Fungi and Protists

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If you have ever seen mold growing on an old loaf of bread or eaten a mushroom, you have encountered a fungus.  Fungi (that’s the plural of fungus) are a group of organisms, or living things, that are all around us.

Fungi have an important job.  They help break down other material, so that living things are able to grow in soil.  This helps make nutritious foods for other organisms.  Fungi are needed for life!

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For many years, scientists thought that all fungi were plants.  Now they know that there are some very important different between these two groups of organisms.  One of the most important differences is that plants are autotrophic.  This means that they can make their own food, just by using the sunlight.  Fungi can’t do this.  They have to “eat” other living things in order to get the energy they need.  This is called being heterotrophic.

Another difference between plants and fungi is that fungi have cell walls like plants do, but their cell walls are made of chitin.  Chitin is a material containing nitrogen that is also found in the shells of animals including beetles and lobsters.

Finally, fungi do not have a vascular system, the system used to transport water and nutrients in plants, but do have hyphae, a structure you will learn about in the next section.

Although mold and mushrooms are easy to see, most fungi are a lot harder to see.  Some are so small they can only be seen with a microscope.

Others are big enough to see, but live in places that make them hard to find.  For example, some fungi live deep in the soil, in decaying logs, inside plants and animals, or even inside or on top of other fungi. A rotting log can be a good location for finding fungi

Fungi have a cell wall.  The cell wall acts as protection for the cells of the fungi from destruction.  Fungi also have hyphae.  These are thread-like structures which are connected to each other and bunched up.  Bunched up hyphae are called mycelium.  Mold on a piece of food or on a rotting piece of wood are really mycelia (that’s the plural of mycelium.

Mycelia absorb nutrients from other organisms they are living in or on, called hosts.  Fungi also have special structures for reproduction.  One example of this is a fruiting body.  Fruiting bodies produce spores.  These are the basic unit of reproduction in fungi, and will be discussed later in this section.

Every organism interacts with other organisms every day.  These interactions can be helpful, harmful, or have no effect at all.  Think about some of the interactions you have had with other organisms.  If you’ve ever been sick with strep throat, you and some bacteria had a harmful interaction. If you pet a dog while you were walking down the street, you had an interaction that probably had no effect at all.  If you sat in the shade under a tree when you were hot you had an interaction that was helpful to you.

Many fungi have relationships that are helpful to them.  These relationships, and any relationship in which at least one organism benefits, are called symbiotic relationships.  A true symbiotic relationship however, is very different that the relationship between you and the tree in the example above.  First, your interaction with the tree was very brief.  Second, although it’s always nice to relax under a tree, you didn’t actually need to sit under it.  In a true symbiotic relationship, the two organisms spend their entire lives together, and it is often very difficult, or even impossible, for one of the organisms to survive without the other.

We said earlier that in symbiosis one of the organisms benefits, but what about the other one?  Does it benefit too?  The answer to this very important question helps determine what type of symbiosis it is.  In parasitism, the other organism is harmed.  A fungi has parasitic relationship with the horse whose leg has an infection.

In commensalism, the other organism is unaffected, and in mutualism the other organism benefits as well.  The moss benefits from sunlight, while the tree is unaffected, an example of commensalism.

Fungi grow close to the roots of many plants.  The plant and the fungus help each other out by “feeding” each other.  The plant undergoes the photosynthesis, a process in which sunlight is changed to the sugars glucose and sucrose.

Fungi need these sugars to survive, and the plants provide this needed energy source.  (Remember that fungi are heterotrophs, so they can’t make the energy on their own.) In return for the food, the fungus provides minerals and water for the roots of the plant.  This is mutualistic symbiosis because each organism benefits from being around the other one, and provides something in return.

Another interesting example of mutualism are lichens.  Lichens are crusty, hard organisms that can be found on trees, walls, logs, and rocks.  What makes lichens so unique is that they are really two organisms in one.  Cells from an algae or bacteria live inside of a fungus.  Both organisms provide nutrients for the other.  Their relationship is mutualistic.

A final example of mutualism amongst fungi involves certain species of ants and termites.  The ants and termites grow fungi in underground “fungus gardens.”  The fungi get a place to live, and the insects get something as well.  When the termites or ants have eaten a large meal of wood or leaves, they eat fungus to help with the digestion of cellulose, a hard to digest protein found in plant material.

Not all symbiotic relationships involving fungi are mutualistic.  Sometimes a fungus benefits at the expense of another organism, making it a parasitic relationship.  Dutch elm disease is one example of such a relationship.  Trees are infected with a fungus, and lose the ability to gain water.  As a result of the lack of water, the tree quickly dies. In humans, fungi are responsible for diseases such as athlete’s foot and ringworm.  In both these diseases, fungi feed on the moist, outer layer of human skin.

We’ve been using mushrooms and mold as examples of fungi, so you might get the idea that fungi don’t do all that much activity, and certainly don’t hunt for their food.  The truth is that there are some fungi that are predators, hunting mainly nematodes.

Nematodes are small worms, and serve as a food source for some fungi.  In these fungi, the hyphae act almost like arms and legs.  The fungi make a circle with their hyphae with a lure inside of it, trapping the nematode in these sticky body structures.

There are two types of reproduction, and fungi engage in both of them.  In asexual reproduction, the first type, only one parent is required.  In one form of asexual reproduction, involving spores, thousands of spores are released from a puffball fungus in a giant “puff.”  These spores then create new fungi.

Budding is another type of asexual reproduction carried out by fungi, in which the fungus grows part of its body, which eventually breaks off.  This broken off piece eventually becomes a new organism.

A final form of asexual reproduction is mycelial fragmentation.  In this form, the mycelia split off, eventually forming a new organism.

The second form of reproduction, sexual reproduction, requires two parents.  It involves cells called diploid cells, that have all of the genetic material of the organism, and haploid cells, which have only half of this information.  Sperm and egg cells are examples of haploid cells in animals.

In animals and plants, two haploid cells come together to form a diploid cell, which eventually becomes the new organism.  But fungi do things very differently.  In fungi, two haploid cells fuse to two other haploid cells, making four haploid cells.  This structure then splits, producing an “adult” organism that has haploid cells.  This is not something that is seen in any other group of organisms.

Scientists have estimated that there are 1.5 million species of fungi, and these organisms live all over.  Most are found on land, although some do live in water.  Some fungi can even live in deserts.  No matter their environment, fungi act as decomposers.  This means that the fungi break down materials to make their environment better for other organisms to grow.

Humans use fungi for many purposes.  One of the most common uses is in food.  Mushrooms are eaten by many people on pizza or in salads. But yeast is used in the fermentation process to make beer, wine, and bread.

Fungi are also important in the production of some antibiotics, including penicillin and the chitin in cell walls has been said to have wound healing properties.

Introduction to Protists

If your kitchen is like most kitchens, you probably have cabinets for cups and pots and pans, along with drawers for silverware and cooking utensils.  You might also have a drawer you call the “junk drawer.”  The things in this drawer aren’t actually “junk.”  If they were, you’d throw them away.  Instead, things usually get put here because they just don’t fit anywhere else.

You might be surprised to learn that the system for classifying organisms has its own “junk drawer.”  It’s called the protist kingdom.  Its members, like the contents of your kitchen junk drawer, are important, but don’t fit nicely in one of the other kingdoms.

Broadly, protists can be classified as animal-like, plant-like, or fungus-like.  It is important to remember that being “animal-like” does not make a protist an animal.  Such and organism, like plant-like or fungus-like protists, are members of an entirely different group of living things.

Protists are so different, it sometimes seems like they have nothing common.  That’s not true.  For example, all protists are eukaryotic, meaning their cell or cells have nuclei.

Also, they prefer to live in aquatic environments.  Many protists are parasites.  Finally, most (although not all) protists are unicellular, meaning they are made of a single cell.  One common protist that is not unicellular is kelp.  In fact, kelp can be over 100 meters long (longer than a football field.)

You can see that kelp looks like a plant.  In fact, it conducts photosynthesis and has many cells like a plant too.  So, why don’t we just call it a plant?  One major reason is that the cells of kelp are not specialized.  Unlike plants or animals, where different cells have different jobs (think about a brain cell versus a blood cell versus a liver cell,) the cells of kelp do not specialize and form tissue.  This is another thing that is true of all protists.  If they are multi-cellular, the cells are not specialized.

Animal-like protists are called protozoa.  Like animals, protozoa can move on their own and are heterotrophic.  Some protozoa eat by wrapping their bodies around their prey, creating a “food storage compartment.”  Toxins are then produced which paralyze the prey, and food moved into the waiting protist.

Other protozoa have flagella, or tails, that assist in feeding.  The flagella whip back and forth creating a current that brings food to the protist.  Still other protozoa are parasites, and get nutrients from a host organism, harming the host in the process.

Animal-like protists can be classified, or placed into groups, based on how they move.  As mentioned above, some move with the aid of a flagellum (that’s the singular form of flagella.)

Others have many small tail-like structures called cilia which they move back and forth to get around.  Still others have what is known as a “fake foot” or pseudopodia.  These protozoa have a part of their cell stretch out, which pulls the rest of the organism along. The amoeba is a common example of this type of protozoan. Some protozoa don’t move at all. (Image here is the use of a false foot allow some protozoa to move.)

Plant-like protists are similar to plants because they are autotrophic, producing their own food through photosynthesis and releasing the oxygen needed for animals to survive.

These protists can be found in aquatic environments as well as in soil and on the outside of plants.

Like fungi, the fungus-like protists are heterotrophs with cell walls that produce by creating spores.  Most of these types of protists cannot moves.  For many years, scientists believed that slime molds were fungi; however they now see that there are enough differences between this organism and true fungi to classify it as a fungus-like protist.

Click here to get started with the experiments for this lesson!

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