Introduction Kingdom Fungi contains all the multi-cellular absorptive heterotrophs; i.e., those living organisms which obtain their nutrients by the extra- cellular, enzymic digestion of organisms and the subsequent absorption of soluble ions and molecules by diffusion.
The importance of fungi in recycling essential nutrient ions for the autotrophs is well-established; and, furthermore, may be of increasing importance because the precipitation of 'acid rain' has resulted in acidic environments and the release of soluble (metal) ions. Moreover, as this Table shows, fungi are significant organisms in everyday-life.
 Fungus or Fungi
 Yeasts used in the baking, brewing, and
 fuel industries; uni-cellular !
 Rhizopus stolonifer
 Usually known as 'black bread mould'.
 Cause 'rust' diseases on crops.
 Used in the manufacture of SCP; nutrient
 source is paper or cereal plant waste.
 Plasmopara viticola
 Causes 'downy mildew' on grapes; aqueous
 copper(II) sulfate will act as a fungicide.
 Cause athlete's foot and ringworm.
 Mucor mieleli
 Biosynthesizes enzyme chymosin (rennin)
 which is suitable for the manufacture of
 vegetarian cheeses.
 Penicillium roqueforti 
 Used to 'ripen' blue-veined cheeses.
 Penicillium notatum
 Biosynthesizes the antibiotic penicillin.
The 'anti-social behaviour' of Rhizopus stolonifer fungus hints at one 
important aspect of fungal growth: namely, food preservation.  Because 
fungi are typical living organisms with cellular metabolism controlled 
by enzymes, most methods of preserving food have focused on directly or 
indirectly inhibiting the activities of these enzymes.
Methods of direct inhibition include refrigeration and 'pickling' in an acidic medium. Thus, low temperatures result in substrate and enzyme particles having less kinetic energy, and so fewer have the required activation energy for successful collisions: whereas, aqueous hydrogen ions [i.e., H1+(aq)], which diffuse into cells across semi-permeable membranes, denature a variety of enzymes.
Methods of indirect inhibition, which focus on the removal of the water required for diffusion of substances in to and out of cells, include immersion in a preservative with a low water potential (e.g., brine or sugar syrup); here, water is removed from fungal cells by exo-osmosis across semi-permeable membranes.
However, as perhaps also hinted at previously, a broader knowledge of the variables involved in the growth and inhibition of fungi is clearly of wider importance ...
In this investigation, using bread as a substrate and (sealed) petri
dishes as safe containers, you are required to examine at least three
variables involved in the inhibition of fungal growth; at least two
of these must be quantitative, and at least one must be qualitative.
1.  A qualitative variable is one that is non-numerical (or purely 
descriptive); e.g., 
(dependent), the colour of the mycelium of hyphae [... black, brown, 
grey, red, white],
(independent), the type of potential inhibitor [... absent ('control'),
copper(II) sulfate, ethanoic acid, phenol, sodium chloride].
2. A quantitative discontinuous variable is one where the data are obtained by counting, and so are whole numbers; e.g., (dependent), the number of spore cases present after 168 hours [... 96, 85, 74, 47, 40], (independent), the number of drops of pond water added to the substrate [... 4, 3, 2, 1, 0].
3. A quantitative continuous variable is one where the data are obtained by measuring, and so can take any value within a continuous range; e.g., (dependent), the area of fungal growth after 168 hours [... 2.07, 1.59, 1.43, 1.42, 1.35 cm²], (independent), the concentration of aqueous copper(II) sulfate added to the substrate [... 1.00, 0.75, 0.50, 0.25, 0.00 mol dm-³ or ... 1.000, 0.100, 0.010, 0.001, 0.000 mol dm-³].
4. For each hypothesis, in order 'to isolate the (chosen) independent variable', it is essential that the values of all the other independent variables are measured and kept constant.
5. Apart from your notes and standard textbooks, sources of scientific knowledge include encyclopaedias and data bases on CD-ROM; it is good practice to include a bibliography in your write-up.
6. You are provided with bread substrates, stock solutions of a wide variety of substances, distilled water, petri dishes, and Sellotape. In addition, you will need to use - within reason - other apparatus you consider to be necessary.
7. The proposed plans of your investigation should be presented in detail; these plans may be modified as the investigation proceeds.
8. Apart from the standard use of safety glasses, and possibly gloves, two safety precautions are vitally important: first, the fully labelled petri dishes, once sealed, must not be opened during the investigation; and second, these dishes must be disposed of by incineration at the end of the investigation.
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