What is Mold? - Definition, Types & Causes

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  • 0:01 Definition of Mold
  • 1:51 Types of Mold
  • 2:27 Causes of Mold
  • 3:33 Benefits of Mold
  • 4:39 Lesson Summary
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Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Mold is a common type of fungus that thrives in moist, warm conditions. It is an important part of our ecosystem and yet can be a nuisance. Learn more about this organism and quiz yourself at the end.

Definition of Mold

Have you ever experienced that moment of horror when you open a sour cream container in your fridge, only to find a patch of gray fuzz resting contentedly on top? If you're like most people, your reactions to this discovery were probably unpleasant. In fact, they were probably along the lines of gross. Most of us don't appreciate finding mold in our refrigerators and bread bags. But what is this hairy organism? And why is it crucial to the survival of an ecosystem? In this lesson, we will learn more about mold and how it functions in our world.

Mold is a living organism that belongs to the kingdom Fungi. Fungi are unique in that although some appear plant-like, they are neither plant nor animal. Mold is heterotrophic, meaning it cannot make its own food like plants do. Mold must gain nutrients from other organic substances. Unlike animals, however, mold does not really 'eat' its food. It must absorb nutrition from other organisms. To do this, mold secretes enzymes that break down the food substance into smaller organic molecules that can then be absorbed. If you've ever grabbed a piece of moldy fruit, you may have felt the soft and mushy area that has essentially been digested.

Mold is composed of thread-like filaments called hyphae. The hyphae then form a conglomerate, which is called a mycelium. You can think of this as like a grassy lawn. Much like individual blades of grass make up a lawn, many hyphae make up a mycelium. This explains its 'hairy' appearance.

Although mold itself has no mobility, its hyphae can grow quite long. This is the primary mode used by the mold to spread more quickly to neighboring organisms. When you see a strawberry in a container that has been engulfed by mold, you can observe the hyphae reaching to the adjoining fruits.

Types of Mold

There are many different varieties of mold. One of the more common types is Rhizopus stolonifer, also known as black bread mold. And don't let the name fool you -- it also enjoys other foods such as fruits and vegetables.

Another well-known type of mold is Penicillium, from which the antibiotic Penicillin is derived. Penicillium can be found in a variety of places, including water-damaged structures as well as various foods.

Aspergillus is a mold which is known to have several carcinogenic varieties, and when found indoors, can cause severe respiratory problems.

Causes of Mold

Mold growth is successful under certain conditions. The four factors that are necessary for mold to thrive are moisture, food, correct temperature and spores.

Let's look at a scenario in which mold grows very well: the fruit bowl on a kitchen counter. In an orange, for example, there is significant moisture. There is food within the orange itself. The temperature in the fruit bowl is about room temperature, which is ideal. And the spores? This is the interesting part. Mold use spores to reproduce, and vast amounts are released into the air. We cannot see them, and therefore we are typically unaware that they are even there. However, when they land on that orange in the fruit bowl that has all the right conditions, the mold will happily germinate and spread.

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