Molds and yeasts are different in many ways. For instance, molds contain hyphae, microscopic stems used in reproduction. Yeast, on the other hand, contains tiny buds which divide in half and multiply for reproduction. Molds and yeasts also contain different pH levels and have different temperature needs, but despite these variations, they also contain some specific similarities.
Mold and yeast are both fungal organisms. They share a similar cell structure, as yeast cells and mold cells both consist of chitin, a glucose derivative. Chitin is the one substance that unifies all fungi, including yeasts, molds, rusts and mushrooms. Since molds and yeasts undergo chemical reactions during their germination processes, they can cause foods to deteriorate or decompose. Some molds can even decompose porous household surfaces, like wood and plaster.
Mold and yeast both require moisture. Without a constant source of moisture, neither organism can germinate. As a result, both organisms are vulnerable to heat, as heat can eliminate moisture and neutralize fungal spores. Likewise, mold and yeast both require oxygen to grow properly. Mold cannot grow at all without oxygen, and yeast will suffer stunted growth in an oxygen-deprived environment. Since yeast simply refers to any unicellular phase in the life of a fungal spore, some yeasts (specifically dimorphic yeasts) can even evolve into mold spores.
Whereas plants need light in order to grow properly, molds and yeasts require no light at all. This is due to the fact that they grow by multiplying their spores, and therefore do not require photosynthesis. Molds and yeasts can grow in complete darkness. In fact, a lack of light can even benefit their growth, because without the heat emitted by light (such as sunlight), moisture can remain for longer periods of time.
Molds and yeasts are both capable of causing allergic reactions. Since they both come from airborne spores, they can pollute the airways of an allergy sufferer and cause symptoms that include rashes, itchiness, sneezing and swelling. Allergic reactions can also occur as a result of ingestion (for example, after eating a food containing mold or yeast). Some types of molds and yeasts can even cause severe infections. If you have no related allergies, you may suffer no symptoms at all after inhalation or ingestion.