Fungus…

Fungi belong to a kingdom of organisms that can look a little like plants, but which do not photosynthesise. Like animals they are heterotrophs – that is they must get their food from organic compounds in their environment. They cannot fix carbon from the atmosphere like plants and algae can. They have a cellular structure with a cell wall, but one made from chitin rather than cellulose (found in plant cell walls).

There are thousands of different species of fungi but they fall into four main groups.

• Moulds
• Yeasts
• Mushrooms/puffballs and toadstools
• An odd assortment of organisms that includes ringworm and athlete’s foot

Chitin is made from a long chain of glucose molecules (similar to starch or cellulose) and is also the material that arthropod exoskeletons are made from. Some animals, like crabs, reinforce the chitin with calcium carbonate (limestone) to make it stronger.

Fungi are very important (along with bacteria) in food webs as decomposers of dead plants and animals. This allows the carbon and other elements to be returned to the environment and recycled. The carbon atoms in your body were once carbon in other living things that have now died – not a pleasant idea but there is a fixed number of carbon atoms on Earth so it is only fair to share!

Fungi can reproduce sexually or asexually. Asexual reproduction does not involve a male and a female so it is very efficient although it leads to less genetic diversity as the offspring are all clones of the parent (except when random mutations occur in the copying process). There are a number of different ways that asexual reproduction can occur.

Yeast uses ‘budding’, where a new yeast cell forms within an adult yeast cell and then pops out through the cell wall. Some moulds can reproduce by ‘fragmentation’ – bits can be broken off and grow into new individuals. Many plants can do this too, for example dandelions, which is why it is important to dig out all parts of the plant when you are weeding your garden. Any bits of dandelion root left in the soil can grow into new plants.

The most common form of asexual reproduction in fungi is the formation of ‘spores’. Different fungi produce different spore forming structures but the most commonly seen are the caps on mushrooms. The spores are dropped from the gills beneath. Spores are a little like seeds and if they land somewhere suitable they will send out roots and start to gather nutrients from their surroundings as they grow. What counts as suitable varies from species to species. If you leave a piece of bread in a sealed plastic bag for a week or so you will see mould begin to grow on it – the spores will have been there before you sealed the bag because they are all around us all the time.

Fungi can also reproduce sexually. There are many different mechanisms for this but they always involve the fusing of gametes to make a new cell – the zygote. These can then be distributed in spores. The advantage of sexual reproduction is that the organism’s genes are shuffled up with their partner’s, potentially giving the offspring a survival advantage. It is this potential for an advantage that drives the process of evolution.

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Questions…

1. What role does cellulose play in the diet of humans?
2. What process allows yeast to produce ethanol?
3. What are fingernails and hair made from? (It’s not chitin as I stupidly told my parents in the car on the way to the airport when they last visited!)
4. What process “fixes carbon from the atmosphere” in plants and certain bacteria?
5. What trophic level do plants occupy in a food chain?
6. Why was the toadstool invited to the party?
7. Why was the toadstool ejected from the party?