Fungi: Introduction, General Characteristics and affinities with plants and animals

 1.1 Introduction

            Dear students, today I am going to start a new unit of your syllabus. What I am going to discuss today is not at all new to us. I assure you that you have read about fungi in your senior secondary classes or you have heard the word mold, mushroom, yeast or penicillium, etc. Yes, today I am going to discuss it.

    Do you know that fungi are a group of lower plants or it is called lower cryptograms (non-flowering plants)? Although it is belonging to the plant kingdom, like other plants such as algae, pteridophytes, or spermatophytes; they cannot prepare their food. Green plants have chlorophyll or photosynthetic apparatus and through these apparatus, they can prepare their food with the help of CO₂, H₂O, and sunlight, such plants are called autotrophs. Perhaps, you might know that as fungi do not have chlorophyll, so they cannot prepare their foods like green plants and are called heterotrophs. As the fungi are belonging to lower plant groups, they cannot be differentiated into root, stem, and leaf. They have a definite pattern of body structure not familiar to other plant groups. Their mode of reproduction and nutrition are also different than other organisms. Today we will discuss mainly fungal characteristics, modes of reproduction, and nutrition. In your next unit will discuss the classification of fungi and their economic importance. Let us go to the next part of today’s discussion.

1.2.1 General Characteristics of Fungi

a.       They are devoid of chlorophyll and so cannot prepare their food,

b.      They grow in air, water, soil, and even outside and inside of living organisms. Fungi like to be in a moist and slightly acidic environment; they can grow with or without light or oxygen,

c.       The cell wall is composed of chitin (a fibrous substance consisting of polysaccharides, which is also the major constituent in the exoskeleton of arthropods).

d.      Fungi can be unicellular, multicellular, or dimorphic, which is when the fungi are unicellular or multicellular depending on environmental conditions.

e.       Some fungi are harmful to other organisms (disease-causing fungi) and some are beneficial for humans, the environment, and other organisms,

f.       Some fungi are drawn their nutrition from dead and decaying organic materials (Eg. Mucor, Agaricus, etc.) and some are drawn from other living organisms (Eg. Puccinia  graminis-trictici, etc.).

g.      Fungi in the morphological vegetative stage consist of a tangle of slender, thread-like hyphae, whereas the reproductive stage is usually more obvious,

h.      Fungi are saprophyte heterotrophs in that they use dead or decomposing organic matter as a source of carbon.

1.2.1 Affinities with Plants

             In the above discussion, we discuss that fungal cell has a cell wall. In all other plant groups, the cell wall is one of the distinguishing characteristics of plants. The reproduction processes of fungi are like other plants. A fungus reproduces by vegetative, asexual, and sexual methods. From the lower cryptograms to higher plants vegetative reproduction is the common method of reproduction. Again, a fungus produces several types of asexual spores. Asexual reproduction is the most common reproductive method of many plants. Higher-class fungi follow advanced reproductive methods which is most commonly seen in higher plants.

         From the above discussion, we have seen that many characteristics of fungi are similar to other plant groups.

      

1.2.2 Affinities with Animals

          Although fungi are placed in the plant kingdom, it has certain dissimilarity with plants and some affinities with animals. Like the animals, a fungus cannot prepare its food. They are dependent on other organisms or dead and decaying organic materials. Like animals, their mode of living is not directly dependent on sunlight. As with animal cells, the polysaccharide of storage is glycogen rather than the starch found in plants. Ergosterol is the steroid molecule in the cell membranes that replaces the cholesterol found in animal cell membranes. Like animals, fungi are heterotrophs: they use complex organic compounds as a source of carbon, rather than fix carbon dioxide from the atmosphere as do some bacteria and most plants. Besides, fungi do not fix nitrogen from the atmosphere. Like animals, they must obtain it from their diet. 

        From the above discussion, it is clear that certain characteristics of fungi have an affinity with animals.

      Check Your Progress

       a) Why fungi are called heterotrophic organisms?

       b) Define thallus?

       c) What are hyphae?

      d)  The fungal cell wall is composed of cellulose/ chitin/ lipids.

      e) Reserve food materials of fungi are starch/ glycogen/ protein