MadSci Network: Microbiology

Re: Why is it that bread mold grows quicker in dark, wet and warm conditions?

Area: Microbiology
Posted By: Hurley Shepherd, Agricultural Research, USDA Southern Regional Center
Date: Wed May 7 14:33:58 1997
Area of science: Microbiology
ID: 860854840.Mi
Like all living things, molds (or fungi) need certain things in order to grow. 
Among these are food, water, and proper temperature.  Molds get their food by 
extracellular (outside the body) digestion, which is another interesting story 
which we won't go into today. 

Just like humans, molds are mostly water.  Water is used in some processes, but
all of the biochemical reactions necessary for life must take place in a watery
solution (the cytoplasm) of the cell.  (More on these reactions later.)  The 
water environment of the cell allows all of the components to move and mix 
properly.  It also prevents the mold from drying out, since water is critical
for normal survival. 

As for temperature, molds are different from us.  We control the temperature of 
our body so that all of the reactions have the proper conditions to go forward. 
We are called "warm blooded".  Molds cannot control their temperature, so they 
must grow and develop at whatever temperature their environment is.  In the 
watery solution inside mold cells are proteins called "enzymes" which direct 
the biochemical processes of the cells.  These enzymes work faster when the 
temperature is higher (as long as it doesn't get too hot!)  If you can do a 
growth experiment at several temperatures, you can probably find the 
temperature at which these enzymes work best for molds.  For humans, it's 98.6 
degrees Fahrenheit, which is our body temperature.  (When biochemists take 
these enzymes out of the body, they still work best at 98.6 degrees.)  For most 
molds, the optimal temperature is around 80 degrees.

Molds do not need light for normal growth, because they do not make their own 
food like plants do.  Molds are eating (using extracellular digestion) food 
that was at some point produced by plants.  At some point in their life, most 
molds do need light to start a different life stage, such as spore production.
The main reason for growing molds in the dark is that they might get dried out 
by sun or other bright lights. 

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