EXPERIMENTAL: AN INVESTIGATION INTO SOME OF THE VARIABLES INVOLVED IN
THE INHIBITION OF FUNGAL GROWTH
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 !
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.
Causes 'downy mildew' on grapes; aqueous
copper(II) sulfate will act as a fungicide.
Cause athlete's foot and ringworm.
Biosynthesizes enzyme chymosin (rennin)
which is suitable for the manufacture of
Used to 'ripen' blue-veined cheeses.
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
(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
(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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