- FUNGI (OR FUNGUSES)
STRUCTURE
| Primitive: relating to, denoting, or preserving the character of an early stage in the evolutionary or historical development of something. Binary: relating to, composed of, or involving two things. |
Fungi are primitive organisms that show very little specialisation. They undergo a sexual and an asexual stage in their life cycles. They usually exist as filaments called hyphae; many hyphae form what is termed the mycelium. Fungal cells are very similar to plant cells with a cell wall nucleus and mitochondria. However, fungi do not possess chloroplasts and cannot carry out photosynthesis. Most fungi are coenocytic which means that they contain many nuclei in one cell which may be an entire hyphae. Cross walls will occur at irregular intervals (see Figure 1 on the following page). Single-celled fungi that do not exist as hyphae are called yeasts.
REPRODUCTION AND GROWTH
The fungi hyphae grow vegetatively and rapidly penetrate the medium in which the fungus is growing. As was previously mentioned, fungal reproduction can be asexual or sexual, however, either reproduction is invariably carried out by spores. These spores can be produced sexually by the fusion of gametes, or asexually by simple mitotic division. When sexually produced spores germinate and undergo meiosis, producing haploid hyphae. Asexually produced spores merely undergo mitosis to produce haploid hyphae. Figure 2 on page 3 illustrates the reproduction in fungi.
- BACTERIA
STRUCTURE
Bacteria are single-celled organisms but are very different from any other type of cell. They are very much smaller, about the size of mitochondria. Bacteria do not possess mitochondria or chloroplasts. See figure 3 on page 4.
REPRODUCTION AND GROWTH
In the case of bacteria, the two terms mentioned above are synonymous. Growth is, in most cases, by a process of binary fission (continued on page 4).
Figure 1: A fungus showing the Mycelial habit with Coenocytic Hyphea.
Figure 2: The production of Fungal Spores, Production of asexually and Sexually formed spores.
Figure 3: Production of sexually produced sporesmin a particular type of fungus
Figure 4: A bacteria cell
This is where the cell divides by splitting in half to form two new cells. Some bacteria produce asexual endospores which are very resistant to severe conditions. An example of this is the bacteria that cause anthrax. Anthrax endospores can survive in the soil for up to 50 years.
- VIRUSES
Viruses are very simple organisms and are much smaller than most bacteria. They have to live and reproduce in other cells, i.e. plants, fungi, bacteria or animals, often causing symptoms of disease. A virus contains some of the chemical that constitutes chromosomes in other cells. i) DNA or ii) RNA. See Figure 4 on page 4.
Viruses are so primitive that they can almost be described as living chemicals and behave just like chemicals in many cases.
GROWTH AND REPRODUCTION
To reproduce, viruses need host cells. The virus attaches itself to a host cell and injects its DNA or RNA into it. The virus DNA then interferes with the host DNA to produce more virus DNA. The host cell then begins to make protein coats to surround the new bits of virus DNA. The host cell eventually ruptures, releasing many new viruses and the cycle continues – See Figure 5 on page 6.
- PARASITISM
This is the term used to describe the situation where one organism lives on or in another organism and will as a result debilitate the host organism. Diseases are mostly parasites of the blood or tissues. The parasite obtains all its food from the host organism.
- SYMBIOSIS
This is very similar to parasitism, however, both organisms actually benefit from the relationship. An example is symbiotic nitrogen fixation by a species of bacteria called Rhizobium. These bacteria live in special nodules in the roots of leguminous crops like soya beans. They take nitrogen from the air and convert it into a type of ‘fertilizer’ that the host plant can use. The net result is that the bacteria are given a home and the host plant gets the nitrogen fertilizer which it needs.
- MODES OF NUTRITION
AUTOTROPHIC
This is a term that describes an organism’s source of carbon. If the source is inorganic, that organism is said to be autotrophic. Such organisms take carbon dioxide in the air as their source of carbon. Plants are an example of this process.
| Debilitate: make very weak and infirm. DNA: Deoxyribonucleic Acid. RNA: Ribonucleic Acid. |
Figure 5: Viruses – Growth and Reproduction
HETEROTROPHIC
This type of nutrition occurs where the source of carbon is organic, i.e. in the form of ready-made carbohydrates and proteins.
Many organisms feed on ready-made food. Organisms such as many fungi that live on dead or decaying organic matter are said to be Saprophytic.
