiting bodies - are visible to the naked eye, but the main part of the fungal structure can only be seen with the aid of a lens or low power microscope.
From a structural point of view, there are two basic forms available to fungi:
* most moulds and higher fungi are filamentous forms - adapted for penetrating solid substrates,
* yeasts are examples of rounded cells - more suited to growth in liquids

Mucor ("the pin-mould") is often taken as an example of a simple filamentous form. This is named after the pin-like structures (sporangia) which project upwards, containing spores.
If such a fungus spore lands on a suitable substrate (i.e. material capable of providing support and nutrition), it may germinate and produce a few filamentous strands (about 10 Ám in diameter) called hyphae (singular - hypha) which are the feeding parts of the fungus and they branch repeatedly. A plant body (thallus) or colony thus builds up consisting of a fine network of hyphae, collectively called mycelium, which runs over the surface of the substrate, penetrating it to some extent.

Structure of fungal mycelium

Labels should include the following - but also read the notes which follow.
mycelium of branched hyphae with nuclei
cytoplasm
surface membrane (cell membrane) and
cell wall


fungal hypha - pass your mouse over this one

There is no chlorophyll (colour whitish grey): photosynthesis is not carried out, and starch is not accumulated: oil or glycogen is the normal food store. The cell wall is made of chitin - a biological polymer containing N (and common with insects). A few fungi have cellulose cell walls. The structure is not divided into cells - it is called a coenocyte - and many minute nuclei are "loose" in the cytoplasm. Hyphae grow out radially to form a mat on a surface due to their need for oxygen, but the hyphae interweave in a complex fashion.

Spore production (sporulation)

Like many other fungi, Mucor produces spores asexually - there is no need for an encounter with another colony. An aerial hypha grows up vertically and the tip swells with cytoplasm containing many nuclei which form into many spores, each containing several nuclei. As the outer layer of the spore case (sporangium) breaks open, either dry spores are shot out and dispersed by air, or spores in a sticky substance are carried away by insects. Alternatively, Mucor can also produce sexual spores, as a result of one colony growing into close contact with another. Special hyphae swell and grow towards each other, then fuse together, and nuclei from each side fuse (fertilisation). The outer wall thickens so that a zygospore is produced, which is fairly long-lived and resistant to adverse conditions. After its dispersal, this will break open to produce a hypha which grows to support a sporangium, which once again releases spores.

Other examples of filamentous fungi mentioned in the syllabus:

Penicillium conidiophores
Penicillium is a genus of filamentous fungi, most of which produce blue-green spores on distinctly shaped slightly different structures. This has many similarities with Mucor, but different species have put put to various uses by Man. For example, Penicillium notatum and Penicillium chrysogenum produce penicillin which has been developed for medical use. See later notes.Other species of Penicillium are responsible for the distinctive taste of blue cheeses, e.g. Stilton, Danish Blue, Roquefort, etc.
Aspergillus is another genus of moulds, and one species is used by Man in the production of soy sauce.
Fusarium is yet another genus of moulds which has been developed into a food (mycoprotein).

Structure of yeast cells Saccharomyces spp.

Yeast is a fungus which has been used by Man for a long time in the brewing and baking processes. Unlike most fungi, yeast cells are round or oval in shape. As they grow, yeast cells extend sections called buds which enlarge and eventually become detached, to lead a separate existence.
Yeast cells with bud

Yeast growth requirements

Like many fungi and bacteria, yeast may be grown on a semi-solid agar medium, but also in liquid media, in which it grows best. See later notes. For a carbon source, yeast may use simple sugars, e.g. glucose, although some species can use other more complex carbohydrates, due to special enzymes they release. This glucose can be respired (used to provide energy) in a similar way to respiration in most other organisms:
C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O + ENERGY
glucose + oxygen --> carbon dioxide + water
(aerobic respiration) but if not enough oxygen is available, then the yeast can respire anaerobically, still using glucose for example:
C6H12O6 --> 2 C2H5OH + 2 CO2 + (LESS) ENERGY
ethanol
"alcohol"
(one form of anaerobic respiration - sometimes called alcoholic fermentation)
Note: Glucose and other sugars are found naturally in fruit juices and cereal products. The ethanol remains in the liquid medium, and most of the carbon dioxide escapes.
This conversion involving fermentation by yeast is the basis of 2 major industries, based on techniques with a long history:

-->
1) Brewing, where ethanol adds an undefinable extra value, acts as a preservative, and also extracts flavours from natural products, e.g.
fruit juices (EEC specifies grapes) -->wine
malted barley extract --> beer
apple juice --> cider
This is also used as a feedstock for distillation industries --> spirits, e.g. brandy, whiskey, gin etc.

2) Baking, where CO2 which is trapped in dough causes bread, etc, to "rise" and so produces a product with lighter texture when cooked.
animated fermentation lock
What functions are performed by a fermentation lock? (several answers) See animation above
Allows CO2 gas to escape (no buildup of pressure) - Prevents entry of oxygen (keeps it anaerobic) - Prevents entry of contaminants (bacteria/other microbes, fruit flies(??), etc) - wine would otherwise turn to vinegar/ ethanoic (acetic) acid - spoiling the wine!
Why is beer often brewed in large vats, practically almost open to the air, whereas a fermentation lock is used in the production of wine?
Beer making is fairly rapid process - yeast growth needs aerobic respiration - high alcohol content not expected in beer. Wine making is opposite - must be more efficient if slower.
What happens to the carbon dioxide produced in the brewing of beer, wine etc?
It bubbles off at the surface of the liquid, but it is collected, piped away and compressed into cylinders for use in industry, e.g. fire extinguishers, fizzy soft drinks and in pressurising keg beers in pubs.
What happens to the yeast in beer, wine etc, after the brewing process? It can be re-used: usually 4-5 times more is produced than put in at the start of the fermentation. But contamination limits re-use; fresh cultures are regularly used. Excess yeast is turned into yeast extracts like Marmite, and used as flavourings and animal feed (high in vitamin B).
Yeast also requires a source of nitrogen and small traces of other elements, although these can easily be provided in the form of ammonium salts which are quite cheap.
Why are these substances quite cheap?
They are inorganic - can be produced by chemical means from cheap resources (air), or dug up.

Unlike other "ingredients" used in brewing and baking, yeast is a living organism, so it must be treated carefully.
"Live" yeast, which can be purchased from bakers' suppliers, needs to be kept cool and used quickly, whereas prepared dried yeast - only a small proportion of which is alive - can easily be kept for a long time. This can be encouraged to grow when mixed with water and sugar and incubated for a few hours.
Different applications may call for different varieties, strains or even species of yeast, e.g. Saccharomyces cereviseae - many different strains for bread, beer and wine, Saccharomyces diastaticus and Saccharomyces carlsbergensis for different types of lager beer.

Growth rate of micro-organisms

Under ideal conditions, bacteria reproduce extremely rapidly, perhaps doubling in numbers every 20 or 30 minutes. Fungi grow more slowly, but yeast population growth shows a similar situation. This exponential growth could theoretically result in incredibly large numbers within a few hours or days.
In order to grow, micro-organisms mainly need a supply of suitable food, usually organic and inorganic, and an appropriate temperature. Unavailability of water or air, and variation of pH away from the optimum, may reduce microbial growth.
As they grow, micro-organisms produce waste products (sometimes extremely complex substances), which accumulate in the surrounding medium, and this may have the effect of poisoning them or at least reducing their activity.
Microbial growth is finally slowed down and practically stopped - due either to lack of nutrients or to the buildup of toxic substances.

bacterial growth curve



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